JP6922866B2 - Stress evaluation method - Google Patents

Description

The present invention relates to a stress evaluation method for evaluating the stress of a structure.

Conventionally, the strain gauge method is the most common method for diagnosing the stress of a structure. This strain gauge method is a method in which a strain gauge is attached to an object, the relative strain is measured as a change in the electrical resistance of the strain gauge, and the strain is converted into stress for evaluation (see Patent Document 1).

Japanese Unexamined Patent Publication No. 2017-049112

Tatsuya Oshio et al., Development of stress stethoscope using friction type strain gauge and application to soundness diagnosis of structures, 60th Annual Scientific Lecture Meeting of Japan Society of Civil Engineers, September 2005

However, in the conventional strain gauge method, it is necessary to remove the non-metal layer such as coating or coating applied to the structure which is the metal substrate, polish the surface of the metal substrate, and then attach the strain gauge. In addition, a dedicated measuring instrument is required to measure the stress of the structure by the strain gauge method. Therefore, when the stress is measured by the strain gauge method for many parts of the structure, there is a problem that both the mounting work and the measuring work become complicated. Further, in the strain gauge method, there is a problem that the stress can be measured only in the portion to which the strain gauge is attached.

The present invention has been made in view of the above, and an object of the present invention is a stress evaluation that can easily evaluate the stress at an arbitrary position on the metal substrate of the lower layer of the non-metal layer without removing the non-metal layer. To provide a method.

(1) In order to solve the above-mentioned problems and achieve the object, the stress evaluation method according to one aspect of the present invention is terahertz at a predetermined position on the surface of an object in which a non-metal layer is provided on an upper layer of a metal substrate. A terahertz wave transmitting means capable of irradiating waves and scanning the surface of the object, and a terahertz wave reflected at the predetermined position of the object can be detected. In a stress evaluation method using a terahertz wave detecting means capable of scanning the surface of an object, the non-metal layer is irradiated with the terahertz wave emitted from the terahertz wave transmitting means, and the terahertz wave is emitted. By detecting the intensity of the terahertz wave transmitted through the non-metal layer by the detecting means, the stress state in the non-metal layer is measured, and the non-metal layer is used as a strain detecting means in the photoelastic method. It is characterized by evaluating the stress state in the substrate.

(2) The stress evaluation method according to one aspect of the present invention is characterized in that, in the above invention (1), the stress states are the principal stress difference and the principal stress direction.

(3) In the stress evaluation method according to one aspect of the present invention, in the invention of (2) above, the stress evaluation apparatus further has an analysis processing means, and the analysis processing means includes the principal stress difference and the principal stress. It is characterized in that the principal stress component and the maximum principal stress direction in the metal substrate are derived based on the direction.

According to the stress evaluation method according to the present invention, by using the non-metal layer as a strain detecting means, the stress at an arbitrary position on the metal substrate under the non-metal layer can be easily applied without removing the non-metal layer. It becomes possible to evaluate.

FIG. 1 is a diagram showing a configuration of a stress measuring device according to an embodiment of the present invention. FIG. 2 is a diagram showing another configuration of the stress measuring device according to the embodiment of the present invention. FIG. 3 is a diagram showing a schematic configuration of a photoelastic stress measuring device for explaining a stress evaluation method by a conventional photoelastic method.

Hereinafter, the stress evaluation method and the stress evaluation device according to the embodiment of the present invention will be described with reference to the drawings. In all the drawings of the following embodiment, the same or corresponding parts are designated by the same reference numerals. Further, the present invention is not limited to the embodiments described below.

First, in explaining the stress evaluation method according to the present invention, the principle of the present invention will be described in order to facilitate understanding of the present invention. Normally, the surface of a structure is coated with a paint or resin having excellent environmental resistance to protect the members. A typical example of these is a non-metal layer as an anticorrosive layer that protects a steel structure from corrosion, such as painting or coating. These non-metal layers are usually in close contact with the surface of the member. Therefore, since this non-metal layer is deformed according to the member, when strain is generated in steel or the like as a metal substrate constituting the structure, the non-metal layer in close contact with the surface is also strained. According to the findings of the present inventor based on Non-Patent Document 1, the strain of the metal substrate and the strain of the non-metal layer on the surface are substantially the same. Therefore, since the strain of the metal substrate can be evaluated by measuring the strain of the non-metal layer, the stress state of the metal substrate can also be evaluated.

Conventionally, a photoelastic method is known as a method for evaluating the stress of a transparent material such as resin or glass. FIG. 3 is a diagram showing a schematic configuration of a photoelastic stress measuring device for explaining a stress measuring method by a conventional photoelastic method.

As shown in FIG. 3, the conventional photoelastic stress evaluation device 100 includes a visible light source 101, a first polarizing plate 102, a λ / 4 wave plate 103, 104, and a second polarizing plate 105, which are arranged linearly with each other. And a camera 106. If necessary, a lens or the like is further provided. Of these, the first polarizing plate 102 and the second polarizing plate 105 are provided so that their polarization directions differ from each other by 90 ° (π / 2). On the other hand, the λ / 4 wave plates 103 and 104 are provided so that their principal axial directions differ from each other by 90 °. On top of that, the polarization direction of the first polarizing plate 102 and the spindle direction of the λ / 4 wave plate 103 are provided so as to be different from each other by 45 ° (π / 4). Similarly, the main axis direction of the λ / 4 wave plate 104 and the polarization direction of the second polarizing plate 105 are provided so as to be different from each other by 45 °. The measurement object 110 made of a photoelastic body whose stress is measured is made of, for example, an epoxy resin that is transparent to visible light. The measurement object 110 is arranged at a predetermined position between the λ / 4 wave plates 103 and 104.

When measuring the stress generated in the object to be measured 110 by the photoelastic stress evaluation device 100, first, visible light is emitted from the visible light source 101. The visible light emitted from the visible light source 101 is linearly polarized by the first polarizing plate 102 and then circularly polarized by the λ / 4 wave plate 103. Visible light that has become circularly polarized light enters the measurement object 110 and causes a phase difference δ due to birefringence caused by the stress field of the measurement object 110, resulting in elliptically polarized light. The elliptically polarized light includes the case of circularly polarized light. Here, the phase difference δ generated by birefringence is proportional to _{the principal stress difference (σ 1} − σ ₂ ) of the object to be measured 110, as shown in the following equation (1).
δ = 2πCt (σ _{1 −σ 2} ) / λ… (1)
Λ is the wavelength of the light to be used, C is the photoelastic coefficient of the material of the object to be measured 110, and t is the thickness of the object to be measured 110.

The light transmitted through the object to be measured 110 is linearly polarized by the λ / 4 wave plate 104, and then the polarized light component passing through the second polarizing plate 105 is imaged by the camera 106. As a result, an image depending on the phase difference generated in the measurement object 110 can be captured by the camera 106, and the stress state of the measurement object 110 can be observed. The image captured by the camera 106 is obtained as an image of photoelastic fringes of a bright and dark pattern on the measurement object 110. The obtained light-dark pattern photoelastic fringes are called isochromatic lines. _{By observing the imaged uniform color lines, the principal stress difference (σ 1} − σ ₂ ) as a photoelastic parameter can be derived from the thickness t of the object to be measured 110 and the photoelastic coefficient C.

On the other hand, in the photoelastic stress evaluation device 100 shown in FIG. 3, the principal stress direction can be further measured by removing the λ / 4 wave plates 103 and 104. That is, in the photoelastic stress evaluation device 100, the λ / 4 wave plates 103 and 104 are removed from the optical axis between the visible light source 101 and the camera 106. In this configuration, the visible light emitted from the visible light source 101 passes through the first polarizing plate 102, is polarized into linearly polarized light, and then enters the measurement object 110. Here, when the polarization direction of the first polarizing plate 102, that is, the polarization direction of the linearly polarized light and the principal stress direction coincide with each other, the linearly polarized light passes through the object to be measured 110 without changing the polarization direction. Since the light transmitted through the object to be measured 110 is linearly polarized light parallel to the polarization direction of the first polarizing plate 102, it hardly transmits through the second polarizing plate 105. In this case, when the light transmitted through the second polarizing plate 105 is observed, a dark line called an isobaric line appears in the measurement object 110. On the other hand, when the polarization direction and the principal stress direction of the first polarizing plate 102 do not match, the light transmitted through the measurement object 110 is transmitted through the measurement object 110 because a phase difference δ is generated by birefringence. The resulting light becomes elliptically polarized light, is polarized by the second polarizing plate 105, and is transmitted. The intensity of the transmitted light changes according to the difference in angle between the polarization direction and the principal stress direction of the first polarizing plate 102. As a result, the angle of the polarization direction of the visible light incident on the measurement object 110 is changed while maintaining the state in which the first polarizing plate 102 and the second polarizing plate 105 are different from each other by 90 °. When the light is rotated to, the light imaged by the camera 106 becomes bright and dark, and the photoelastic fringes are imaged corresponding to the rotation angle of the first polarizing plate 102. Based on the image of the photoelastic fringes captured by the camera 106, the principal stress direction in the measurement object 110 can be derived.

As described above, the photoelastic stress evaluation device 100 _{can detect the principal stress difference (σ 1} − σ ₂ ) and the principal stress direction as photoelastic parameters. When the principal stress difference (σ ₁ − σ ₂ ) and the principal stress direction are obtained, for example, the principal stress components σ ₁ and σ ₂ in the object to be measured 110 are derived in a separated state by the shear stress difference integration method. Will be possible. If the principal stress components σ ₁ and σ ₂ and the principal stress direction (maximum principal stress direction) are obtained, the state of the measurement object 110, that is, the soundness of the measurement object 110 can be evaluated in detail.

However, in the stress measurement by the photoelastic method using the conventional photoelastic stress evaluation device 100 described above, since the visible light emitted from the visible light source 101 is used, the visible light is used as the measurement object 110. It was limited to those made of transparent transparent materials. On the other hand, the resin material that constitutes the coating or coating used for corrosion protection against actual structures is opaque to visible light, so the photoelastic method using visible light is used. Was difficult to apply.

Therefore, the present inventor conceived to use a terahertz wave, which is a kind of electromagnetic wave, instead of visible light, and studied a stress evaluation method by a photoelastic method using a terahertz wave. Most of the terahertz waves are transmitted when irradiated with a non-metal material such as resin, while most of them are reflected when irradiated with a metal material. The present inventor pays attention to the property of the terahertz wave, and by using it in combination with the principle of the photoelastic method in which the birefringence phenomenon of electromagnetic waves is generated by strain, even in a resin which is a non-metal layer opaque to visible light. I came up with the idea that strain can be measured. Furthermore, the present inventor has found that by evaluating the strain of the non-metal layer, the strain of an object such as a steel material in the lower layer can be evaluated. Such a method is referred to as a "terahertz wave photoelastic method" in the present specification. That is, the present inventor has come up with the idea of using a non-metal layer made of resin such as an anticorrosion layer in close contact with the surface of a structure as a strain sensor for measuring the strain of the structure by using a terahertz wave. .. By measuring the strain of the non-metal layer using the terahertz wave photoelastic method described above by the inventor's diligent study, the strain at an arbitrary position of the structure can be measured in a non-contact manner. Furthermore, the present inventor has conceived that a terahertz wave photoelastic method using reflection is also possible because most of the terahertz waves are reflected when the metal material is irradiated. The present invention described below has been devised by the above diligent studies.

(Stress evaluation device)
FIG. 1 is a diagram showing a configuration of a stress evaluation device according to an embodiment of the present invention. As shown in FIG. 1, the stress evaluation device 1 according to the embodiment includes an analysis control unit 10, a terahertz wave transmitter 11, and a terahertz wave detector 12. In the steel structure 15 to be measured in one embodiment, an anticorrosion layer 15b of a non-metal layer made of various resins such as coating and coating is provided on the surface of the steel material 15a as a metal substrate.

The stress evaluation device 1 is configured so that the terahertz wave L ₁ is polarized so that the surface of the steel structure 15 can be irradiated, and the terahertz wave L ₂ reflected from the steel structure 15 is polarized and then detected. It consists of a reflective terahertz wave measuring device. That is, the stress evaluation device 1 has both a terahertz wave transmitting means and a terahertz wave detecting means. Here, the terahertz wave is an electromagnetic wave belonging to the so-called terahertz region, which is a frequency region of about ^{1 terahertz (1 THz = 10 12} Hz), specifically, a frequency region on the order of 100 GHz to 10 THz (10 ¹¹ to 10 ^{13 Hz).} The terahertz region is a frequency region that has both straightness of light and transmission of radio waves. In one embodiment, the frequency of the terahertz wave can be selected according to conditions such as the material and thickness of the anticorrosion layer 15b, and is selected according to the attenuation (transmittance) of the terahertz wave in the anticorrosion layer 15b. You may.

The terahertz wave transmitter 11 as a terahertz wave transmitting means includes, for example, a terahertz wave generating element 11a equipped with a resonance tunneling diode (RTD) or the like, a hemispherical lens 11b, a collimating lens 11c, and an objective lens 11d. It is composed. A light conducting antenna (PCA) may be used instead of the resonance tunnel diode. On the transmitting side of the terahertz wave transmitter 11, a first polarizing plate 13a as a transmitting side linearly polarizing means and a λ / 4 wave plate 14a as a transmitting side phase conversion means are provided. The linearly polarized light means functions as a phase conversion means for performing phase conversion on electromagnetic waves. The polarization direction of the first polarizing plate 13a and the main axis direction of the λ / 4 wave plate 14a are provided so as to be different from each other by 45 °. The terahertz wave transmitter 11, the first polarizing plate 13a, and the λ / 4 wavelength plate 14a constitute an optical system arranged linearly, but are not necessarily limited to the case where they are arranged linearly. , An optical system that bends the terahertz wave may be further provided with a reflection mirror or the like that reflects the terahertz wave L _1. Terahertz wave transmitter 11, outgoing optical system composed of the first polarizing plate 13a, and lambda / 4 wave plate 14a is a terahertz wave L _1p is linearly polarized light of the terahertz wave L _1, the plane of the steel structures 15 It is configured so that it can be irradiated at a predetermined angle α.

The terahertz wave detector 12 as the terahertz wave detecting means includes, for example, a terahertz wave detecting element 12a made of an RTD, a hemispherical lens 12b, and a condenser lens 12c. The terahertz wave detector 12 is provided in the stress evaluation device 1 in a state where the reflected waves of the terahertz waves (terahertz waves L ₂ , L _{2p) can be received by the terahertz wave detection element 12a.} On the detection side of the terahertz wave detector 12, a second polarizing plate 13b as a detection-side linearly polarizing means and a λ / 4 wave plate 14b as a detection-side phase conversion means are provided. The main axis direction of the λ / 4 wavelength plate 14b and the polarization direction of the second polarizing plate 13b are provided so as to be different from each other by 45 °.

The first polarizing plate 13a and the second polarizing plate 13b are provided so that the polarization directions differ from each other by 90 ° (π / 2). Here, the fact that the first polarizing plate 13a and the second polarizing plate 13b are provided so that the polarization directions differ by 90 ° means that the λ / 4 wavelength plates 14a and 14b are not provided and the compound refraction phenomenon occurs. After being linearly polarized by the first polarizing plate 13a in a state where no member is provided, the terahertz wave reflected on a predetermined surface does not pass through the second polarizing plate 13b without changing the polarization state. Is to become. Further, due to the relationship between the first polarizing plate 13a and the second polarizing plate 13b described above, the λ / 4 wave plates 14a and 14b differ from each other by 90 ° in the main axis direction.

In the stress evaluation device 1 configured as described above, the terahertz including at least a terahertz wave transmitter 11, a terahertz wave detector 12, a first polarizing plate 13a, a second polarizing plate 13b, and a λ / 4 wave plate 14a, 14b. The wave optical system is integrally traversable with respect to the steel structure 15. As a result, the stress evaluation device 1 is configured to be able to irradiate the _{terahertz wave L 1} and detect the reflected terahertz wave while scanning a predetermined range on the surface of the steel structure 15. As a scanning mechanism for scanning the terahertz wave optical system of the stress evaluation device 1 relative to the surface of the steel structure 15, various conventionally known scanning mechanisms can be adopted, and further, scanning is performed manually. It is also possible to let it.

The analysis control unit 10 as an analysis means and a control means includes a signal amplification unit 10a, a bias generation unit 10b, a lock-in detection unit 10c, and an analysis processing unit 10d. The analysis control unit 10 performs various controls on the terahertz wave transmitter 11. Further, the analysis control unit 10 performs various processes on the terahertz wave signal detected by the terahertz wave detector 12. The signal amplification unit 10a amplifies the signal detected by the terahertz wave detector 12 and outputs it to the lock-in detection unit 10c as terahertz wave reception data. The bias generation unit 10b generates a bias voltage to bias the terahertz wave generating element 11a and the terahertz wave detecting element 12a, thereby changing the transmitted terahertz wave or the detected terahertz wave according to the bias voltage. The terahertz wave transmitted or detected by the terahertz wave generating element 11a and the terahertz wave detecting element 12a may be weak. In this embodiment, an example is shown in which the transmitted or detected terahertz wave is weak, and lock-in detection is used to detect the terahertz wave. At the time of lock-in detection, the terahertz wave transmitter 11 uses a modulated reference signal as the bias voltage of the terahertz wave generating element 11a, so that the noise component of the terahertz wave detection signal is removed. As a result, even if the transmitted or detected terahertz wave is weak, the detection can be performed with high accuracy. The analysis processing unit 10d as an analysis processing means includes a predetermined recording unit (not shown) for storing the detected terahertz wave reception data, and performs analysis processing on the terahertz wave reception data.

(Stress evaluation method)
(Measurement method of principal stress difference)
Next, the measurement of stress by the stress evaluation device 1 configured as described above will be described. As described above, the steel structure 15 is provided with the anticorrosion layer 15b on the steel surface 15as of the steel material 15a. The steel material 15a is a typical material generally used in structures such as bridges and pipes. The steel structure 15 includes, for example, a steel structure having an anticorrosion layer, and a non-metal layer on the upper layer of the base, with a predetermined surface of a metal base such as aluminum (Al) or stainless steel (SUS) as a base. It can be a variety of formed objects.

The anticorrosion layer 15b functions as an anticorrosion layer on the steel surface 15as of the underlying steel material 15a, and also includes an adhesive and the like. The anticorrosion layer 15b is provided in close contact with the steel surface 15as of the steel material 15a. Therefore, when strain is generated in the steel material 15a provided with the anticorrosion layer 15b due to stress, most of the generated strain is propagated to the anticorrosion layer 15b, and strain similar to that of the steel material 15a is also generated in the anticorrosion layer 15b. Therefore, the steel structure 15 is inspected in advance by a conventionally known method, for example, a peeling test or the like to check whether the anticorrosion layer 15b is in close contact with the steel surface 15as of the steel material 15a.

In steel structure 15 in a state where anticorrosion layer 15b is in close contact with the steel surface 15As, it emits the terahertz wave L ₁ toward the terahertz wave oscillator 11 of the stress evaluation apparatus 1 to the surface 15bs of the anticorrosion layer 15b. Specifically, the terahertz wave generated in the terahertz wave generating element 11a is emitted as a _{terahertz wave L 1} via the hemispherical lens 11b, the collimating lens 11c, and the objective lens 11d. Here, the transmitted terahertz wave L ₁ is typically a continuously transmitted terahertz continuous wave, but may be an intermittently transmitted terahertz pulse wave or tone burst wave.

Terahertz wave L ₁ originating from the terahertz wave oscillator 11 is linearly polarized by the first polarizer 13a. The linearly polarized terahertz wave that has passed through the first polarizing plate 13a passes through the λ / 4 wave plate 14a and becomes circularly polarized light. The circularly polarized terahertz wave L _1p is incident on the anticorrosion layer 15b of the steel structure 15 at an incident angle of a predetermined angle α. As described above, the anticorrosion layer 15b is strained according to the strain of the lower steel material 15a. Therefore, the terahertz wave L _1p incident on the anticorrosion layer 15b is completely reflected by the steel surface 15as while birefringence corresponding to the strain occurs in the anticorrosion layer 15b. _{The terahertz wave L 2} completely reflected on the steel surface 15 as is emitted from the surface 15 bs while birefringence corresponding to the strain occurs in the anticorrosion layer 15 b.

_{The terahertz wave L 2} emitted from the surface 15 bs has a phase difference δ due to birefringence and is elliptically polarized light or circularly polarized light. _{The terahertz wave L 2p,} which is a polarized component that has passed through the plate 13b, is detected by the terahertz wave detector 12. As a result, the terahertz wave detector 12 detects the ellipticity of the terahertz wave transmitted through the anticorrosion layer 15b and the intensity of the terahertz wave depending on the phase difference generated by the birefringence in the anticorrosion layer 15b.

_{As described above, the intensity of the terahertz wave L 2p} detected by the terahertz wave detector 12 is a physical quantity proportional to the principal stress difference (σ ₁ − σ _{2) of the anticorrosion layer 15b.} On the other hand, the photoelastic modulus and the thickness of the anticorrosion layer 15b are measured in advance. Then, when the intensity of the terahertz wave L _{2p is detected, the principal stress difference (σ 1} − σ ₂ ) of the anticorrosion layer 15b can be derived from the intensity, photoelastic coefficient, and thickness of the terahertz wave L _2p. As described above, the strain of the anticorrosion layer 15b conforms to the strain of the lower steel material 15a. Therefore, when the principal stress difference (σ ₁ − σ ₂ ) of the anticorrosion layer 15b is obtained, the principal strain difference (ε ₁ −ε ₂ ) of the anticorrosion layer 15b can be derived from this principal stress difference. Since the principal strain difference (ε ₁ −ε ₂ ) of the anticorrosion layer 15b is equivalent to the principal strain difference of the steel surface 15as, the principal stress difference (σ ₁ ′ −) of the steel surface 15as is based on various parameters of the steel material 15a. σ ₂ ′) can be derived. That is, the principal stress difference (σ ₁ ′ − σ ₂ ′) of the steel material 15a can be derived from the derived principal stress difference (σ ₁ − σ _{2) of the anticorrosion layer 15b.} These derivations are executed by the analysis processing unit 10d of the analysis control unit 10.

Further, the above-mentioned scanning mechanism causes the above-mentioned terahertz wave optical system to scan a predetermined range along the surface of the steel structure 15. As a result, the terahertz wave intensity distribution according to the strain of the anticorrosion layer 15b is detected by the terahertz wave detector 12 in a predetermined range of the surface 15bs of the steel structure 15. The intensity distribution of the terahertz wave detected by the terahertz wave detector 12 is obtained as the distribution of the color lines of the photoelastic fringes of the terahertz wave in the anticorrosion layer 15b. As described above, the distribution of the color matching lines of the photoelastic stripes in the obtained anticorrosion layer 15b is the distribution of the color matching lines in the steel material 15a. That is, the anticorrosion layer 15b functions as a strain sensor for detecting _{the principal stress difference (σ 1} − σ _{2) of the strain of the steel material 15a.} By detecting the strain state in the anticorrosion layer 15b as the strain sensor using a terahertz wave, it is possible to measure the strain state of the steel material 15a.

Here, the direction of the principal stress generated in the steel material 15a of the steel structure 15 described above may be clear depending on the shape, design, or the like. In this case, based on the principal stress difference (σ ₁ − σ ₂ ) obtained as described above and the principal stress direction that is clear from the shape, design, etc., the principal stress is applied by a conventionally known shear stress difference integration method. It is possible to obtain the components σ ₁ and σ ₂ in a form separated from each other. These derivations are executed by the analysis processing unit 10d of the analysis control unit 10 and the like. By deriving the principal stress components σ ₁ and σ ₂ and the principal stress direction, it is possible to evaluate in detail the state of the steel structure 15, that is, the soundness of the steel material 15a which is the object to be measured.

Furthermore, when the stress generated in the steel material 15a is a uniaxial stress field or a stress field close to the uniaxial stress field, the principal stress difference (σ ₁ − σ ₂ ) should be regarded as approximately the maximum principal stress component σ _1. Can be done. In this case, the derivation process for separating the principal stress components σ ₁ and σ _{2 from the principal stress difference (σ 1} − σ ₂ ) by the shear stress difference integration method can be omitted. That is, it is possible to evaluate the soundness of the steel material 15a in detail only by measuring the principal stress difference (σ ₁ − σ _2).

(Measuring device in the direction of principal stress)
On the other hand, when the principal stress direction generated in the steel material 15a which is the above-mentioned structure is unknown, it is necessary to measure the principal stress direction in addition to the measurement of _{the principal stress difference (σ 1} − σ _2). FIG. 2 is a diagram showing another configuration of the stress evaluation device according to one embodiment for measuring the principal stress direction.

As shown in FIG. 2, the stress evaluation device 2 has a configuration in which the λ / 4 wave plates 14a and 14b are not provided in the stress evaluation device 1. Further, the stress evaluation device 2 is provided with a first polarizing plate 21a and a second polarizing plate 21b, respectively, corresponding to the first polarizing plate 13a and the second polarizing plate 13b in the stress evaluation device 1. The first polarizing plate 21a and the second polarizing plate 21b are each composed of disk-shaped polarizing plates having the same diameter, and have a disk gear shape in which outer teeth having the same pitch are formed on the outer peripheral portion of the disk shape. Have.

In the stress evaluation device 2, a polarizing plate synchronous rotation mechanism 22 is provided between the first polarizing plate 21a and the second polarizing plate 21b. The polarizing plate synchronous rotation mechanism 22 includes a turning head 22a, a gearbox 22b, and a polarizing plate synchronous rotation gear 22c. The swivel head 22a is connected to the polarizing plate synchronous rotary gear 22c via a conventionally known gear box 22b. By rotating the swivel head 22a around the rotation shaft O, the polarizing plate synchronous rotary gear 22c is rotated via a plurality of gears in the gearbox 22b. External teeth that mesh with the external teeth of the outer peripheral portions of the first polarizing plate 21a and the second polarizing plate 21b are formed on the outer peripheral portion of the polarizing plate synchronous rotary gear 22c. By engaging the outer teeth of the polarizing plate synchronous rotation gear 22c with the outer teeth of the first polarizing plate 21a and the second polarizing plate 21b, the first polarizing plate 21a and the first polarizing plate 21a and the first polarizing plate 21a and the first polarizing plate 21a and the first polarizing plate 21a 2 The polarizing plate 21b rotates in the same rotation direction. Here, the outer diameter of the first polarizing plate 21a and the outer diameter of the second polarizing plate 21b are the same as each other. Therefore, when the turning head 22a is rotated to rotate the polarizing plate synchronous rotation gear 22c, the first polarizing plate 21a and the second polarizing plate 21b differ in the polarization direction by a predetermined polarization direction, here, by an angle of 90 °. It rotates in the same direction of rotation while maintaining its state. Other configurations are the same as those of the stress evaluation device 1.

(Measuring method in the direction of principal stress)
Next, a method of measuring the principal stress direction using the stress evaluation device 2 described above will be described. That is, as shown in FIG. 2, the terahertz wave L ₁ emitted from the terahertz wave oscillator 11, after being polarized into linearly polarized light passes through the first polarizer 21a, enters the anticorrosion layer 15b of steel structures 15 Will be done. _{The terahertz wave L 1p} incident on the anticorrosion layer 15b is completely reflected by the steel surface 15as of the steel material 15a and is emitted through the anticorrosion layer 15b. In this state, the polarizing plate synchronous rotation gear 22c is rotated to rotate the first polarizing plate 21a and the second polarizing plate 21b while maintaining a state in which the polarization directions differ from each other by 90 °.

As the first polarizing plate 21a and the second polarizing plate 21b rotate, the polarization direction of the first polarizing plate 21a, that is, the polarization direction of the linearly polarized terahertz wave L _1p and the principal stress direction of the anticorrosion layer 15b coincide with each other. A condition arises. In this state, the terahertz wave L _1p passes through the anticorrosion layer 15b without changing the linearly polarized light. The terahertz wave L _1p is further reflected by the steel surface 15as and passes through the anticorrosion layer 15b. _{The terahertz wave L 2} transmitted through the anticorrosion layer 15b is linearly polarized light along the polarization direction of the first polarizing plate 21a. Therefore, the terahertz wave L ₂ hardly transmits the second polarizing plate 21b in the polarization direction different from the polarization direction of the first polarizing plate 21a by 90 °, and the intensity of the _{terahertz wave L 2p transmitted through the second polarizing plate 21b.} Becomes minimal. In this case, when the terahertz wave L _2p transmitted through the second polarizing plate 21b is observed, a dark line called an isobaric line in which the intensity of the terahertz wave L _{2p is minimized appears in the anticorrosion layer 15b.}

As described above, when the polarization direction of the first polarizing plate 21a and the principal stress direction of the anticorrosion layer 15b coincide with each other, the terahertz wave L ₂ hardly passes through the second polarizing plate 21b. That is, in the X-axis and Y-axis shown in FIG. 2 and the second polarizing plate 21b, when the terahertz wave L _2p transmitted through the second polarizing plate 21b becomes the minimum, the polarization direction of the second polarizing plate 21b and the polarization direction of the second polarizing plate 21b. The direction orthogonal to the polarization direction of the second polarizing plate 21b is the principal stress direction. Thereby, the main stress direction of the anticorrosion layer 15b can be derived.

On the other hand, when the polarization direction and the principal stress direction of the first polarizing plate 21a do not match, the terahertz wave L _1p incident on the anticorrosion layer 15b causes a phase difference due to the birefringence generated in the anticorrosion layer 15b. Therefore, the terahertz wave L ₂ emitted from the anticorrosion layer 15b is elliptically polarized light or circularly polarized light. The elliptically polarized terahertz wave L ₂ passes through the second polarizing plate 21b and is polarized into linearly polarized light. The linearly polarized terahertz wave L _2p is detected by the terahertz wave detector 12. The intensity of the terahertz wave L _2p in this state is larger than the intensity of _{the terahertz wave L 2p} when the polarization direction of the first polarizing plate 21a and the principal stress direction of the anticorrosion layer 15b coincide with each other.

Based on the above changes in the state, _{the intensity of the terahertz wave L 2p} detected by the terahertz wave detector 12 alternates between maximum and minimum while the first polarizing plate 21a and the second polarizing plate 21b rotate by 90 °. Expresses in. _{As a result, when the principal stress direction is derived, the principal stress direction of the maximum principal stress component σ 1} can be derived based on the shape of the steel material 15a, the operating conditions of the external force, and the like by a conventionally known method.

Further, by the scanning mechanism (not shown) described above, a second terahertz wave optical system in which at least a terahertz wave transmitter 11, a terahertz wave detector 12, a first polarizing plate 21a, and a second polarizing plate 21b are integrated is provided. A predetermined range is scanned along the surface of the steel structure 15. As a result, the terahertz wave detector 12 detects the intensity distribution of _{the terahertz wave L 2p} along the principal stress direction of the anticorrosion layer 15b in a predetermined range of the steel structure 15. _{The intensity distribution of the terahertz wave L 2p} detected by the terahertz wave detector 12 is obtained as an isotonic line of the terahertz wave in the anticorrosion layer 15b. Since the anticorrosion layer 15b is in close contact with the steel surface 15as of the steel material 15a, the isotonic lines of the obtained photoelastic stripes are the isotonic lines of the steel material 15a. That is, the anticorrosion layer 15b functions as a strain sensor for detecting the principal stress direction of the strain of the steel material 15a.

Based on the above method, the principal stress difference (σ ₁ − σ ₂ ) and the principal stress direction are derived. Based on the derived principal stress difference (σ ₁ − σ ₂ ) and the principal stress direction, the principal stress components σ ₁ and σ ₂ can be derived in a form separated from each other by a conventionally known shear stress difference integration method. .. These derivations are executed by the analysis processing unit 10d of the analysis control unit 10 and the like. Once the photoelastic parameters such as the principal stress components σ ₁ and σ ₂ and the principal stress direction in the anticorrosion layer 15b are known, the strain of the anticorrosion layer 15b can be derived based on the elastic constant of the anticorrosion layer 15b. As described above, the anticorrosion layer 15b and the steel material 15a constituting the underlying structure are usually in close contact with each other, and the strain of the anticorrosion layer 15b conforms to the strain of the steel material 15a. Therefore, the stress generated in the steel material 15a can be derived based on the strain of the steel material 15a obtained as the strain of the anticorrosion layer 15b and the elastic modulus of the steel material 15a.

That is, the stress evaluation devices 1 and 2 can measure the strain state of the steel material 15a by detecting the strain state of the anticorrosion layer 15b serving as a strain sensor using a terahertz wave. As a result, when the principal stress components σ ₁ and σ ₂ and the principal stress direction are derived, the stress state of the steel structure 15, that is, the stress state of the steel material 15a, which is the object to be measured, can be measured. In other words, by deriving the stress state of the anticorrosion layer 15b by the terahertz wave photoelastic method, the stress state of the lower steel material 15a can be finally derived and the soundness can be evaluated in detail.

(Modification example)
Next, a modified example of the method for measuring the principal stress difference and the method for measuring the principal stress direction according to the above-described embodiment will be described. That is, in the stress evaluation device 2 shown in FIG. 2, the first polarizing plate 21a and the second polarizing plate 21b rotate in the same rotation direction while maintaining a state in which the polarization directions differ by 90 °. In this case, the detection intensity at a position in an arbitrary polarization direction and the position where the first polarizing plate 21a and the second polarizing plate 21b are rotated by 45 ° while maintaining a state in which the polarization directions differ by 90 ° from that position. detects the intensity of the terahertz wave L _2p, by adding the intensities of the two detected terahertz waves L _2p, it is also possible to determine the principal stress difference (σ ₁ -σ _2).

More specifically, the intensity I ₁ of the light detected in an arbitrary polarization direction is the angle φ between the phase difference δ caused by birefringence and the principal stress direction and the polarization direction, as shown in the following equation (2). Depends on.
I ₁ = A ² sin ² 2φ ・ sin ² (δ / 2)… (2)
Note that A is the amplitude of the incident light.

Further, from the position where the light intensity I ₁ is detected, the first polarizing plate 21a and the second polarizing plate 21b are detected at a position rotated by 45 ° while maintaining a state in which the polarization directions differ from each other by 90 °. _{The intensity I 2} of the light to be obtained is as shown in Eq. (3).
I ₂ = A ² sin ² 2 (φ + π / 4) ・ sin ² (δ / 2)… (3)

When the light intensities I ₁ and I ₂ detected as described above are combined, the combined light intensity I is the angle φ between the principal stress direction and the polarization direction, as shown in the following equation (4). Does not depend on.
I = I ₁ + I ₂ = A ² sin ² (δ / 2)… (4)
This means that in the photoelastic method, the same result as when circularly polarized light is used can be obtained. As a result, the stress evaluation device 2 can derive both the principal stress direction and the principal stress difference (σ ₁ − σ _{2) of the anticorrosion layer 15b.} Other configurations are the same as those of the above-described embodiment.

According to the stress evaluation method according to the above-described embodiment, the terahertz wave is applied to the steel structure 15, and the principal stress difference of the anticorrosion layer 15b is based on the intensity of the terahertz wave reflected by the steel surface 15as of the steel material 15a. Both σ ₁ − σ ₂ ) and the principal stress direction can be derived. Therefore, the anticorrosion layer 15b can be used as a strain detecting means for the steel material 15a, and the stress state of the steel material 15a can be measured and evaluated by a simple configuration.

Although one embodiment of the present invention has been specifically described above, the present invention is not limited to the above-mentioned one embodiment, and various modifications based on the technical idea of the present invention are possible. For example, the configurations of the stress evaluation devices 1 and 2 described in the above embodiment are merely examples, and the principal stress difference and the principal stress direction of the non-metal layer in close contact with the surface of the structure are measured using a terahertz wave. If possible, a device having a different configuration may be used if necessary. Further, the present invention is not limited by the description and drawings which form a part of the disclosure of the present invention according to the above-described embodiment.

For example, in the above-described embodiment, the steel structure 15 is irradiated with a terahertz wave in a spot manner and reflected in a spot by the steel surface 15as of the steel material 15a, but the steel structure 15 is not necessarily irradiated and reflected in a spot manner. Not limited to. For example, instead of the terahertz wave transmitter 11, a terahertz wave light source capable of emitting a terahertz wave in a planar manner is used, and instead of the terahertz wave detector 12, a terahertz wave L _2p can be detected as a planar distribution. It is also possible to use a wave detection array or the like. According to this configuration, the stress distribution of the steel material 15a in the steel structure 15 as the measurement object, that is, the principal stress difference (σ ₁ − σ ₂ ) and the principal stress direction can be derived and evaluated. ..

For example, in the above-described embodiment, in the stress evaluation device 2, as a mechanism for rotating the first polarizing plate 21a and the second polarizing plate 21b while maintaining a state in which the polarization directions differ from each other by 90 °. Although the polarizing plate synchronous rotation mechanism 22 is used, the mechanism is not necessarily limited to this mechanism. If it is possible to rotate the first polarizing plate 21a and the second polarizing plate 21b while maintaining a state in which the polarization directions differ from each other by 90 °, various conventionally known rotation mechanisms can be adopted. be.

For example, in the above-described embodiment, in the stress evaluation devices 1 and 2, the optical system for emitting the terahertz wave toward the steel structure 15 and the terahertz wave reflected on the steel surface 15as are detected. Although the optical system is configured to be non-coaxial with a different optical axis, it is not necessarily limited to non-coaxial. Specifically, by using a half mirror or the like, it is possible to configure the optical system that emits the terahertz wave and the optical system that detects the reflected terahertz wave to be coaxial in which the optical axes overlap.

1,2 Stress evaluation device 10 Analysis control unit 10a Signal amplification unit 10b Bias generation unit 10c Lock-in detection unit 10d Analysis processing unit 11 Terrahertz wave transmitter 11a Terrahertz wave generator 11b, 12b Hemispherical lens 11c Collimating lens 11d Objective lens 12 Terrahertz Wave detector 12a Terrahertz wave detection element 12c Condensing lens 13a, 21a First polarizing plate 13b, 21b Second polarizing plate 14a, 14b λ / 4 wavelength plate 15 Steel structure 15a Steel material 15as Steel surface 15b Anticorrosion layer 15bs Surface 22 Polarization Plate synchronous rotation mechanism 22a Swivel head 22b Gear box 22c Plate plate synchronous rotation gear 23 Polarization direction L ₁ , L _1p , L ₂ , L _2p Terrahertz wave

Claims (3)

A terahertz wave transmitting means capable of irradiating a terahertz wave at a predetermined position on the surface of an object provided with a non-metal layer on the upper layer of the metal substrate and scanning the surface of the object, and the object. In a stress evaluation method using a stress evaluation device including a terahertz wave detecting means capable of detecting a terahertz wave reflected at a predetermined position and capable of scanning the surface of the object.
The stress in the non-metal layer is obtained by irradiating the non-metal layer with the tera-hertz wave emitted from the tera-hertz wave transmitting means and detecting the intensity of the tera-hertz wave transmitted through the non-metal layer by the tera-hertz wave detecting means. A stress evaluation method characterized by evaluating a stress state in a metal substrate by measuring a state and using the non-metal layer as a strain detecting means in a photoelastic method. The stress evaluation method according to claim 1, wherein the stress state is a principal stress difference and a principal stress direction. The stress evaluation device further has an analysis processing means, and the stress evaluation device has an analysis processing means.
The stress evaluation method according to claim 2, wherein the analysis processing means derives a principal stress component and a maximum principal stress direction in the metal substrate based on the principal stress difference and the principal stress direction.

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