CN111323454B - Method for detecting cracks through laser thermal imaging - Google Patents
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Abstract
The invention discloses a method for detecting cracks by laser thermal imaging. The laser spots are two groups of point spots arranged into parallel line arrays, and the two line arrays are staggered with a certain distance along the length direction of the line. When scanning detection is carried out, each spot generates heat flow in the direction parallel to the scanning direction and the direction perpendicular to the scanning direction, and cracks in all directions near the spot can be detected. If the crack is parallel to the scanning direction and is just positioned at the center of a certain light spot, or the crack is positioned on the symmetrical line of the centers of two adjacent point light spots of a certain line array, the crack can not cause temperature abnormity when the light spot of the line array passes through the crack, but can certainly cause temperature abnormity when the light spot of another line array passes through the crack, and therefore the crack is detected. By adopting the laser spot design, cracks in special positions and special directions cannot be omitted, and the scanning detection mode only needs to execute one-dimensional scanning once, so that the detection efficiency is higher.
Description
Technical Field
The invention relates to a nondestructive testing method for surface cracks, in particular to a crack testing method based on laser thermal imaging.
Background
Laser thermal imaging is a nondestructive testing method mainly aiming at surface and near-surface cracks, and the basic principle is that heat transfer is caused by local heating of laser, and when cracks exist on a heat transfer path, due to high thermal resistance brought by air gaps at the cracks, the spatial distribution rule or the time change rule of the temperature at two sides of the cracks is abnormal.
The spatial distribution mode of the power density of the light spots during laser thermal imaging detection has great influence on the detection effect. Common current laser modes include: point laser, line laser, and dot matrix laser.
The spot laser heating can cause heat transfer in all directions simultaneously, so that the heat transfer is hindered except for the crack which passes through the center of the light spot and expands along the radial direction, and the detection can be realized. However, the spot laser can only detect a small area close to the spot at a time, and if the whole surface is to be detected, two-dimensional scanning needs to be performed, so that the detection efficiency is low. The detection system using the point laser structurally comprises a point laser and a two-dimensional scanning system, and the structure is relatively simple. The spot laser scanning detection method is also called as a flying spot technology, and is the most original working form of the laser thermal imaging detection technology, and various improved versions of the method generally aim to improve the detection effect on various position and form cracks or improve the signal to noise ratio.
The light spot of the linear laser is linear, and only one-dimensional scanning is needed for detecting the whole surface. However, the heat transfer caused by the linear laser is mainly performed in the direction perpendicular to the linear light spot, so that only the cracks with small included angles with the linear light spot can cause large influence on the heat transfer and are easy to detect, and the small cracks perpendicular to the linear light spot can be usually missed to detect. To solve this problem, two one-dimensional scans can be performed using a line-spot laser, the two scanning directions being perpendicular to each other. The detection system based on the line laser generally comprises two line lasers and two one-dimensional scanning devices, or a line laser, a one-dimensional line scanning device and a 90-degree steering device; compared with a point laser heating mode, the system complexity mainly lies in the difference between a line laser and a point laser.
The lattice laser is simultaneously used for testing a plurality of point lasers which are distributed in an array. Although each point laser can only detect a small area near the light spot, a large number of point lasers can simultaneously detect, and a large area can be detected at a time. Point lasers may not need to be scanned; or after one area is detected, the detection is switched to the next area in a jumping mode, and therefore the detection efficiency is highest. During the lattice laser detection, the light spots are generally static during the detection, so that the detection sensitivity has obvious difference along with the difference of the distance from the light spots, and the test result has stronger non-uniformity. Moreover, if the crack position is very coincidentally just along the heat flow direction, such as passing through the center of the light spot, or just on the symmetrical line of the middle of the two light spots, the detection is easy to miss. In the detection system based on the lattice laser, a moving device can be omitted, but the number of the point lasers is large, or a single high-power laser and a complex optical device are used for generating the point laser array, so that the design is relatively more complex.
Disclosure of Invention
The technical problem to be solved by the invention is as follows: the laser thermal imaging detection method is provided, cracks in all positions and trends can be detected, and meanwhile detection speed and system complexity are represented well.
The technical scheme adopted by the invention for solving the technical problem is as follows:
the laser spot used for thermal excitation during testing is composed of a plurality of spot light spots, the spot light spots are arranged into two linear arrays which are parallel to each other, and the spot light spots in the two linear arrays are staggered with each other by a certain distance.
During testing, the laser light spot moves in a one-dimensional uniform motion relative to the surface of a test sample, and the motion direction is along the direction of a vertical linear array; the thermal imaging device records the temperature field evolution of the test surface caused by heating, and generates an image showing the cracks through data processing.
Further, when the test sample surface is planar:
and setting the temperature field evolution data recorded by the thermal imager as T (T, x, y), wherein T is discrete time, and x and y are discrete coordinates of the position of the temperature measurement point on the surface of the sample. For any point P on the surface of the sample, the coordinates are (x, y), and the data are processed as follows:
calculating an X-direction temperature gradient GX1(X, Y) and a Y-direction temperature gradient GY1(X, Y) of the point P at the time t1- Δ t1, and an X-direction temperature gradient GX2(X, Y) and a Y-direction temperature gradient GY2(X, Y) at the time t2- Δ t2, wherein t1 is the time when the first linear array passes through the point P, t2 is the time when the second linear array passes through the point P, and Δ t1 and Δ t2 are constant values.
The temperature gradient abnormality of the point P in the X direction and the Y direction at the time t 1-delta t1 and the temperature gradient abnormality in the X direction and the Y direction at the time t 2-delta t2 are calculated.
And solving the square sum of all temperature gradient anomalies, and carrying out square root calculation on the square sum to obtain a test result for representing the test surface of the sample.
Further, when the test sample surface is a cylindrical surface:
and setting the temperature field evolution data recorded by the thermal imager as T (T, x, y), wherein T is discrete time, and x and y are discrete coordinates of the position of the temperature measurement point on the surface of the sample. For any point P on the surface of the sample, the coordinates are (x, y), and the data are processed as follows:
calculating an X-direction temperature gradient GX1(X, Y) and a Y-direction temperature gradient GY1(X, Y) of the point P at the time t1- Δ t1, and an X-direction temperature gradient GX2(X, Y) and a Y-direction temperature gradient GY2(X, Y) at the time t2- Δ t2, wherein t1 is the time when the first linear array passes through the point P, t2 is the time when the second linear array passes through the point P, and Δ t1 and Δ t2 are constant values.
And calculating the temperature gradient anomalies of the point P in the X direction and the Y direction at the time t 1-delta t1 and the temperature gradient anomalies in the X direction and the Y direction at the time t 2-delta t2, and taking absolute values of all the temperature gradient anomalies.
And selecting the maximum temperature gradient abnormal absolute values of the point P in the X direction and the Y direction at two moments, and adding the maximum temperature gradient abnormal absolute values and the maximum temperature gradient abnormal absolute values to obtain a test result representing the test surface of the sample.
The invention has the beneficial effects that: by adopting the laser arrangement array, cracks in special positions and special directions cannot be omitted, and the scanning detection mode only needs to execute one-dimensional scanning once, so that the detection efficiency is higher.
Drawings
Fig. 1 is a basic structure of a laser thermal imaging detection system in embodiment 1 of the present invention;
fig. 2 shows a laser spot and a crack on the surface of a sample in embodiment 1 of the present invention;
fig. 3 is a basic structure of a laser thermal imaging detection system in embodiment 2 of the present invention;
fig. 4 shows a laser spot and a crack on the surface of a sample in embodiment 2 of the present invention;
Detailed Description
For the purpose of making the objects, technical solutions and advantages of the present invention clear, the present invention will be further described with reference to the accompanying drawings, in which:
The basic structure of the test system is shown in FIG. 1: the surface of a sample 1 to be measured is provided with a crack 2, a laser 3 projects laser 4 to the surface of the sample, and the laser 4 heats the sample 1 to cause the sample to face a heat flow 5; the thermal imager 6 is positioned above the sample 1, observes and records the evolution of the surface temperature field of the sample 1, and transmits data to the data analysis software 7; the laser 3 is arranged on the displacement table 8 and can perform one-dimensional scanning, so that the laser 4 can continuously scan and heat the surface of the sample 1.
The surface condition of the sample 1 is shown in FIG. 2, and the laser 4 used for thermal excitation during the test consists of 18 spot spots 411-419 and 421-429, wherein the diameter of each spot is about 1 mm; wherein, 9 light spots such as 411 to 419 are arranged in a line array L1 with equal interval, and the interval between adjacent light spots is about 3 mm; the spot light spots 421-429 and other 9 light spots are arranged at equal intervals to form a line array L2, and the interval between adjacent light spots is about 3 mm; spot arrays L1 and L2 were parallel and spaced approximately 8mm apart from each other. For convenience of description, a coordinate system is established with the line array direction as the x-axis and the perpendicular array direction as the y-axis. The spot light spots in the line arrays L1 and L2 were misaligned in the X-axis direction, being offset from each other by about 1mm in the X-coordinate. During testing, the laser 4 moves on the surface of the sample 1 from one end to the other end at a constant speed of about 5mm/s along the y-axis direction. The surface of sample 1 had cracks of various orientations, with crack 21 oriented approximately parallel to the wire array and cracks 22, 23, and 24 oriented approximately perpendicular to the wire array at different locations.
The thermal imager is assumed to record temperature field evolution data as T (T, x, y), where T is discrete time and x and y are discrete coordinates of the location of the sample surface thermometry point. For any point P on the surface of the sample, the coordinate is (x, y), and the test result RLT (x, y) is calculated as follows:
step 1: assuming that the time when the straight line where the line array L1 is located (i.e. the straight lines where the centers of all the light spots of the point in the line array are located) moves through the point is t1, the time when the straight line where the line array L2 is located moves through the point is t2, and t1 and t2 can be obtained by calculation according to the P point coordinate, the spot movement speed, the scanning start time and other prior knowledge; calculating an X-direction temperature gradient GX1(X, Y) and a Y-direction temperature gradient GY1(X, Y) of the P point at the time t1- Δ t1, and an X-direction temperature gradient GX2(X, Y) and a Y-direction temperature gradient GY2(X, Y) at the time t2- Δ t 2:
GX1(x,y)＝T(t1-Δt,x,y)-T(t1-Δt,x-1,y)
GY1(x,y)＝T(t1-Δt,x,y)-T(t1-Δt,x,y-1)
GX2(x,y)＝T(t2-Δt,x,y)-T(t2-Δt,x-1,y)
GY2(x,y)＝T(t2-Δt,x,y)-T(t2-Δt,x,y-1)
wherein the parameters Δ t1 and Δ t1 relate to the diameter of the light spot, the scanning speed, the thermal diffusion speed of the material, etc., and generally correspond to the time difference between the time when the temperature gradient at the point takes the maximum value when no crack exists and the time when the light spot passes through the point, such as 20ms (the thermal imaging frame frequency is 50Hz, and corresponds to the sampling interval of 1 frame); the optimal value of the parameter can be determined empirically or by limited heuristics in practice; the parameter has little effect on the test effect if the deviation is not too large, even if it is not an optimal value.
Step 2: calculating the temperature gradient anomaly:
and step 3: all temperature gradient anomalies were combined:
the image described by the RLT matrix reflects the test results for the test face of sample 1 and the crack appears as a bright line.
One of the advantages of the test scheme is that cracks in any direction and any position can be detected, which is described as follows:
1) for a crack 21 parallel to the line array, the motion tracks of the light spots 423, 424 and 414 vertically pass through the crack, so that the temperature gradient in the Y direction of the crack position is larger than that in the normal area before the corresponding light spot arrives, and the temperature gradient can be detected;
2) for a crack 22 perpendicular to the line array, when the line array L2 is scanned to the vicinity of the crack, the crack will cause an obstruction to the X-direction thermal flow due to the closer spot 425 is to the crack, resulting in an abnormal X-direction temperature gradient and thus being detected;
3) for the crack 23 perpendicular to the line array, it is just located on the motion track of the light spot 426, so when the line array L2 scans to the vicinity of the crack 23, there is no X-direction heat flow at this position, and no abnormal temperature gradient is caused; however, since the light spots of the line array L1 are staggered by 1mm from the line array L2 in the X direction, when the L1 array moves to the vicinity of the crack 23, the light spot 416 is 1mm away from the crack position, and the light spot 417 is 2mm away from the crack, so that X-direction heat flow is generated and blocked by the crack, and the X-direction temperature gradient is abnormal, so that the X-direction heat flow can still be successfully detected;
4) for a crack 24 perpendicular to the line array, which is just on the line of symmetry of spot 426 and spot 427, there is no X-direction heat flow when the line array L2 is moved to the vicinity of the crack 24, and the crack does not cause a temperature gradient anomaly; however, since the spots of line array L1 are offset from line array L2 by 1mm in the X-direction, when the L1 array is moved to the vicinity of crack 24, spot 416 will be 2.5mm from crack 24, while spot 417 will be only 0.5mm from crack 24, so the crack is no longer at the center line of symmetry of two adjacent spots, which would normally have X-direction heat flow that would be impeded by crack 24 to cause an anomaly.
In the aspect of detection efficiency, because the scanning directions of the two line arrays are consistent, all cracks can be detected only by performing one-dimensional scanning; and if line facula laser is used, two times of one-dimensional scanning with mutually vertical directions need to be executed, and the detection efficiency of the scheme is higher.
The basic structure of the test system is shown in FIG. 3: the surface of a cylindrical sample to be detected 11 is provided with a crack 2, a laser 14 projects laser 16 to the surface of the sample, and a laser 15 projects laser 17 to the surface of the sample; laser heating causes a facing heat flow 5; the thermal imager 6 is positioned above the sample 1, observes and records the evolution of the surface temperature field of the sample 11, and transmits data to the data analysis software 7; the cylindrical sample 11 is placed on a cylindrical roller 12 and a roller 13, and the roller 12 and the roller 13 rotate at the same speed at a constant speed, so that the cylindrical sample 11 rotates at the same speed, and laser thermal imaging scanning detection of the whole outer circular surface of the sample 11 is realized.
The surface condition of the sample 11 is shown in fig. 4, the outer circumferential surface of the cylindrical sample 11 is unfolded into a rectangle; for convenience of expression, a coordinate system is established on the expansion plane as follows: the axial direction of the cylindrical sample is a Z axis, and the axial direction of the outer cylindrical surface is a theta axis. During testing, the laser 16 used for thermal excitation consists of 9 point light spots 411-419, the laser 17 consists of 9 point light spots 421-429, and the diameters of all 18 light spots are about 1 mm; wherein, the spot light spots 411 to 419 and the like are arranged in a linear array L1 with 9 light spots in equal interval, L1 is parallel to the Z axis, and the distance between the light spots of the adjacent points in L1 is about 3 mm; the spot light spots 421-429 and 9 light spots are arranged at equal intervals to form a line array L2, the L2 is parallel to the Z axis, and the interval between the adjacent spot light spots is about 3 mm; spot arrays L1 and L2 were circumferentially spaced about 8mm apart. The spot light spots in the line arrays L1 and L2 were misaligned in the Z-axis direction, being approximately 1mm out of Z-coordinate from each other. During testing, the sample rotates at a constant speed of 5mm/s, so that the two laser spot line arrays can be equivalently scanned at a constant speed along the outer cylindrical surface of the sample 11. The surface of sample 11 had cracks of various orientations, with crack 21 oriented approximately parallel to the array of wires, and cracks 22, 23, and 24 oriented approximately perpendicular to the array of wires, at various locations.
The thermal imager is assumed to record temperature field evolution data as T (T, x, y), where T is discrete time and x and y are discrete coordinates of the location of the sample surface thermometry point. For any point P on the surface of the sample, the coordinate is (x, y), and the test result RLT (x, y) is calculated as follows:
step 1: assuming that the time when the straight line where the line array L1 is located (i.e. the straight lines where the centers of all the light spots of the point in the line array are located) moves through the point is t1, the time when the straight line where the line array L2 is located moves through the point is t2, and t1 and t2 can be obtained by calculation according to the P point coordinate, the spot movement speed, the scanning start time and other prior knowledge; calculating an X-direction temperature gradient GX1(X, Y) and a Y-direction temperature gradient GY1(X, Y) of the P point at the time t1- Δ t1, and an X-direction temperature gradient GX2(X, Y) and a Y-direction temperature gradient GY2(X, Y) at the time t2- Δ t 2:
GX1(x,y)＝T(t1-Δt,x,y)-T(t1-Δt,x-1,y)
GY1(x,y)＝T(t1-Δt,x,y)-T(t1-Δt,x,y-1)
GX2(x,y)＝T(t2-Δt,x,y)-T(t2-Δt,x-1,y)
GY2(x,y)＝T(t2-Δt,x,y)-T(t2-Δt,x,y-1)
wherein the parameters Δ t1 and Δ t1 relate to the diameter of the light spot, the scanning speed, the thermal diffusion speed of the material, etc., and generally correspond to the time difference between the time when the temperature gradient at the point takes the maximum value when no crack exists and the time when the light spot passes through the point, such as 20ms (the thermal imaging frame frequency is 50Hz, and corresponds to the sampling interval of 1 frame); the optimal value of the parameter can be determined empirically or by limited heuristics in practice; the parameter has little effect on the test effect if the deviation is not too large, even if it is not an optimal value.
Step 2: calculating the temperature gradient anomaly:
and step 3: all temperature gradient anomalies were combined:
RLT (x, y) ═ max { GX1E (x, y), GX2E (x, y) } + max { GY1E (x, y), GY2E (x, y) } where max denotes the maximum of several pieces of data in parentheses.
In the image described by the RLT matrix, the detection result of the cylindrical sample 11 after the outer cylindrical surface is unfolded is reflected, and the crack appears as a bright line.
The embodiments described in this specification are merely illustrative of implementations of the inventive concept and the scope of the present invention should not be considered limited to the specific forms set forth in the embodiments but rather by the equivalents thereof as may occur to those skilled in the art upon consideration of the present inventive concept.
Claims (2)
1. A laser thermal imaging detection method is characterized in that:
the laser spot used for thermal excitation during testing consists of a plurality of spot light, the spot light is arranged into two linear arrays which are parallel to each other, and the spot light in the two linear arrays are staggered with each other by a certain distance;
during testing, the laser light spot moves in a one-dimensional uniform motion relative to the surface of a test sample, and the motion direction is along the direction of a vertical linear array; recording the evolution of a test surface temperature field caused by heating by a thermal imaging device, and generating an image for displaying the crack through data processing;
when the test sample surface is planar:
setting temperature field evolution data recorded by a thermal imager as T (T, x, y), wherein T is discrete time, and x and y are discrete coordinates of the position of a temperature measuring point on the surface of a sample; for any point P on the surface of the sample, the coordinates are (x, y), and the data are processed as follows:
calculating an X-direction temperature gradient GX1(X, Y) and a Y-direction temperature gradient GY1(X, Y) of the point P at the time t 1-delta t1, and an X-direction temperature gradient GX2(X, Y) and a Y-direction temperature gradient GY2(X, Y) at the time t 2-delta t2, wherein t1 is the time when the first linear array passes through the point P, t2 is the time when the second linear array passes through the point P, and delta t1 and delta t2 are constant values;
calculating the temperature gradient abnormity of the point P in the X direction and the Y direction at the time t 1-delta t1 and the temperature gradient abnormity of the point P in the X direction and the Y direction at the time t 2-delta t 2;
and solving the square sum of all temperature gradient anomalies, and carrying out square root calculation on the square sum to obtain a test result for representing the test surface of the sample.
2. A laser thermal imaging detection method is characterized in that:
the laser spot used for thermal excitation during testing consists of a plurality of spot light, the spot light is arranged into two linear arrays which are parallel to each other, and the spot light in the two linear arrays are staggered with each other by a certain distance;
during testing, the laser light spot moves in a one-dimensional uniform motion relative to the surface of a test sample, and the motion direction is along the direction of a vertical linear array; recording the evolution of a test surface temperature field caused by heating by a thermal imaging device, and generating an image for displaying the crack through data processing;
when the test sample surface is cylindrical:
setting temperature field evolution data recorded by a thermal imager as T (T, x, y), wherein T is discrete time, and x and y are discrete coordinates of the position of a temperature measuring point on the surface of a sample; for any point P on the surface of the sample, the coordinates are (x, y), and the data are processed as follows:
calculating an X-direction temperature gradient GX1(X, Y) and a Y-direction temperature gradient GY1(X, Y) of the point P at the time t 1-delta t1, and an X-direction temperature gradient GX2(X, Y) and a Y-direction temperature gradient GY2(X, Y) at the time t 2-delta t2, wherein t1 is the time when the first linear array passes through the point P, t2 is the time when the second linear array passes through the point P, and delta t1 and delta t2 are constant values;
calculating the temperature gradient anomalies of the point P in the X direction and the Y direction at the time t 1-delta t1 and the temperature gradient anomalies in the X direction and the Y direction at the time t 2-delta t2, and taking absolute values of all the temperature gradient anomalies;
and selecting the maximum temperature gradient abnormal absolute values of the point P in the X direction and the Y direction at two moments, and adding the maximum temperature gradient abnormal absolute values and the maximum temperature gradient abnormal absolute values to obtain a test result representing the test surface of the sample.
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