JP7429385B2 - Internal defect detection method - Google Patents

Description

The present invention relates to an internal defect detection method for nondestructively inspecting internal defects such as cavities, cracks, junkers, and delaminations existing inside a structure such as a concrete structure.

Conventionally, as prestressed concrete using the post-tension method, after the concrete has hardened, a prestressed steel material such as a steel wire or steel rod is inserted into a sheath that has been buried in advance, and tension is introduced into the prestressed steel material. A PC structure is known in which prestress is introduced by injecting grout into the gap between the sheath and the PC steel material.

The grout is used to fill the gap between the prestressing steel and the sheath in order to prevent corrosion of the prestressed steel and to integrate the concrete member and the prestressed steel. Therefore, if a cavity is created in the sheath due to insufficient filling of grout, the PC steel material will corrode or break due to the infiltration of water and chlorides, causing significant deformation of the PC structure and jeopardizing the safety of the structure. There is a risk that it will not be possible to secure the required amount.

In a previous application, the applicant proposed a striking device for use in the impact elastic wave method (Patent Document 1 below) and an internal defect detection method (Patent Document 2 below). We have been developing a method to detect defects non-destructively.

In particular, in Patent Document 2 listed below, vibrations on the surface of a structure are received when an elastic wave is input to the surface, the received wave is Hilbert-transformed to calculate the instantaneous phase, and the change in the instantaneous phase per unit time is After calculating the instantaneous phase difference, which is a quantity, and obtaining the time from the input of the elastic wave until a change occurs in the instantaneous phase difference, the internal We established a method to estimate the location of internal defects from the round trip distance to the defect.

Japanese Patent Application Publication No. 2017-133936 JP2018-119845A

However, in the method described in Patent Document 2, there are problems in terms of reproducibility and accuracy in actual measurements due to noise and errors in processing analysis.

Therefore, the main object of the present invention is to provide an internal defect detection method that allows internal defects to be detected with higher accuracy.

In order to solve the above problems, the present invention according to claim 1 provides a method for detecting internal defects by receiving surface vibrations when an elastic wave is input to the surface of a structure, comprising:
Multiple measurement points are placed equidistant from the elastic wave input point, and the received waves measured at each measurement point are separated into amplitude information and phase information by Hilbert transform, and the amplitude difference waveform between the received waves is calculated. and a phase difference waveform, and then estimating the position of the internal defect from both or either of the amplitude difference waveform and the phase difference waveform .

The invention according to claim 1 defines a method for estimating the position of an internal defect on a plane. Specifically, each received wave measured at each measurement point is subjected to Hilbert transform and separated into amplitude information and phase information, and after obtaining an amplitude difference waveform and a phase difference waveform between the received waves, the amplitude difference waveform is The position of the internal defect on the plane is estimated from both or one of the phase difference waveform and the phase difference waveform.

As a second aspect of the present invention, there is provided an internal defect detection method according to the first aspect, wherein measurement points are arranged at symmetrical positions equidistant from the input point of the elastic wave.

In the invention according to claim 2, the input point and the measurement point are arranged on a straight line. That is, measurement points are placed at positions symmetrical to the input point of the elastic wave, and both or either of the amplitude difference waveform and phase difference waveform of the received waves measured at the paired measurement points is obtained. The position of the internal defect is estimated by determining .

As a third aspect of the present invention, there is provided an internal defect exploration method according to the first aspect, in which a plurality of measurement points are arranged on a circle centered on the input point of the elastic wave in plan view.

In the third aspect of the invention, the plurality of measurement points are arranged on a circle centered on the input point of the elastic wave when viewed in plan. This makes it possible to three-dimensionally detect the position of the internal defect.

As the present invention according to claim 4 , the amplitude difference waveform and the phase difference waveform when the surface of the structure is sequentially moved and a plurality of points are measured while maintaining the relative positional relationship between the input point and the measurement point. There is provided an internal defect exploration method according to any one of claims 1 to 3 , wherein the position of the internal defect on a plane is estimated from the state of both or one of the amplitude changes.

In the invention according to claim 4 , the amplitude difference waveform and the phase difference waveform are formed by sequentially moving the surface of the structure and measuring a plurality of points while maintaining the relative positional relationship between the input point and the measurement point. At measurement positions where the amplitude change of both or one of the above-mentioned amplitude difference waveform and/or phase difference waveform is symmetrical, with zero when the input point of the elastic wave is directly above the internal defect. The positive and negative are reversed. Therefore, the position of the internal defect on the plane can be estimated from the state of amplitude change in both or one of the amplitude difference waveform and the phase difference waveform .

As the present invention according to claim 5 , the delay time between the received waves measured at each measurement point is detected, and the distance between each measurement point and the input point is corrected from the delay time . A method for detecting an internal defect according to any of the above is provided.

In the invention as set forth in claim 5 , there are several possible causes of errors in actual measurements, but distance correction of the measurement position, which is considered to be a relatively large factor among them, is defined. Specifically, the delay time between the received waves measured at each measurement point is detected, and the distance between each measurement point and the input point is corrected based on the delay time. The delay time detection methods include a method that focuses on the first amplitude of the received wave and detects the time when the amplitude becomes 1/2 of the maximum value, a method that detects the rise time of the received wave, and a paired measurement. Examples include a method of detecting delay time from cross-correlation between received waves at a point.

As the present invention according to claim 6 , the received waves measured at each measurement point are each subjected to Hilbert transform to calculate the phase, and the phase difference which is the amount of change in the phase per unit time is calculated, and the elastic wave is calculated. After obtaining the time from the time of input until a change occurs in the phase difference between each measurement point, the internal defect is calculated from the round trip distance to the internal defect obtained by multiplying the time by the propagation speed of the elastic wave inside the structure. There is provided an internal defect exploration method according to any one of claims 1 to 5 , which estimates the position of an internal defect.

The invention according to claim 6 defines a method for estimating the depth of an internal defect. The depth of the internal defect can be estimated based on the time from the input of the elastic wave to the arrival of the reflected wave at the internal defect at each measurement point. In order to specify the time when the reflected wave from the internal defect arrives, the phase difference, which is the amount of change in the phase of the received wave per unit time, is calculated. Then, the geometric positional relationship of the measurement point arrangement is calculated from the round trip distance to the internal defect obtained by multiplying the time from the input of the elastic wave to the arrival of the reflected wave by the propagation speed of the elastic wave inside the structure. The depth of internal defects can be determined by taking this into account.

As the present invention according to claim 7 , in the structure, after the concrete has hardened, a prestressing steel material is inserted into a sheath buried in advance, and after tension is introduced into the prestressing steel material, the inside of the sheath is A post-tensioned PC structure formed by injecting grout into the gap between the PC steel material,
There is provided an internal defect exploration method according to any one of claims 1 to 6 , wherein the exploration method is used to confirm the filling state of the grout.

The seventh aspect of the invention indicates that the present inspection method is particularly suitable for checking the filling state of grout in a post-tensioned PC structure.

As described in detail above, according to the present invention, internal defects can be detected with higher accuracy.

1A is a cross-sectional view and FIG. 1B is a plan view showing a configuration for carrying out an internal defect detection method according to the present invention. FIG. It is a graph showing the received waves at each measurement point when there is no internal defect D. FIG. 2 is a configuration diagram for implementing the internal defect detection method according to the present invention. It is a graph showing the received waves at each measurement point when there is an internal defect D. FIG. 2 is a side view showing the striking device 1. FIG. FIG. 2 is a two-dimensional schematic diagram showing a propagation situation of elastic waves. It is a graph showing the relationship between the distance from the internal defect D and the amplitude of the difference waveform. It is a graph showing the relationship between the real part and the imaginary part by Hilbert transformation. FIG. 3 is a cross-sectional view showing the arrangement of input points and measurement points. It is a graph showing the phase difference for each input point. FIG. 7 is a plan view showing a configuration for implementing an internal defect detection method according to a second embodiment.

Embodiments of the present invention will be described in detail below with reference to the drawings.

As shown in FIG. 1, the method for searching for internal defects D according to the present invention is a method for detecting internal defects D by receiving surface vibrations when an elastic wave is input to the surface of a structure. In detail, as shown in Fig. 1, a plurality of measurement points P1 and P2 are arranged at positions equidistant from the elastic wave input point F, and the received waves measured at each measurement point P1 and P2 are different from each other. It is characterized by estimating the position of internal defects while removing the influence of surface waves propagating on the surface of the structure by determining the differential waveform.

[First form example]
In the exploration method according to the first embodiment, as shown in FIG. By determining the difference waveform between the received waves measured by the method, the position of the internal defect is estimated while removing the influence of the surface waves propagating on the surface of the structure.

In this exploration method, in order to eliminate the influence of surface waves propagating on the surface of a structure, in the method of detecting the internal defect D by receiving surface vibrations when an elastic wave is input to the surface of the structure, elastic Measurement points P1 and P2 are placed at symmetrical positions equidistant from the wave input point F, respectively. Due to this arrangement, the surface wave components at the pair of measurement points P1 and P2 have the same phase and the same amplitude, and by taking the difference waveform of the received waves of both, the surface wave components are removed (cancelled) and the internal Only the reflected wave from the defect D remains. The reflected wave from the internal defect D that appears in the received waves measured at the paired measurement points P1 and P2 is an elastic wave input when the internal defect D is not at the center of symmetry with respect to the measurement points P1 and P2. Since there is a time lag between when the received waves arrive and when they arrive, this effect appears on the difference waveform between the received waves. In this way, by finding the difference waveform between the received waves measured at the paired measurement points P1 and P2 and removing the influence of the surface waves, the influence of the reflected wave from the internal defect D can be clearly seen in the difference waveform. Therefore, the position of the internal defect D can be estimated with higher accuracy.

To further explain the outline of this exploration method, as shown in Figure 1, measurement points P1 and P2 were placed at symmetrical positions equidistant from the elastic wave input point F, and elastic waves were input to the input point F. Then, the received waves at measurement points P1 and P2 are measured. If there is no internal defect D in the structure, only surface waves are observed at each measurement point P1 and P2, as shown in Figure 2, so the received wave measured at measurement point P1 and the received wave measured at measurement point P2 are The waveform is almost the same as the received wave.

Next, received waves when the structure has an internal defect D as shown in FIG. 3 will be described. FIG. 3 shows a configuration example in which the surface of a structure is sequentially moved to measure multiple points while maintaining the relative positional relationship between the input point and the measurement point. In this configuration example, in the area to the left of internal defect D, the received wave at input point F is measured at measurement points P1 and P2, and in the area to the right of internal defect D, the received wave at input point F' is measured at measurement point P1. ', P2'. The received wave measured at each measurement point has a waveform in which a reflected wave from the internal defect D is superimposed on a surface wave, as shown in FIG. The influence of this reflected wave is greater on the received waves measured at measurement points closer to the internal defect D (measurement point P2 at input point F, measurement point P1' at input point F'). Therefore, by sequentially moving the surface of the structure and measuring multiple points while maintaining the relative positional relationship between the input point and the measurement point, the tendency of the waveform measured before and after the internal defect D changes. , it can be estimated that the internal defect D exists at the changed position. If the input point is located directly above the internal defect D, the surface wave and the reflected wave from the internal defect D overlap under approximately the same conditions, so that approximately the same received wave can be obtained.

Therefore, by arranging measurement points P1 and P2 at symmetrical positions equidistant from the elastic wave input point F, and finding the difference waveform between the received waves measured at the pair of measurement points P1 and P2, it is possible to The influence of the surface waves propagating on the surface of the object can be removed, and the influence of the reflected waves from the internal defect D can clearly appear in the difference waveform, so that the position of the internal defect D can be estimated with higher accuracy. Become. Further, by sequentially moving the surface of the structure and measuring at a plurality of points and observing the state of change in the amplitude of the received wave, the position of the internal defect D on the plane can be estimated.

The principle of this exploration method will be explained in detail below.
The input elastic waves may be generated by impulse excitation that strikes a structure, or by tone burst excitation having a constant steady amplitude portion. In generating elastic waves, it is particularly preferable to use a striking device used in the impact elastic wave method disclosed in JP-A-2017-133936. As shown in FIG. 5, this striking device 1 is a steel rod of a predetermined length manufactured with a constant cross-sectional area over its entire length, and the tip surface 2a that is made to collide with the object to be inspected has a spherical crown shape. , the upper end surface 2b consists of a plunger 2 having a planar shape, and a steel ball 3 of a predetermined diameter that collides with the upper end surface 2b of the plunger 2.

In the striking device 1, the steel ball does not directly strike the object to be inspected, but the steel ball 3 of a predetermined diameter strikes the object through the plunger 2 (one-dimensional rod) having the above structural condition. to give an elastic wave. By using such indirect impact, the impact force is generated by the steel ball 3 colliding with the plunger upper end surface 2b, and the impact force is generated by the steel ball 3 colliding with the plunger upper end surface 2b, and the impact force is applied to the inspection object through the tip surface 2a of the plunger 2 that is in contact with the inspection object. transmitted to things. Since these are materials (steel) with a stable elastic modulus, the contact time between the steel ball 3 and the plunger 2 is always constant, making it possible to stably give waves of a constant frequency.

In addition, since it is known that plunger 2 with a specified length generates a specific frequency that resonates with the length of the steel rod (multiple reflection theory of elastic waves), the frequency caused by the impact and the resonance of plunger 2 By matching the frequencies within a certain range, an elastic wave with a resonant frequency corresponding to the plunger length will be generated within the plunger, and this can be transmitted to the object to be inspected. .

Therefore, when the plunger 2 made of the same material is hit with the steel ball 3 of the same diameter and the same mass while sequentially moving the inspection target point, an elastic wave of a constant frequency is stably applied to the inspection target. becomes possible.

The measurement points in this embodiment are two points symmetrical with respect to the input point F of the elastic wave, or an even number of points symmetrical with respect to the input point F. One or more pairs of measurement points are arranged, each pair being arranged at a point symmetrical position with respect to the input point F. Although they are arranged in one set in FIG. 1, they may be arranged in one or more sets on the extension of the straight line connecting the input point F and the measurement points P1 and P2, or at any angle with respect to this straight line. One or more sets may be arranged on a straight line extending in a linear direction having a difference. By arranging pairs of measurement points on a plurality of straight lines with different directions and measuring at multiple sets of measurement points, it is possible to detect the position of the internal defect D three-dimensionally, and detect internal defects with higher accuracy. The position of defect D can now be estimated.

As shown in FIG. 1, the elastic wave measuring means is such that structures are placed at measuring points on the same plane as the elastic wave input surface and a predetermined distance apart from the elastic wave input point F. An elastic wave sensor 4 is arranged to measure response elastic waves. The elastic wave sensor 4 measures elastic waves in response to waves applied to the structure by the striking device 1 while in contact with the surface of the structure. As this elastic wave sensor 4, for example, an acceleration sensor, a vibration sensor, etc. can be used. Although the receiving direction of the elastic wave sensor 4 is not particularly limited, it is preferably the input direction of the elastic wave (vertical direction (vertical direction) with respect to the measurement surface).

The vibrations measured by the elastic wave sensor 4 are converted from analog to digital by an A/D converter (not shown), and then input to a waveform recording device (not shown), and are also input to an evaluation device (not shown). sent to ).

The waveform recording device is a device for recording wave electrical signals measured by the elastic wave sensor 4. This waveform recording device is composed of a server for storing electrical signals, a storage such as a hard disk, a recording medium such as a CD or a DVD, a memory, and the like. The waveform recording device can receive signals from the elastic wave sensor 4 and store them in chronological order.

The evaluation device is composed of an electronic device such as a PC (personal computer), a smartphone, a tablet terminal, or a wearable terminal. This evaluation device analyzes waveforms stored in the waveform recording device. This evaluation device may be configured to be able to calculate the phase and the phase difference, which is the amount of change per unit time, by applying, for example, Hilbert transform to the recorded waveform. , P2 may be obtained. Further, by applying FFT (Fast Fourier Transform), time-axis waveform data may be converted to frequency-axis spectrum data. Further, the evaluation device may be configured to be able to determine the presence or absence of internal defects in a structure such as concrete based on the presence or absence of a change in phase difference, or a change in phase difference occurs from the time of input of an elastic wave. Alternatively, the position of the internal defect may be calculated by obtaining the time until the end.

This evaluation device can display each data via a display section consisting of, for example, a display (not shown). Furthermore, the evaluation device can record each of these data in the storage, display these data on the display unit, or write these data in a portable memory based on a command from the user. The user can take out this portable memory from the evaluation device and carry it around freely. Furthermore, the evaluation device can also transfer these data to other electronic devices via a public communication network.

As shown in FIG. 1, the distance L between the input point F and the measurement points P1 and P2 is arbitrary, but from the viewpoint of measurement accuracy, it is preferably 20 to 100 mm, preferably 30 to 70 mm. .

As shown in FIG. 6, the input elastic waves propagate to the paired measurement points P1 and P2 as surface waves (R waves), and also as longitudinal elastic waves (P waves) or transverse elastic waves (S waves). This becomes a wave that propagates inside the structure. If there is an internal defect (void), the elastic wave propagated inside the structure is reflected there, reaches measurement points P1 and P2, and overlaps with the surface wave.

Since the pair of measurement points P1 and P2 are arranged at symmetrical positions equidistant from the elastic wave input point F, substantially the same surface waves propagate at these measurement points P1 and P2. Therefore, when the difference between the received waves measured at the paired measurement points P1 and P2 is calculated, the surface wave components are canceled out, and only the influence of the reflected wave component from the internal defect D remains.

Since the amplitude of this difference waveform reflects the influence of the reflected wave from the internal defect D, it can be easily inferred that the amplitude differs depending on the distance between the measurement points P1, P2 and the internal defect D. Utilizing this principle, by sequentially moving the surface of the structure and measuring multiple points while maintaining the relative positional relationship between the input point F and the measurement points P1 and P2, it is possible to The location can be estimated. In order to measure a plurality of points in this way, it is preferable to divide the surface of the structure vertically and horizontally into a mesh shape of a predetermined size, and sequentially measure at the intersection points. Although the size of the mesh can be set as appropriate, it is preferably about 20 to 100 mm, preferably about 30 to 70 mm, from the viewpoint of suppressing an increase in workload due to an increase in the number of measurement points and improving measurement accuracy.

FIG. 7 shows the distance on a plane from the input point F directly above the internal defect D when the internal defect D is present, and the amplitude of the difference waveform between the received waves measured at the paired measurement points P1 and P2. This is a graph obtained by theoretical calculation of the relationship. In FIG. 7, the amplitude of the difference waveform on the vertical axis is shown in absolute value, and the amplitude of one of the difference waveforms becomes negative when the distance on the plane from directly above the internal defect is zero. When the input point F of the elastic wave is located directly above the internal defect, that is, when the distance on the plane from directly above the internal defect is zero, the reflected wave from the internal defect D is transmitted to the paired measurement points P1 and P2, respectively. Since the waves are transmitted equally, the amplitude of the difference waveform between the received waves at both measurement points P1 and P2 becomes zero. On the other hand, when the input point F moves away from the position directly above the internal defect D, the amplitude of the difference waveform increases symmetrically around the position directly above the internal defect D, and when the input point F moves away beyond a certain point, the amplitude of the difference waveform tends to decrease. becomes. As a result, while maintaining the relative positional relationship between input point F and measurement points P1 and P2, it is possible to change the state of the amplitude change of the difference waveform when sequentially moving the surface of the structure and measuring multiple points on the plane. It becomes possible to estimate the position of the internal defect D at . Specifically, the position where the absolute value of the amplitude of the difference waveform is the minimum or the position where the positive/negative of the amplitude of the difference waveform is reversed is determined by measurement, and this position is determined as the position directly above the internal defect D on the plane. It can be estimated.

In actual measurements, the results often do not match the theoretical calculations at the pair of measurement points P1 and P2 due to differences in sensitivity of the elastic wave sensor 4, differences in coupling impedance due to grounding conditions, minute errors in measurement positions, etc. . Therefore, it is preferable to perform signal processing to remove these errors and extract necessary information.

Among these error factors, the one that has a relatively large influence is the error in the measurement position. If the size of the ground plane of the elastic wave sensor 4 occupies a large proportion of the distance between the input point F and the measurement points P1 and P2, a minute error in the installation position of the elastic wave sensor 4 will affect the measurement results. becomes big. Therefore, it is preferable to perform distance correction of this measurement position according to the following procedure.

The procedure for distance correction is to detect the delay time between the received waves measured at the paired measurement points P1 and P2, and to correct the distance between each measurement point P1 and P2 and the input point F based on the delay time. I do. As a method for detecting the delay time, any known method used for correcting the delay time between two waveforms can be adopted without limitation. It is preferable to use a method of detecting the time when . In addition, a method of detecting the rise time of the received wave, a method of detecting the delay time from the cross-correlation between the received waves of the pair of measurement points P1 and P2, etc. may be used.

After correcting the distance of the measurement positions as described above, the received waves measured at the pair of measurement points P1 and P2 are each subjected to Hilbert transformation and separated into amplitude information and phase information. In the Hilbert transform, after passing through a frequency filter that passes only the positive frequency region in the complex Fourier transform, a time axis waveform is obtained by inverse Fourier transform, and its amplitude and phase can be separated and determined. (See the formula below and Figure 8).

x(t)=Acos(ωt)=Acos(θ)
x(t)=xr(t)+i×xi(t)
xr(t)=A(t)cosθ(t), xi(t)=A(t)sinθ(t)
Amplitude A(t)=(xr(t) ² +xi(t) ² ) ^1/2
Phase θ(t)=tan ^-1 (xi(t)/xr(t))
Then, an amplitude difference waveform, which is a difference waveform between the amplitude information of the pair of measurement points P1 and P2, and a phase difference waveform, which is a difference waveform between the phase information, are obtained. When obtaining these differential waveforms, one (for example, P1) of the pair of measurement points P1 and P2 is always subtracted from the other (P2).

As shown in FIG. 9, while maintaining the relative positional relationship between input point F and measurement points P1 and P2, the surface of the structure is moved sequentially to obtain multiple input points (F11 to F15 in the illustrated example). When measured, as shown in the illustrated example, the measurement position may have a relationship that is bilaterally symmetrical with respect to the position (F13) of the internal defect D on the plane. For example, in FIG. 9, input points F11 and F15 are symmetrical with respect to the area directly above internal defect D (F13), and the measurement point on the other side of input point F11 (F11-P2) and the input point The measurement point on one side of F15 (F15-P1) is the same measurement point. That is, when comparing the difference waveforms, the difference waveform at the input point F11 and the difference waveform at the input point F15 are theoretically waveforms whose polarity is inverted. In this way, when the surface of the structure is sequentially moved and measured at multiple input points, the center position between the input points where a waveform with reversed sign is obtained can be estimated to be the position directly above the internal defect D, and the plane It becomes possible to estimate the position of the internal defect D on the surface. Further, as shown in FIG. 7, even when an input point where the amplitude of the difference waveform becomes zero is obtained, it is possible to estimate that the input point is a position directly above the internal defect D.

Estimation of the position of internal defect D on such a plane can be performed using either the amplitude difference waveform or the phase difference waveform described above. Alternatively, either the amplitude difference waveform or the phase difference waveform may be used.

Next, a method for estimating the depth of the internal defect D will be explained. The depth of this internal defect D can be estimated based on the time from the input of the elastic wave to the arrival of the reflected wave at each of the pair of measurement points P1 and P2. Specifically, the received waves measured at the pair of measurement points P1 and P2 are each subjected to Hilbert transform to calculate the phase, and the phase difference, which is the amount of change in the phase per unit time, is calculated.

Here, if there is an internal defect D, the reflected wave reflected by the internal defect is added (interferes) to the received wave at a predetermined time, so the measured vibration waveform becomes a waveform with disordered amplitude and phase. The phase obtained by Hilbert transformation of is a sawtooth waveform with different folding intervals. Further, the phase difference begins to change depending on the time when the reflected wave reaches the surface of the structure.

In this way, by calculating the phase by subjecting the received wave to Hilbert transformation and calculating the phase difference, which is the amount of change in the phase, it is possible to prevent the waves propagating on the surface and the reflected waves from interfering with each other. Since the time at which the phase difference changes can be obtained by multiplying the time from the input of the elastic wave until the phase difference changes by the propagation speed of the longitudinal elastic wave inside the structure, the time to the internal defect D can be calculated. You will be able to get the round trip distance. Furthermore, the position of the internal defect D can be estimated from the round trip distance to the internal defect D.

To explain with an example, when the elastic wave input point F and the pair of measurement points P1 and P2 are arranged as shown in FIG. 9, the received wave is measured when the elastic wave is input to the input point F11. Measurements were taken at points F11-P1 and F11-P2, and received waves when an elastic wave was input to input point F15 were measured at measurement points F15-P1 and F15-P2. Note that the measurement point F11-P2 on the right side of the input point F11 and the measurement point F15-P1 on the left side of the input point F15 are at the same position. This measurement point F11-P2 (F15-P1) is located almost directly above the internal defect D, and the input points F11 and F15 are arranged at almost symmetrical positions.

At this time, if the phase difference is calculated from the phase obtained by Hilbert transformation of the received waves measured at each measurement point, the result will be as shown in FIG. 10. As shown in FIG. 10, in both cases of input point F11 and input point F15, a difference occurs in the phase difference between P1 and P2 around the time of 0.035 ms. This difference in phase difference has a reverse relationship such that in the case of the input point F11, the P1 side has a large value, and in the case of the input point F15, the P2 side has a large value. This means that the direction of the trend is reversed, and that the reflected wave from internal defect D is reflected from the measurement point far from internal defect D (measurement point F11-P2 at input point F11, measurement point F11-P2 at input point F15). Since this shows that the reflected wave has also reached point F15-P1), this time of 0.035 ms can be determined to be the time when the reflected wave arrived.

By multiplying this time of 0.035 ms by the propagation speed of elastic waves inside the structure (4000 m/s in the case of concrete), the round trip distance from the surface of the structure to the internal defect D can be obtained. Thereafter, as shown in FIG. 9, an elliptical arc-shaped defect position assumption curve 9 having a round trip distance equivalent to the round trip distance to the internal defect D is drawn on the cross-sectional view of the structure. If the defect position assumption curves 9, 9 are similarly drawn for each input point F11, F15, the position of the internal defect D can be estimated from the position where these curves intersect or are close to each other. The defect position assumption curves 9, 9 indicate that the distance from the input point of the elastic wave to the measurement point through the inside of the structure is the distance of the elastic wave inside the structure in the time until the above-mentioned phase difference changes. It is an elliptical arc-shaped curve centered on two points, the input point and the measurement point of the elastic wave, drawn so as to have the round trip distance to the internal defect D obtained by multiplying the propagation speed. The round trip distance to the internal defect D obtained from the measurement results for each input point indicates that the internal defect D exists on the defect position assumption curve 9. Therefore, by measuring the vibration at a plurality of input points and/or by measuring the vibration at a plurality of paired measurement points, the position of the internal defect D can be estimated more precisely. Furthermore, it is also possible to estimate the size of the internal defect D from the range where a plurality of defect position assumption curves 9 intersect.

The internal defects of structures targeted by this exploration method can include a wide range of defects that occur inside concrete structures, such as cavities, cracks, junkers, and delaminations.

In particular, in the structure, after the concrete has hardened, a prestressed steel material is inserted into a sheath buried in advance, and after tension is introduced into the prestressed steel material, a gap between the prestressed steel material and the prestressed steel material in the sheath is inserted. When the target is a post-tensioned PC structure in which grout is injected into the grout, it is suitable to use this inspection method to confirm the filling state of the grout.

Grout is filled in the sheath pipes of prestressed steel materials in prestressed concrete structures such as bridges, viaducts, and buildings, but if a grout filling failure occurs in the sheath pipes of such prestressed steel materials, it may be caused by this. Corrosion and breakage of the PC steel may occur. Therefore, it is very important to investigate the degree of grout filling in the sheath pipe of PC steel materials using the impact elastic wave method.

By using this detection method to detect internal defects in PC structures, it is possible to accurately confirm insufficient grout filling, and to take appropriate measures such as refilling unfilled areas. Become.

In addition, unlike the conventional impact elastic wave method, this exploration method is based on the assumption that the input elastic wave is reflected at the opposing surface and undergoes multiple reflections in the thickness direction of the structure. Since this is not a targeted technology, it can be applied to structures with a cross-sectional shape other than a plate shape, such as the lower flange of a T girder, or to thick structures, and internal defects can be detected with high accuracy.

The effects of this exploration method can be summarized as follows. (1) Simple waveform processing that reflects actual conditions eliminates arbitrariness, making it possible to detect internal defects with higher accuracy. (2) Reproducibility and reliability can be improved by sequentially moving the measurement position and observing the amplitude change state of the differential waveform. (3) Accuracy and reliability can be further improved by observing the phase state in addition to the amplitude. (4) Detection accuracy can be further improved by providing multiple pairs of measurement points.

[Second form example]
In the first embodiment described above, the measurement points P1 and P2 are placed at symmetrical positions equidistant from the elastic wave input point F, but even if the distance between the elastic wave input point F and the measurement point is equidistant, For example, the surface wave components at each measurement point have the same phase and amplitude, and by taking the difference waveform between the two measurement points, the surface wave component is removed (cancelled) and the influence of only the reflected wave from the internal defect D is eliminated. In the second embodiment, as shown in FIG. 11, a plurality of measurement points P1, three in the illustrated example, are placed on a circle centered on the input point F of the elastic wave in plan view. P2 and P3 are arranged. Then, by finding the difference waveform between the received waves measured at each measurement point P1 to P3, the position of the internal defect D is estimated with the influence of the surface waves propagating on the surface of the structure removed. . In this way, by arranging the plurality of measurement points P1 to P3 on a circle centered on the input point F in plan view, the position of the internal defect can be detected three-dimensionally.

To explain in more detail below, the measurement points in the second embodiment are two or more, preferably three or more, at a predetermined interval in the circumferential direction on a circle centered on the elastic wave input point F. They are arranged at 10 locations, more preferably at 3 to 5 locations. Although the arrangement interval in the circumferential direction is arbitrary, it is preferable to arrange the arrangement at approximately equal intervals. The number of measurement points may be an even number or an odd number.

When there are three or more measurement points, when calculating the difference waveform between the received waves measured at each measurement point, calculate the difference waveform between the received waves measured at two of the multiple measurement points. demand. When calculating the difference waveform, you can calculate the difference waveform for all combinations that can be extracted from multiple measurement points, or you can calculate the difference waveform only for the measurement point where the amplitude is the maximum and the measurement point where the amplitude is the minimum among the multiple measurement points. You can.

The smaller the distance between the internal defect D and the measurement point, the greater the effect of the reflected wave from the internal defect D, and the larger the distance, the smaller the effect of the reflected wave. Therefore, by determining the difference waveform of each measurement point using the method described above, and by determining the combination of measurement points with the largest difference waveform among multiple measurement points, the relative relationship between these measurement points and input point F can be calculated. From the positional relationship, it becomes possible to understand three-dimensionally in which direction the internal defect D exists with respect to the input point F. Thereby, by increasing the density of input points of elastic waves in that direction, the position of the internal defect D can be estimated with higher accuracy.

When measuring multiple points by sequentially moving the surface of a structure while maintaining the relative positional relationship between the input point F and the measurement point, it is necessary to sequentially move the surface of the structure while maintaining the relative angle between the input point F and the measurement point. Make it move.

1...Blowing device, 2...Plunger, 3...Steel ball, 4...Elastic wave sensor, 9...Defect position assumed curve, D...Internal defect, F...Input point, P1, P2, P3...Measurement point

Claims (7)

A method of detecting internal defects by receiving surface vibrations when an elastic wave is input to the surface of a structure,
Multiple measurement points are placed equidistant from the elastic wave input point, and the received waves measured at each measurement point are separated into amplitude information and phase information by Hilbert transform, and the amplitude difference waveform between the received waves is calculated. and a phase difference waveform, and then estimating the position of the internal defect from both or either of the amplitude difference waveform and the phase difference waveform . 2. The internal defect detection method according to claim 1, wherein the measurement points are arranged at symmetrical positions equidistant from the input point of the elastic wave. 2. The internal defect exploration method according to claim 1, wherein a plurality of measurement points are arranged on a circle centered on the input point of the elastic wave in plan view. While maintaining the relative positional relationship between the input point and the measurement point, change in amplitude of both or either of the amplitude difference waveform and the phase difference waveform when the surface of the structure is sequentially moved and multiple points are measured. The internal defect exploration method according to any one of claims 1 to 3 , wherein the position of the internal defect on a plane is estimated from the state. Exploration of internal defects according to any one of claims 1 to 4 , wherein the delay time between the received waves measured at each measurement point is detected, and the distance between each measurement point and the input point is corrected based on the delay time. Method. The received waves measured at each measurement point are each subjected to Hilbert transformation to calculate the phase, and the phase difference, which is the amount of change in the phase per unit time, is calculated. After obtaining the time until a change occurs in the phase difference, the position of the internal defect is estimated from the round trip distance to the internal defect obtained by multiplying the time by the propagation speed of an elastic wave inside the structure. 5. The internal defect detection method according to any one of 5 . In the structure, after the concrete has hardened, a prestressed steel material is inserted into a sheath buried in advance, tension is introduced into the prestressed steel material, and then grout is inserted into the gap between the prestressed steel material and the prestressed steel material within the sheath. It is a post-tensioned PC structure made by injecting
The internal defect exploration method according to any one of claims 1 to 6 , wherein the exploration method is used to confirm the filling state of the grout.

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