JP6746870B2 - Crack detection method - Google Patents

Description

The present invention relates to a crack detecting method for detecting a crack generated inside a reinforced concrete structure using an electromagnetic wave radar.

As disclosed in Patent Document 1 and Non-Patent Documents 1 and 2, various methods for inspecting concrete structures using electromagnetic waves have been proposed. Non-Patent Documents 1 and 2 outline the nondestructive tests applied to reinforced concrete structures.

Further, Patent Document 1 describes a method of discriminating the type of a defective portion such as a cavity or a junker generated inside a concrete structure from the measurement result of a reflected wave of an electromagnetic wave radar.

JP, 2005-43197, A

Yoshihiro Masuda, 2 others, “Establishment of Japan Nondestructive Inspection Association Standard for Reinforcement Exploration by Electromagnetic Wave Radar and Induction”, Concrete Engineering, Vol.49, No.4, 2011.4, pp.15-21 Masashi Mori, Kazuhiro Kuzume, "Theory and Practice of Nondestructive Testing of Concrete (2) Theory and Practice of Electromagnetic Wave Method", Concrete Engineering, Vol.51, No.3, 2013.3, pp.278-282

However, the technology to detect internal defects that spread over a relatively wide range (thickness of 10 cm to 20 cm) such as cavities and junkers by the electromagnetic wave radar method has been established and applied in various fields. It is not possible to detect small cracks such as hair cracks (width 0.3 mm or less) that occur inside concrete.

That is, if the target object to be detected is large or has a large difference from the relative permittivity of concrete, the maximum amplitude of the electromagnetic wave waveform increases or decreases at the boundary of substances having different relative permittivities, but it is easy to detect concrete. It is difficult to detect an object with a small width such as a horizontal crack generated inside by the electromagnetic wave radar method. On the other hand, if the horizontal cracks that are generated inside and are not exposed on the surface are left as they are, the response to peeling or dropping of concrete pieces may be delayed.

Therefore, an object of the present invention is to provide a crack detection method capable of detecting even a small crack generated inside a reinforced concrete structure.

In order to achieve the above-mentioned object, the crack detection method of the present invention is a crack detection method for detecting a crack generated inside a reinforced concrete structure using an electromagnetic wave radar, and a crack is detected on the surface of the reinforced concrete structure to be inspected. On the other hand, the measurement step of measuring the electromagnetic wave propagation characteristics by the electromagnetic wave radar at different times, the difference extraction step of comparing the electromagnetic wave waveform data at different times and extracting the difference, and estimating the crack state from the extracted difference And a determination step.

Here, it is preferable that the measuring step is performed before and after the state of the reinforced concrete structure is changed. For example, the measurement is performed before and after the state changes due to rust (such as before corrosion before rust occurs and after corrosion after rust occurs).

Further, in the difference extracting step, when comparing the electromagnetic wave waveform data of the same measurement point at different times, the scanning plane positions of the electromagnetic wave radar are made to coincide, and the positions where the amplitude of the electromagnetic wave waveform data becomes 0 coincide. In addition, it is possible to perform processing for adjusting the scale of the electromagnetic wave waveform data.

Further, in the difference extraction step, it is possible to perform a process of extracting a portion exceeding a threshold value as a state change portion with respect to the waveform extracted by the difference between the electromagnetic wave waveform data of the same measurement point at different times.
Then, in the determination step, the state change point can be estimated from the extracted difference, and the crack state can be estimated from the distribution of the state change points.

The crack detection method of the present invention thus configured estimates the crack state by comparing the electromagnetic wave waveform data at different times measured from the surface of the reinforced concrete structure and extracting the difference.

If these different times indicate the state before and after the occurrence of cracks, the extracted difference is specified by the cracks or factors related thereto, so even if the cracks generated inside the reinforced concrete structure are minute. You will be able to detect.

It is a flowchart explaining the crack detection method of this Embodiment. It is a figure explaining the experiment performed in order to confirm the thing which can be detected with an electromagnetic wave radar, (a) is a side view of the sample used for an experiment, and (b) is a top view of the sample. It is a figure explaining the measurement data used for detection of rust, (a) is image data which showed the amplitude value of an electromagnetic wave waveform in the section of a specimen, (b) is an electromagnetic wave waveform measured at a plurality of measurement points. It is the figure which showed. It is a figure explaining the detection result of rust, (a) is the image data which showed the amplitude value of the electromagnetic wave waveform in the section of the sample, and (b) is the figure which showed the electromagnetic wave waveform measured at a plurality of measurement points. Is. It is a figure explaining the experiment performed in order to confirm that an internal crack can be detected, (a) is a side view of a specimen used for an experiment, and (b) is a top view of a specimen. FIG. 4 is a plan view for explaining a cross section of a sample of interest in an experiment conducted to confirm that an internal crack can be detected. It is a distribution chart of the actual corrosion rate based on the measurement result which confirmed the state of the internal crack which arose in the specimen in the section cut. It is a figure explaining the difference extraction method, (a) is a figure for explaining the step which adjusts a scanning surface position and depth, and (b) is a figure showing electromagnetic wave waveform data after adjustment. It is a figure explaining a difference extraction method, (a) is a figure showing a difference result, and (b) is a figure for explaining distribution processing by a threshold. It is an example in which the distribution of the difference values of the electromagnetic wave waveform extracted by the difference extraction method is three-dimensionally illustrated in the range above the reinforcing bar where cracks have occurred. It is the figure which compared the distribution of the difference value of the electromagnetic wave waveform extracted by the difference extraction method with a plane, and was compared with the actual crack distribution, (a) is a distribution diagram showing the result by the difference extraction method, (b) Is a distribution map of cracks measured. FIG. 7 is a diagram in which the result of the crack detection method of the present embodiment and the actual crack occurrence state are vertically arranged and compared, and is a comparison diagram at the cut surface I position in FIG. 6. FIG. 7 is a diagram in which the result of the crack detection method of the present embodiment and the actual crack occurrence state are vertically arranged and compared, and is a comparison diagram at the cutting plane III position in FIG. 6. FIG. 7 is a diagram in which the result of the crack detection method of the present embodiment and the actual crack occurrence state are arranged side by side and compared, and is a comparison diagram at the cutting surface V position in FIG. 6. It is an example in which the distribution of the difference values of the electromagnetic wave waveform extracted by the difference extraction method is three-dimensionally illustrated within the entire specimen including the periphery of the reinforcing bar.

Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a flow chart for explaining the crack detection method of this embodiment.

According to the crack detection method of the present embodiment, even minute cracks that occur inside a reinforced concrete structure can be detected. For example, it is possible to detect internal cracks, such as horizontal cracks, which are generated from inside the reinforced concrete structure and have no cracks reaching the surface.

Some of these internal cracks are generated and progress due to the corrosion of the reinforcing bars. The mechanism is that rust is first generated by corrosion of the reinforcing bar, and the expansion pressure causes cracks to occur around the reinforcing bar. Furthermore, when rust is generated, the rust exudes during the crack, and the movement of the rust affects the progress of the crack. That is, in many cases, it is considered that rust is present in the internal cracks.

Therefore, in the crack detection method of the present embodiment, cracks occurring inside the reinforced concrete structure are detected by detecting rust or a change in state caused by it. Therefore, the fact that rust can be detected by the electromagnetic wave radar will be described first.

FIG. 2 shows a test piece K used in an experiment conducted for confirming an object detectable by an electromagnetic wave radar. The specimen K is a rectangular parallelepiped made of concrete, and an embedded object M to be detected is embedded at a depth of 50 mm from the surface, and an electromagnetic wave radar machine is made to scan along a scanning point S on the surface of the specimen K. Thus, the electromagnetic wave propagation characteristic is measured.

The electromagnetic wave radar machine is a device capable of detecting the position and size of the embedded object M inside the sample K using the electromagnetic wave radar. The electromagnetic wave radar device can detect the position and size of the buried object M or the like by emitting the electromagnetic wave and receiving the electromagnetic wave reflected by the boundary surface or the like where the physical properties of the buried object M or the cavity change. it can. For example, the electromagnetic wave radar has a vehicle-like appearance, and can freely travel at a desired inspection point by a plurality of wheels.

The electromagnetic wave radar used in this experiment has a measurement depth of 2mm-300mm, a frequency of 2.6GHz at the center of the antenna, and a parallel radiation interval of 1.25mm-2.5mm. Further, the test piece K had a length of 600 mm, a width of 600 mm, and a height of 200 mm.

3 and 4 show changes in the electromagnetic wave waveform in concrete measured by an electromagnetic wave radar. Here, FIG. 3 shows a case where there is no buried object M, and the buried object M of FIG. 4 is rust.

These results are output when the set value of the relative permittivity of concrete of the electromagnetic wave radar is set to 6. In the visible images of FIGS. 3A and 4A, the amplitude value of the electromagnetic wave waveform is visualized by the grayscale of black and white with the vertical axis as the depth and the horizontal axis as the scanning distance.

In the electromagnetic wave waveforms of FIGS. 3(b) and 4(b), the vertical axis represents the amplitude value and the horizontal axis represents the depth, and the maximum or minimum value of the electromagnetic wave waveform (see the arrow position in FIG. 4) is embedded in concrete. It represents the boundary with the object M. 3(b) and 4(b) show a scanning distance of 150 mm, which is the center of the installation position of the buried object M, and a total of 3 points 25 mm before and after the center.

First, looking at the visible image (Fig. 3(a)) and the electromagnetic wave waveform (Fig. 3(b)) without the buried object, the amplitude value from the surface layer to a depth of about 25 mm is affected by the reflection of electromagnetic waves from the concrete surface. Increase or decrease greatly. Even in the visible image, the black and white layers can be confirmed in the same range. In the other range, such a peculiar change is not confirmed.

Based on the results without this buried object, we focus on the changes in electromagnetic wave propagation characteristics due to substances with different relative dielectric constants. In the visible image of rust (Fig. 4(a)), a change appeared as a white reaction near the scanning distance of 160 mm-220 mm and the depth of 50 mm, and although it moved from the original burial position (center), it protruded. A white reaction can be seen. In the electromagnetic wave waveform (FIG. 4(b)), a clear positive maximum amplitude value is shown at a depth of 55 mm at a scanning distance of 175 mm (dashed line), and it can be seen that it is buried at this point.

Also, from the results of the visible image and the electromagnetic wave waveform, it can be inferred that the relative permittivity of this rust is larger than that of concrete. Here, as a result of performing a similar experiment using the buried object M as a reinforcing bar, the maximum amplitude value of the reinforcing bar is a positive value which is about 2 to 3 times that of rust, and the two can be easily discriminated.

Furthermore, it was found that the water content of concrete affects the electromagnetic wave propagation characteristics. For example, it has been found that the electromagnetic wave propagation speed of a reinforcing bar changes greatly only when the natural drying period after placing concrete differs by one day. Specifically, it was confirmed that when the natural drying period increases, the depth of the entire electromagnetic wave waveform shifts to a shallower direction as compared with that before the drying. In short, the difference in water content of concrete affects the amplitude value (reaction value) and depth of the entire electromagnetic wave waveform.

As a result of the above experiment, it was confirmed that the presence of the buried object M having a relative dielectric constant different from that of concrete, particularly rust, can be detected. Further, although detailed description is omitted, in an experiment using the embedded material M in which the relative permittivity around the reinforcing bar in which the outer periphery of the reinforcing bar is wound with styrofoam is smaller than that of concrete, the characteristics of the substance around the reinforcing bar (styrofoam) (negative It was confirmed experimentally that the minimum amplitude value) can be detected. That is, if a state change occurs inside the reinforced concrete structure, it can be detected.

Therefore, the reinforced concrete structure is gradually corroded, and the electromagnetic wave propagation characteristics are measured by the electromagnetic wave radar, and the difference of the electromagnetic wave waveform in the concrete cross section at the same measurement point before and after the corrosion is calculated as the distribution of rust in the internal crack (state change). It will be explained that cracks can be detected by considering the distribution of points).

FIG. 5 shows an outline of the test piece 1 simulating a reinforced concrete structure used in the experiment. The sample 1 had a width of 500 mm, a length of 1000 mm, and a height of 200 mm, and one deformed bar 2 (D19) having a cover thickness of 30 mm was arranged at the center position in the width direction. This rebar 2 was entirely embedded in concrete in order to prevent rust juice from flowing out of the sample 1 along the rebar.

In order to partially corrode the entire length of the reinforcing bar 2 instead of uniformly corroding the reinforcing bar 2, a water tank 3 having a length of 200 mm and a width of 80 mm was installed immediately above the central portion of the reinforcing bar 2 for the electrolytic corrosion test. The electrolytic corrosion test was performed by placing a 3% NaCl aqueous solution and a copper plate in the water tank 3, and connecting the copper plate and the reinforcing bar 2 so as to form a series circuit via a power supply device. Then, energization was performed stepwise until the cumulative total of the assumed mass reduction amounts of the reinforcing bars 2 became 1, 2, 4, 6, 8, 10%, and the reduced amount was defined as the assumed corrosion rate.

On the other hand, the measurement by the electromagnetic wave radar machine is performed before the energization and after the energization corresponding to each assumed corrosion rate, in the axial direction of the reinforcing bar 2 and the axis orthogonal to the measurement range 12 shown in FIG. Scanning was performed at intervals of 25 mm. In order to unify the environmental conditions in any of the measurements, the measurement was made 24 hours after the target corrosion rate was reached and the water tank 3 was removed.

Then, for confirmation, after completion of all electrolytic corrosion tests and measurements by the electromagnetic wave radar machine, the specimen 1 was cut by the wet concrete cutter at the broken line position in the direction perpendicular to the rebar axis at 50 mm intervals in FIG. The property was confirmed. At that time, the crack width was measured on a crack scale.

First, there were no visually observable cracks on the surface 11 of the sample 1 after the electrolytic corrosion test. No vertical cracks were present inside the sample 1. The width of the horizontal crack observed inside the test piece 1 was 0.05 mm (the minimum scale of the crack gauge) to 0.4 mm at the maximum, which was a crack with a very small width (corresponding to a hair crack).

Further, the reinforcing bars 2 were squeezed out from the cut specimen 1 and immersed in an aqueous 10% ammonium citrate solution to remove corrosion products, and then the mass was measured to calculate the mass reduction rate.

FIG. 7 shows the actual corrosion rate distribution created based on the horizontal cracks actually measured by the crack gauge from the cut specimen 1. The actual corrosion rate was 18.5% at maximum, and the corrosion rate was about 15% or more directly under the water tank 3. Then, the amount of corrosion drastically decreased as the distance from the water tank 3 increased, and became almost zero at a distance of about 100 mm-150 mm.

Next, a difference extraction method for extracting the difference between the electromagnetic wave waveforms in the concrete cross section of the same measurement point at different times will be described. Here, the data measured by the electromagnetic wave radar device is digitized for each depth at each measurement point.

In the difference extraction method, electromagnetic wave waveform data in two different states obtained at the same measurement point of a substance (specimen 1) are compared, and the difference is extracted to extract the numerical difference as a state change. That is, if the electromagnetic wave waveform data measured at two different times is one time before the corrosion of the sample 1 and the other time after the corrosion of the sample 1, the extracted difference is the corrosion (state change point). ).

Therefore, the difference extraction method will be described in detail with reference to FIGS. FIG. 8A shows electromagnetic wave waveform data measured by an electromagnetic wave radar before and after corrosion at a certain measurement point. From this electromagnetic wave waveform data, attention is paid to the amplitude value at point A that represents the reflection of the electromagnetic wave on the surface 11 of the sample 1. Since the depth of this amplitude value corresponds to the position of the surface 11 (scanning surface position) of the sample 1, this position is adjusted with the reference (depth 0 mm) (see FIG. 8B).

Then, a depth shift due to a subtle difference in the condition of the sample 1 caused by a difference in environmental conditions such as the temperature at the time of measurement and the dry state (water containing state) of the concrete is adjusted. Paying attention to the position (point B in FIG. 8A) of the intersection point with the amplitude value 0 appearing immediately after the reference position (scanning surface position), the scale in the depth direction of the entire electromagnetic wave waveform is adjusted, and this point is adjusted. Match before and after corrosion.

That is, as shown in FIG. 8B, the scale reduction in the depth direction is performed in order to match the points B before and after corrosion. Here, the depth at which the amplitude value becomes 0 is adjusted to 10 mm. It is easy to understand that the depth adjustment is performed by focusing on the point that the positional relationship between the B1 point and the B2 point showing the peak is different from the positional relationship in FIG. 8A. The depth information is affected by the relative permittivity of concrete, which causes some errors. In short, this adjustment is a process for reducing the error in the measurement environment.

Then, by taking the difference between the electromagnetic wave waveform data before and after the corrosion after the adjustment, the waveform data having the difference value shown in FIG. 9A is obtained. Further, in order to make this difference value clearer, a threshold value (dashed line) that can eliminate the influence of an error between measurements is provided as shown in FIG. To extract.

The calculated difference value shows a positive value when a substance having a relative permittivity larger than concrete is newly present after the state change by the experiment of the buried object M described above, and a substance having a relative permittivity smaller than concrete exists. It has been understood that a negative value is shown when doing. Since the purpose of this experiment is to detect the rust inside the crack, a positive difference value is extracted as the difference value extracted from the two measurement data before and after the corrosion.

FIG. 10 shows an example of a result obtained by three-dimensionally visualizing the difference value of the electromagnetic wave waveform at each measurement point obtained by the difference extraction method in the range above the reinforcing bar 2 where the crack has occurred. Here, FIG. 10 shows the difference value between the corrosion rate of 0% (before corrosion) and the assumed corrosion rate of 10% (after corrosion).

At the assumed corrosion rate of 10%, it is possible to confirm the spread of the distribution of the difference value at the position away from the reinforcing bar 2. Also, although detailed explanation is omitted, when looking at the three-dimensionally visualized difference values of the assumed corrosion rates of 1, 2, 4, 6, and 8%, the internal damage is understood from the small stage of the assumed corrosion rates of 1% and 2%. In addition, it was confirmed that the distribution of the difference value gradually spreads from around the reinforcing bar 2 as the assumed corrosion rate increases (corrosion progresses). Moreover, the value of the difference value also gradually increases as the assumed corrosion rate increases, and it can be confirmed that the corrosion is progressing.

FIG. 11 shows the planar distribution of rust indicated by the difference value when the assumed corrosion rate is 10%. FIG. 11A is a distribution chart showing the result of the difference extraction method described in the present embodiment, and FIG. 11B is measured by actual measurement from the sample 1 cut at each cross section shown in FIG. It is a distribution map of a horizontal crack.

The distribution of rust (state change points) indicated by the difference value (FIG. 11( a )) and the distribution of horizontal cracks (FIG. 11( b )) are roughly the same although the range of rust indicated by the difference value is slightly larger. You can check what you are doing. Here, the actually-measured crack width of the horizontal crack is 0.05 mm to 0.4 mm, and it can be said that the distribution of the difference value detects a crack having a minute width that is difficult to detect by other estimation methods.

12 to 14 show a distribution of rust (upper diagram) and a distribution of actually measured horizontal cracks (lower diagram) indicated by the difference value in the cross section of the specimen 1 when the assumed corrosion rate is 10%. Here, FIG. 12 shows a comparison in the I section of FIG. 6, FIG. 13 shows a comparison in the III section of FIG. 6, and FIG. 14 shows a comparison in the V section of FIG.

It can be seen from FIG. 12 that the distribution of difference values does not appear in the cross section in which no crack was actually measured. 13 and 14, cracks propagate obliquely from the reinforcing bar 2 toward the surface 11, whereas the rust distribution indicated by the difference value is partially interrupted and overlapped at some points. However, they show almost the same shape, and it can be said that they can be used for detection by estimating cracks.

On the other hand, FIG. 15 shows the result of three-dimensional visualization of the positive value of the difference between the corrosion rate before corrosion of 0% and the assumed corrosion rate after corrosion of 10% (actual corrosion rate of 18.5%). In FIG. 10, the output is limited to the range where cracks occur, but in FIG. 15, the visualization range is expanded to the periphery of the reinforcing bar 2 where no crack exists. From this result, it can be seen that a distribution such as a mountain-like shape peculiar to the reinforcing bar appears from the position of the reinforcing bar 2. The distribution of the difference value peculiar to this reinforcing bar is considered to be due to the change in the water content around the reinforcing bar. However, by indirectly examining the distribution of the difference value excluding the influence of this reinforcing bar 2 (for example, FIG. 10), It will be possible to visualize the internal crack area.

Next, the crack detection method of the present embodiment and its operation will be described. FIG. 1 is a flowchart for explaining each step of the crack detection method according to the present embodiment.

First, in step S1, the surface of the reinforced concrete structure to be inspected is measured by an electromagnetic wave radar before corrosion. A typical example of the measurement before corrosion is immediately after the construction of the reinforced concrete structure, but the measurement is not limited thereto, and the measurement result at the time when it can be assumed to be sound can be measured before the corrosion. Also, the explanation as "before corrosion" is to make two different times convenient for "before corrosion" and "after corrosion", and even if the measurement result at the time when corrosion begins, It is possible to detect the spread and progress of cracks by the difference (change in state) by comparing with the measurement result at the time when the corrosion has progressed.

Step S2 is carried out after a lapse of time from step S1 to the extent that corrosion occurs. For example, after step S1, it is possible to use the result of regular measurement performed for maintenance. The steps S1 and S2 are measurement steps.

Then, in step S3, as described above with reference to FIG. 8, the electromagnetic wave waveform data at two times to be compared are adjusted by moving the scanning plane position and depth by changing the scale in the depth direction. To do.

Further, the difference processing of the electromagnetic wave waveform data adjusted in step S3 is performed to generate a waveform having a difference value as shown in FIG. 9A (step S4). Then, an extraction process using a threshold value suitable for the physical property (rust) desired to be extracted as a state change is performed from the waveform of the difference value (step S5, see FIG. 9B). The steps from S3 to S5 thus far are the difference extraction steps.

The output of the extraction result (step S6) can be visualized three-dimensionally as shown in FIG. 10 or can be shown in plan as shown in FIG. Further, as shown in the upper diagrams of FIGS. 12 to 14, it is also possible to output for each cross section. Then, in the determination step, the state of cracks inside the reinforced concrete structure can be estimated by looking at these output results.

The crack detection method of the present embodiment configured as described above estimates the crack state by comparing the electromagnetic wave waveform data at different times measured from the surface of the reinforced concrete structure and extracting the difference.

If these different times indicate the states before and after the occurrence of cracks and before and after the progress of cracks, the extracted difference is identified as the state change due to the cracks or the factors related to the cracks, and therefore the cracks generated inside the reinforced concrete structure. Can be detected even if it is minute.

For example, in the detection of the internal defect of the concrete based on the conventional elastic wave method, it could not be detected unless the thickness is 5 mm or more, but if it is the crack detection method of the present embodiment, it is very 0.05 mm-0.4 mm. Even cracks with a narrow width can be detected.

Further, even when comparing the measurement results at different times after the occurrence of the crack, a difference (state change) will occur if there is progress of the crack, so that the crack can be detected. That is, even if there is no measurement data at the time of soundness, it can be detected if there is a change over time.

The embodiment of the present invention has been described in detail above with reference to the drawings. However, the specific configuration is not limited to this embodiment, and a design change that does not depart from the gist of the present invention is Included in the invention.

For example, in the above-described embodiment, the case of comparing electromagnetic wave waveform data at the same measurement point at different times has been described, but the same measurement point is not limited to completely the same location, but the measurement points in the vicinity It is possible to compare even the measurement results.

1 Specimen (reinforced concrete structure)
11 Surface 2 Rebar

Claims (5)

A crack detection method for detecting cracks generated inside a reinforced concrete structure using an electromagnetic wave radar,
For the surface of the reinforced concrete structure to be inspected, a measurement step of measuring the electromagnetic wave propagation characteristics by the electromagnetic wave radar at different times,
A difference extracting step of comparing the electromagnetic wave waveform data at different times to extract a difference,
And a determination step of estimating the state of cracks from the extracted difference ,
In the difference extraction step, when comparing the electromagnetic wave waveform data of the same measurement point at different times, the scanning plane positions of the electromagnetic wave radar are made coincident by aligning the first peak positions, and the coincident scanning plane positions are made. A crack detection method, characterized in that the scale of the electromagnetic wave waveform data is adjusted so that the position where the amplitude of the electromagnetic wave waveform data that appears immediately after is matched . The crack detecting method according to claim 1, wherein the measuring step is performed before and after the state of the reinforced concrete structure is changed. The crack detecting method according to claim 2, wherein the change in the state of the reinforced concrete structure is caused by rust. The said difference extraction step WHEREIN: With respect to the waveform extracted by the difference of the electromagnetic wave waveform data of the same measurement point of different time, the part which exceeds a threshold value is extracted, and it is set as a state change part. crack detection method according to any one of 3. Wherein in the determination step estimates the state change point from the extracted difference, crack detection method according to any one of claims 1 to 4, characterized in that for estimating the the distribution of the state change point cracking conditions ..