CN113804127A - Bending failure strain measurement method - Google Patents
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- 229910000831 Steel Inorganic materials 0.000 claims description 44
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Abstract
A bending failure strain measuring method is characterized in that bending failure strains corresponding to different bending radii are found out through an image processing system based on a CCD camera. According to the bending failure strain measuring method, an image processing system based on a CCD camera is added on the basis of an original test system, the image processing system is matched with the original test system, corresponding bending failure strains under different bending male die radiuses are found out, a corresponding data set of radius-bending failure strain is established, and an index for pre-judging the formability of bending deformation of parts is formed.
Description
Technical Field
The invention belongs to the technical field of metal sheet forming, and particularly relates to a bending failure strain measurement method.
Background
Bending deformation generally exists in sheet metal parts, including stamping parts, bent parts and the like of automobiles, household appliances and the like, in the process of part design or material selection, unreasonable characteristic definition or improper material selection can cause unnecessary bending cracking in the forming process, especially with the urgent need of light weight and the extreme pursuit of product modeling, advanced high-strength steel with relatively poor formability is brought into a material selection system, and the evaluation or prediction of the bending formability of materials or parts is more important.
Currently, two standards are implemented for evaluating the bending formability of sheet metal, namely series of standards GB/T15825.5-2008 "sheet metal forming performance and test method, part 5: in a bending test and GB/T232-2010 metal material bending test method, the minimum relative bending radius Rmin/T corresponding to the outer side of a bending fillet before cracking is specified as an evaluation index in the standard, the used test method comprises pure bending modes such as three-point bending, 90-degree V bending, 180-degree press bending and bending, and the like, and the radius R of a bending male die is changed until the outer side surface of a deformation area cracks or obviously dents during the test.
However, in many cases the actual forming process is not a right angle or 180 degrees as described in the above standards, and the bending process is not a pure bending process, and most cases include some pre-deformation or stack forming, and it is difficult to evaluate the formability of the part using only the minimum relative bending radius, or the index can only reflect the bending performance of the material itself. Bending deformation is common in press and bend parts such as automobile bodies in white and home appliances, and evaluation of bending formability is important. The minimum relative bending radius index specified in the existing standard does not consider deformation history and nonstandard part characteristics, so that the formability of an actual part is difficult to effectively evaluate, and the minimum relative bending radius index can only be used for evaluating the bending performance of the material.
The invention application No. 99803699.4 discloses a forming method and device for bending steel plates, which comprises the following three parts: one part is to build and utilize a database containing data on flat steel plates, target curved steel plates, steel plates being formed and their forming information, another part is to derive new forming information by means of an artificial neural network system, and the third part is to obtain forming information by calculating in-plane and bending strains. In the third section, initial forming information is obtained by calculating strain from the relationship between the flat steel plate and the target bent steel plate, and new forming information is generated by calculating strain from the relationship between the partially formed bent steel plate and the target bent steel plate; the final target steel sheet is reached by repeatedly performing the measurement of the difference between the steel sheet in the performing step and the final target steel sheet and the calculation of the new strain in each process.
The invention application with the application number of 201410513455.3 discloses a method and a device for acquiring bending strain of a pipeline, which are used for acquiring a three-dimensional path of a pipeline surveying and mapping device passing through the pipeline to be measured; acquiring a corresponding relation between a time vector and each coordinate axis in a three-dimensional coordinate system according to the three-dimensional path, wherein the three-dimensional path is a curve in the three-dimensional coordinate system; and acquiring the bending curvature of the pipeline to be tested according to the curved surface calculation model and the corresponding relation, and acquiring the bending strain of the pipeline to be tested according to the bending curvature of the pipeline to be tested.
Disclosure of Invention
In order to solve the problems, the invention provides a bending failure strain measuring method which takes deformation history into consideration, obtains bending strain of cracking limit of materials under different fillet radii and takes the bending strain as an index for predicting formability in a part bending deformation process, and the technical scheme is as follows:
a bending failure strain measurement method is characterized in that:
through the image processing system based on the CCD camera, the bending failure strain corresponding to different bending radii is found out, and the method specifically comprises the following steps:
s1: taking a rectangular steel plate as a piece to be measured, setting the width direction of the steel plate as the Y direction, setting the length direction of the steel plate as the X direction, taking each point on the central line of the length direction of the steel plate as the center, carrying out colored calibration of the adjacent domain radius of each X axial direction along the X direction according to a set value, determining the straight line where the calibrated radius is positioned as a reference calibration line pair, and setting the distance between the reference calibration line pairs as an initial distance a0，
S2: at an initial distance a0Carrying out colored calibration of 2-5 groups of calibration line pairs along the X direction for the distance standard to form colored calibration lines which are equidistantly arranged along the X direction,
s3: loading the steel plate to be measured based on the current bending radius, shooting and tracking the plate to be measured in the loading process in real time through a CCD camera,
s4: carrying out image information digital conversion on each frame of image shot by the CCD camera through an image processing unit in the image processing system;
s5: extracting characteristic image information of each frame of converted image to form tracking of the colored calibration line;
s6: comparing the gray signal values of the front frame image and the rear frame image of the characteristic image to find out a pixel missing frame;
s7: comparing the pixel missing difference values of the previous and the next frames to the pixel missing frame,
s8: comparing the difference comparison result with a set value Q to find a next missing pixel frame corresponding to the first difference comparison result being equal to or greater than Q, and designating the missing pixel frame as a target frame,
s9: calculating the distance a in the X direction between the reference calibration line pair of the target frame according to the shot time of the target frame in the image processing unitf，
S10: according to afAnd calculating the bending failure strain corresponding to the current bending radius.
The bending failure strain measurement method is characterized by comprising the following steps:
the color calibration in step S1 is performed by using one of electrochemical etching, laser marking, and inkjet to form a significant contrast with the color of the steel sheet.
The bending failure strain measurement method is characterized by comprising the following steps:
the initial distance a described in step S100.25R or less, R: the bending punch radius was tested.
The bending failure strain measurement method is characterized by comprising the following steps:
the load loading in step S3 is performed by an action execution unit in the image processing system, specifically, by a controlled-movement bending male die, and when the bending male die moves to the experimental position where the steel plate to be measured is located, the movement after the bending male die and the CCD camera are arranged in a relatively stationary manner.
The bending failure strain measurement method is characterized by comprising the following steps:
the feature image information extraction described in step S5 is completed by edge detection.
The bending failure strain measurement method is characterized by comprising the following steps:
in step S6, the missing pixel frame is found by establishing a covariance matrix of the feature image and matching the mahalanobis distance between the previous and next frames.
The bending failure strain measurement method is characterized by comprising the following steps:
the bending failure strain in step S10 is specifically:
wherein the content of the first and second substances,
εbf: bending failure strain;
If: the arc length between the reference calibration line pairs corresponding to the shooting time of the target frame;
a0: initial distance of the reference calibration line pair, unit: mm;
r: current bending punch radius, unit: mm;
t: thickness of the steel plate to be measured, unit: mm;
af: the distance between the reference calibration line pairs of the target frame in the X direction is as follows, the unit: mm.
The bending failure strain measurement method is characterized by comprising the following steps:
the edge detection is an edge detection operator based on first order differential.
According to the bending failure strain measuring method, an image processing system based on a CCD camera is added on the basis of an original test system, the image processing system is matched with the original test system, bending failure strains corresponding to different bending radii are found out, a corresponding data set of radius-bending failure strain is established, and a formability pre-judgment index of bending deformation of a part is formed.
Drawings
FIG. 1 is a measurement step sequence diagram of the present invention;
FIG. 2 is a schematic diagram of the system of the present invention;
FIG. 3 is a schematic view of a colored calibration line in the present invention;
FIG. 4 is a schematic diagram of the deformation of the steel sheet to be measured according to the present invention.
Detailed Description
Hereinafter, a bending failure strain measurement method according to the present invention will be described in further detail with reference to the drawings and embodiments of the specification.
A bending failure strain measurement method as shown in figures 1 and 2,
through the image processing system based on the CCD camera, the bending failure strain corresponding to different bending radii is found out, and the method specifically comprises the following steps:
s1: taking a rectangular steel plate as a piece to be measured, setting the width direction of the steel plate as the Y direction, setting the length direction of the steel plate as the X direction, taking each point on the central line of the length direction of the steel plate as the center, carrying out colored calibration of the adjacent domain radius of each X axial direction along the X direction according to a set value, determining the straight line where the calibrated radius is positioned as a reference calibration line pair, and setting the distance between the reference calibration line pairs as an initial distance a0，
S2: at an initial distance a0Carrying out colored calibration of 2-5 groups of calibration line pairs along the X direction for the distance standard to form colored calibration lines which are equidistantly arranged along the X direction,
s3: based on the current bending radius, the plate to be measured is loaded, and the CCD camera is used for shooting and tracking the steel plate to be measured in real time in the loading process,
s4: carrying out image information digital conversion on each frame of image shot by the CCD camera through an image processing unit in the image processing system;
s5: extracting characteristic image information of each frame of converted image to form tracking of the colored calibration line;
s6: comparing the gray signal values of the front frame image and the rear frame image of the characteristic image to find out a pixel missing frame;
s7: comparing the pixel missing difference values of the previous and the next frames to the pixel missing frame,
s8: comparing the difference comparison result with a set value Q to find a next missing pixel frame corresponding to the first difference comparison result being equal to or greater than Q, and designating the missing pixel frame as a target frame,
s9: calculating the distance a in the X direction between the reference calibration line pair of the target frame according to the shot time of the target frame in the image processing unitf，
S10: according to afAnd calculating the bending failure strain corresponding to the current bending radius.
Wherein the content of the first and second substances,
the color calibration in step S1 is performed by using one of electrochemical etching, laser marking, and inkjet to form a significant contrast with the color of the steel sheet.
Wherein the content of the first and second substances,
the initial distance a described in step S100.25R or less, R: the bending punch radius was tested.
Wherein the content of the first and second substances,
the load loading in step S3 is performed by an action execution unit in the image processing system, specifically, by a controlled-movement bending male die, and when the bending male die moves to the experimental position where the steel plate to be measured is located, the movement after the bending male die and the CCD camera are arranged in a relatively stationary manner.
Wherein the content of the first and second substances,
the feature image information extraction described in step S5 is completed by edge detection.
Wherein the content of the first and second substances,
in step S6, the missing pixel frame is found by establishing a covariance matrix of the feature image and matching the mahalanobis distance between the previous and next frames.
Wherein the content of the first and second substances,
the bending failure strain in step S10 is specifically:
wherein the content of the first and second substances,
εbf: bending failure strain;
If: the arc length between the reference calibration line pairs corresponding to the shooting time of the target frame;
a0: initial distance of the reference calibration line pair, unit: mm;
r: current bending punch radius, unit: mm;
t: thickness of the steel plate to be measured, unit: mm;
af: the distance between the reference calibration line pairs of the target frame in the X direction is as follows, the unit: mm.
Wherein the content of the first and second substances,
the edge detection is an edge detection operator based on first order differential.
Working principle and embodiment
1. The original experimental system comprises a bending male die controlled by a computer, a platform to be tested and a steel plate to be tested, and a CCD camera and a corresponding image processing unit are additionally arranged in the original experimental system; the CCD camera is arranged on the bending outer side of a sample to be tested, and bending performed by an experiment is not limited to three-point bending related in national standards, namely: 90-degree V bending, 180-degree press bending and roll bending, and also comprises other non-standard bending tests, such as a stretch bending test with tensile deformation superposition; to form test measurements at different bend radii.
2. Sample preparation. The selected sample is generally regular rectangle, and the bending deformation area can be regular rectangle in special case, and other areas of the sample are irregular, but the normal bending of the sample is not influenced. Before the test, marking lines are arranged at equal intervals on the bending area of the surface of the sample along the central position to two sides of the length direction of the sample, the marking line range covers the fillet deformation area, the central line position is not marked, the used marking lines are differentiated from the color of the sample, the marking mode does not damage the surface of the sample, and the modes of slight electrochemical corrosion, laser marking, ink jet and the like can be considered; the process is the forming process of the reference calibration line pair and the colored calibration line, the marking is to track the fillet deformation area of the steel plate, further to track the pixel change of the center line position, and the tracking of the pixel change of the center line position is to construct an edge detection operator for the area with a certain area radius; the color calibration is used for forming a gray value difference which is obvious to a steel plate and serving for edge detection.
3. During testing, the side of the sample marking is arranged on the outer side of the bending fillet, the bending male die is in contact with the sample and applies force to bend the sample, and meanwhile, a computer sends a trigger signal to a servo drive assisted by a CCD camera and a traction camera to synchronously move with the bending male die; and the moving speed of the camera is consistent with that of the bending male die, so that the camera and the bending surface of the sample are always kept at a constant working distance, a real-time image of a bending area is obtained, and the bending failure is realized when the width value of the crack on the outer side of the fillet reaches a set value, and the moment is recorded as tf.
4. The camera can shoot at a certain frequency while moving, the shooting frequency can be set according to the bending performance of a test sample, the frequency of not less than 5 frames/second is set at the moment of 0-t1 due to less deformation, and the shooting frequency of not less than 10 frames/second is set at the moment of t1-tf due to close bending failure; the time tf may be obtained from a first test, setting the time t1 to not exceed 2/3 tf. In addition, image acquisition allows for delayed start, i.e., image acquisition is started after the curve reaches a set displacement.
5. The image processing system analyzes and processes each frame of acquired image, and edge identification is carried out on the marking line with the width by adopting an edge detection technology so as to form real-time tracking of the reference calibration line pair; and the real-time changing X-direction distance between the reference calibration line pairs is calibrated as atAnd the distance measured at the time when t is 0, i.e. when the specimen has not been bent, is denoted as a0. The method specifically comprises the following steps: carrying out gray level identification and binarization processing on the image with the marked lines on each frame, and obtaining the image by an edge identification algorithm
Is marked as Xb t、Xc t....... The coordinates of all discrete points are in the same coordinate system; accordingly, the distance between adjacent mark lines and the maximum distance at time t can be calculated.
Wherein the content of the first and second substances,
6. And the image processing unit realizes automatic identification of the cracks on the outer surface of the sample through judgment of pixel missing. Comparing the difference value of the pixel value Pn of the measured area of the image of the nth frame with the pixel value Pn-1 of the measured area of the image of the nth-1 frame, and if Pn < Pn-1, determining that a flaw (microcrack) is generated; when the difference between Pn and Pn-1 is continuously increased, which means that the crack is enlarged, when the difference obtained by subtracting Pn-1 from Pn reaches or exceeds a Q value, the crack is not acceptable, and the bending reaches a limit state, namely tf, the corresponding maximum center distance af is recorded; the pixel missing is completed by means of the operation of covariance matrixes among the characteristic images and the comparison of the Mahalanobis distance of the previous frame and the next frame; wherein the Q value is determined by means of one-time manual detection of the set of equipment.
7. Strain of bending failure epsilonbfAnd (4) calculating. The method is completed according to the following formula;
wherein the content of the first and second substances,
εbf: bending failure strain;
If: the arc length between the reference calibration line pairs corresponding to the shooting time of the target frame;
a0: initial distance of the reference calibration line pair, unit: mm;
r: current bend radius, unit: mm;
t: thickness of the plate to be measured, unit: mm;
af: the distance between the reference calibration line pairs of the target frame in the X direction is as follows, the unit: mm.
Examples section
1. FIG. 2 shows the application of the technical scheme in the three-point bending test method mentioned in the national standard GB/T15825.5-2008. The bending force, i.e. the bending punch radius R, is 2mm in this example. The invention adds image collection and processing on the basis of the original bending test system, wherein the image collection is completed by a CCD camera, and the whole image collection and test process is controlled by a computer. The dotted line shows the positions of the bending male die and the sample steel plate after the steel plate to be measured is bent. H in the figure is the distance of movement. After image acquisition, an image processing unit for completing corresponding data operation, and movement control over the bending male die and movement control over the CCD camera are integrated in a computer, the computer controls the movement speed and displacement of the bending male die, meanwhile, the computer receives starting signal feedback of the bending test device and triggers the CCD camera to take pictures, and image acquisition frequency and the number of the pictures are set by the computer.
2. And (4) preparing a sample. FIG. 3 shows a sample of this example, which is made of high-strength steel for automobile parts and has a thickness of 1.2mm, and is cut into a rectangle with a length of 200mm and a width of 40mm on a plate shearing machine, wherein the length direction is the rolling direction, and edge burrs are ground. Marking lines with the space of 0.5mm and the line width of 0.1mm are marked from the middle part to two sides of the sample in a laser marking mode, and the width of a marking area is 5 mm;
3. during the test, the test sample is placed on a bending test platform, and the side of the marking line is arranged below. Adjusting the height and focal length of the camera to the optimum working position, i.e. the image is clear and the whole marked area can be shot, in this case the distance between the camera and the lower surface of the sample is 150mm, and measuring the distance by using an image processing system, i.e. a0＝0.5mm；
4. When the test is started, the computer system presses a start button, the bending male die 3 starts to move downwards at a constant speed at the speed of 1mm/s, meanwhile, the image acquisition system receives a trigger signal, starts to acquire images at the frequency of 5 frames/s, and meanwhile, the camera synchronously moves downwards at the speed of 1 mm/s;
5. the image processing system has the function of automatic crack identification, the cracks or flaws are defined according to the missing of pixels in the comparison measurement range of the front and back collected images, the images reaching the threshold value are automatically detected through the defined threshold value Q, namely, the cracks on the outer side of the bending fillet reach the bending failure state, a stop signal is fed back to the computer, the computer outputs the stop signal, the movement of the bending male die is stopped, and the test is stopped;
6. in the test, the tracking of the reference calibration line pair is formed through edge detection so as to form the real-time tracking of the distance change of the reference calibration line pair and the reference calibration line pair, thereby finding the distance a corresponding to the target framefIn this example af＝0.7mm；
7. Calculating the bending failure strain epsilonbfThe bending failure strain was calculated to be 0.405 according to the above formula.
Thus, the bending failure strain can make up for the shortfall in the evaluation of bending performance with the minimum relative bending radius specified in the existing standards. The method for measuring the bending failure strain provided by the invention transmits an image captured by a camera in real time to a computer control system, the computer processes the image by shooting a sample with regular marking lines, automatically judges whether cracks appear on the outer side of the bend or not by setting a certain threshold value, obtains the distance between two adjacent marking lines with the largest distance after the bend by processing and analyzing a target image, and obtains the bending failure strain by calculating through a geometric formula. By using the method, the bending crack judgment is automated, the influence of human factors is avoided, and the result is stable; meanwhile, the obtained bending failure strain is based on a boundary recognition algorithm of image processing, so that the artificial measurement error is avoided, and meanwhile, the bending failure strain belongs to on-line measurement rather than off-line manual measurement, so that the springback after bending is avoided, and the measurement precision is improved.
According to the bending failure strain measuring method, an image processing system based on a CCD camera is added on the basis of an original test system, the image processing system is matched with the original test system, bending failure strains corresponding to different bending radii are found out, a corresponding data set of radius-bending failure strain is established, and a formability pre-judgment index of bending deformation of a part is formed.
Claims (8)
1. A bending failure strain measurement method is characterized in that:
through the image processing system based on the CCD camera, the bending failure strain corresponding to different bending radii is found out, and the method specifically comprises the following steps:
s1: taking a rectangular steel plate as a piece to be measured, setting the width direction of the steel plate as the Y direction, setting the length direction of the steel plate as the X direction, taking each point on the central line of the length direction of the steel plate as the center, carrying out colored calibration of the adjacent domain radius of each X axial direction along the X direction according to a set value, determining the straight line where the calibrated radius is positioned as a reference calibration line pair, and setting the distance between the reference calibration line pairs as an initial distance a0，
S2: at an initial distance a0Carrying out colored calibration of 2-5 groups of calibration line pairs along the X direction for the distance standard to form colored calibration lines which are equidistantly arranged along the X direction,
s3: based on the current bending radius, the steel plate to be measured is loaded, and the CCD camera is used for shooting and tracking the steel plate to be measured in real time in the loading process,
s4: carrying out image information digital conversion on each frame of image shot by the CCD camera through an image processing unit in the image processing system;
s5: extracting characteristic image information of each frame of converted image to form tracking of the colored calibration line;
s6: comparing the gray signal values of the front frame image and the rear frame image of the characteristic image to find out a pixel missing frame;
s7: comparing the pixel missing difference values of the previous and the next frames to the pixel missing frame,
s8: comparing the difference comparison result with a set value Q to find a next missing pixel frame corresponding to the first difference comparison result being equal to or greater than Q, and designating the missing pixel frame as a target frame,
s9: calculating the distance a in the X direction between the reference calibration line pair of the target frame according to the shot time of the target frame in the image processing unitf，
S10: according to afAnd calculating the bending failure strain corresponding to the current bending radius.
2. The bending failure strain measurement method according to claim 1, wherein:
the color calibration in step S1 is performed by using one of electrochemical etching, laser marking, and inkjet to form a significant contrast with the color of the steel sheet.
3. The bending failure strain measurement method according to claim 1, wherein:
the initial distance a described in step S100.25R or less, R: the bending punch radius was tested.
4. The bending failure strain measurement method according to claim 1, wherein:
the load loading in step S3 is performed by an action execution unit in the image processing system, specifically, by a controlled-movement bending male die, and when the bending male die moves to the experimental position where the steel plate to be measured is located, the movement after the bending male die and the CCD camera are arranged in a relatively stationary manner.
5. The bending failure strain measurement method according to claim 1, wherein:
the feature image information extraction described in step S5 is completed by edge detection.
6. The bending failure strain measurement method according to claim 1, wherein:
in step S6, the missing pixel frame is found by establishing a covariance matrix of the feature image and matching the mahalanobis distance between the previous and next frames.
7. The bending failure strain measurement method according to claim 1, wherein:
the bending failure strain in step S10 is specifically:
wherein the content of the first and second substances,
εbf: bending failure strain;
If: the arc length between the reference calibration line pairs corresponding to the shooting time of the target frame;
a0: initial distance of the reference calibration line pair, unit: mm;
r: current bending punch radius, unit: mm;
t: thickness of the steel plate to be measured, unit: mm;
af: the distance between the reference calibration line pairs of the target frame in the X direction is as follows, the unit: mm.
8. The bending failure strain measurement method according to claim 5, wherein:
the edge detection is an edge detection operator based on first order differential.
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