JP7718682B2 - Non-destructive testing methods - Google Patents

Description

The present invention relates to a non-destructive testing method. More specifically, it relates to a non-destructive testing method that uses a magnetic flux leakage method to detect fractures in reinforcing bars, steel rods, steel wires, etc. installed within concrete structures.

The magnetic leakage flux method has long been used as a non-destructive testing method for detecting damaged areas in reinforcing bars, steel rods, steel wires, etc. (hereinafter referred to as reinforcing bars, etc.) installed within concrete structures. With this method, a magnet such as a permanent magnet is moved along the surface of the concrete to magnetize the reinforcing bars, etc., and the magnetic flux density leaking from the surface of the concrete is then measured. The presence or absence of damage to the reinforcing bars, etc. is then detected based on the results of the magnetic flux density measurement. Technologies for detecting damaged areas in reinforcing bars, etc. using this magnetic leakage flux method are disclosed in Patent Documents 1 to 6.

Patent No. 3734822 Patent No. 5946638 WO2020/027028 publication WO2020/027043 publication Patent No. 6305860 JP 2020-148565 A

The techniques disclosed in Patent Documents 1 to 6 can be used to detect damaged areas in rebars and other structures with a certain degree of accuracy, but there is a demand for methods that can determine damaged areas with greater accuracy.

In light of the above circumstances, the present invention aims to provide a non-destructive testing method that can accurately estimate the presence or absence of fractures in reinforcing bars and other structures within concrete structures.

<Fracture determination method>
The non-destructive testing method of the first invention is a method for measuring the magnetic flux density of an object to be tested that is embedded in a concrete structure and extends in a first direction outside the concrete structure along the first direction of the object to be tested, and for estimating the presence or absence of a fracture in the object to be tested based on fluctuations in the measured magnetic flux density, characterized in that the measured magnetic flux density is the magnetic flux density in a second direction that is perpendicular to the surface of the concrete structure and the first direction of the object to be tested, and it is estimated that a fracture has occurred in the object to be tested if the measured magnetic flux density satisfies predetermined conditions.

Given conditions:
When the distance from the measurement position in the second direction to the test object is distance Za, (1) the difference between the maximum value and the minimum value in the fluctuation curve along the first direction of the measured magnetic flux density in the second direction is included in the range of the difference ΔB(α, Za, Gap) between the maximum value and the minimum value in the fluctuation curve of the magnetic flux density value B(x, α, Za, Gap) obtained when the magnetic flux density correction coefficient α and the fracture length Gap are changed in equation (1).
(2) The difference in the positions in the first direction between the maximum value and the minimum value in the fluctuation curve along the first direction of the measured magnetic flux density in the second direction is included in the range of the difference ΔX(α, Za, Gap) between the positions in the first direction between the maximum value and the minimum value in the fluctuation curve of the magnetic flux density value B(x, α, Za, Gap) obtained when the magnetic flux density correction coefficient α and the fracture length Gap are changed in equation (1).

Formula (1)
△B (α, Za, Gap) = B (x1, α, Za, Gap) - B (x2, α, Za, Gap)
△X (α, Za, Gap) = x2 (α, Za, Gap) - x1 (α, Za, Gap)
B (x, α, Za, Gap) = Bp (x, Za, Gap) + α・x
Bp (x, Za, Gap) = (Br・S/4π)・(Za/((x+Gap/2)2+y2+Za2)3/2)−(Br・S/4π)・(Za/((x-Gap/2)2+y2+Za2)3/2)
αR≦α≦αL

Bp: theoretical formula of magnetic flux density from a fracture of an object to be inspected, measured outside the concrete structure along a first direction of the object to be inspected; x1: position in the first direction at which B(x, α, Za, Gap) has a maximum value; x2: position in the first direction at which B(x, α, Za, Gap) has a minimum value; α·x: correction term for correcting magnetic flux density along the first direction other than the magnetic flux density from the fracture of the object to be inspected; α: correction coefficient for magnetic flux density along the first direction; αR: minimum correction coefficient at which B(x) has a maximum value and a minimum value; αL: maximum correction coefficient at which B(x) has a maximum value and a minimum value; Br: residual magnetic flux density of the object to be inspected; S: cross-sectional area of the object to be inspected; Gap: fracture length; x: coordinate of the measurement position of the magnetic flux density in the first direction when the fracture position of the object to be inspected is taken as the origin; y: coordinate of the measurement position of the magnetic flux density in a third direction perpendicular to both the first direction and the second direction when the fracture position of the object to be inspected is taken as the origin.

The non-destructive testing method of the second invention is the same as that of the first invention, and includes measuring the magnetic flux density along a first direction of a test object by varying a distance Za and a fracture length Gap in a test structure having a test object with a fracture that is a member equivalent to the test object, and creating an approximate formula ΔBgF(0, Za, Gap) of the difference between the maximum value and the minimum value and an approximate formula ΔXgF(0, Za, Gap) of the difference in position in the first direction between the maximum value and the minimum value based on a fluctuation curve formed using the measured magnetic flux density, and calculating a difference ΔBg(α, Za, Gap) obtained by converting the difference ΔB(α, Za, Gap) between the maximum value and the minimum value in the fluctuation curve of the magnetic flux density value B(x) using formula (2) based on the approximate formula ΔBgF(0, Za, Gap) and the approximate formula ΔXgF(0, Za, Gap) The method is characterized in that the difference ΔX(α, Za, Gap) between the positions of the maximum and minimum values in the first direction of the fluctuation curve of (x) is converted by equation (3) to calculate a difference ΔXg(α, Za, Gap), and if the difference between the maximum and minimum values in the fluctuation curve along the first direction of the measured magnetic flux density in the second direction is within the range of the difference ΔBg(α, Za, Gap) obtained when the magnetic flux density correction coefficient α and the fracture length Gap are changed in equation (1), and if the difference between the positions of the maximum and minimum values in the first direction of the fluctuation curve along the first direction of the measured magnetic flux density in the second direction is within the range of the difference ΔXg(α, Za, Gap) obtained when the magnetic flux density correction coefficient α and the fracture length Gap are changed in equation (1), it is estimated that a fracture has occurred in the inspection object.

Formula (2)
ΔBg(α, Za, Gap) = (ΔBgF(0, Za, Gap)/ΔB(0, Za, Gap)・ΔB(α, Za, Gap)
ΔBgF(0, Za, Gap)=EXP((V1+V2)・Za2+V0)・(U・LN(Gap/Za)+1): Gap<Za
ΔBgF(0, Za, Gap)=EXP((V1+V2)・Za2+V0):Gap=Za
ΔBgF (0, Za, Gap) = EXP (V1・Za2+V2・Gap2+V0): Gap>Za

Formula (3)
ΔXg (α, Za, Gap) = ((ΔXgF (0, Za, Gap) - V3)・ΔX (α, Za, Gap)) / ΔX (0, Za, Gap) + V3
ΔXgF (0, Za, Gap) = V4・Za+V3: Gap≦Za
ΔXgF (0, Za, Gap) = V4・Gap+V3: Gap≧Za

V0 to V4: Coefficients obtained by experiment U: Coefficient obtained by experiment

The non-destructive testing method of the third invention is the same as that of the second invention, in which a cross reinforcing bar is provided so that a first reinforcing bar and a second reinforcing bar are perpendicular to each other with a gap between them and the test object, and the test object is arranged so that a fracture position overlaps with the first reinforcing bar when viewed parallel to the second reinforcing bar and from the normal direction of a plane parallel to the first and second reinforcing bars, the method comprising: measuring the magnetic flux density in the normal direction of the plane parallel to the first and second reinforcing bars along a first direction of the test object with the first reinforcing bar arranged between the test object and the first reinforcing bar; calculating the difference ΔB0 between the maximum value and the minimum value in the variation curve of the measured magnetic flux density and the difference ΔX0 between the positions of the maximum value and the minimum value in the first direction; and calculating the difference ΔBg(α, Za, Gap) based on the difference ΔB0 and the difference ΔX0 using the formula The method is characterized in that the difference ΔBm(α, Za, Gap) converted by (4) and the difference ΔXg(α, Za, Gap) converted by equation (5) are calculated to obtain a difference ΔXm(α, Za, Gap), and if the difference between the maximum value and the minimum value in the fluctuation curve of the measured magnetic flux density along the first direction is within the range of the difference ΔBm(α, Za, Gap) obtained when the magnetic flux density correction coefficient α and the fracture length Gap are changed in equation (1), and if the difference in the positions of the maximum value and the minimum value in the first direction in the fluctuation curve of the measured magnetic flux density along the first direction is within the range of the difference ΔXm(α, Za, Gap) obtained when the magnetic flux density correction coefficient α and the fracture length Gap are changed in equation (1), it is estimated that a fracture has occurred in the test object.

Formula (4)
ΔBm (α, Za, Gap) = V5・ΔBg (α, Za, Gap)
Formula (5)
ΔXm (α, Za, Gap) = V6・(ΔXg (α, Za, Gap) - V3) + V3

V5: A coefficient set so that the difference ΔB0 between the maximum value and the minimum value in the variation curve of the magnetic flux density measured along the first direction of the test object matches the difference ΔBm (0, Za, Gap) when α = 0. V6: A coefficient set so that the difference ΔX0 between the positions of the maximum value and the minimum value in the first direction in the variation curve of the magnetic flux density measured along the first direction of the test object matches the difference ΔXm (0, Za, Gap) when α = 0.

The non-destructive testing method of the fourth invention is characterized in that, in the third invention, the range of the difference ΔXm(α, Za, Gap) is expanded to the range of formula (6).

Formula (6)
ΔXmL (α, Za, Gap, θ) ≦ΔXm (α, Za, Gap) ≦ ΔXmR (α, Za, Gap, θ)
ΔXmL(α, Za, Gap, θ) = (ΔXm(0, Za, Gap)+(NL(θ)/2)・Dm(θ)-V3)・(ΔXm(α, Za, Gap)-V3)/(ΔXm(0, Za, Gap)-V3)+V3
ΔXmR(α, Za, Gap, θ) = (ΔXm(0, Za, Gap) + (NR(θ)/2)・Dm(θ)-V3)・(ΔXm(α, Za, Gap)-V3)/(ΔXm(0, Za, Gap)-V3)+V3

Dm (θ) = V6・D2/cos (θ): 0 degrees ≦ θ ≦ θ0
Dm(θ)=V6・D1/sin(θ): θ0≦θ≦90 degrees θ0=arctan(D1/D2)

D1: Distance between adjacent first rebars of crossing rebars D2: Distance between adjacent second rebars of crossing rebars θ: Angle between the first rebar of crossing rebars and the object to be inspected NL(θ), NR(θ): Integer values obtained by experiment

The non-destructive testing method of the fifth invention is characterized in that, in any of the first to fourth inventions, the magnetic flux density in the second direction is measured along the first direction of the test object at a plurality of positions at different distances from the test object in the third direction or the second direction, and if the magnetic flux densities measured at the plurality of measurement positions satisfy the specified condition, it is estimated that a fracture has occurred in the test object.

<Fracture determination method>
According to the first and second aspects of the present invention, even if there is an obstacle between the concrete structure and the inspection object, it is possible to improve the accuracy of detecting a fracture in the inspection object based on the measured magnetic flux density.
According to the third and fourth aspects of the present invention, it is possible to improve the accuracy of detecting fractures in an inspection object based on the measured magnetic flux density when the obstacle embedded in a concrete structure is a crossed reinforcing bar.
According to the fifth aspect of the present invention, it is possible to further increase the accuracy of detecting a fracture in an inspection object based on the measured magnetic flux density.

1A and 1B are schematic explanatory diagrams of a nondestructive inspection device 1 used in the nondestructive inspection method of the present embodiment, in which (A) is a side view and (B) is a plan view. 1 is a schematic block diagram of a nondestructive inspection device 1 used in the nondestructive inspection method of the present embodiment. (A) is a schematic diagram of a model in which two bar magnets are arranged coaxially with the north pole of one and the south pole of the other separated by a predetermined distance (corresponding to the fracture length Gap), and (B) to (D) are diagrams showing the variation curves of magnetic flux density in each axial direction along the axial direction of the bar magnet in the model of (A). 1 is an example of a graph showing a decision region 1 that satisfies formula (1). (A) is a graph obtained based on the fluctuation curve of magnetic flux density measured along the first direction of the test object when the distance Za is changed while keeping the fracture length Gap constant (Gap = 10 mm), (A-1) is an example of a graph showing the relationship between the distance Za and the difference ΔBgF (0, Za, Gap) between the maximum and minimum values in the fluctuation curve of magnetic flux density, and (A-2) is an example of a graph showing the relationship between the distance Za and the difference ΔXgF (0, Za, Gap) between the positions of the maximum and minimum values in the first direction in the fluctuation curve of magnetic flux density. (B) is a graph obtained based on the fluctuation curve of magnetic flux density measured along the first direction of the test object when the fracture length Gap is changed while keeping the distance Za constant (Za = 100 mm), (B-1) is an example of a graph showing the relationship between the difference ΔBgF (0, Za, Gap) between the maximum and minimum values in the fluctuation curve of magnetic flux density and the fracture length Gap, and (B-2) is an example of a graph showing the relationship between the difference ΔXgF (0, Za, Gap) between the positions of the maximum and minimum values in the first direction in the fluctuation curve of magnetic flux density and the fracture length Gap. 10 is an example of a graph showing a decision region 2 that satisfies equations (2) and (3). 10 is an example of a graph showing a decision region 3 that satisfies equations (4) and (5). (A) is an example of a graph showing the judgment region 4 that satisfies equation (6) created for a test structure TM with a distance Za (Za = 110 mm) and a breaking length Gap (Gap = 10 mm) and the test results for this test structure TM, and (B) is an example of a graph showing the judgment region 4 that satisfies equation (6), and is an example of a graph showing the judgment region 4 when the breaking length Gap is changed at a specific distance Za. 10A and 10B are examples of graphs showing the judgment area 4 obtained from the measurement values of two magnetic sensors at different positions in the second direction, where (A) is a graph showing the judgment area 4 obtained from the measurement values of magnetic sensor 6, which has a short distance Za in the second direction, and (B) is a graph showing the judgment area 4 obtained from the measurement values of magnetic sensor 7, which has a long distance Za. This is an example of a graph in which the judgment area 4 obtained from the measurement values of two magnetic sensors in the second direction is classified by the fracture length gap, where (A), (B), and (C) are graphs showing the judgment area 4 obtained from the measurement values of magnetic sensor 6 with a short distance Za in the second direction, and (D), (E), and (F) are graphs showing the judgment area 4 obtained from the measurement values of magnetic sensor 7 with a long distance Za in the second direction. 1A and 1B are schematic explanatory diagrams of a test structure TM, in which (A) is a schematic cross-sectional view taken along line AA in (B), and (B) is a schematic cross-sectional view taken along line BB in (A). 1A and 1B are schematic explanatory views of another non-destructive inspection device 1 used in the non-destructive inspection method of the present embodiment, in which (A) is a side view and (B) is a plan view.

The non-destructive testing method of this embodiment uses the magnetic flux leakage method to estimate fractures in test objects buried inside concrete structures. This method can improve the accuracy of detecting fractures in test objects that extend in one direction, such as reinforcing bars, steel rods, and steel wires.

The concrete structure in which the inspection target is buried and inspected using the nondestructive inspection method of this embodiment (hereinafter sometimes simply referred to as the concrete structure to be inspected) is not particularly limited. For example, bridge girders, piers, deck slabs, etc. for roads and railways can be cited as examples of concrete structures that can be inspected using the nondestructive inspection method of this embodiment.

Concrete structures that can be inspected using the non-destructive inspection method of this embodiment are not limited to those with flat surfaces, but also include those with cylindrical surfaces. For example, pillars with cylindrical surfaces can also be listed as concrete structures that can be inspected using the non-destructive inspection method of this embodiment.

The object to be inspected using the non-destructive inspection method of this embodiment is not particularly limited, and may be any object extending in one direction parallel to the surface of the concrete structure being inspected, where there is a possibility of damage such as fracture. For example, the object to be inspected may be a prestressing steel rod (high-strength steel with a diameter of 10 mm or more), a prestressing steel wire (high-strength steel wire with a diameter of 8 mm or less), a prestressing steel strand (a strand of prestressing steel wire), or a steel bar for reinforced concrete.

The following description will focus on the case where the surface of the concrete structure inspected by the non-destructive inspection method of this embodiment is flat.

In the following explanation, the surface of the concrete structure to be inspected includes the tangent plane of a structure with a cylindrical surface (curved concrete structure). In the case of a cylindrical curved concrete structure, the rebars to be inspected extend along the axial direction of the surface of the curved concrete structure (the central axis direction of the cylindrical surface). For this reason, the device used in the non-destructive inspection method of this embodiment measures the magnetic flux density while moving along the axial direction of the surface of the curved concrete structure. Furthermore, the magnetic sensor (described below) that measures the magnetic flux density measures the magnetic flux density while remaining parallel to the tangent plane of the surface of the curved concrete structure when the device moves along the axial direction of the surface of the curved concrete structure. Therefore, in the following explanation, when the surface of the concrete structure is used as a reference, this means that the tangent plane is used as a reference for a curved concrete structure.

<Concrete structure C>
First, a concrete structure C to be inspected by the non-destructive inspection method of this embodiment will be briefly described.

The concrete structure C to be inspected is, for example, a bridge girder, pier, deck slab, or other structure, with an inspection object P, such as rebar, steel rod, or steel wire, buried inside the structure (see Figure 1). Specifically, the concrete structure C to be inspected has a planar surface CF, and the inspection object P is embedded so that it extends in one direction parallel to this surface CF. The direction in which this inspection object P extends, i.e., the axial direction of the inspection object P (the left-right direction in Figure 1), will be referred to below as the first direction of the inspection object P.

In the following, the first direction of the inspection object P may be referred to as the X-axis direction, the direction parallel to the surface CF of the concrete structure C and perpendicular to the X-axis direction as the Y-axis direction, and the normal direction of the surface CF of the concrete structure C as the Z-axis direction (see Figure 1). The Y-axis direction is the third direction referred to in the claims, and the Z-axis direction is the second direction referred to in the claims.

Note that the concrete structure C being inspected may contain embedded components other than the object of inspection P, such as reinforcing bars and spacers.

Furthermore, the concept that the surface CF of the concrete structure C and the inspection object P are parallel not only refers to the case where the two are completely parallel over the entire inspection object P, but also includes the case where part of the inspection object P is slightly inclined relative to the surface CF of the concrete structure C being inspected.

<Non-destructive inspection device 1 of this embodiment>
Next, before describing the nondestructive inspection method of this embodiment, a nondestructive inspection device 1 used in the nondestructive inspection method of this embodiment will be described.

<Mobile object 2>
As shown in Figure 1, the non-destructive inspection device 1 has a moving body 2. This moving body 2 is structured so that it can move smoothly in one direction along the surface CF of the concrete structure C to be inspected. More specifically, the moving body 2 is structured so that it can move linearly along the direction of its central axis B (see Figure 1 (B)). Moreover, the moving body 2 has the function of being able to move while maintaining a constant distance H between the surface CF of the concrete structure C to be inspected and a magnetic flux measuring unit 5, which will be described later. Note that, hereinafter, the direction of the central axis B of the moving body 2 (i.e., the direction of movement of the moving body 2) may be referred to as the x-axis direction (see Figure 1).

Specifically, the moving body 2 comprises a main body 2a and a plurality of wheels 2r rotatably attached to the main body 2a (see Figure 1(A)). The wheels 2r are arranged so that their rotation axes are perpendicular to the central axis B of the moving body 2. Furthermore, the wheels 2r are arranged so that their rotation axes are parallel to the surface CF of the concrete structure C to be inspected when the moving body 2 is placed on the surface CF of the concrete structure C to be inspected. Note that below, the direction parallel to the rotation axes of the wheels 2r may be referred to as the y-axis direction, and the direction perpendicular to the x-axis and y-axis directions may be referred to as the z-axis direction (see Figure 1). The z-axis direction is parallel to the normal to the surface CF of the concrete structure C when the moving body 2 is placed on the surface CF of the concrete structure C.

The multiple wheels 2r are also arranged so that they can move while maintaining a constant distance H (distance in the Z-axis direction) between the magnetic flux measuring unit 5 (more specifically, the magnetic sensor 6) and the surface CF of the concrete structure C to be inspected. In other words, by rolling the multiple wheels 2r while they are in contact with the surface CF of the concrete structure C to be inspected, the mobile body 2 can be moved along the X-axis direction on the surface CF of the concrete structure C to be inspected while maintaining a constant distance H in the Z-axis direction between the magnetic flux measuring unit 5 and the surface CF of the concrete structure C to be inspected. The following description assumes that when the mobile body 2 is moved, its movement direction is along the X-axis direction.

The number of wheels 2r provided on the moving body 2 is not particularly limited. As mentioned above, as long as the moving body 2 can be moved in the X-axis direction while maintaining a constant distance in the Z-axis direction between the magnetic flux measuring unit 5 and the surface CF of the concrete structure C to be inspected, the number of wheels 2r may be three, four or more.

Furthermore, the structure of the mobile body 2 is not particularly limited as long as it can maintain a constant distance H in the Z-axis direction between the magnetic flux measurement unit 5 and the surface CF of the concrete structure C to be inspected, and can move smoothly along the X-axis direction on the surface CF of the concrete structure C to be inspected. In the above example, the mobile body 2 has wheels 2r, but the above function can also be achieved by laying a guide rail or the like parallel to the surface CF of the concrete structure C to be inspected along the X-axis direction and moving the mobile body 2 along this guide rail or the like. Note that maintaining a constant distance H in the Z-axis direction between the magnetic flux measurement unit 5 and the surface CF of the concrete structure C to be inspected also includes cases where the distance H between them varies by approximately 20 mm as the mobile body 2 moves along the surface CF of the concrete structure C.

Furthermore, if the surface CF of the concrete structure C to be inspected is uneven, an imaginary plane obtained by averaging the surface unevenness in the area corresponding to the magnetic flux measurement unit 5 (area of width W in Figure 1(B)) corresponds to the surface CF of the concrete structure C to be inspected. Therefore, if the surface CF of the concrete structure C to be inspected is uneven, the distance H in the Z-axis direction between the magnetic flux measurement unit 5 and the surface CF of the concrete structure C to be inspected is the distance from the imaginary plane in the Z-axis direction to the magnetic flux measurement unit 5. Note that, hereinafter, when referring to the surface CF of the concrete structure C to be inspected, this also includes an imaginary plane obtained by averaging the surface unevenness in the area corresponding to the magnetic flux measurement unit 5.

Furthermore, if the concrete structure to be inspected is a cylindrical pillar (equivalent to the curved concrete structure described above), the distance between the magnetic flux measurement unit 5 and the surface of the concrete structure will vary depending on the position in the direction perpendicular to the pillar's axial direction. However, the mobile body 2 of the non-destructive inspection device 1 has the function of being able to move while maintaining a constant distance between each magnetic sensor 6 of the magnetic flux measurement unit 5 and the surface CF of the concrete structure C when the mobile body 2 is moved in the pillar's axial direction (i.e., the axial direction of the object to be inspected, which corresponds to the first direction of the object to be inspected in the claims). For example, if a tangent plane is set that is tangent to the intersection of a plane passing through the central axis B of the mobile body 2 and the central axis of the pillar with the surface of the pillar, the mobile body 2 of the non-destructive inspection device 1 has the function of being able to move while maintaining a constant distance between this tangent plane and each magnetic sensor 6 of the magnetic flux measurement unit 5.

<Magnetic flux measuring unit 5>
The magnetic flux measuring unit 5 measures the magnetic flux density on the surface CF of the concrete structure C to be inspected, and is equipped with a magnetic sensor 6. The magnetic sensor 6 is capable of measuring the magnetic flux density at least in the z-axis direction, and is disposed on the base member 5a of the magnetic flux measuring unit 5 (see FIG. 1(B)). Note that the magnetic sensor 6 may be any known magnetic sensor capable of measuring the magnetic flux density at least in the z-axis direction, and various known magnetic sensors may be used. For example, a Hall element sensor, an MR sensor, an MI sensor, a TMR sensor, etc. may be used as the magnetic sensor 6.

While Figure 1(B) shows multiple magnetic sensors 6 (three in Figure 1(B)), the number of magnetic sensors 6 is not particularly limited. When multiple magnetic sensors 6 are provided, it is desirable to arrange the multiple magnetic sensors 6 on the base member 5a of the magnetic flux measurement unit 5 so that they are spaced apart along the y-axis direction (see Figure 1(B)). Specifically, it is preferable to arrange the magnetic sensors 6 so that they are spaced apart along the y-axis direction so that the magnetic flux density in the z-axis direction of the inspection target P can be measured when the moving object 2 is placed on the surface CF of the concrete structure C to be inspected. In particular, it is desirable to arrange the magnetic sensors 6 so that their z-axis measurement axes are located in the same plane parallel to both the z-axis and y-axis directions. Here, "the z-axis measurement axes of all magnetic sensors 6 being located in the same plane parallel to both the z-axis and y-axis directions" includes cases where the z-axis measurement axes of all magnetic sensors 6 are located completely in the same plane and cases where there is a slight misalignment between the z-axis measurement axes of each magnetic sensor 6. Misalignment between the measurement axes in the z-axis direction of each magnetic sensor 6 includes both cases where the measurement axes in the z-axis direction of each magnetic sensor 6 are misaligned in the x-axis direction by approximately ±20 mm or less, and cases where the measurement axes in the z-axis direction of each magnetic sensor 6 are slightly tilted from each other.

Furthermore, when multiple magnetic sensors 6 are installed, there are no particular limitations on the spacing between the magnetic sensors 6 or the width W of the area in which the magnetic sensors 6 are installed. The spacing and width of the area can be set appropriately depending on the depth and spacing at which the inspection object P is buried. For example, if the distance in the second direction (Z-axis direction) from the inspection object P to the surface CF of the concrete structure C to be inspected is approximately 100 to 200 mm, then the spacing between the magnetic sensors 6 in the y-axis direction should preferably be approximately 50 to 100 mm, and the width of the area in which the magnetic sensors 6 are installed should preferably be approximately 300 to 1000 mm.

Furthermore, when one magnetic sensor 6 is provided, it is desirable to position the magnetic sensor 6 so that it is located on the center line B of the mobile body 2. This makes it easier to determine whether the magnetic sensor 6 has broken. When multiple magnetic sensors 6 are provided, it is desirable to arrange the multiple magnetic sensors 6 symmetrically with respect to the center line B of the mobile body 2. In other words, it is desirable to arrange the multiple magnetic sensors 6 so that the same number of magnetic sensors 6 are located on both sides of the center line B of the mobile body 2. For example, if a plane that includes the center line B of the mobile body 2 and is perpendicular to the y-axis direction is defined as a reference plane SA, it is desirable to arrange the multiple magnetic sensors 6 so that they are symmetrical with respect to this reference plane SA. For example, when three magnetic sensors 6 are provided as shown in Figure 1 (B), it is desirable to arrange one magnetic sensor 6 at a position that intersects with the reference plane SA, and to arrange the other two magnetic sensors 6 one on each side of the reference plane SA.

<Control unit 4>
2, the control unit 4 has a position calculation function that calculates the position of the magnetic sensor 6 of the magnetic flux measurement unit 5, and an operation control function that controls the operation of the magnetic sensor 6 of the magnetic flux measurement unit 5. The control unit 4 also has a storage function that associates data on the measurement values of the magnetic flux density measured by the magnetic sensor 6 of the magnetic flux measurement unit 5 with the position of the magnetic sensor 6 of the magnetic flux measurement unit 5 calculated by the position calculation function and stores the associated data in a storage unit such as a memory. If the magnetic flux measurement unit 5 has multiple magnetic sensors 6, the control unit 4 has a storage function that associates the measurement values of each magnetic sensor 6 with the position of each magnetic sensor 6 and stores the associated data in the storage unit.

Furthermore, the control unit 4 has an analysis function that uses the data stored in the storage unit to create graphs and maps showing fluctuations in measured values of magnetic flux density.
Furthermore, if the control unit 4 is provided with a data communication function so that measurement data can be sent and received between an external personal computer or cloud computer, the memory function, analysis function, etc. of the control unit 4 can be made to work in conjunction with an external personal computer, etc.

<Position calculation function>
The control unit 4 has a position calculation function that calculates the amount of movement of the mobile object 2, in other words, the amount of movement of the magnetic sensor 6 of the magnetic flux measurement unit 5. This position calculation function calculates the movement distance of the magnetic sensor 6 of the magnetic flux measurement unit 5 from the initial position (the position where the mobile object 2 is placed on the surface CF of the concrete structure C to be inspected) to the current position, in other words, the current position of the magnetic sensor 6 of the magnetic flux measurement unit 5 based on the initial position. For example, if the position of the magnetic sensor 6 relative to a reference position of the mobile object 2 (e.g., the position of the wheel 2r) is stored in the storage unit, the position calculation function can calculate the movement distance of the magnetic sensor 6 in the X-axis direction and the position of the magnetic sensor 6 after the movement (position in the X-axis direction) based on the amount of movement of the mobile object 2 from the initial position in the x-axis direction and the position of the magnetic sensor 6 relative to the reference position, using the reference position at the initial position. When multiple magnetic sensors 6 are provided, the movement distance of each magnetic sensor 6 in the X-axis direction and the position of each magnetic sensor 6 after the movement (position in the X-axis direction) can be calculated based on the amount of movement of the moving body 2 in the X-axis direction and the relative position of each magnetic sensor 6 with respect to the reference position.

The method for calculating the movement distance of the magnetic sensor 6 of the magnetic flux measurement unit 5 in the X-axis direction from the initial position is not particularly limited. As shown in FIG. 2, the mobile unit 2 may be provided with a detector 4c that detects the movement distance from the initial position of the mobile unit 2. For example, the detector 4c may be an encoder that can detect the amount of rotation (rotation angle) of the wheel 2r. In this case, the position calculation function can calculate the movement distance of the magnetic sensor 6 of the magnetic flux measurement unit 5 in the X-axis direction from the initial position and the position (position in the X-axis direction) of the magnetic sensor 6 of the magnetic flux measurement unit 5 relative to the initial position after the movement, based on the rotation angle of the wheel 2r detected by the detector 4c and the diameter of the wheel 2r.

Furthermore, the detector 4c that detects the distance traveled from the initial position of the moving object 2 described above is not limited to a detector that detects the distance traveled in the X-axis direction based on the number of rotations of the wheel 2r, such as the encoder described above. An optical mouse, an accelerometer, or the like may also be used as the detector 4c that determines the distance traveled in the X-axis direction.

Furthermore, the method for determining the current position of the magnetic sensor 6 (in other words, the position where the magnetic sensor 6 is measuring the magnetic flux density) is not limited to the method described above. For example, a position marker may be provided on or near the concrete structure C, and the position of the magnetic flux measuring unit 5 relative to this position marker at the time the magnetic flux density is measured may be measured to determine the position where the magnetic flux measuring unit 5 measured the magnetic flux density (position in the X-axis direction).

<Operation control function>
The control unit 4 has an operation control function that controls the operation of the magnetic sensor 6 of the magnetic flux measurement unit 5. This operation control function has the functions of determining the timing at which the magnetic sensor 6 of the magnetic flux measurement unit 5 measures the magnetic flux density, causing the magnetic sensor 6 of the magnetic flux measurement unit 5 to measure the magnetic flux density at that timing, and transmitting the measured value to the storage function. For example, when a measurement start signal is input by an operation button or the like, the operation control function then causes the magnetic sensor 6 to measure the magnetic flux density at predetermined time intervals (e.g., every 10 milliseconds) and transmit the measured value to the storage function (hereinafter, the term "causing the magnetic sensor 6 to measure the magnetic flux density and transmitting the measured value to the storage function" may be used), or causes the magnetic sensor 6 to measure the magnetic flux density at predetermined distances based on the amount of movement of the mobile object 2 in the x-axis direction calculated by the position calculation function. Of course, the control unit 4 may also be configured to cause the magnetic sensor 6 to continuously measure the magnetic flux density and continuously transmit the measured value to the storage function when a measurement start signal is input by an operation button or the like. When a plurality of magnetic sensors 6 are provided, the control unit 4 may control all of the magnetic sensors 6 to perform measurements at the same timing, or may control each magnetic sensor 6 to perform measurements at an appropriate timing. For example, each magnetic sensor 6 may be controlled to perform measurements at the timing when the movement amount of each magnetic sensor reaches a predetermined movement amount.

In addition, if the position calculation function has the detector 4c described above, the signal emitted by the detector 4c may be supplied directly to the operation control function, and the operation control function may cause the magnetic sensor 6 to measure the magnetic flux density based on the signal emitted by the detector 4c. For example, if the detector 4c is an encoder, the operation control function may cause the magnetic sensor 6 to measure the magnetic flux density in accordance with the rotation angle of the wheel 2r detected by the detector 4c (i.e., when the wheel 2r rotates by a predetermined angle). The timing at which the magnetic sensor 6 starts measuring the magnetic flux density may also be determined based on the signal emitted by the detector 4c. For example, when the detector 4c detects that the wheel 2r has started to rotate from a non-rotating state and emits a signal, the magnetic sensor 6 may start measuring the magnetic flux density based on that signal.

Furthermore, the magnetic sensor 6 of the magnetic flux measurement unit 5 may be in a state where it constantly measures magnetic flux density, regardless of commands from the operation control function. In this case, the operation control function only needs to have the function of transmitting a signal from the magnetic sensor 6 to the memory function at the above-mentioned time intervals or after a predetermined movement distance after a measurement start signal is input.

<Memory function>
The storage function is a function for storing, in association with each other, the movement distance of the magnetic flux measurement unit 5 calculated by the position calculation function and the measurement value of the magnetic flux density measured by the magnetic sensor 6 of the magnetic flux measurement unit 5. Specifically, the storage function has a function for storing, in association with each other, the measurement value of the magnetic flux density measured by the magnetic sensor 6 of the magnetic flux measurement unit 5, the time at which the magnetic sensor 6 measured the magnetic flux density, and the position of the magnetic sensor 6 of the magnetic flux measurement unit 5 at that time (the movement amount of the mobile object 2 in the x-axis direction, the signal of the detector 4c, etc.).

In addition, if the magnetic flux measurement unit 5 has multiple magnetic sensors 6, it has a function of storing in the memory unit the measured magnetic flux density value measured by each magnetic sensor 6, the time at which each magnetic sensor 6 measured the magnetic flux density, and the position of each magnetic sensor 6 of the magnetic flux measurement unit 5 at that time (such as the amount of movement of the mobile object 2 in the x-axis direction or the signal from the detector 4c).

The memory unit also stores information about the position (e.g., initial position) at which the magnetic sensor 6 started measurement and the start time. If the magnetic flux measurement unit 5 has multiple magnetic sensors 6, information about the position at which each magnetic sensor 6 started measurement and the start time is stored in association with each magnetic sensor 6.

Therefore, by obtaining the information stored in the memory unit, it is possible to identify the position at which the magnetic flux density measured by the magnetic sensor 6 of the magnetic flux measurement unit 5 was measured. Note that if the magnetic flux measurement unit 5 has multiple magnetic sensors 6, it is possible to identify the position at which the magnetic flux density measured by each magnetic sensor 6 was measured.

<Analysis function>
The control unit 4 also has an analysis function for analyzing the data stored in the memory unit. This analysis function uses the data stored in the memory unit to determine whether or not the inspection object P has a fracture, and to estimate the distance from the fractured portion of the inspection object P to the magnetic sensor 6, i.e., the buried depth of the fractured portion of the inspection object P in the concrete structure C, the fracture length Gap, etc. Details of the analysis function will be described later.

The control unit 4 does not have to have the above-mentioned analysis function, and may have only the above-mentioned position calculation function, operation control function, and storage function. In this case, a function for supplying the data stored in the control unit 4 to an external device may be provided so that the data can be analyzed by an analysis device provided separately from the nondestructive inspection device 1. In this case, the data may be supplied from the control unit to the analysis device via a wired or wireless connection, or the data may be stored in a storage device such as a USB and supplied from the control unit to the analysis device.
Furthermore, the analysis function may perform preprocessing such as smoothing or flattening of the data as needed to improve the accuracy of the analysis. In other words, when forming a variation curve of magnetic flux density as described below, the analysis function may have a function for performing preprocessing such as smoothing or flattening of the data to improve the accuracy of the analysis based on the variation curve of magnetic flux density. The method for smoothing or flattening the data is not particularly limited, and well-known methods (for example, moving average method, filtering, slope correction using an approximate straight line, etc.) can be used.

<Non-destructive inspection method of this embodiment>
A method for inspecting for fractures in an inspection target P embedded in a concrete structure C to be inspected using the non-destructive inspection device 1 of this embodiment described above will be described. Note that the following description will be given on the assumption that the surface CF of the concrete structure C to be inspected is a horizontal plane. Of course, even if the surface CF of the concrete structure C to be inspected is a vertical plane or an inclined plane, inspection can be performed in a similar manner using the non-destructive inspection device 1 of this embodiment.

First, a magnet is moved along the surface of the concrete structure C to be inspected to magnetize the inspection object P embedded in the concrete structure C to be inspected. Note that the method for magnetizing the inspection object P is not particularly limited. In the following explanation, we will explain the case where the magnet is moved along the first direction of the inspection object P with the north and south poles of the magnet aligned in the first direction of the inspection object P (the X-axis direction in Figure 1). Furthermore, it is desirable to demagnetize the inspection object P after it has been magnetized. The method for demagnetizing the inspection object P is also not particularly limited.

After magnetizing the inspection object P, the movable body 2 of the non-destructive inspection device 1 is placed on the surface CF of the concrete structure C to be inspected (see Figure 1(A)). At this time, the movable body 2 is positioned so that the center line B of the movable body 2 is positioned vertically above the central axis of the inspection object P (see Figure 1(B)). Then, by moving the movable body 2 along the surface CF of the concrete structure C to be inspected and the first direction of the inspection object P, the magnetic sensor 6 can be moved along the first direction of the inspection object P while remaining positioned vertically above the inspection object P.

When multiple magnetic sensors 6 are provided, they can be positioned so that the central axis of the inspection object P is included in the reference plane SA (see Figure 1 (B)). When the mobile body 2 is moved along the surface CF of the concrete structure C to be inspected and the first direction of the inspection object P, the relative positions of the multiple magnetic sensors 6 and the inspection object P can be maintained constant while the mobile body 2 is moved.

The moving body 2 may also be positioned so that the magnetic sensor 6 is not vertically above the inspection object P. Furthermore, if multiple magnetic sensors 6 are provided, they may be positioned so that the central axis of the inspection object P is slightly offset from the reference plane SA. For example, in the Y-axis direction, the magnetic sensor 6 or the reference plane SA of the moving body 2 may be offset from the central axis of the inspection object P by approximately 0 to 10 mm.

Once the mobile object 2 is positioned, a measurement start signal is input using an operation button or the like, and the mobile object 2 is moved along the first direction (X-axis direction) of the inspection object P. The magnetic sensor 6 of the magnetic flux measurement unit 5 (or each magnetic sensor 6 if multiple magnetic sensors 6 are present) then measures the magnetic flux density along the movement path of the magnetic sensor 6. In other words, the magnetic flux density along the first direction of the inspection object P at the position of the magnetic sensor 6 is measured. The measured value of the magnetic flux density measured by the magnetic sensor 6 is then stored in the memory function, associated with the measurement position and measurement time.

The inspection is completed when the mobile object 2 has been moved to the area where the inspection object P is to be inspected, i.e., the distance over which the inspection object P is to be inspected.

Once the measurement is completed, the control unit 4 uses its analysis function to determine whether the test object P is broken or not.
Furthermore, if necessary, it is possible to estimate the distance from the surface of the concrete structure to the fractured portion of the inspection object, that is, the buried depth of the inspection object P where the fracture has occurred and the fracture length Gap.

<Regarding the magnetic sensor 6>
In the above example, the magnetic sensor 6 used was one capable of measuring magnetic flux density in at least the z-axis direction. However, the magnetic sensor 6 may be one capable of measuring magnetic flux density in three axial directions. In this case, since the magnetic flux density can be measured not only in the z-axis direction but also in the x-axis and y-axis directions, these measured values can be used for fracture detection, etc. When measuring the magnetic flux density in three axial directions, multiple magnetic sensors that measure magnetic flux density in one axial direction may be used to measure multiple axes. For example, multiple magnetic sensors that measure magnetic flux density in one axial direction may be arranged adjacent to each other to measure magnetic flux density in multiple axes.

<Detailed explanation of analysis functions>
As described above, the non-destructive inspection device 1 of this embodiment can determine whether or not there is a fracture in the inspection object P by utilizing the magnetic flux density measured by the magnetic sensor 6 of the magnetic flux measurement unit 5. In addition, the magnetic flux density measured by the magnetic sensor 6 of the magnetic flux measurement unit 5 may also be utilized to estimate the embedded depth of the inspection object P in the concrete structure C and the fracture length Gap.

The following describes how the analysis function of the control unit 4 determines whether or not the inspection object P has fractured, how it estimates the buried depth of the fractured portion of the inspection object P in the concrete structure C, and how it estimates the fracture length Gap.

<Method for determining whether or not the inspection object P is broken>
The analysis function calculates the difference ΔB between the maximum value and the minimum value (see FIG. 3D) and the difference ΔX between the positions of the maximum value and the minimum value in the first direction (the distance ΔX in the first direction between the position where the maximum value of the magnetic flux density occurs and the position of the minimum value) (see FIG. 3D) in a variation curve of the magnetic flux density along the first direction formed based on the measured magnetic flux density, and estimates that a break has occurred in the test object P if the difference ΔB and the difference ΔX satisfy predetermined conditions.

The specified conditions are the following two conditions, and if both conditions are met, it is possible to infer that a fracture has occurred in the object of inspection P.

When the distance from the measurement position (i.e., the magnetic sensor 6) in the second direction (Z-axis direction) to the inspection object P is distance Za (the distance from the magnetic sensor 6 to the central axis of the inspection object P, see FIG. 1A ),
(1) The difference ΔB between the maximum value and the minimum value in the fluctuation curve of the measured magnetic flux density along the first direction is included in the range of the difference ΔB(x, α, Za, Gap) between the maximum value and the minimum value in the fluctuation curve of the magnetic flux density value B(α, Za, Gap) obtained when the magnetic flux density correction coefficient α and the fracture length Gap are changed in equation (1).
(2) The difference ΔX between the positions of the maximum value and the minimum value in the first direction on the fluctuation curve of the measured magnetic flux density along the first direction is included in the range of the difference ΔX(α, Za, Gap) between the positions of the maximum value and the minimum value in the first direction on the fluctuation curve of the magnetic flux density value B(x, α, Za, Gap) obtained when the magnetic flux density correction coefficient α and the fracture length Gap are changed in equation (1).

Formula (1)
△B (α, Za, Gap) = B (x1, α, Za, Gap) - B (x2, α, Za, Gap)
△X (α, Za, Gap) = x2 (α, Za, Gap) - x1 (α, Za, Gap)
B (x, α, Za, Gap) = Bp (x, Za, Gap) + α・x
Bp (x, Za, Gap) = (Br・S/4π)・(Za/((x+Gap/2) ² +y ² +Za ² ) ^3/2 )−(Br・S/4π)・(Za/((x-Gap/2) ² +y ² +Za ² ) ^3/2 )
αR≦α≦αL

Bp: theoretical formula of magnetic flux density from a fracture of an object to be inspected, measured outside the concrete structure along a first direction of the object to be inspected; x1: position in the first direction at which B(x, α, Za, Gap) has a maximum value; x2: position in the first direction at which B(x, α, Za, Gap) has a minimum value; α·x: correction term for correcting magnetic flux density along the first direction other than the magnetic flux density from the fracture of the object to be inspected; α: correction coefficient for magnetic flux density along the first direction; αR: minimum correction coefficient at which B(x) has a maximum value and a minimum value; αL: maximum correction coefficient at which B(x) has a maximum value and a minimum value; Br: residual magnetic flux density of the object to be inspected; S: cross-sectional area of the object to be inspected; Gap: fracture length; x: coordinate of the measurement position of the magnetic flux density in the first direction when the fracture position of the object to be inspected is taken as the origin; y: coordinate of the measurement position of the magnetic flux density in a third direction perpendicular to both the first direction and the second direction when the fracture position of the object to be inspected is taken as the origin.

Note that x and y correspond to the distance from the fractured portion of the inspection object P to the magnetic sensor 6 in the first and third directions, and y = 0 when the magnetic sensor 6 is positioned vertically above the inspection object P.
Furthermore, the origin of the fractured portion of the inspection object P means a position midway between one edge and the other edge of the fractured portion of the inspection object P in the first direction.

When creating a graph with the difference ΔX (α, Za, Gap) on the horizontal axis and the difference ΔB (α, Za, Gap) on the vertical axis, the range that satisfies this formula (1) is the range enclosed by the lines (hereinafter sometimes referred to as judgment area 1) (see Figure 4). In other words, if the difference ΔB and difference ΔX obtained from the measured magnetic flux density are plotted on a graph, and if that point is within the judgment area (point A), it can be assumed that a break has occurred in the test object P, and if that point is outside the judgment area (point B), it can be assumed that no break has occurred in the test object P (see Figure 4).

For example, let us assume that the magnetization strength of the inspection object P is Br = 1.36 T (tesla), the cross-sectional area of the inspection object P is S = 256 mm ² (16 mm square), the distance from the inspection object P to the measurement position is Za = 110 mm, αR = -2000 μT/mm, and αL = 2000 μT/mm. Under these conditions, when the gap is changed from 1 to 400 mm and α is changed within the above range, the judgment region (judgment region 1) obtained from equation (1) will be the range shown in FIG.

Note that the above formula B(x, α, Za, Gap) is in the form of the above formula Bp(x, Za, Gap) plus a correction value (α x), for the following reasons:

First, the above formula Bp(x, Za, Gap) is a theoretical formula for calculating the magnetic flux density formed based on a model (see Figure 3(A)) in which two bar magnets are arranged coaxially with the north pole of one magnet and the south pole of the other magnet spaced a predetermined distance apart (corresponding to the fracture length Gap). In other words, this formula calculates the magnetic flux density detected by the magnetic sensor 6 by considering only the magnetic field formed by the magnetized test object P.

However, in the concrete structure C in which the inspection object P is buried, there is a high possibility that objects that cause disturbances in the inspection, such as crossed rebars, are buried within the concrete structure C (see Figure 1(A)). In other words, the magnetic field generated by magnetized crossed rebars and the like is likely to affect the magnetic flux density detected by the magnetic sensor 6. As a result, the magnetic flux density measured by the magnetic sensor 6 is affected by the magnetic field of the objects that cause disturbances, resulting in a difference between the theoretical magnetic flux density calculated using the above formula Bp(x, Za, Gap) and the actually measured magnetic flux density.

Therefore, in the above Bp(x, Za, Gap), a correction value (α x) is added to the theoretical magnetic flux density calculated using the above formula Bp(x, Za, Gap) to reduce the difference between the measured magnetic flux density and the theoretical magnetic flux density, thereby improving the accuracy of estimating the fracture of the inspection object P under the above specified conditions.

The reason for adopting α・x as the correction value is that, although the background magnetic flux density, which is a disturbance factor in the measured magnetic flux density, can take various functional forms, it was considered simple and appropriate to approximate the background magnetic flux density with a linear equation (α・x + β), assuming that the background magnetic field is continuous near the fracture site and that smoothing processes such as moving averages are performed as pre-processing of the measurement data. Furthermore, as for β, the constant term in the linear equation, the difference △B, which is the evaluation index, is the difference between the maximum and minimum values in the fluctuation curve, there is no loss of generality even if β = 0.

The distance Za (see FIG. 1A) used for the above Bp(x, Za, Gap) can be the sum of the cover depth ZB (see FIG. 1A) of the inspection object P obtained from the design drawings or the like of the concrete structure C to be inspected, the distance from the surface CF of the concrete structure C to the magnetic sensor 6 when the non-destructive inspection device 1 is placed on the surface CF of the concrete structure C (distance H in FIG. 1A), and the radius of the inspection object P (or the distance from the top surface to the middle position in the vertical direction in FIG. 1A).
In addition, the cross-sectional area S of the inspection object P can be a value estimated from the design drawings of the concrete structure C to be inspected.
In addition, the distance Za and the cross-sectional area S of the inspection object P may be values measured or estimated using various rebar inspection methods, instead of values estimated from design drawings of the concrete structure C, etc.

The residual magnetic flux density Br of the inspection object P can be a value obtained from an experiment using a test structure in which a test object (preferably made of the same material as the inspection object P) is embedded in concrete, the test object being a component equivalent to the inspection object. In other words, the test object can be embedded in concrete to the same depth as the inspection object P in the concrete structure C, and the test object can be magnetized and magnetized using the same method as in the inspection. Then, the magnetic flux density obtained by measuring the test object with the magnetic sensor 6 of the nondestructive inspection device 1 can be used to calculate the residual magnetic flux density Br of the inspection object P using the above formula Bp(x, Za, Gap). In this case, the value obtained by multiplying the residual magnetic flux density Br of the inspection object P by the cross-sectional area S of the inspection object P can be calculated using the above formula Bp(x, Za, Gap).

Furthermore, since the magnetic permeability of dry concrete is generally nearly the same as that of air, the test structure does not necessarily have to be a structure in which the test object is buried in concrete; a pseudo-concrete-embedded structure can also be used as the test structure. Examples of pseudo-concrete-embedded structures include test structures in which the test object is simply installed to have a predetermined structure or shape, i.e., a structure in which the test object is installed in an exposed state, or a structure in which the test object is installed to have a predetermined structure or shape and a member (e.g., a plate-shaped member) is provided to cover the test object. In the case of a structure in which a member is provided to cover the test object, the surface of the member covering the test object corresponds to the surface of the concrete in a test structure in which the test object is embedded in concrete. Hereinafter, the surface of the concrete in the test structure and the surface of the member covering the test object in the test structure may be referred to as the surface of the test structure.

In the following, the term "test structure" includes both structures in which test objects or other structures (for example, crossed rebars, etc.) are embedded in concrete, and pseudo-concrete-embedded structures that contain test objects or other structures. In the following, "between the test object and the surface of the concrete" in a test structure in which the test object is embedded in concrete corresponds to "between the test object and the magnetic sensor 6 of the non-destructive testing device 1" in a pseudo-concrete-embedded structure in which the test object is exposed, and corresponds to "between the test object and the member covering the test object" in a pseudo-concrete-embedded structure that contains a member covering the test object.

<Correction based on test results from test structure (1)>
In the above example, the accuracy of estimating fracture of the inspection object P was improved by introducing a correction value α·x into the magnetic flux density obtained by the theoretical formula Bp(x, Za, Gap) to set up a judgment region 1 that takes disturbance into consideration. In order to further improve the accuracy of estimating fracture of the inspection object P, it is desirable to set up a judgment region in which the results obtained from a test using a test structure are reflected in formula (1).

For example, multiple test structures are created with different distances Za (i.e., cover depth ZB) and fracture lengths Gap, and the magnetic flux density along the first direction of the test object (i.e., the axial direction of the test object) is measured for each test structure. Note that no magnetic material other than the test object is embedded (or installed) in the test structure. In other words, this corresponds to the case where α = 0 in ΔB(α, Za, Gap).

After measuring the magnetic flux density for each test structure with varying distance Za and fracture length Gap, a magnetic flux density fluctuation curve is created based on the obtained magnetic flux density. The approximate formulas ΔBgF(0, Za, Gap) and ΔXgF(0, Za, Gap) are then created using the difference between the maximum and minimum values on this magnetic flux density fluctuation curve and the difference in position in the first direction between the maximum and minimum values on the magnetic flux density fluctuation curve. Using the approximate formulas ΔBgF(0,Za,Gap) and XgF(0,Za,Gap), the difference ΔB(α,Za,Gap) and the difference ΔX(α,Za,Gap) are converted to Δdifference Bg(α,Za,Gap) and the difference ΔXg(α,Za,Gap), and judgment region 2 is formed using these differences ΔBg(α,Za,Gap) and ΔXg(α,Za,Gap). Since this judgment region 2 reflects the test results of the test structure, it can be used as a judgment region (judgment region 2) that can more appropriately determine fracture in the concrete structure C in which the actual inspection target P is buried.

The conversion of the differences ΔB(α, Za, Gap) and ΔX(α, Za, Gap) to the differences ΔBg(α, Za, Gap) and ΔXg(α, Za, Gap) using the above-mentioned approximate formulas ΔBgF(0, Za, Gap) and ΔXgF(0, Za, Gap) can be performed using the following formulas (2) and (3), respectively.

Formula (2)
ΔBg(α, Za, Gap) = (ΔBgF(0, Za, Gap)/ΔB(0, Za, Gap)・ΔB(α, Za, Gap)
ΔBgF(0, Za, Gap)=EXP((V1+V2)・Za ² +V0)・(U・LN(Gap/Za)+1): Gap<Za
ΔBgF(0, Za, Gap)=EXP((V1+V2)・Za ² +V0): Gap=Za
ΔBgF (0, Za, Gap) = EXP (V1・Za ² +V2・Gap ² +V0): Gap>Za

Formula (3)
ΔXg (α, Za, Gap) = ((ΔXgF (0, Za, Gap) - V3)・ΔX (α, Za, Gap)) / ΔX (0, Za, Gap) + V3
ΔXgF (0, Za, Gap) = V4・Za+V3: Gap≦Za
ΔXgF (0, Za, Gap) = V4・Gap+V3: Gap≧Za

V0 to V4: Coefficients obtained by experiment U: Coefficient obtained by experiment

For example, for the conditions used to determine the judgment region 2 described above, if the difference ΔB (α, Za, Gap) and the difference ΔX (α, Za, Gap) are converted to the difference ΔBg (α, Za, Gap) and the difference ΔXg (α, Za, Gap), the judgment region (judgment region 2) will be the range shown in Figure 6.

The approximate formulas ΔBgF(0, Za, Gap) and ΔXgF(0, Za, Gap) can be obtained in various ways. For example, as shown in Figure 5, the difference between the maximum and minimum values and the difference in position in the first direction between the maximum and minimum values can be calculated from the magnetic flux density variation curve along the first direction of the test object obtained for each test structure. The relationship between the measurement results of each test structure and the distance Za and the fracture length Gap can then be graphed to obtain an approximate formula, from which the above-mentioned approximate formulas ΔBgF(0, Za, Gap) and ΔXgF(0, Za, Gap) can be obtained.

Here, the difference ΔBg(0,Za,Gap) is converted to ΔBg(α,Za,Gap) using a simple proportional formula, while the difference ΔXg(0,Za,Gap) is converted to the difference ΔX(α,Za,Gap) using a proportional formula that includes V3. This is because while the difference ΔX(0,Za,Gap) in equation (1) is proportional to Za and Gap, the corresponding difference ΔXg(0,Za,Gap) is affected by the deviation of the magnetized position from the fracture point in the first direction of the test object, and therefore the difference ΔXg(0,Za,Gap) (ΔXg(0,Za,Gap) - V3) is proportional to Za and Gap. In other words, V3 can be considered the deviation of the magnetized position of the test object in the first direction from the fracture point, so the difference ΔXg(α, Za, Gap) corresponds to (ΔXg(α, Za, Gap) - V3).

<Correction based on test results from test structure (2)>
In the correction (1) based on the test results of the test structure described above, it is assumed that no magnetic material other than the test object is embedded in the test structure. In the actual concrete structure C, magnetic material other than the test object is embedded, so in order to further improve the fracture estimation accuracy of the inspection object P, it is desirable to reflect the results obtained from the test structure in which magnetic material other than the test object is embedded in formula (1).

For example, as shown in Figure 11, a test structure TM is used in which a test object Q is embedded in concrete, with crossed rebars CR provided between the surface of the test structure TM and the test object Q (or between the test object Q and the magnetic sensor 6 of the non-destructive inspection device 1). In other words, a test structure TM is used in which crossed rebars CR, in which multiple first rebars R1 and multiple second rebars R2 are arranged so that they are perpendicular to each other, are provided between the surface of the test structure TM and the test object Q. Furthermore, the test structure TM is arranged so that the test object Q is parallel to the second rebars R2 and the fracture position GA overlaps with the first rebar R1 in the second direction.

In such a test structure TM, the test object Q is magnetized and the magnetic flux density along the first direction is measured. Then, a magnetic flux density fluctuation curve is created based on the obtained magnetic flux density, and the difference ΔB0 between the maximum and minimum values on this magnetic flux density fluctuation curve and the difference ΔX0 between the positions of the maximum and minimum values on the magnetic flux density fluctuation curve in the first direction are calculated.

The difference ΔBg(0, Za, Gap) is calculated for this test structure TM using the method described above, and V5 is determined so that the value of the difference ΔBg(0, Za, Gap) matches the value of the difference ΔB0 (i.e., V5 such that ΔB0 = V5 · ΔBg(0, Za, Gap)). Then, using equation (4) using this V5, the difference ΔBg(α, Za, Gap) is converted to the difference ΔBm(α, Za, Gap).
Similarly, the difference ΔXg(0, Za, Gap) is calculated for this test structure TM using the method described above, and V6 is found where the value of the difference ΔXg(0, Za, Gap) and the value of the difference ΔX0 match (that is, V6 where ΔXg=V6·(ΔXg(0, Za, Gap)−V3)+V3). Then, using equation (5) using this V6, the difference ΔXg(α, Za, Gap) is converted to the difference ΔXm(α, Za, Gap).

Formula (4)
ΔBm (α, Za, Gap) = V5・ΔBg (α, Za, Gap)
Formula (5)
ΔXm (α, Za, Gap) = V6・(ΔXg (α, Za, Gap) - V3) + V3

Once these equations (4) and (5) are formed, the difference ΔBg(α, Za, Gap) is converted using equation (4) to obtain the difference ΔBm(α, Za, Gap), and the difference ΔXg(α, Za, Gap) is converted using equation (5) to obtain the difference ΔXm(α, Za, Gap). A judgment region is then formed using these differences ΔBm(α, Za, Gap) and ΔXm(α, Za, Gap). This judgment region then reflects the test results of the test structure TM, and can be used as a judgment region (judgment region 3) suitable for a concrete structure C in which cross rebars CR are buried between the surface CF of the concrete structure C and the inspection target P.

For example, in the above-mentioned test structure TM, if the cover depth of the crossing rebars CR is 50 mm, the spacing D1 of the first rebars R1 is 125 mm, the spacing D2 of the second rebars R2 is 250 mm, the length of the test object Q is 2 m, the gap is 10 mm, and the fracture position GA is the middle position in the axial direction of the test object Q, V5 and V6 are determined using the above method.
In the calculation using equation (1), the following are used: magnetization strength of test object Q Br = 1.36 T, cross-sectional area of inspection object P S = 256 mm ² (16 mm square), distance from inspection object P to the measurement position Za = 110 mm, αR = -2000 μT/mm, αL = 2000 μT/mm, and Gap = 10 mm.

In this case, the difference ΔBg (α = 0, Za = 110 mm, Gap = 10 mm) is 740 μT, and the difference ΔXg (α = 0, Za = 110 mm, Gap = 10 mm) (V3 = 150 mm) is 250 mm. On the other hand, the difference ΔB0 = 260 μT and the difference ΔX0 = 220 mm, so under these conditions, V5 = 0.35 and V6 = 0.70.

<Verification of V5 and V6>
Here, the appropriateness of V5 and V6 was verified using a verification test structure in which the position of test object Q was changed as the test structure. In the verification test structure, test object Q was placed parallel to and directly below second rebar R2. Under this condition, the magnetic fields of test object Q and second rebar R2 overlap, and therefore, among the arrangements of test object Q and crossed rebar CR, the difference between the maximum and minimum values of the fluctuation waveform of magnetic flux density along the first direction of test object Q is the smallest.

The differences ΔBL and ΔXL obtained from the fluctuation waveforms of the magnetic flux density along the first direction of test object Q of this verification test structure were compared with the differences ΔBm (α = αL, Za = 110 mm, Gap = 10 mm) and ΔXm (α = αL, Za = 110 mm, Gap = 10 mm) obtained using V5 = 0.35 and V6 = 0.70. The results showed that the differences ΔBL and ΔBm (α = αL, Za = 110 mm, Gap = 10 mm) were nearly identical, and the differences ΔXL and ΔXm (α = αL, Za = 110 mm, Gap = 10 mm) were also nearly identical.

In other words, it was confirmed that by using V5 and V6 obtained using the above method, an appropriate judgment region (judgment region 3) that takes into account the influence of cross-rebar CR can be obtained (see Figure 7).

The coefficients V5 and V6 may be determined for each condition by creating multiple test structures TM with different distances Za and fracture length gaps, conducting tests on each test structure TM, and determining the coefficients V5 and V6 for each condition based on the results. In this case, an appropriate judgment region 3 can be set by using coefficients V5 and V6 appropriate to the conditions of the concrete structure C in which the actual inspection target P is buried, that is, coefficients V5 and V6 obtained from a test structure TM under conditions similar to those of the concrete structure C in which the actual inspection target P is buried. However, if tests are conducted on a test structure TM using a representative distance Za and representative fracture length gap (a representative fracture length gap expected during inspection) for the concrete structure C in which the actual inspection target P is buried, coefficients V5 and V6 with high versatility can be obtained.

It is also possible to create test structures TM with different cover depths for the crossing rebars CR (i.e., the distance from the surface of the test structure TM to the crossing rebars CR), different spacing between the multiple first rebars R1, and different spacing between the multiple second rebars R2, and determine coefficients V5 and V6 for each test structure TM. In this case, too, an appropriate judgment region 3 can be set by using appropriate coefficients V5 and V6 to match the conditions of the concrete structure C in which the actual inspection target P is buried, i.e., coefficients V5 and V6 obtained from a test structure TM with conditions similar to those of the concrete structure C in which the actual inspection target P is buried. However, if tests are conducted using a test structure TM in which representative crossing rebars CR are buried in the concrete structure C in which the actual inspection target P is buried, it is possible to obtain coefficients V5 and V6 with high versatility.

<Correction of the effects of crossed rebars>
When crossing rebars are embedded in the concrete structure C, the installation intervals of the first and second rebars in the crossing rebars affect the fluctuations in the measured magnetic flux density. Therefore, by setting a judgment region that takes into account the influence of the installation intervals of the first and second rebars in the crossing rebars, the accuracy of fracture judgment can be improved.

If the installation spacing of the first rebars in the crossing rebars is D1 and the installation spacing of the second rebars is D2, by expanding the range of the difference ΔXm(α, Za, Gap) to the range of equation (6), an appropriate judgment region (judgment region 4) can be set that also includes the influence of the distance D1 between the first rebars and the distance D2 between the second rebars (see Figure 8 (B)).

Formula (6)
ΔXmL (α, Za, Gap, θ) ≦ΔXm (α, Za, Gap) ≦ ΔXmR (α, Za, Gap, θ)
ΔXmL(α, Za, Gap, θ) = (ΔXm(0, Za, Gap)+(NL(θ)/2)・Dm(θ)-V3)・(ΔXm(α, Za, Gap)-V3)/(ΔXm(0, Za, Gap)-V3)+V3
ΔXmR(α, Za, Gap, θ) = (ΔXm(0, Za, Gap) + (NR(θ)/2)・Dm(θ)-V3)・(ΔXm(α, Za, Gap)-V3)/(ΔXm(0, Za, Gap)-V3)+V3

Dm (θ) = V6・D2/cos (θ): 0 degrees ≦ θ ≦ θ0
Dm(θ)=V6・D1/sin(θ): θ0≦θ≦90 degrees θ0=arctan(D1/D2)

D1: Distance between the first adjacent cross bars D2: Distance between the second adjacent cross bars θ: Angle between the first cross bar and the object NL(θ), NR(θ): Integer values obtained by experiment

It should be noted that if θ is not a single value but is within a range such as θ1≦θ≦θ2, ΔXm(α, Za, Gap) is set within the maximum range.
Furthermore, the installation interval D1 between adjacent first reinforcing bars of the crossing reinforcing bars and the installation interval D2 between adjacent second reinforcing bars of the crossing reinforcing bars refer to the distance between the central axes of each reinforcing bar (see FIG. 11).

The validity of expanding the range of the difference ΔXm (α, Za, Gap) to the range of Equation (6) was verified for a test structure TM. The test structure TM had a cover depth of 50 mm for the crossing rebars CR, a spacing D1 between the first rebars R1 of 125 mm, a spacing D2 between the second rebars R2 of 250 mm, a length of 2 m for the test object Q, and a gap of 10 mm (the midpoint in the axial direction of the test object Q). The magnetic flux density along the first direction of the test object Q was measured for multiple test structures TM in which the position of the test object Q and the angle relative to the crossing rebars were changed between 15 and 90 degrees. The differences ΔBm and ΔXm obtained from the fluctuating waveforms of this magnetic flux density were then measured. Then, when judgment region 4 (θ = 45 degrees, NL(θ) = -1, NR(θ) = 4) was created for test structure TM, it was confirmed that the differences ΔBm and ΔXm obtained for all test structures TM were all contained within judgment region 4 (see Figure 8 (A)).

<When multiple magnetic sensors 6 are provided>
Furthermore, when multiple magnetic sensors 6, 7 are provided at different distances from the test object P in the second direction (see FIG. 12), it is desirable to form judgment regions 1 to 4 for each magnetic sensor 6, 7 based on the position (distance Za) of each magnetic sensor 6, 7 (see FIG. 9). In this case, it is verified whether the difference ΔB between the maximum and minimum values obtained based on the fluctuation curve of the magnetic flux density measured by each magnetic sensor 6, 7 and the difference ΔX in the first direction between the positions of the maximum and minimum values are included in the judgment regions 1 to 4 of each magnetic sensor 6, 7. Then, by determining that a break has occurred when the difference ΔB and the difference ΔX of each magnetic sensor 6, 7 are both included in the judgment regions 1 to 4 of each magnetic sensor 6, 7, the accuracy of break detection can be improved.

Also, as shown in Figure 10, the range of the fracture length gap may be divided to form judgment regions. In this case, depending on which judgment region divided by the fracture length gap the difference ΔB between the maximum and minimum values obtained based on the fluctuation curve of the measured magnetic flux density and the difference ΔX in the first direction between the positions of the maximum and minimum values fall into, it becomes possible to estimate the fracture length gap as well as determine whether or not a fracture has occurred in the test object P. For example, in Figure 10, if the difference ΔB and the difference ΔX do not fall within the judgment region where the fracture length gap is 1 mm ≦ Gap ≦ 10 mm, but fall within the judgment region where the fracture length gap is 10 mm ≦ Gap ≦ 100 mm, it becomes possible to determine that a fracture has occurred in the test object P and that the fracture length gap is approximately 10 mm to 100 mm.

<Method for estimating the buried depth of the fractured portion of the inspection object P>
In general, in addition to the inspection object P such as reinforcing bars, steel rods, steel wires, etc., other components (obstructing components) such as reinforcing bars for reinforcing bars, spacers, etc. are embedded in the concrete structure C. If such obstructing components are located closer to the surface CF of the concrete structure C than the inspection object P, that is, if the buried depth of the obstructing components is shallower than the buried depth of the inspection object P, the obstructing components will become an obstacle to inspecting the inspection object P for fractures, which may cause an erroneous diagnosis of the fracture of the inspection object P.

Even if the amount of magnetization (residual magnetic flux density) of the inspection object P is the same, the magnetic flux density detected by the magnetic sensor 6 will differ depending on the distance ZB from the surface CF of the concrete structure C to the inspection object P, and the amount of variation will also differ.

To prevent such misdiagnosis, the analysis function may have the ability to calculate distance Za based on the measured magnetic flux density and estimate distance ZB, which is the cover depth. If the estimated distance ZB is then approximately the same as the cover depth of the inspection object P obtained from design drawings, etc., it can be determined that the detected fracture is a fracture in the inspection object P.

Note that the cover depth, distance ZB, is the distance from the surface CF of the concrete structure C to the fracture position of the inspection object P, and is the length obtained by subtracting the fracture depth Za from the distance from the surface CF of the concrete structure C in the second direction to the magnetic sensor 6 and the radius of the inspection object P (or the distance from the top surface to the midpoint in the vertical direction in Figure 1(A)).

Specifically, the fracture depth Za can be estimated by the following equation (7) based on a graph showing the fluctuation of the magnetic flux density at the position of the magnetic sensor 6 (fluctuation curve of the magnetic flux density).

Formula (7) Za=((LN(ΔB)-A0)/A1) ^1/2

ΔB: difference between the maximum and minimum values of the magnetic flux density in the second direction in the variation curve of the magnetic flux density along the first direction of the object to be inspected; A0, A1: coefficients determined by the shape of the object to be inspected, the residual magnetic flux density, and the fracture length Gap of the object to be inspected.

The coefficients A0 and A1 can be found from the relationship between the difference ΔB between the maximum and minimum values in a variation curve of magnetic flux density obtained by an experiment using a test structure and the fracture depth Za. In other words, as shown in Figure 5 (A-1), when the fracture length Gap is kept constant and the fracture depth Za is changed, an approximate formula can be formed based on the fracture depth Za and the difference ΔB between the maximum and minimum values in the variation curve of magnetic flux density measured at each fracture depth Za, and the coefficients A0 and A1 can be determined from this approximate formula.

If the fracture depth Za can be determined using the above formula (7), it is possible to determine whether the detected fracture is a fracture in the object of inspection P by comparing that depth with the position of the object of inspection P described in the design drawings, etc. This makes it possible to prevent false detection of a fracture in the object of inspection P.

Furthermore, when a plurality of magnetic sensors 6 are provided, by comparing the fracture depth Za obtained based on the measurement results of the plurality of magnetic sensors 6, it becomes easier to prevent erroneous detection of fracture in the inspection object P.
For example, the accuracy of estimating the fracture depth Za can be improved by calculating the average value of the fracture depths Za obtained by the magnetic sensors 6 and using this as the fracture depth Za, or by performing statistical processing or curve approximation.

Furthermore, the fracture depth Za affects not only the amount of magnetic flux density detected by the magnetic sensor 6, but also the fluctuation curve along the first direction. Therefore, the fracture depth Za may be estimated by the following equation (8) based on a graph showing the fluctuation of magnetic flux density at the position of the magnetic sensor 6 (flux density fluctuation curve). In other words, the fracture depth Za may be estimated based on the difference ΔX between the positions in the first direction between the maximum and minimum values in the fluctuation curve of the magnetic flux density in the second direction along the first direction of the inspection object P.

Formula (8) Za=(ΔX-a0)/a1
ΔX: difference in position between the maximum value and the minimum value in the first direction in the variation curve of the magnetic flux density in the second direction along the first direction of the object to be inspected; a0, a1: coefficients determined by the shape of the object to be inspected and the fracture length Gap

The coefficients a0 and a1 can be found from the relationship between the difference ΔX in the positions in the first direction between the maximum value and the minimum value in a variation curve of magnetic flux density obtained by an experiment using a test structure and the fracture depth Za. In other words, as shown in Figure 5 (A-2), when the fracture length Gap is kept constant and the fracture depth Za is changed, an approximate formula can be formed based on the fracture depth Za and the difference ΔX in the positions in the first direction between the maximum value and the minimum value in the variation curve of magnetic flux density measured at each fracture depth Za, and the coefficients a0 and a1 can be determined from this approximate formula.

<Measurement of multiple positions in the third direction>
When a plurality of magnetic sensors 6 are provided lined up along the third direction, a magnetic sensor 7 may be provided at each position in the third direction, located above the magnetic sensor 6 (above in the second direction) (see FIG. 12 ). In this case, the accuracy of estimating the fracture depth Za can be improved by estimating the fracture depth Za based on the following equation (9).

Formula (9) Za=(-c・d+((c・d ² -γ・(c-1)) ^1/2 )/(c-1)
γ=(LN(ΔB1/ΔB2))/An+ε ²・(c-1)
c = Af/An

ΔB1: difference between the maximum value and the minimum value in the variation curve of the magnetic flux density in the second direction measured at a measurement position (magnetic sensor 6) close to the inspection object P in the second direction, along the first direction of the inspection object P. ΔB2: difference between the maximum value and the minimum value in the variation curve of the magnetic flux density in the second direction measured at a measurement position (magnetic sensor 7) far from the inspection object P in the second direction, along the first direction of the inspection object P. d: distance between the two measurement positions (between magnetic sensors 6 and 7) in the second direction. ε: distance from the inspection object P to the measurement position (magnetic sensors 6 and 7) in the third direction. An: coefficient determined by the distance from the inspection object P and the concrete structure C to the measurement position, and applied to the measurement position (magnetic sensor 6) close to the inspection object P in the second direction. Af: coefficient determined by the distance from the inspection object P and the concrete structure C to the measurement position, and applied to the measurement position (magnetic sensor 7) far from the inspection object P in the second direction.

The coefficients An and Af can be determined from the relationship between the difference ΔB between the maximum and minimum values in a variation curve of magnetic flux density obtained by an experiment using a test structure and the fracture depth Za. In other words, as shown in Figure 5 (A-1), when the fracture length Gap is kept constant and the fracture depth Za is changed, an approximate equation can be formed based on the fracture depth Za and the difference ΔB between the maximum and minimum values in the variation curve of magnetic flux density measured at each fracture depth Za, and the coefficients An and Af can be determined from this approximate equation.

<Method for estimating the break length gap>
Furthermore, it is also possible to estimate the size of the fracture, i.e., the fracture length Gap, based on the difference ΔB between the maximum value and the minimum value and the difference ΔX between the positions of the maximum value and the minimum value in the first direction. If the fracture length Gap can be estimated, it is possible to grasp the extent of the progression of deterioration due to corrosion or the like of the inspection object P, which can be used as information for determining whether prompt repair is necessary, etc.

Even if the amount of magnetization (residual magnetic flux density) of the test object P is the same, if the fracture length Gap is different, the magnetic field at the fractured portion will change, and the fluctuation state of the magnetic flux density fluctuation curve detected by the magnetic sensor 6 will also change.

Therefore, the fracture length Gap can be estimated by the following equation (10) based on a graph (fluctuation curve of magnetic flux density) showing the fluctuation of magnetic flux density at the position of the magnetic sensor 6. In other words, the fracture length Gap can be estimated based on the difference ΔB between the maximum value and the minimum value in the fluctuation curve of magnetic flux density in the second direction along the first direction of the inspection object P.
Whether to use the equation Gap≦Za or Gap≧Za can be determined by comparing the estimation results of Gap from both equations with Za.

Formula (10)
Gap=EXP((ΔB-C0)/C1): Gap≦Za
Gap=((LN(ΔB)-C2)/C3) ^1/2 : Gap≧Za

ΔB: difference between the maximum and minimum values in the variation curve of the magnetic flux density in the second direction along the first direction of the object to be inspected; C0 to C3: coefficients determined by the shape of the object to be inspected and the distance Za from the object to the measurement position in the second direction.

The coefficients C0 to C3 can be found from the relationship between the fracture length Gap and the difference ΔB between the maximum and minimum values in a variation curve of magnetic flux density obtained by an experiment using a test structure. That is, as shown in Figure 5 (B-1), by keeping the fracture depth Za constant and varying the fracture length Gap, an approximate formula can be formed based on the fracture length Gap and the difference ΔB between the maximum and minimum values in the variation curve of magnetic flux density measured at each fracture length Gap, and the coefficients C0 to C3 can be determined from this approximate formula.

The fracture length Gap may also be estimated by the following equation (11) based on a graph (fluctuation curve of magnetic flux density) showing the fluctuation of magnetic flux density at the position of the magnetic sensor 6. In other words, the fracture length Gap may be estimated based on the difference ΔX between the positions in the first direction of the maximum value and the minimum value in the fluctuation curve of the magnetic flux density in the second direction along the first direction of the inspection object P (the distance ΔX in the first direction between the position where the maximum value of magnetic flux density occurs and the position of the minimum value).
If the estimated result of the fracture length Gap does not exceed the fracture depth Za, the estimation result is determined to be "the fracture length Gap does not exceed the fracture depth Za."

Formula (11)
Gap=c1・ΔX−c0: Gap≧Za

ΔX: difference between the position of the maximum value and the position of the minimum value on the variation curve of the magnetic flux density in the second direction along the first direction of the object to be inspected; c0, c1: coefficients determined by the shape of the object to be inspected.

The coefficients c0 and c1 can be found from the relationship between the fracture length Gap and the difference ΔX in the positions in the first direction between the maximum and minimum values in a variation curve of magnetic flux density obtained by an experiment using a test structure. That is, as shown in Figure 5 (B-2), by keeping the fracture depth Za constant and varying the fracture length Gap, an approximation formula can be formed based on the fracture length Gap and the difference ΔX in the positions in the first direction between the maximum and minimum values in the variation curve of magnetic flux density measured at each fracture length Gap, and the coefficients c0 and c1 can be determined from this approximation formula.

<Regarding the position of the magnetic sensor 6 (measurement position)>
In the third direction, the number of positions at which the magnetic flux density is measured is not limited to three, but may be four or more, or may be one or two. When measuring at multiple positions, it is desirable that the measurement positions be arranged symmetrically with respect to the central plane.

When measuring magnetic flux density at multiple positions in the second direction, the number of measurement positions is not limited to two, and can be three or more. Using three or more positions is preferable because it allows for increased estimation accuracy through statistical processing and curve approximation. When measuring at multiple positions in the second direction, it is desirable that all measurement positions aligned along the second direction (measurement positions aligned vertically) be positioned on a plane parallel to the reference plane SA (see Figure 1(B)). In other words, it is desirable to arrange measurement positions aligned along the second direction so that they are aligned along the normal direction of the plane, as this simplifies estimation when performing statistical processing, curve approximation, etc.

The non-destructive testing method of the present invention is suitable for detecting fractures in reinforcing bars, steel rods, steel wires, etc. installed within concrete structures.

REFERENCE SIGNS LIST 1 Non-destructive inspection device 2 Mobile body 5 Magnetic flux measuring unit 6 Magnetic sensor 7 Magnetic sensor A First measurement direction B Central axis of mobile body 2 SA Reference plane C Concrete structure CF Surface of concrete structure C CR Crossed rebar R1 First rebar R2 Second rebar P Inspection object Q Test object

Claims (5)

A method for estimating the presence or absence of a fracture in an inspection object embedded in a concrete structure, the method comprising: measuring a magnetic flux density of the inspection object extending in a first direction outside the concrete structure along the first direction of the inspection object; and estimating the presence or absence of a fracture in the inspection object based on a variation in the measured magnetic flux density,
the magnetic flux density to be measured is a magnetic flux density in a second direction that is a direction perpendicular to the first direction on the surface of the concrete structure and the inspection target,
A non-destructive testing method characterized in that, when the measured magnetic flux density satisfies both of predetermined conditions (1) and (2), it is estimated that a fracture has occurred in the test object.

Given conditions:
When the distance from the measurement position in the second direction to the test object is distance Za, (1) the difference between the maximum value and the minimum value in the fluctuation curve along the first direction of the measured magnetic flux density in the second direction is included in the range of the difference ΔB(α, Za, Gap) between the maximum value and the minimum value in the fluctuation curve of the magnetic flux density value B(x, α, Za, Gap) obtained when the magnetic flux density correction coefficient α and the fracture length Gap are changed in equation (1).
(2) The difference in the positions in the first direction between the maximum value and the minimum value in the fluctuation curve along the first direction of the measured magnetic flux density in the second direction is included in the range of the difference ΔX(α, Za, Gap) between the positions in the first direction between the maximum value and the minimum value in the fluctuation curve of the magnetic flux density value B(x, α, Za, Gap) obtained when the magnetic flux density correction coefficient α and the fracture length Gap are changed in equation (1).

Formula (1)
△B (α, Za, Gap) = B (x1, α, Za, Gap) - B (x2, α, Za, Gap)
△X (α, Za, Gap) = x2 (α, Za, Gap) - x1 (α, Za, Gap)
B (x, α, Za, Gap) = Bp (x, Za, Gap) + α・x
Bp (x, Za, Gap) = (Br・S/4π)・(Za/((x+Gap/2) ² +y ² +Za ² ) ^3/2 )−(Br・S/4π)・(Za/((x-Gap/2) ² +y ² +Za ² ) ^3/2 )
αR≦α≦αL

Bp: theoretical formula of magnetic flux density from a fracture of an object to be inspected, measured outside the concrete structure along a first direction of the object to be inspected; x1: position in the first direction at which B(x, α, Za, Gap) has a maximum value; x2: position in the first direction at which B(x, α, Za, Gap) has a minimum value; α·x: correction term for correcting magnetic flux density along the first direction other than the magnetic flux density from the fracture of the object to be inspected; α: correction coefficient for magnetic flux density along the first direction; αR: minimum correction coefficient at which B(x) has a maximum value and a minimum value; αL: maximum correction coefficient at which B(x) has a maximum value and a minimum value; Br: residual magnetic flux density of the object to be inspected; S: cross-sectional area of the object to be inspected; Gap: fracture length; x: coordinate of the measurement position of the magnetic flux density in the first direction when the fracture position of the object to be inspected is taken as the origin; y: coordinate of the measurement position of the magnetic flux density in a third direction perpendicular to both the first direction and the second direction when the fracture position of the object to be inspected is taken as the origin.
In a test structure having a test object with a fracture that is a component equivalent to the inspection object, the distance Za and the fracture length Gap are varied to measure the magnetic flux density along the first direction of the test object, and based on a fluctuation curve formed using the measured magnetic flux density, an approximate formula ΔBgF(0, Za, Gap) of the difference between the maximum value and the minimum value and an approximate formula ΔXgF(0, Za, Gap) of the difference in position in the first direction between the maximum value and the minimum value are created;
Based on the approximate formulas ΔBgF(0, Za, Gap) and ΔXgF(0, Za, Gap), a difference ΔBg(α, Za, Gap) obtained by converting the difference ΔB(α, Za, Gap) between the maximum value and the minimum value in the fluctuation curve of the magnetic flux density value B(x) using formula (2) and a difference ΔXg(α, Za, Gap) obtained by converting the difference ΔX(α, Za, Gap) between the positions of the maximum value and the minimum value in the first direction in the fluctuation curve of the magnetic flux density value B(x) using formula (3) are calculated;
The difference between the maximum value and the minimum value in the variation curve of the measured magnetic flux density in the second direction along the first direction is within the range of the difference ΔBg(α, Za, Gap) obtained when the magnetic flux density correction coefficient α and the fracture length Gap in Equation (1) are changed,
2. A non-destructive testing method according to claim 1, characterized in that it is estimated that a fracture has occurred in the test object when the difference in the positions in the first direction between the maximum value and the minimum value in a variation curve along the first direction of the measured magnetic flux density in the second direction is within the range of the difference ΔXg(α, Za, Gap) obtained when the magnetic flux density correction coefficient α and the fracture length Gap in equation (1) are changed.

Formula (2)
ΔBg(α, Za, Gap) = (ΔBgF(0, Za, Gap)/ΔB(0, Za, Gap)・ΔB(α, Za, Gap)
ΔBgF(0, Za, Gap)=EXP((V1+V2)・Za ² +V0)・(U・LN(Gap/Za)+1): Gap<Za
ΔBgF(0, Za, Gap)=EXP((V1+V2)・Za ² +V0): Gap=Za
ΔBgF (0, Za, Gap) = EXP (V1・Za ² +V2・Gap ² +V0): Gap>Za

Formula (3)
ΔXg (α, Za, Gap) = ((ΔXgF (0, Za, Gap) - V3)・ΔX (α, Za, Gap)) / ΔX (0, Za, Gap) + V3
ΔXgF (0, Za, Gap) = V4・Za+V3: Gap≦Za
ΔXgF (0, Za, Gap) = V4・Gap+V3: Gap≧Za

V0 to V4: Coefficients obtained by experiment U: Coefficient obtained by experiment
In a test structure in which a cross rebar is provided at a distance from the test object so that the first rebar and the second rebar are perpendicular to each other, and the test object is arranged so that the fracture position overlaps with the first rebar when viewed parallel to the second rebar and from the normal direction of a plane parallel to the first rebar and the second rebar, the magnetic flux density is measured along the first direction of the test object in the normal direction of the plane parallel to the first rebar and the second rebar with the first rebar arranged between the test object and the first rebar, and the difference ΔB0 between the maximum value and the minimum value in the variation curve of the measured magnetic flux density and the difference ΔX0 between the positions of the maximum value and the minimum value in the first direction are calculated,
Based on the difference ΔB0 and the difference ΔX0, the difference ΔBg(α, Za, Gap) is converted by equation (4) to obtain a difference ΔBm(α, Za, Gap), and the difference ΔXg(α, Za, Gap) is converted by equation (5) to obtain a difference ΔXm(α, Za, Gap),
The difference between the maximum value and the minimum value in the variation curve of the measured magnetic flux density along the first direction is within the range of the difference ΔBm(α, Za, Gap) obtained when the magnetic flux density correction coefficient α and the fracture length Gap in Equation (1) are changed,
3. A non-destructive testing method according to claim 2, characterized in that it is estimated that a fracture has occurred in the test object when the difference in the positions of the maximum and minimum values in the first direction on a variation curve of the measured magnetic flux density along the first direction is within the range of the difference ΔXm(α, Za, Gap) obtained when the magnetic flux density correction coefficient α and the fracture length Gap in equation (1) are changed.

Formula (4)
ΔBm (α, Za, Gap) = V5・ΔBg (α, Za, Gap)
Formula (5)
ΔXm (α, Za, Gap) = V6・(ΔXg (α, Za, Gap) - V3) + V3

V5: A coefficient set so that the difference ΔB0 between the maximum value and the minimum value in the variation curve of the magnetic flux density measured along the first direction of the test object matches the difference ΔBm (0, Za, Gap) when α = 0. V6: A coefficient set so that the difference ΔX0 between the positions of the maximum value and the minimum value in the first direction in the variation curve of the magnetic flux density measured along the first direction of the test object matches the difference ΔXm (0, Za, Gap) when α = 0.
4. A non-destructive inspection method according to claim 3, wherein the range of the difference ΔXm(α, Za, Gap) is expanded to the range of formula (6).

Formula (6)
ΔXmL (α, Za, Gap, θ) ≦ΔXm (α, Za, Gap) ≦ ΔXmR (α, Za, Gap, θ)
ΔXmL(α, Za, Gap, θ) = (ΔXm(0, Za, Gap)+(NL(θ)/2)・Dm(θ)-V3)・(ΔXm(α, Za, Gap)-V3)/(ΔXm(0, Za, Gap)-V3)+V3
ΔXmR(α, Za, Gap, θ) = (ΔXm(0, Za, Gap) + (NR(θ)/2)・Dm(θ)-V3)・(ΔXm(α, Za, Gap)-V3)/(ΔXm(0, Za, Gap)-V3)+V3

Dm (θ) = V6・D2/cos (θ): 0 degrees ≦ θ ≦ θ0
Dm(θ)=V6・D1/sin(θ): θ0≦θ≦90 degrees θ0=arctan(D1/D2)

D1: Distance between the first adjacent rebars of the crossing rebars D2: Distance between the second adjacent rebars of the crossing rebars θ: Angle between the first rebar of the crossing rebars and the object to be inspected NL(θ), NR(θ): Integer values obtained by experiment
measuring a magnetic flux density in the second direction along the first direction of the object to be inspected at a plurality of positions at different distances from the object to be inspected in the third direction or the second direction;
5. The non-destructive testing method according to claim 1, wherein it is estimated that a fracture has occurred in the test object when the magnetic flux densities measured at a plurality of measurement positions satisfy the predetermined condition.