Damage inspection method

Abstract

To provide a damage inspection method that can accurately detect damaged areas. [Solution] A thermal image acquisition step ST1 acquires multiple thermal images by sequentially imaging the object to be inspected after applying a thermal load at predetermined time steps; a temperature difference calculation step ST2 extracts a temperature profile from each of the multiple thermal images and calculates the temperature difference dT of adjacent evaluation points; and calculates the rate of change dTr of the temperature difference dT at each time step and the average value dTr of the rate of change dTr. ave Step ST3 calculates the temperature difference rate of change, and Step ST4 identifies the candidate damage site Pc as the evaluation point corresponding to the temperature difference dT if the absolute value of the temperature difference dT calculated for the first acquired thermal image exceeds the threshold R, and the average value dTr corresponding to the candidate damage site Pc ave Of these, the smallest average value dTr ave The system includes a damage site identification step ST5 which identifies a candidate damage site Pc corresponding to the damage site as the damage site.

Description

[Technical Field]

[0001] The present invention relates to a damage inspection method that can accurately detect damaged areas such as cracks present in an object to be inspected, such as a welded joint, in distinction from uneven areas such as welds. [Background technology]

[0002] Conventionally, as shown in Patent Documents 1 to 3, for example, a method has been proposed to detect damaged areas such as cracks present in an object under inspection using an infrared imaging device.

[0003] The method described in Patent Document 1 involves determining the surface temperature distribution of a defect from a thermal image acquired after heating or cooling the object to be inspected, determining the spatial derivative of this surface temperature distribution, and detecting the defect based on its inflection point. The method described in Patent Document 2 involves creating samples of difference images (samples of sound and unsound) of the heat distribution images of the exposed portion of the steel material before and after heating in sound and unsound steel joints, and determining whether the object to be inspected is sound or unsound by comparing the difference image of the exposed portion of the steel material to be inspected with the samples. The method described in Patent Document 3 is a method for determining the presence or absence of cracks based on changes in the intensity of infrared radiation in an infrared image obtained by artificially irradiating a structure with infrared radiation and receiving the reflected infrared radiation.

[0004] However, in the methods described in Patent Documents 1 to 3, if the object to be inspected is a welded joint and has uneven areas such as welds, there is a risk of mistakenly detecting areas that are actually sound (where no cracks or other damaged areas exist) as damaged areas. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 2-12045 [Patent Document 2] Japanese Patent Publication No. 2004-37201

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0006] The present invention has been made in view of the problems of the above prior art, and an object thereof is to provide a damage inspection method capable of accurately detecting damage sites such as cracks existing in an inspection target such as a welded joint, distinguishing them from uneven portions such as welded portions.

Means for Solving the Problems

[0007] In order to solve the above problems, the present inventors have made a hypothesis and conducted intensive studies. As a prerequisite for the hypothesis, the present inventors considered acquiring a plurality of thermal images showing the temperature distribution of the inspection target by imaging the inspection target after applying a heat load at each predetermined time step using an infrared imaging device. Then, for each of the plurality of thermal images, a temperature profile, which is the temperature distribution on a predetermined straight line, was extracted, and based on the temperature profile, the temperature difference dT between adjacent evaluation points located on the straight line was calculated.

[0008] FIG. 1 is a diagram for explaining the hypothesis established by the present inventors. FIG. 1(a) is a diagram schematically showing the temperature profile obtained immediately after applying a heat load to the inspection target (in the example shown in FIG. 1(a), immediately after the heating is completed). The lower diagram in FIG. 1(a) shows the inspection target, and the upper diagram shows the temperature profile at the corresponding position of the inspection target shown in the lower diagram. FIG. 1(b) is a diagram schematically showing the temperature profiles obtained immediately after applying a heat load to the inspection target (in the example shown in FIG. 1(b), immediately after the heating is completed) and after a predetermined time has elapsed. The lower diagram in FIG. 1(b) shows the inspection target, and the upper diagram shows the temperature profile at the corresponding position of the inspection target shown in the lower diagram. The present inventors have established the following hypotheses (1) to (3). (1) As shown in Figure 1(a), it was assumed that in damaged areas such as cracks, the temperature difference dT immediately after applying a heat load would be large due to the insulating effect of the voids. On the other hand, it was also assumed that the temperature difference dT immediately after applying a heat load would be large in uneven areas such as welded joints. (2) As shown in Figure 1(b), the temperature difference dT caused by uneven areas such as welds is temporary (the temperature difference dT is large only immediately after the heat load is applied), and it was assumed that the temperature difference dT changes significantly over time, and becomes smaller after a predetermined time (for example, about 10 seconds). On the other hand, in damaged areas such as cracks, the insulating effect of the void is the cause of the temperature difference dT, so it was assumed that the change in the temperature difference dT is small over a time period of about 10 seconds, and that the temperature difference dT remains large after a predetermined time, similar to immediately after the heat load is applied. (3) Therefore, we thought that by evaluating the rate of change of the temperature difference dT, it would be possible to distinguish between damaged areas such as cracks with a small rate of change and uneven areas such as welds with a large rate of change, and to accurately detect damaged areas without misdetecting uneven areas.

[0009] Next, the inventors verified the validity of the above hypothesis by performing a heat transfer analysis. Figure 2 shows the numerical analysis model (finite element analysis model) of the welded joint that was used to perform heat transfer analysis to verify the validity of the hypothesis. Figure 2(a) is an overall diagram of the numerical analysis model, and Figure 2(b) is a magnified view of the area enclosed by the dashed line in Figure 2(a). As shown in Figure 2, a crack extending in the thickness direction (vertical direction in Figure 2) was defined as a damaged area near the arc weld in the numerical analysis model using double nodes. In Figure 2(a), the "heated area" is defined as having a volumetric heat generation of 0.023 W / mm². 3 The heat load (heating) was set to 0.5°C, and the conditions were to allow 10 seconds of air cooling (natural cooling) after heating was completed. For the thermal properties (thermal conductivity, density, specific heat) of the welded joint, data taken from 590 MPa class steel plates were used. The air cooling conditions were an ambient temperature of 20°C, an emissivity of 0.8 for the welded joint, and a convection heat transfer coefficient of 11.628 W / (m³) assuming natural convection. 2 ·K) Under the conditions described above, heat transfer analysis was performed using the general-purpose non-linear finite element analysis program "Abaqus" manufactured by SIMULIA as software, and the temperature profile, which is the distribution of the temperature T on line A shown by the broken line in Fig. 2(b), was calculated for each time step at an interval of 1 second from immediately after the heating ended until 10 seconds had elapsed.

[0010] Fig. 3 is a diagram showing the results obtained from the above heat transfer analysis. The horizontal axis of Fig. 3 indicates the position on line A (the distance from the left end of line A shown in Fig. 2(b)). The vertical axis of Fig. 3 indicates the average value dTr of the temperature T and the change rate dTr to be described later. ave In Fig. 3, only the temperature profiles immediately after heating, 1 second later, 2 seconds later, 5 seconds later, and 10 seconds later are plotted as the temperature profile, but actually 11 temperature profiles in total are obtained for each time step at an interval of 1 second. The average value dTr of the change rate dTr shown in Fig. 3 ave When calculating, first, for each of the 11 temperature profiles calculated for each time step at an interval of 1 second, the temperature difference dT between adjacent evaluation points (adjacent nodes of the numerical analysis model) was calculated. Next, based on the following formula (1), the change rate dTr for each time step of the 11 temperature differences dT was calculated. dTr = |(dT i - dT i-1 ) / dT i | ···(1) In the above formula (1), the subscript i of dT means the time step and is an integer from i = 1 to 10. dT0 means the temperature difference dT immediately after heating, dT1 means the temperature difference dT 1 second after heating, and dT 10 means the temperature difference dT 10 seconds after heating. The same applies to other subscripts. A total of 10 change rates dTr will be calculated.

[0011] And in Fig. 3, the value obtained by averaging the change rate dTr for each time step of this temperature difference dT (the value obtained by dividing the sum of the 10 change rates dTr by 10) is plotted as the average value dTr of the change rate dTr ave . As shown in Figure 3, in a welded joint, the average value of the rate of change dTr is dTr ave The value of the minimum occurs at the location where the crack (damage site) exists, and the average value of the rate of change dTr at the location where the arc weld exists is compared to this minimum. ave Because the value is large, it was found that the inventors' hypothesis mentioned above is valid.

[0012] This invention was completed based on the findings of the inventors described above. In other words, to solve the above problem, the present invention provides a thermal image acquisition step of acquiring a plurality of thermal images showing the temperature distribution of an object to be inspected by sequentially imaging the object to be inspected at predetermined time steps after applying a heat load using an infrared imaging device; a temperature difference calculation step of extracting a temperature profile, which is a temperature distribution on a predetermined straight line, from each of the plurality of thermal images, and calculating the temperature difference dT of adjacent evaluation points located on the straight line based on the temperature profile; and calculating the rate of change dTr of the temperature difference dT calculated for each of the plurality of thermal images at each time step, and the average value dTr of the rate of change dTr at all of the time steps. ave A step to calculate the temperature difference change rate, and a step to identify a candidate damage site Pc, in which, if the absolute value of the temperature difference dT calculated for the first thermal image acquired among the plurality of thermal images exceeds a predetermined threshold R, the evaluation point corresponding to the temperature difference dT is identified as a candidate damage site Pc, and the average value dTr corresponding to the candidate damage site Pc ave Of these, the smallest of the above average values ​​dTr ave The present invention provides a damage inspection method comprising: a damage site identification step of identifying the candidate damaged site Pc corresponding to as a damaged site.

[0013] In the present invention, "the evaluation point corresponding to the temperature difference dT" means an evaluation point located on either side of a predetermined set of adjacent evaluation points from which the temperature difference dT was calculated. According to the damage inspection method of the present invention, the thermal image acquisition step, the temperature difference calculation step, and the temperature difference change rate calculation step are performed, as described above with reference to Figures 2 and 3, to obtain the average value of the change rate dTr dTrave This is calculated. Furthermore, according to the damage inspection method of the present invention, in the damage site candidate identification step, if the absolute value of the temperature difference dT calculated for the first acquired thermal image (for example, a thermal image acquired for the object under inspection immediately after applying a thermal load) exceeds a predetermined threshold R, the evaluation point corresponding to the temperature difference dT is identified as a damage site candidate Pc. This damage site candidate Pc may include not only damaged areas such as cracks, but also uneven areas such as welded joints. However, according to the damage inspection method of the present invention, in the damage site identification step, the average value dTr of the rate of change dTr corresponding to the candidate damage site Pc is obtained. ave Of these, the smallest average value dTr ave The candidate Pc for the damaged site corresponding to this is identified as the damaged site. Therefore, as the inventors' findings above indicate, the average value of the rate of change dTr (the rate of change dTr) is small. ave Damage sites such as small cracks and large change rates (average value of change rate dTr) ave It is expected that this will allow for the distinction between uneven areas such as large welds and other irregularities, and that damaged areas can be detected with high accuracy without falsely detecting uneven areas.

[0014] Here, although it is possible to use the damaged area identified in the damaged area identification step of the damage inspection method according to the present invention as the final inspection result, it is preferable to perform a further visual inspection on the identified damaged area in order to ensure the reliability of the inspection. Also, the damaged area identified in the damaged area identification step is the smallest average value dTr ave Since it only includes the candidate Pc for the corresponding injury site, the next smallest average value dTr ave If the corresponding candidate PC is indeed the injured site, there is a risk of overlooking it.

[0015] To avoid the above problems, the damage inspection method according to the present invention includes: a visual inspection step of visually inspecting whether or not damage actually exists at the identified damaged area; a soundness determination step of determining that the identified damaged area is sound if it is not possible to confirm the presence of damage in the visual inspection step; and, if it is possible to confirm the presence of damage in the visual inspection step, determining that damage exists at the identified damaged area and determining the average value dTr corresponding to the candidate damaged area Pc. ave Of these, the next smallest average value dTr ave The second damage site identification step involves identifying the candidate damage site Pc corresponding to the second damage site as a new damage site, and in the visual inspection step, until it is no longer possible to confirm the actual presence of damage, or in the second damage site identification step, the next smallest average value dTr ave It is preferable to repeatedly perform the visual inspection step and the second damage location identification step until the presence of the damage is eliminated.

[0016] According to the preferred method described above, the visual inspection step visually checks whether or not damage actually exists at the identified damaged area, thus ensuring the reliability of the inspection. If the presence of actual damage cannot be confirmed in the first visual inspection step, the soundness determination step determines that the identified damaged area is sound (i.e., the damaged area identified in the damaged area identification step is determined to be a false detection (a false detection caused by the attachment of foreign matter or a depression that is not damage), and the inspection result is corrected to the correct value). On the other hand, if the presence of actual damage can be confirmed in the first visual inspection step, the second damaged area identification step determines that damage exists at the identified damaged area, and also determines the next smallest average value dTr ave The candidate Pc for the corresponding injury site is identified as the new injury site. Then, in the visual inspection step, until the presence of actual damage can no longer be confirmed, or in the second injury site identification step, the next smallest average value dTr aveSince the visual inspection step and the second damage identification step are repeated until no damage remains, the risk of missing a damaged area is reduced. [Effects of the Invention]

[0017] According to the present invention, damaged areas such as cracks present in objects to be inspected, such as welded joints, can be accurately detected and distinguished from uneven areas such as welds. [Brief explanation of the drawing]

[0018] [Figure 1] This is a diagram illustrating the hypothesis put forward by the inventors. [Figure 2] This document presents a numerical analysis model (finite element analysis model) of a welded joint, which was used to perform a heat transfer analysis to verify the validity of the hypothesis. [Figure 3] This figure shows the results obtained from a heat transfer analysis using the analytical model shown in Figure 2. [Figure 4] This is a flowchart illustrating the steps of a damage inspection method according to one embodiment of the present invention. [Figure 5] This is a perspective view showing the schematic configuration of a welded joint used as the object to be inspected in an embodiment of the present invention. [Figure 6] The thermal image initially acquired in step ST1 of the embodiment of the present invention is shown. [Figure 7] Figure 6 shows the temperature profile (distribution of temperature T) and temperature difference dT extracted from the first thermal image acquired. [Figure 8] In the embodiment of the present invention, the temperature profile (distribution of temperature T) extracted from the first acquired thermal image and the average value dTrave of the rate of change dTr calculated using all thermal images are shown. [Modes for carrying out the invention]

[0019] The following describes a damage inspection method according to one embodiment of the present invention, with reference to the attached drawings as appropriate. Figure 4 is a flowchart illustrating the steps of the damage inspection method according to this embodiment. As shown in Figure 4, the damage inspection method according to this embodiment includes a thermal image acquisition step ST1, a temperature difference calculation step ST2, a temperature difference change rate calculation step ST3, a candidate damage site identification step ST4, and a damage site identification step ST5. Furthermore, in a preferred embodiment, the damage inspection method according to this embodiment includes a visual inspection step ST6, a soundness determination step ST8, and a second damage site identification step ST9 following the thermal image acquisition step ST1 to the damage site identification step ST5. Each step will be described in order below.

[0020] <Thermal image acquisition step ST1> In the thermal image acquisition step ST1, an infrared imaging device is used to sequentially image the object under inspection after applying a heat load at predetermined time steps, thereby acquiring multiple thermal images showing the temperature distribution of the object under inspection, such as those shown in Figure 6 described later. The thermal load is not limited to this, but for example, it may be applied to the object under inspection using a heat gun. The thermal load is not limited to this, but for example, a heat source that is 50°C or more above room temperature may be used to apply a temperature distribution higher than room temperature to the area within the field of view of the infrared imaging device of the object under inspection. The imaging of the object under inspection is not limited to this, but for example, it may be performed from immediately after the thermal load is applied until 10 seconds have elapsed. For example, if the imaging of the object under inspection is performed at 1-second intervals from immediately after the thermal load is applied until 10 seconds have elapsed, a total of 11 thermal images will be acquired.

[0021] <Temperature difference calculation step ST2> In the temperature difference calculation step ST2, a temperature profile, which is the temperature distribution along a predetermined straight line, is extracted for each of the multiple thermal images. Based on this temperature profile, the temperature difference dT of adjacent evaluation points located on the straight line is calculated, for example, as shown in Figure 7 below. The evaluation points are, for example, pixels that make up the thermal image, and in this case, adjacent evaluation points are adjacent pixels. The setting of the predetermined straight line and the extraction of the temperature profile can be performed using predetermined image processing software. The predetermined straight line can be set to extend at any position in the thermal image and in any direction, such as vertically or horizontally. However, if it is possible to predict in advance which parts of the object under inspection are likely to have damage, it is preferable to set the line to pass through these parts. Furthermore, the number of predetermined straight lines is not limited to one; it is also possible to set multiple straight lines and repeatedly perform the extraction of the temperature profile and the calculation of the temperature difference dT for each set straight line.

[0022] <Temperature difference change rate calculation step ST3> In step ST3, the temperature difference rate of change calculation step, the rate of change dTr for each time step of the temperature difference dT calculated for each of the multiple thermal images is calculated based on the following equation (1). dTr=|(dT i -dT i-1 ) / dT i | ···(1) In equation (1) above, the subscript i in dT represents the time step. For example, if imaging of the subject under examination is performed at 1-second intervals from immediately after the application of a heat load until 10 seconds have elapsed, and a total of 11 thermal images are acquired, then i = an integer from 1 to 10. dT0 represents the temperature difference dT immediately after the application of the heat load, dT1 represents the temperature difference dT 1 second after the application of the heat load, and dT 10 This represents the temperature difference dT from immediately after the heat load is applied to 10 seconds later. The same applies to the other subscripts. In the above case, a total of 10 rate of change dTr values ​​will be calculated.

[0023] Then, in the temperature difference rate of change calculation step ST3, for example, as shown in Figure 8 described later, the average value dTr of the rate of change dTr over all time steps is calculated.ave The average value of the 10 rate of change dTr (the sum of the 10 rate of change dTr divided by 10) is calculated as the average rate of the rate of change dTr. ave It will be calculated as follows.

[0024] <Step ST4: Identifying potential injury sites> In the injury site candidate identification step ST4, if the absolute value of the temperature difference dT calculated for the first thermal image acquired among multiple thermal images (if imaging of the subject under examination is performed at 1-second intervals from immediately after the application of a thermal load until 10 seconds have elapsed, and a total of 11 thermal images are acquired, the thermal image acquired immediately after the application of a thermal load) exceeds a predetermined threshold R, the evaluation point corresponding to that temperature difference dT is identified as the injury site candidate Pc, as shown in Figure 7 below. A predetermined threshold is determined, for example, by the following equation (2). R[℃] = 2.5 × L[mm] ... (2) In equation (2) above, L represents the distance between adjacent evaluation points from which the temperature difference dT was calculated.

[0025] <Step ST5: Identifying the site of injury> In the injury site identification step ST5, for example, as shown in Figure 8 described later, the average value of the rate of change dTr corresponding to the candidate injury site Pc is dTr ave Of these, the smallest average value dTr ave The candidate Pc corresponding to the injury site is identified as the injury site.

[0026] By performing the thermal image acquisition step ST1 to the damage site identification step ST5 described above, the average value of the rate of change dTr (dTr) is small. ave Damage sites such as small cracks and large change rates (average value of change rate dTr) ave It is expected that this will allow for the distinction between uneven areas such as large welds and other irregularities, and that damaged areas can be detected with high accuracy without falsely detecting uneven areas. While it is possible to use the damaged area identified in the damaged area identification step ST5 as the final inspection result, it is preferable to perform a further visual inspection of the identified damaged area to ensure the accuracy of the inspection. Furthermore, the damaged area identified in the damaged area identification step ST5 is the smallest average value dTr ave Since it only includes the candidate Pc for the corresponding injury site, the next smallest average value dTr ave If the corresponding candidate PC is indeed the injured site, there is a risk of overlooking it. Therefore, as described above, the damage inspection method according to this embodiment, in a preferred embodiment, includes a visual inspection step ST6, a soundness determination step ST8, and a second damage location identification step ST9.

[0027] <Visual inspection step ST6, soundness determination step ST8> In the visual inspection step ST6, the identified damaged area is visually inspected to determine whether or not actual damage exists. This ensures the accuracy of the inspection. Then, in the visual inspection step ST6, if the presence of actual damage cannot be confirmed (i.e., the result is "No" in step ST7), the identified damaged area is determined to be sound in the soundness determination step ST8. In other words, the identified damaged area is determined to be a false positive (a false positive caused by foreign matter adhesion or a depression that is not damage), and the inspection result is corrected to the correct result. After executing the soundness determination step ST8, the inspection is terminated. In the initial visual inspection step ST6, a visual inspection is performed to determine whether or not damage actually exists at the location identified in the damage identification step ST5. If the presence of actual damage cannot be confirmed, the soundness determination step ST8 determines that the identified damage area is sound, and therefore, the area located on a predetermined straight line in the thermal image of the object being inspected is sound.

[0028] <Step 2: Identifying the Injury Site (ST9)> On the other hand, if the presence of actual damage is confirmed in the visual inspection step ST6 (i.e., "Yes" in step ST7), then in the second damage site identification step ST9, it is determined that damage exists at the identified damage site, and the average value dTr corresponding to the candidate damage site Pc is calculated. ave Of these, the next smallest mean value dTr ave The candidate Pc site corresponding to this is identified as a new injury site.

[0029] Then, in the visual inspection step ST6, until it is no longer possible to confirm the presence of actual damage (i.e., until the result is "No" in step ST7 and the soundness determination step ST8 is executed to end the inspection), or in the second damage site identification step ST9, the next smallest average value dTr ave The visual inspection step ST6 and the second injury site identification step ST9 are repeatedly performed until the condition is no longer present (until the result is "No" in step ST10). (If the result is "Yes" in step ST10, the process returns to the visual inspection step ST6.) In the second and subsequent visual inspection steps ST6, a visual inspection is performed to determine whether or not damage actually exists at the injury site identified in the second injury site identification step ST9. By repeatedly performing the visual inspection step ST6 and the second injury site identification step ST9 in this way, the risk of missing an injury site can be reduced. The next smallest average value is dTr. ave The condition ceases to exist (resulting in "No" in step ST10) when a visual inspection step ST6 is performed on all candidate Pc sites and all candidate Pc sites have been identified as damaged sites in the second damaged site identification step ST9.

[0030] The following describes an example in which the thermal image acquisition step ST1 to the damage location identification step ST5 of the damage inspection method according to this embodiment are performed. Figure 5 is a perspective view showing the schematic configuration of the welded joint used as the object of inspection in this embodiment. The welded joint in this embodiment is an lap joint formed by arc welding two 2.6 mm thick steel plates together, and a crack, which is a damaged area, is present near the arc weld.

[0031] In this embodiment, in thermal image acquisition step ST1, a heat gun was used to apply hot air at approximately 80°C to the area around the crack to heat it. From immediately after the end of heating until 10 seconds had elapsed, an infrared imaging device was used to sequentially image the area around the crack in the welded joint at 1-second time steps, thereby acquiring multiple (11) thermal images showing the temperature distribution around the crack in the welded joint. Figure 6 shows the thermal image initially acquired in the thermal image acquisition step ST1 of this embodiment.

[0032] Next, in the temperature difference calculation step ST2, for each of the multiple thermal images, a temperature profile, which is the temperature distribution on line B (see Figures 5 and 6), was extracted using predetermined image processing software attached to the infrared imaging device. Then, based on this temperature profile, the temperature difference dT of the evaluation points (pixels that make up the thermal image) located on line B was calculated. Figure 7 shows the temperature profile (distribution of temperature T) and temperature difference dT extracted from the initial thermal image shown in Figure 6. The horizontal axis in Figure 7 indicates the position on line B (distance from the left end of line B shown in Figure 5). In Figure 7, temperature T is plotted as "◆", and temperature difference dT is plotted as "○" and "●".

[0033] Next, in the temperature difference rate of change calculation step ST3, the rate of change dTr for each time step at 1-second intervals of the temperature difference dT calculated for each of the multiple thermal images was calculated based on the equation (1) described above. In this example, since 11 thermal images were acquired, a total of 10 rate of change dTr were calculated. Then, in the temperature difference rate of change calculation step ST3, the average value dTr of the rate of change dTr for all time steps over 10 seconds was calculated. ave The result was calculated. Figure 8 shows the temperature profile (distribution of temperature T) extracted from the first thermal image acquired and the average value of the rate of change dTr calculated using all thermal images. ave This is shown. The horizontal axis in Figure 8 indicates the position on line B (distance from the left end of line B shown in Figure 5). In Figure 8, temperature T is represented by "◆", and the average value of the rate of change dTr is dTr. ave These are plotted using "□" and "■".

[0034] Next, in the damage site candidate identification step ST4, if the absolute value of the temperature difference dT calculated for the first thermal image acquired among multiple thermal images, as shown in Figure 6, exceeds a predetermined threshold R, the evaluation point corresponding to that temperature difference dT is identified as the damage site candidate Pc, as shown in Figure 7. In this embodiment, since the distance L between adjacent evaluation points is 0.457 mm, the threshold R was set to 1.1425°C according to equation (2) above. In Figure 7, the temperature difference dT corresponding to the evaluation points identified as candidate damaged site Pc is plotted with "●", and the other temperature difference dTs are plotted with "○". In this example, five evaluation points were identified as candidate damaged site Pc.

[0035] Finally, in the injury site identification step ST5, as shown in Figure 8, the average value of the rate of change dTr corresponding to the five candidate injury sites Pc is dTr. ave (The smallest mean value among the points plotted with "■" in Figure 8) ave The candidate Pc for the corresponding injury site was identified as the injury site. As shown in Figure 8, the minimum average value dTr ave This occurs at point c, and a visual inspection of the welded joint revealed the presence of a crack at the location corresponding to point c. Next, the smallest average value dTr ave The location of the crack is point d, and although the welded joint was visually inspected, no crack was found in the area corresponding to point d. Based on the embodiments described above, it has been shown that the damage inspection method according to this embodiment can accurately detect cracks present in welded joints, distinguishing them from uneven areas in the arc weld. [Explanation of Symbols]

[0036] ST1... Thermal image acquisition step ST2...Temperature difference calculation step ST3... Step for calculating the rate of change due to temperature difference ST4... Step to identify potential damaged areas ST5... Step to identify the damaged area ST6...Visual inspection step ST8... Health Assessment Step ST9...Second step in identifying the damaged area

Claims

1. A thermal image acquisition step involves using an infrared imaging device to sequentially image the object under inspection after applying a heat load at predetermined time steps, thereby acquiring multiple thermal images showing the temperature distribution of the object under inspection. A temperature difference calculation step involves extracting a temperature profile, which is a temperature distribution along a predetermined straight line, for each of the plurality of thermal images, and calculating the temperature difference dT of adjacent evaluation points located on the straight line based on the temperature profile. The rate of change dTr of the temperature difference dT calculated for each of the plurality of thermal images is calculated for each time step, and the average value dTr of the rate of change dTr at all time steps is calculated. ａｖｅ A step to calculate the temperature difference rate of change, A step to identify a candidate damage site, in which, if the absolute value of the temperature difference dT calculated for the first thermal image acquired among the plurality of thermal images exceeds a predetermined threshold R, the evaluation point corresponding to the temperature difference dT is identified as a candidate damage site Pc, The average value dTr corresponding to the candidate damaged site Pc. ａｖｅ Of these, the minimum of the above average value dTr ａｖｅ The process includes a step of identifying a candidate damaged area Pc corresponding to the damaged area as the damaged area, Damage inspection methods.

2. A visual inspection step to determine whether or not actual damage exists at the identified damaged area, If, in the visual inspection step, it is not possible to confirm the presence of actual damage, a soundness determination step is performed in which the identified damaged area is determined to be sound. If the presence of actual damage is confirmed in the visual inspection step, it is determined that damage exists at the identified damaged area, and the average value dTr corresponding to the candidate damaged area PC is also determined. ａｖｅ Of these, the next smallest average value dTr ａｖｅ The second step of identifying a damage site is to identify the candidate damage site Pc corresponding to the first step as a new damage site. In the visual inspection step, until it is no longer possible to confirm the presence of actual damage, or in the second damage location identification step, the next smallest average value dTr ａｖｅ The visual inspection step and the second damage location identification step are repeatedly performed until the damage is no longer present. The damage inspection method according to claim 1.

Images