Damage inspection method

Abstract

To provide a damage inspection method that can accurately detect damaged areas. [Solution] The method includes: step ST1 acquiring multiple thermal images by sequentially capturing images while moving an infrared imaging device relative to the object to be inspected; step ST2 extracting a temperature profile from each of the multiple thermal images; step ST3 calculating an average temperature profile by averaging the temperature profiles; step ST4 calculating the temperature difference dT of adjacent evaluation points based on the average temperature profile; step ST5 identifying evaluation points where the absolute value of the temperature difference dT exceeds a threshold Ra as candidate damaged area Pa; step ST6 identifying evaluation points where the absolute value of the temperature difference dT is less than or equal to a threshold Rb1 as healthy areas; and step ST7 identifying candidate damaged area Pa where the absolute value of the difference between the average temperature corresponding to the candidate damaged area Pa and the average healthy area temperature Tp exceeds a predetermined threshold Rb2 as the damaged area.

Description

[Technical Field]

[0001] The present invention relates to a damage inspection method that can detect damaged areas such as cracks present in an object to be inspected by using multiple thermal images obtained by sequentially imaging the object to be inspected while moving an infrared imaging device relative to the object to be inspected. In particular, the present invention relates to a damage inspection method that does not require forcibly inducing stress fluctuations in the object to be inspected and can accurately detect damaged areas present in the object to be inspected. [Background technology]

[0002] Conventionally, a method has been proposed, for example, in Patent Document 1, that uses multiple thermal images obtained by sequentially imaging an object under inspection while moving an infrared imaging device relative to the object under inspection to detect damaged areas such as cracks present in the object under inspection.

[0003] The method described in Patent Document 1 is a method for detecting defects in a structure (girder) on which a moving body (trolley) that generates a moving load travels, and involves installing an infrared imaging device on the moving body, using the infrared imaging device to image the structure on which stress fluctuations occur as the moving body travels, measuring the temperature distribution fluctuations on the surface of the structure as a thermal image, and thereby detecting defects present in the structure. In other words, the method described in Patent Document 1 requires that stress fluctuations be forcibly generated in the structure being inspected while the infrared imaging device is moving, as a prerequisite for detecting defects, which are damaged areas. Therefore, there is a problem that the objects being inspected are limited to structures on which a moving body that generates moving loads travels, such as material handling machinery. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2008-8705 [Overview of the project] [Problems that the invention aims to solve]

[0005] This invention has been made in view of the problems of the prior art described above, and aims to provide a damage inspection method that does not require forcibly inducing stress fluctuations in the object to be inspected, and that can accurately detect damaged areas present in the object to be inspected. [Means for solving the problem]

[0006] To solve the aforementioned problems, the inventors diligently studied a damage inspection method capable of detecting damaged areas such as cracks present in an object to be inspected by using multiple thermal images obtained by sequentially imaging the object to be inspected while moving an infrared imaging device relative to the object to be inspected. Specifically, the inventors considered extracting a temperature profile, which is the temperature distribution on the evaluation line segment in the thermal image corresponding to the same virtual line segment of the object under inspection, for each of the multiple thermal images, and calculating an average temperature profile, which is the average temperature distribution on the evaluation line segment, by averaging the temperature profiles extracted for each of the multiple thermal images. Then, they considered calculating the temperature difference dT of adjacent evaluation points located on the evaluation line segment based on the average temperature profile. Furthermore, it was found that when the object under inspection is subjected to heat from sunlight or ambient heat, the absolute value of the temperature difference dT increases at damaged areas such as cracks due to the insulating effect of the voids. In addition, temperature fluctuations caused by minute irregularities other than the damaged areas of the object under inspection occur in the temperature profile, which can become noise in detecting damaged areas. However, it was found that this noise can be reduced by using an average temperature profile obtained by averaging the temperature profiles (the absolute value of the temperature difference dT caused by minute irregularities tends to be sufficiently small compared to the absolute value of the temperature difference dT at the damaged areas).

[0007] 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 multiple thermal images showing the temperature distribution of an object to be inspected by sequentially imaging the object to be inspected while moving an infrared imaging device relative to the object to be inspected; a temperature profile extraction step of extracting a temperature profile, which is the temperature distribution on the evaluation line segment in the thermal image corresponding to the same virtual line segment of the object to be inspected, for each of the multiple thermal images; an average temperature profile calculation step of calculating an average temperature profile, which is the average temperature distribution on the evaluation line segment, by averaging the temperature profiles extracted for each of the multiple thermal images; a temperature difference calculation step of calculating the temperature difference dT of adjacent evaluation points located on the evaluation line segment based on the average temperature profile; and the temperature difference d A damage inspection method is provided, comprising: a damage site candidate identification step in which, when the absolute value of T exceeds a predetermined threshold Ra, the evaluation point corresponding to the temperature difference dT is identified as a candidate damaged site Pa; a healthy site identification step in which, when the absolute value of the temperature difference dT is less than or equal to a predetermined threshold Rb1 which is smaller than the threshold Ra, the evaluation point corresponding to the temperature difference dT is identified as a healthy site; and a damage site identification step in which, the average value of the average temperature corresponding to the healthy site in the average temperature profile is defined as the healthy site temperature average value Tp, and when the absolute value of the difference between the average temperature corresponding to the candidate damaged site Pa in the average temperature profile and the healthy site temperature average value Tp exceeds a predetermined threshold Rb2, the candidate damaged site Pa is identified as a damaged site.

[0008] 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, in the thermal image acquisition step and the temperature profile extraction step, the temperature profile on the evaluation line segment in the thermal image corresponding to the same virtual line segment of the object under inspection is extracted for each of the multiple thermal images acquired by sequentially imaging the object under inspection while moving the infrared imaging device relative to the object under inspection. For example, when the infrared imaging device is moved in one direction relative to the object under inspection, the evaluation line segment in the thermal image corresponding to the same virtual line segment of the object under inspection will move in the opposite direction of the one direction in each sequentially acquired thermal image. The amount of movement of the evaluation line segment between each thermal image is determined by the relative movement speed, frame rate, and imaging resolution of the infrared imaging device. Using these values, the position of the evaluation line segment in each thermal image corresponding to the same virtual line segment of the object under inspection can be calculated, and the temperature profile on the calculated evaluation line segment can be extracted. In the thermal image acquisition step of the damage inspection method according to the present invention, it is only necessary to move the infrared imaging device relative to the object under inspection, and there is no constraint of forcibly causing stress fluctuations in the object under inspection.

[0009] Furthermore, according to the damage inspection method of the present invention, the average temperature profile is calculated by averaging the temperature profiles extracted for each of the multiple thermal images in the average temperature profile calculation step, which is the average temperature distribution on the evaluation line segment. By using this average temperature profile, the damage inspection method of the present invention can reduce noise caused by minute irregularities and other factors other than the damaged area of ​​the object being inspected, as described above.

[0010] Furthermore, according to the damage inspection method of the present invention, the temperature difference calculation step calculates the temperature difference dT of adjacent evaluation points located on the evaluation line segment based on the average temperature profile. The damage inspection method of the present invention can detect damaged areas such as cracks, as described above, based on the magnitude of the absolute value of this temperature difference dT. However, evaluation points with large absolute values ​​of temperature difference dT may include areas other than the damaged area. Therefore, in order to accurately detect the damaged area, the damage inspection method according to the present invention, in the damaged area candidate identification step, identifies the evaluation point corresponding to the temperature difference dT as a candidate damaged area Pa when the absolute value of the temperature difference dT exceeds a predetermined threshold Ra, rather than immediately identifying the evaluation point as the damaged area. Next, in the healthy area identification step, if the absolute value of the temperature difference dT is less than or equal to a predetermined threshold Rb1 which is smaller than the threshold Ra, the evaluation point corresponding to the temperature difference dT is identified as a healthy area. Finally, in the damaged area identification step, the average value of the average temperature corresponding to the healthy area in the average temperature profile is defined as the healthy area temperature average value Tp, and only when the absolute value of the difference between the average temperature corresponding to the candidate damaged area Pa in the average temperature profile and the healthy area temperature average value Tp exceeds a predetermined threshold Rb2 is the candidate damaged area Pa identified as the damaged area. This makes it possible to identify (detect) the damaged area with greater accuracy compared to immediately identifying the evaluation point corresponding to the temperature difference dT that exceeds the predetermined threshold Ra as the damaged area. As described above, the damage inspection method according to the present invention does not require the forced generation of stress fluctuations in the object under inspection, and it is possible to accurately detect damaged areas present in the object under inspection.

[0011] In the damage inspection method according to the present invention, if it is possible to predict in advance the areas in the object to be inspected where damage is likely to exist, it is conceivable to set one virtual line segment to pass through these areas (and therefore, set one evaluation line segment for each thermal image). However, setting only one virtual line segment (setting one evaluation line segment) may lead to missing damaged areas when the above prediction is difficult, or when damage exists in an area slightly outside the predicted area. To reduce the risk of missing damaged areas, it is preferable to set multiple virtual line segments (set multiple evaluation line segments).

[0012] In other words, in the damage inspection method according to the present invention, in the temperature profile extraction step, a temperature profile is extracted for each of the multiple evaluation line segments in the thermal image corresponding to each of the multiple virtual line segments of the object to be inspected; in the average temperature profile calculation step, the average temperature profile is calculated for each of the multiple evaluation line segments; in the temperature difference calculation step, the temperature difference dT is calculated for each of the multiple evaluation line segments; in the damage site candidate identification step, it is preferable to determine whether there is a temperature difference dT for each of the multiple evaluation line segments whose absolute value exceeds the threshold Ra; in the damage site candidate identification step, for evaluation line segments in which there is a temperature difference dT whose absolute value exceeds the threshold Ra, the evaluation point corresponding to the temperature difference dT is identified as a damage site candidate Pa, and then the healthy site identification step and the damage site identification step are executed; and in the damage site candidate identification step, for evaluation line segments in which there is no temperature difference dT whose absolute value exceeds the threshold Ra, it is preferable to determine that there is no damage site, and then the healthy site identification step and the damage site identification step are not executed.

[0013] According to the preferred method described above, for each of the multiple evaluation line segments in the thermal image corresponding to each of the multiple virtual line segments of the object under inspection, the steps of temperature profile extraction, average temperature profile calculation, temperature difference calculation, and damage site candidate identification are performed. Then, for evaluation line segments in which a candidate for a damaged area has been identified (evaluation line segments in which there is a temperature difference dT whose absolute value exceeds the threshold Ra), the healthy body part identification step and the damaged area identification step are executed as described above. On the other hand, for evaluation line segments in which a candidate for a damaged area has not been identified (evaluation line segments in which there is no temperature difference dT whose absolute value exceeds the threshold Ra), there is no point in executing the healthy body part identification step and the damaged area identification step. Therefore, after determining that there is no damaged area, the healthy body part identification step and the damaged area identification step are not executed. According to the preferred method described above, multiple virtual line segments are set (multiple evaluation line segments are set), thus reducing the risk of missing damaged areas.

Advantages of the Invention

[0014] According to the present invention, there is no restriction on forcibly generating stress fluctuations in the inspection target, and damage sites existing in the inspection target can be accurately detected.

Brief Description of the Drawings

[0015] [Figure 1] It is a flowchart schematically showing steps of a damage inspection method according to an embodiment of the present invention. [Figure 2] It is a diagram schematically explaining the outline of the thermal image acquisition step ST1 shown in FIG. 1. [Figure 3] It shows an example of the first thermal image acquired in the thermal image acquisition step ST1 shown in FIG. 1. [Figure 4] It is a diagram showing an example of a temperature profile on an evaluation line segment corresponding to line A, which is extracted in the temperature profile extraction step ST2 shown in FIG. 1. [Figure 5] It is a diagram showing an example of an average temperature profile on an evaluation line segment corresponding to line A, which is calculated in the average temperature profile calculation step ST3 shown in FIG. 1. [Figure 6] It is a diagram showing an example of the results of executing the temperature difference calculation step ST4, the damage site candidate identification step ST5, and the healthy site identification step ST6 shown in FIG. 1 for the evaluation line segment corresponding to line A. [Figure 7] It is a diagram showing an example of the results of executing the damage site identification step ST7 shown in FIG. 1 for the evaluation line segment corresponding to line A. [Figure 8] It is a diagram showing an example of a temperature profile on an evaluation line segment corresponding to line B, which is extracted in the temperature profile extraction step ST2 shown in FIG. 1. [Figure 9] It is a diagram showing an example of an average temperature profile on an evaluation line segment corresponding to line B, which is calculated in the average temperature profile calculation step ST3 shown in FIG. 1. [Figure 10] It is a diagram showing an example of the results of executing the damage site candidate identification step ST5 shown in FIG. 1 for the evaluation line segment corresponding to line B. [Modes for carrying out the invention]

[0016] The following describes a damage inspection method according to one embodiment of the present invention, with reference to the attached drawings as appropriate. Figure 1 is a flowchart illustrating the steps of the damage inspection method according to this embodiment. As shown in Figure 1, the damage inspection method according to this embodiment includes a thermal image acquisition step ST1, a temperature profile extraction step ST2, an average temperature profile calculation step ST3, a temperature difference calculation step ST4, a candidate damage site identification step ST5, a healthy site identification step ST6, and a damage site identification step ST7. Each step will be described in order below.

[0017] <Thermal image acquisition step ST1> Figure 2 is a schematic diagram illustrating the overview of the thermal image acquisition step ST1. Figure 2(a) is a schematic diagram showing the overview of the apparatus configuration for performing the thermal image acquisition step ST1. Figure 2(b) is a schematic diagram showing the thermal images acquired sequentially in the thermal image acquisition step ST1. In the thermal image acquisition step ST1, as shown in Figure 2(a), the infrared imaging device is moved relative to the object under inspection (in the example shown in Figure 2(a), a welded joint), and the object under inspection is successively imaged to acquire multiple thermal images showing the temperature distribution of the object under inspection. The acquired multiple thermal images are input to the image processing device shown in Figure 2(a). The image processing device consists of, for example, a computer on which predetermined image processing software or programs are installed and which is capable of performing image processing and various calculation processes. In the example shown in Figure 2(a), the object to be inspected is kept stationary, while the infrared imaging device is mounted on a mobile cart and moved in one direction (the X direction) relative to the object to be inspected. However, the present invention is not limited to this, and it is also possible to sequentially image the object to be inspected while the infrared imaging device is kept stationary and the object to be inspected is moved, or while both the infrared imaging device and the object to be inspected are moved (for example, in opposite directions).

[0018] The relative movement speed of the infrared imaging device is preferably up to 1000 mm / sec, although it also depends on the size of the inspection area of the inspection target. Further, the frame rate of the infrared imaging device is preferably 3 frames / sec or more, more preferably 10 frames / sec or more. Furthermore, the imaging resolution of the infrared imaging device is preferably 5.0 mm / pixel or less. When executing the thermal image acquisition step ST1, it is not necessary to apply an artificial thermal load to the inspection target, and it is sufficient if heat due to sunlight or ambient environmental heat is applied. However, for example, it is also possible to apply a thermal load using a halogen lamp or the like.

[0019] Here, as shown in FIG. 2(a), consider setting two virtual line segments (line A, line B) in the inspection target. The line A shown in FIG. 2(a) is a virtual line segment passing through the crack, which is the damage site, and the line B is a virtual line segment not passing through the crack, which is the damage site. As shown in FIG. 2(b), consider sequentially acquiring a total of n thermal images in the thermal image acquisition step ST1. Then, as shown in FIG. 2(b), in the first thermal image, the evaluation line segments corresponding to the virtual line segments line A and line B are line A1 and line B1, respectively, and in the m(1 < m < n)-th thermal image, the evaluation line segments corresponding to the virtual line segments line A and line B are line A m , line B m respectively, and in the n-th thermal image, the evaluation line segments corresponding to the virtual line segments line A and line B are line A n , line B n respectively. Then, the evaluation line segments in each thermal image will move in the reverse direction of the one direction (X direction). And if a plurality of thermal images are continuously acquired according to the frame rate, the amount of movement of the evaluation line segments between each of the acquired thermal images is determined by the relative movement speed, frame rate, and imaging resolution of the infrared imaging device.

[0020] Figure 3 shows an example of the first thermal image acquired in thermal image acquisition step ST1. The thermal image shown in Figure 3 was acquired under the following inspection conditions. [Testing conditions] (1) Items to be inspected: A lap joint formed by arc welding two 2.6 mm thick steel plates together, as shown in Figure 2(a), in which a crack exists as a damaged area in one of the steel plates. (2) Relative movement speed of the infrared imaging device: As shown in Figure 2(a), the infrared imaging device was mounted on a mobile cart and moved in the X direction at a relative movement speed of 12 mm / sec. (3) Frame rate of the infrared imaging device: 10 frames / sec (4) Imaging resolution of the infrared imaging device: 0.6 mm / pixel (5) Imaging time for infrared imaging device: 1 sec (imaging continuously according to the frame rate) According to the above inspection conditions, the infrared imaging device moves 1.2 mm between each acquired thermal image (12 [mm / sec] / 10 [frames / sec] = 1.2 [mm / frame]), so the amount of movement of the evaluation line segment between each thermal image is 1.2 mm (= equivalent to 2 pixels).

[0021] <Temperature Profile Extraction Step ST2> In the temperature profile extraction step ST2, using the image processing device shown in Figure 1, a temperature profile is extracted for each of the multiple thermal images shown in Figures 2(b) and 3, which is the temperature distribution on the evaluation line segment in the thermal image corresponding to the same virtual line segment of the object being inspected. Specifically, in this embodiment, an image processing device is used to set evaluation line segments (line A1, line B1) corresponding to the virtual line segments (line A, line B) of the object under inspection in the first thermal image, and extract a temperature profile, which is the temperature distribution on these evaluation line segments. Next, for each subsequent thermal image, the temperature profile is first extracted on the line segment at the same position as the evaluation line segment (line A1, line B1) in the first thermal image (the line segment whose coordinates in each thermal image are the same as the evaluation line segment in the first thermal image). The temperature profile extracted in this way for each subsequent thermal image does not correspond to the position of the virtual line segments (line A, line B) of the object under inspection, but rather is the temperature profile on a line segment that has shifted in the relative movement direction (X direction) of the infrared imaging device. Therefore, by correcting this shift (performing position adjustment), the temperature profile on the evaluation line segment corresponding to the virtual line segments (line A, line B) of the object under inspection is extracted for the subsequent thermal images as well. In the following, we will first explain what to do with the evaluation line segment corresponding to line A, and then explain what to do with the evaluation line segment corresponding to line B.

[0022] Figure 4 shows an example of a temperature profile on the evaluation line segment corresponding to line A, extracted in step ST2 under the aforementioned inspection conditions. Figure 4(a) shows the temperature profile extracted for the 1st to 3rd thermal images before correction of positional displacement, and Figure 4(b) shows the temperature profile extracted for the 1st to 3rd thermal images after correction of positional displacement. In Figure 4(a), the horizontal axis represents the position on the evaluation line segment (line A1) of the first thermal image (the distance from the left end of line A1 shown in Figure 2(b), which corresponds to the left end of line A shown in Figure 2(a)), since the positional displacement has not been corrected. In Figure 4(b), the horizontal axis represents the position on the evaluation line segment corresponding to line A (line A1 for the first thermal image, line A2 for the second thermal image, and line A3 for the third thermal image) (the distance from the left end of each line A1 to A3), since the positional displacement has been corrected. As can be seen by comparing Figure 4(a) and Figure 4(b), the temperature profile of the first thermal image remains unchanged, the temperature profile of the second thermal image is shifted 1.2 mm (equivalent to 2 pixels) to the left (in the X direction) before and after correction, and the temperature profile of the third thermal image is shifted 2.4 mm (equivalent to 4 pixels) to the left (in the X direction) before and after correction. In this embodiment, for each thermal image from the second to the next, the temperature profile on the line segment at the same position as the evaluation line segment of the first thermal image is first extracted, and then the positional shift is corrected. However, the present invention is not limited to this. Since the amount of movement of the evaluation line segment between each thermal image can be calculated in advance using the relative movement speed, frame rate, and imaging resolution of the infrared imaging device, it is also possible to determine the position (coordinates in each thermal image) of the evaluation line segment of each thermal image corresponding to the virtual line segment of the object under inspection from the beginning, and then directly extract the temperature profile on the determined evaluation line segment.

[0023] <Average temperature profile calculation step ST3> In the average temperature profile calculation step ST3, the temperature profiles extracted from multiple thermal images as shown in Figure 4(b) are averaged using the image processing device shown in Figure 1, thereby calculating the average temperature T on the evaluation line segment. ave The mean temperature profile, which is the distribution of the temperature, is calculated. Figure 5 shows an example of the average temperature profile on the evaluation line segment corresponding to line A, calculated in the average temperature profile calculation step ST3 under the aforementioned inspection conditions. Although Figure 4(b) only shows the temperature profiles extracted for the first to third thermal images, under the aforementioned inspection conditions, 10 thermal images (10 [frames / sec] × 1 [sec] = 10 [frames]) are actually acquired per second of imaging time by the infrared imaging device. Therefore, under the aforementioned inspection conditions, the average temperature profile is calculated in the average temperature profile calculation step ST3 by averaging the temperature profiles extracted for each of the 10 thermal images.

[0024] <Temperature difference calculation step ST4> In the temperature difference calculation step ST4, the temperature difference dT of adjacent evaluation points located on the evaluation line segment is calculated using the image processing device shown in Figure 1, based on the average temperature profile shown in Figure 5. The evaluation points are, for example, pixels that make up the thermal image, and in this case, adjacent evaluation points are adjacent pixels. In this embodiment, pixels are used as evaluation points.

[0025] <Step ST5: Identifying potential damaged areas> In the injury site candidate identification step ST5, using the image processing device shown in Figure 1, if the absolute value of the temperature difference dT exceeds a predetermined threshold Ra, the evaluation point corresponding to the temperature difference dT is identified as the injury site candidate Pa. Specifically, it is determined whether or not there is a temperature difference dT whose absolute value exceeds a predetermined threshold Ra (step ST51 in Figure 1). If there is a temperature difference dT whose absolute value exceeds the threshold Ra (if "Yes" is found in step ST51 in Figure 1), the evaluation point corresponding to that temperature difference dT is identified as a candidate damaged site Pa (step ST52 in Figure 1). The threshold value Ra is determined, for example, by the following equation (1). Ra[℃] = 0.1 × L[mm] ···(1) In equation (1) above, L represents the distance between adjacent evaluation points from which the temperature difference dT was calculated. Under the aforementioned inspection conditions, the imaging resolution of the infrared imaging device is 0.6 mm / pixel, and since pixels are used as evaluation points, L = 0.6 mm, and from equation (1) above, Ra = 0.06 °C.

[0026] Figure 6 shows an example of the results obtained when performing the temperature difference calculation step ST4, the candidate damage site identification step ST5, and the healthy part identification step ST6 (described later) for the evaluation line segment corresponding to line A, under the aforementioned inspection conditions. In Figure 6, the average temperature T calculated in the average temperature profile calculation step ST3 is shown. ave The values ​​plotted with "◆" are shown, and the absolute values ​​of the temperature difference dT calculated in step ST4 are plotted with "+", "○", and "●". The data plotted with "◆" are the same as those shown in Figure 5. As shown in Figure 6, since the absolute value of the temperature difference dT plotted with "+" exceeds the threshold Ra, the six evaluation points corresponding to the temperature difference dT plotted with "+" are identified as candidate damage sites Pa.

[0027] <Healthy Body Identification Step ST6> In the healthy tissue identification step ST6, using the image processing device shown in Figure 1, if the absolute value of the temperature difference dT is less than or equal to a predetermined threshold Rb1 which is smaller than the threshold Ra, the evaluation point corresponding to the temperature difference dT is identified as a healthy tissue. The threshold Rb1 is determined, for example, by the following equation (2). Rb1[℃]=0.067×L[mm] ···(2) In equation (2) above, L represents the distance between adjacent evaluation points from which the temperature difference dT was calculated. Under the aforementioned inspection conditions, the imaging resolution of the infrared imaging device is 0.6 mm / pixel, and since pixels are used as evaluation points, L = 0.6 mm, and from equation (2) above, Rb1 = 0.04 °C.

[0028] As shown in Figure 6, since the absolute value of the temperature difference dT plotted with "○" is less than or equal to the threshold Rb1, the 19 evaluation points corresponding to the temperature difference dT plotted with "○" are identified as healthy tissues.

[0029] <Step ST7: Identifying the Damaged Area> In the injury site identification step ST7, the average value of the average temperature corresponding to a healthy area in the average temperature profile is calculated as the healthy area temperature average value Tp using the image processing device shown in Figure 1. Then, if the absolute value of the difference between the average temperature corresponding to the candidate injury site Pa in the average temperature profile and the healthy area temperature average value Tp exceeds a predetermined threshold Rb2, the candidate injury site Pa is identified as an injury site. The threshold Rb2 is determined, for example, by the following equation (3). Rb1[℃]=0.417×L[mm] ···(3) In equation (3) above, L represents the distance between adjacent evaluation points from which the temperature difference dT was calculated. Under the aforementioned inspection conditions, the imaging resolution of the infrared imaging device is 0.6 mm / pixel, and since pixels are used as evaluation points, L = 0.6 mm, and from equation (3) above, Rb2 = 0.25°C.

[0030] Figure 7 shows an example of the results obtained by performing the damage identification step ST7 on the evaluation line segment corresponding to line A under the inspection conditions described above. In Figure 7, the average temperature T calculated in the average temperature profile calculation step ST3 is shown. ave The plotted points are marked with "×" and "◆", and the plotted data is the same as that shown in Figure 5. Of these, the data plotted with "×" corresponds to the six candidate injury sites Pa identified in the injury site candidate identification step ST5. As shown in Figure 7, among the data corresponding to the six candidate injury sites Pa plotted with "×", the average temperature T corresponding to the candidate injury site Pa is aveThere is one instance where the absolute value of the difference between the healthy body temperature average Tp and the Pa site exceeds a predetermined threshold Rb2. Therefore, this candidate Pa site is identified as the damaged site. It was found that the identified damaged site coincided with the location of a crack actually present in the inspected object (see Figure 2(a)).

[0031] The above explanation described what to do for the evaluation line segment corresponding to line A. Next, we will explain what to do for the evaluation line segment corresponding to line B.

[0032] For the evaluation line segment corresponding to line B, the same steps as for the evaluation line segment corresponding to line A are performed: temperature profile extraction step ST2, average temperature profile calculation step ST3, temperature difference calculation step ST4, and damage site candidate identification step ST5. Figure 8 shows an example of a temperature profile on the evaluation line segment corresponding to line B, extracted in step ST2 under the aforementioned inspection conditions. Figure 8(a) shows the temperature profile extracted for the 1st to 3rd thermal images before correction of positional displacement, and Figure 8(b) shows the temperature profile extracted for the 1st to 3rd thermal images after correction of positional displacement. In Figure 8(a), the horizontal axis represents the position on the evaluation line segment (line B1) of the first thermal image (the distance from the left end of line B1 shown in Figure 2(b), which corresponds to the left end of line B shown in Figure 2(a)), since the positional displacement has not been corrected. In Figure 8(b), the horizontal axis represents the position on the evaluation line segment corresponding to line B (line B1 for the first thermal image, line B2 for the second thermal image, and line B3 for the third thermal image) (the distance from the left end of each line B1 to B3), since the positional displacement has been corrected. As can be seen by comparing Figure 8(a) and Figure 8(b), the temperature profile of the first thermal image remains unchanged, the temperature profile of the second thermal image is shifted 1.2 mm (equivalent to 2 pixels) to the left (in the X direction) before and after correction, and the temperature profile of the third thermal image is shifted 2.4 mm (equivalent to 4 pixels) to the left (in the X direction) before and after correction.

[0033] FIG. 9 is a diagram showing an example of an average temperature profile on an evaluation line segment corresponding to line B, which is calculated in the average temperature profile calculation step ST3 under the above-described inspection conditions. In FIG. 8(b), only the temperature profiles extracted from the first to third thermal images are shown, but under the above-described inspection conditions, actually, 10 thermal images (10 [frames / sec] × 1 [sec] = 10 [frames]) are acquired in 1 second of the imaging time of the infrared imaging device. Therefore, under the above-described inspection conditions, in the average temperature profile calculation step ST3, the average temperature profile is calculated by averaging the temperature profiles extracted for every 10 thermal images.

[0034] Regarding the evaluation line segment corresponding to line B, in the temperature difference calculation step ST4, the temperature difference dT between adjacent evaluation points located on the evaluation line segment is calculated based on the average temperature profile as shown in FIG. 9 using the image processing device shown in FIG. 1. The evaluation points are, for example, pixels constituting the thermal image, and in this case, the adjacent evaluation points are adjacent pixels. In the present embodiment, pixels are used as the evaluation points.

[0035] FIG. 10 is a diagram showing an example of the results of executing the temperature difference calculation step ST4 and the damage site candidate identification step ST5 for the evaluation line segment corresponding to line B under the above-described inspection conditions. In FIG. 10, the average temperature T calculated in the average temperature profile calculation step ST3 ave is plotted with "◆", and the absolute value of the temperature difference dT calculated in the temperature difference calculation step ST4 is plotted with "○". The data plotted with "◆" is the same as that shown in FIG. 9. As shown in Figure 10, the absolute values ​​of the temperature difference dT plotted with "○" do not exceed the threshold Ra. That is, for the evaluation line segment corresponding to line B, there are no temperature difference dT values ​​whose absolute value exceeds the threshold Ra (resulting in "No" in step ST51 of Figure 1), so it is determined that there are no damaged areas for the evaluation line segment corresponding to line B (step ST53 of Figure 1). Unlike the evaluation line segment corresponding to line A, the inspection is terminated without performing the healthy area identification step ST6 and the damaged area identification step ST7. It was found that there are actually no cracks or other damage in line B, the subject being inspected, which was determined to have no damaged areas.

[0036] As described above, the damage inspection method according to this embodiment does not require forcibly generating stress fluctuations in the object under inspection, and it is possible to accurately detect damaged areas present in the object under inspection. In this embodiment, the case where two virtual line segments (line A and line B) are set in the object under inspection was used as an example for explanation, but the present invention is not limited to this, and can be similarly applied to cases where only one virtual line segment or three or more virtual line segments are set. Furthermore, although this embodiment describes the case where the object to be inspected is a welded joint (see Figure 2(a)), the objects to which the present invention can be applied are not limited to this. The present invention can be applied to a wide range of objects to be inspected, such as automotive structural members other than welded joints, home appliances, materials such as slabs, crane runway girders inside factory buildings, belt conveyors, building walls and roofs, and social infrastructure such as highways. [Explanation of Symbols]

[0037] ST1... Thermal image acquisition step ST2...Temperature profile extraction step ST3...Average temperature profile calculation step ST4...Temperature difference calculation step ST5... Step to identify potential damaged areas ST6...Step to identify a healthy body part ST7... Step to identify the damaged area

Claims

1. A thermal image acquisition step involves sequentially imaging the object under inspection while moving an infrared imaging device relative to the object under inspection, thereby acquiring multiple thermal images showing the temperature distribution of the object under inspection. A temperature profile extraction step for each of the plurality of thermal images, which is the temperature distribution on the evaluation line segment in the thermal image corresponding to the same virtual line segment of the object under inspection, The average temperature profile calculation step involves calculating an average temperature profile, which is the average temperature distribution on the evaluation line segment, by averaging the temperature profiles extracted for each of the plurality of thermal images. A temperature difference calculation step that calculates the temperature difference dT of adjacent evaluation points located on the evaluation line segment based on the average temperature profile, A step to identify a candidate damage site, in which, if the absolute value of the temperature difference dT exceeds a predetermined threshold Ra, the evaluation point corresponding to the temperature difference dT is identified as a candidate damage site Pa. A healthy body part identification step in which, if the absolute value of the temperature difference dT is less than or equal to a predetermined threshold Rb1 which is smaller than the threshold Ra, the evaluation point corresponding to the temperature difference dT is identified as a healthy body part, The method includes a step of identifying a damaged area where, if the average value of the average temperature corresponding to the healthy area in the average temperature profile is defined as the average healthy area temperature value Tp, and the absolute value of the difference between the average temperature corresponding to the candidate damaged area Pa in the average temperature profile and the average healthy area temperature value Tp exceeds a predetermined threshold Rb2, the candidate damaged area Pa is identified as a damaged area. Damage inspection methods.

2. In the temperature profile extraction step, for each of the multiple evaluation line segments in the thermal image corresponding to each of the multiple virtual line segments of the object under inspection, the temperature profile is extracted. In the average temperature profile calculation step, the average temperature profile is calculated for each of the multiple evaluation line segments. In the step of calculating the temperature difference, the temperature difference dT is calculated for each of the multiple evaluation line segments. In the step of identifying candidate damaged areas, it is determined whether there is a temperature difference dT whose absolute value exceeds the threshold Ra for each of the multiple evaluation line segments. In the step of identifying candidate damaged areas, for evaluation line segments where the temperature difference dT exceeds the threshold Ra in absolute value, the evaluation point corresponding to the temperature difference dT is identified as a candidate damaged area Pa, and then the healthy area identification step and the damaged area identification step are executed. In the step of identifying candidate damaged areas, for evaluation line segments where there is no temperature difference dT whose absolute value exceeds the threshold Ra, it is determined that there are no damaged areas, and the healthy area identification step and the damaged area identification step are not performed. The damage inspection method according to claim 1.

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