CN101319977A - True stress-true strain calculation model and test system - Google Patents

Abstract

Aiming at the actual problem of true stress-true strain measurement, the present invention studies the essential characteristics of material deformation, and discloses a true stress-true strain calculation model and a practical test system: 1. On the basis of the deformation analysis of the tensile test piece, Taking the unit body with special features as the object of stress-strain analysis, a unique full-deformation unit model was created, firstly solving the theoretical analysis problem of true stress and true strain, establishing a theoretical basis, and establishing a new true stress-true strain calculation formula:

,

. 2. Adopting machine vision technology and image processing method to develop a practical software and hardware system suitable for it, and realizing the non-contact true stress-true strain direct measurement in the true sense. 3. The hardware of the test system includes a light source (1), a CCD camera (2), an image acquisition card (3), a data acquisition card (4), and a material testing machine (5). 4. The software includes image capture, load data collection and comprehensive calculation and analysis programs.

Description

True stress-true strain calculation model and test system

(1) Technical field

The invention belongs to experimental mechanics, and specifically creates a theoretical model based on mechanics principles, and utilizes vision technology and image analysis technology to form a non-contact measurement method and applies it to material mechanical performance testing.

(2) Background technology

The stress-strain response characteristics of materials under various conditions are the main mechanical performance indicators reflecting the performance of materials. The research on these characteristics and the confirmation of various mechanical performance indicators are based on experiments. Therefore, in the mechanical properties of materials In research, testing experiments are very important.

The test of material stress-strain relationship is usually realized by using a material testing machine to carry out tensile experiments, and the final result is derived by calculation based on the original size of the sample (cross-sectional area A ₀ , gauge length L ₀ ). This stress-strain is called engineering stress-strain.

Engineering stress-strain curves do not represent the intrinsic behavior of a material. Because the cross-sectional area A and the gauge length l of the sample are constantly changing at any moment of stretching, the relationship between the true stress σ _i and true strain ε _i at any instant and the corresponding engineering stress σ and engineering strain ε There is a difference, especially when the material enters the plastic deformation stage. Especially when the sample has local deformation (necking), the engineering stress-strain curve (σ-ε curve) shows a trend that the stress decreases significantly with the increase of strain. This is because the cross-section at the neck of the sample shrinks rapidly when the necking occurs, so the load required to continue the deformation of the sample decreases, but the corresponding engineering stress is still calculated with the original area A ₀ , resulting in the stress-strain The operational deviation is exacerbated. In fact, although the applied load F decreases after necking, the material is still hardening during the whole necking process, so the stress required for further deformation is still increasing. The error between the test parameters and the real parameters needs to be resolved with more accurate and reliable theoretical models and applicable test methods.

So far, the fundamental reason why the true stress and true strain test problems have not been fundamentally solved is that there has been no breakthrough in the relevant measurement theory analysis models. Secondly, the traditional measurement methods mainly use contact sensors for dynamic measurement of diameter and gauge length, or directly use the displacement of the movable chuck as the deformation of the gauge length of the specimen. This method cannot accurately measure the true stress of the material- The main reasons for the true strain are: the uncertainty of the position of the necking point, the randomness of the position of the fracture surface, the unpredictability of the fracture moment and the breakage phenomenon at the moment of fracture bring many insurmountable difficulties to the contact measurement. Therefore, exploring a new theoretical model and seeking a non-contact measurement method is the fundamental way to solve the problem.

(3) Contents of the invention

Aiming at the practical problem of true stress-true strain measurement, the invention studies the essential characteristics of material deformation, and discloses a true stress-true strain calculation model and a test system. Based on the analysis of the deformation of the tensile test piece, the present invention takes the unit body with special features as the object of stress-strain analysis, creates a unique full-deformation unit model, and first establishes the theoretical basis of true stress and true strain. Established the corresponding deformation evaluation and calculation theory, first solved the analysis problem of true stress and true strain theory, and then used machine vision technology and image processing method to create an applicable software and hardware system suitable for non-contact measurement, realizing the true sense of The true stress-true strain measurement of .

The true stress-true strain measurement method and calculation model, on the basis of the deformation analysis of the tensile test piece, take the unit body with special characteristics as the object of stress-strain analysis, and create a full deformation unit model; when the test piece is stretched When stretching, the tiny unit at the minimum diameter of the final fracture goes through all stages of the entire deformation process until it breaks, and the mechanical behavior of the tiny unit truly and completely reflects the real constitutive relationship of the material; the stress and strain measured on the tiny unit are the true stress , true strain; the deformation geometry of this unit is that during the tensile test, whether it is uniform elastic, plastic deformation, or local deformation (necking) stage, the deformation of the specimen is axisymmetric, and the plane projection of the necking figure The characteristic is that the outer contour line of the necking part is always a curve symmetrical and concave inward with the axis. If two straight lines are respectively tangent to the two outer contour curves on the inner side of the planar projection figure of the specimen, the During the whole test process, with the development of necking, the tangential relationship is maintained, and at the same time, the distance between the two straight lines is dynamically measured in real time, and the minimum diameter within the gauge range of the specimen at any time is measured.

When the material is necked, the material near the minimum diameter is basically a geometrically symmetrical body, where a tiny length of l ₀ is taken as the analysis unit, and in the extreme case where the length tends to zero, the curvature change of the busbar can be ignored. As a pie-shaped unit body, the true stress and true strain at any time can be expressed as:

${\mathrm{<span\; class="notranslate">\; \&sigma;</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; 4</span>}\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}}{{\mathrm{<span\; class="notranslate">\; \&pi;d</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}$ ${\mathrm{<span\; class="notranslate">\; \&epsiv;</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; d</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}{{\mathrm{<span\; class="notranslate">\; d</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}$

Where: F _i - tensile load at any time, d _i - minimum diameter at any time, d ₀ - original diameter, σ _i - true stress at any time, ε _i - true strain at any time;

As long as F _i and d _i are measured at any instant, accurate true stress and true strain at any time can be obtained. If _li is measured at the same time, basic mechanical parameters such as elongation and elastic modulus can be obtained at the same time.

2. True stress and true strain measurement system, including specimen (1), light source (2), CCD camera (3), material testing machine (4), image and load capture unit (5), image and load analysis processing unit;

On the one hand, the image of the deformation process of the sample is collected in a timely manner by the camera method, and the rich functions of the visual technology are used to track the change of the cross-section size at any time, find the minimum cross-section position, calculate the minimum diameter, and simultaneously record the load change process, and use the minimum cross-section and load The data calculates the true stress and true strain of each moment; on the other hand, set the mark point at the determined position, and take the displacement image of the mark point in real time, which is used for the calculation of elongation, and can also calculate the engineering stress-strain at the same time, and finally compile the processing software Output various detection data and draw true stress-true strain relationship curve and engineering stress-strain curve; information capture includes two parts: image acquisition and load acquisition. Part of it is to monitor the entire stretching process of the tensile sample through the visual system to collect images in preparation for the measurement of the smallest diameter. The hardware for image measurement mainly includes: camera, CCD, image acquisition card, and lighting system. The load signal is directly obtained from the load output end of the material tensile testing machine through the data acquisition card; the acquisition of the image and the load are two parallel main lines, which enter the computer for processing; complete the image capture and image analysis processing of the model object, and obtain Real material mechanical parameters and true stress-true strain curve.

The software process of the true stress and true strain measurement system is as follows: use the CCD sensor to collect images during the deformation process of the sample, convert the original images into digital image information through the image acquisition card and send them to the computer, and use image processing software to process the digital images And get the cross-sectional area of the neck and the axial displacement of the sample; the load change signal during the stretching process is converted into a voltage change signal through the pressure sensor on the equipment, and the voltage signal is collected by a data acquisition card and transmitted to the computer. In order to ensure that the image acquisition card and the data acquisition card acquire at the same time, the bus trigger mode is used to run the two signals synchronously. The whole process is controlled and operated only by the software system compiled by LabVIEW.

The image acquisition part of the true stress and true strain measurement system includes four parts: equipment initialization, acquisition channel setting, image acquisition and image reading; the whole process of image acquisition is first started by initialization, using IMAQInit.vi module, the function of the module It is to configure the IMAQ file and initialize the external equipment for image acquisition; next, set the channel. The setting of the acquisition channel is realized through a property node. The channel is pre-set when the image acquisition card is installed, and the channel names of the two are corresponding Consistent, the image acquisition is realized by the IMAQ Grab Setup.vi module. The function of this function is to continuously acquire the images collected from the image acquisition card; it is used in conjunction with the IMAQ Init.vi module; to read the image to be selected for application, Realized by the IMAQ Acquire.vi module; the minimum diameter capture and gauge distance measurement program of the full deformation unit includes the following functional sub-modules: image acquisition, median filter, threshold segmentation, closed operation of binary image, boundary extraction and dimension measurement; The front part of the binarization subVI is an image acquisition module. After the image is acquired, the binarization subVI performs median filtering and threshold segmentation to make the image clearer and easier for boundary detection; before boundary extraction, according to the edge shape to be detected , first set the search area for diameter measurement and length measurement in the image by using the area delineation subVI; execute the Clamp Horizontal Min command in the set search area to measure the minimum distance on the boundary in the horizontal direction, that is, to track the neck in real time At the same time, execute the Clamp Vertical Min command in the search area in the vertical direction to scan the change of the gauge length in real time to obtain the minimum diameter and real-time gauge length.

The load data processing program of the true stress and true strain measurement system is:

During the stretching process, the load signal is obtained from the DC voltage output of the tensile testing machine port, and the frequency of writing the load to the memory is the same as the frequency of image acquisition. The data acquisition program is written using the DAQmx function; set the function DAQmx CreateChannel to create a voltage analog input channel; in the function DAQmx Read, select Anolog DBL 1 Chan1Samp, that is, one channel reads one sample, and the read voltage The value is multiplied by the voltage equivalent to get the load value. Voltage equivalent refers to the load value represented by each volt of voltage. The functions and settings of each function are:

(1) DAQmx Create Channel: This function is used to create a DAQmx data acquisition channel. Here, select the voltage analog input channel. Its main parameters are: Physical channels - set the physical channel; Minimum value - the minimum input voltage value, here is set to - 10V; Maximum value- the maximum voltage input value, here is set to +10V;

(2) DAQmx Timming DAQmx: Select the sample clock subVI here. It can set the number of samples, the sample rate, and set the buffer if necessary. Its main parameters are: Rate-set the sampling rate of each channel; Source-set the clock signal source when sampling, you can choose the default setting; Active edge-sample at the rising edge and falling edge of the clock; Sample mode-sampling mode, select continuous sampling; Samples per channel-the number of samples per channel when limited sampling;

(3) DAQmx Start Task: start the DAQmx task function;

(4) DAQmx Trigger: By configuring this function, define the action when the trigger signal is received. Select start digital edge here, that is, start collecting data when a digital trigger signal is received;

(5) DAQmx Read: DAQmx read data function, choose Anolog Wfm 1Chan Nsamp here, it returns the one-dimensional waveform data of analog input, including 1 channel, each channel has N samples. Its main parameters are: Task/Channel in - input task name or virtual channel name, here we set the input channel as Channel8; Number of samples per channel - execute the amount of data retrieved from each channel once; Timeout - timeout, Set the waiting time for sampling. If you do not collect enough data, you will return as much as you want and report an error. If you set it to -1, you will wait indefinitely. Set the waiting time to 10 seconds according to the sampling frequency and number of samples.

In the system that adopts the image processing technology to measure in the present invention, the intermediate result of the calculation is represented by the number of pixels, and the unit of the measurement result can only be calculated by finding the corresponding relationship between the unit length (mm) on the object surface and the number of pixels on the image surface. Convert to mm length.

The calibration process is basically the same as the full deformation unit test procedure. It also collects the sample image, selects a calibration target range, filters and binarizes the original diameter image in the range, and finally measures the vertical and horizontal distances. The sub-VI measures the number of pixels occupied by the diameter of the sample, divides the diameter value (mm) of the sample by the measurement result, and obtains the actual length represented by each pixel, which is the pixel equivalent. In the process of calibration, how many pixels are required at this diameter position, and the ratio of the original diameter value to the corresponding number of pixels is the pixel equivalent. Once the pixel equivalent is obtained, it will be used as a standard value. In the subsequent calculation program, it will be used as the calculation reference quantity to accurately calculate the diameter and gauge length at any time.

(4) Description of drawings

Fig. 1 Full deformation unit model and deformation characteristic diagram;

Fig. 2 Schematic diagram of minimum diameter capture measurement;

Fig. 3 schematic diagram of system structure principle;

Figure 4 is: system software block diagram;

Figure 5 image acquisition and minimum diameter capture program of full deformation unit;

Fig. 6 true stress-true strain calculation program;

Figure 7 load data processing program;

Figure 8 Calibration program flow diagram.

(5) Specific implementation methods

The basic idea of the total denaturation unit analysis method of the present invention is that when the sample is stretched, only the micro unit at the minimum diameter of the final fracture has experienced each stage of the whole deformation process until fracture, so only the mechanical properties of this micro unit The behavior can truly and completely reflect the real constitutive relationship of the material. Therefore, only the stress and strain measured on this unit are the real stress and strain. From the point of view of deformation geometric characteristics, when the material is necked, the material near the minimum diameter is basically a geometrically symmetrical body, where a tiny length with a length of l ₀ is taken as the analysis unit. In the limit case where the length tends to zero , the curvature change of the generatrix can be ignored, and it can be regarded as a pie-shaped unit body, and its model and deformation characteristics are shown in Figure 1.

During the whole tensile test process, whether it is uniform elastic, plastic deformation, or local deformation (necking) stage, the deformation of the specimen is always axisymmetric, and from the characteristics of the plane projection of the necking figure, the necking The outer contour line of the part is a curve that is symmetrical and concave inward with the axis. As long as two straight lines are respectively tangent to the two outer contour curves on the inner side of the planar projection figure of the specimen, the This tangent relationship is maintained, and the distance between the two straight lines is dynamically measured in real time, which can effectively ensure the minimum diameter within the gauge length range of the specimen at any time. Figure 2 is the basic principle diagram of the minimum diameter capture.

The combination of modern vision and image processing technology provides an effective guarantee for the realization of this measurement method. This project uses vision technology and image analysis methods to establish a test system suitable for full deformation unit analysis, and completes image capture and measurement of model objects. Image analysis and processing to obtain real material mechanical parameters and true stress-true strain curves.

The hardware design of the test system mainly includes two parts: image acquisition and load acquisition. One part is to collect images through the visual system to monitor the entire stretching process of the tensile sample in real time and prepare for the measurement of the smallest diameter. The hardware for image measurement mainly includes: camera, CCD, image acquisition card, and lighting system. The load signal is directly obtained from the load output end of the material tensile testing machine through the data acquisition card. The acquisition of images and payloads are two parallel main lines, and finally they all enter the computer for processing. Figure 3 is a simplified diagram of the system structure.

The software development of this system adopts the LabVIEW of NI Company of the United States as the development platform. The image acquisition card and data acquisition card in the system hardware, as well as the IMAQ Vision software package are all NI products, and the hardware has good compatibility. In addition, because the LabVIEW development platform does not have a ready-made sub-pixel processing module, and MATLAB has powerful data processing capabilities, so in the process of sub-pixel processing, the image is converted into an array and processed by MATLAB. Finally, two The software is well combined, so that the data processed by MATLAB is fed back to the LabVIEW development platform. Figure 4 is a flow diagram of the system software.

Advantages of the invention

(1) The true stress-true strain full-deformation element analysis method mentioned in this project can truly describe the deformation process of the material. Using this model, the true stress and true strain can be effectively measured, which is better than the traditional true stress -True strain and engineering stress-strain can more accurately represent the mechanical behavior characteristics of materials.

(2) The non-contact testing method based on machine vision technology can realize the real-time monitoring of the whole stretching process. It completely solves the problem that the deformation cannot be accurately detected in the whole process, so as to obtain a correct and high-precision true stress-true strain curve.

(3) This system uses LabVIEW as the development platform and uses modular design ideas for software design. The combination of the two makes the system have the characteristics of short development cycle, high precision, flexible application, clear structure, easy maintenance and expansion, etc. Insufficiency of traditional testing methods.

(4) Since the testing process is completely non-contact, this system can be used for high and low temperature performance testing of materials.

(5) Appropriate hardware systems can be established to meet the needs of special material performance testing, such as true stress-true strain testing of superplastic materials, through techniques such as lens stitching.

Claims (6)

1. True stress-true strain measurement method and calculation model: On the basis of the deformation analysis of the tensile test piece, the unit body with special features is taken as the object of stress-strain analysis, and the full deformation unit model is created; when the test When the sample is stretched, only the tiny unit at the smallest diameter of the final fracture goes through all stages of the entire deformation process until it breaks, and its mechanical behavior truly and completely reflects the real constitutive relationship of the material, so the stress measured on the tiny unit and strain are true stress and true strain; the deformation geometry of this unit is that during the tensile test, whether it is uniform elastic, plastic deformation, or local deformation (necking) stage, the deformation of the specimen is always axisymmetric , the feature of the plane projection of the necking figure is that the outer contour line of the necking part is always a curve that is symmetrical and concave inward with the axis. The tangent straight line, during the whole test process, with the development of necking, makes this tangent relationship maintained. As long as the distance between the two straight lines is measured dynamically and in real time, the distance within the gauge length of the specimen at any time can be accurately measured. Minimum diameter. Theoretical model and calculation formula: When the material is necked, the material near the minimum diameter is basically a geometrically symmetrical body, where a tiny length with a length of l ₀ is taken as the analysis unit. In the limit case where the length tends to zero, The curvature change of the busbar can be ignored, and it can be regarded as a pie-shaped unit body. The true stress and true strain at any time can be expressed as: ${\mathrm{<span\; class="notranslate">\; \&sigma;</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; 4</span>}\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}}{{\mathrm{<span\; class="notranslate">\; \&pi;d</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}$ ${\mathrm{<span\; class="notranslate">\; \&epsiv;</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; d</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}{{\mathrm{<span\; class="notranslate">\; d</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}$ Where: F _i - tensile load at any time, d _i - minimum diameter at any time, d ₀ - original diameter, σ _i - true stress at any time, ε _i - true strain at any time; As long as F _i and d _i are measured at any instant, accurate true stress and true strain at any time can be obtained. If _li is measured at the same time, basic mechanical parameters such as elongation and elastic modulus can be obtained at the same time. 2. True stress and true strain measurement system, including specimen (1), light source (2), CCD camera (3), material testing machine (4), image and load capture unit (5), image and load analysis processing unit; On the one hand, the image of the deformation process of the sample is collected in a timely manner by the camera method, and the rich functions of the visual technology are used to track the change of the cross-section size at any time, find the minimum cross-section position, calculate the minimum diameter, and simultaneously record the load change process, and use the minimum cross-section and load The data calculates the true stress and true strain at each instant; on the other hand, it is also possible to set marker points at certain positions and take real-time images of the displacement of the marker points for elongation calculation. It can also calculate engineering stress-strain at the same time, and finally compile The processing software outputs various detection data and draws the true stress-true strain relationship curve and engineering stress-strain curve; information capture includes two parts: image acquisition and load acquisition. Part of it is to monitor the entire stretching process of the tensile sample through the visual system to collect images in preparation for the measurement of the smallest diameter. The hardware for image measurement mainly includes: camera, CCD, image acquisition card, and lighting system. The load signal is directly obtained from the load output end of the material tensile testing machine through the data acquisition card; the acquisition of the image and the load are two parallel main lines, which enter the computer for processing; complete the image capture and image analysis processing of the model object, and obtain Real material mechanical parameters and true stress-true strain curve. 3. true stress as claimed in claim 2, true strain measurement system, the software process of its system is: gather the image in the sample deformation process with CCD sensor, convert original image into digital image information and transmit by image acquisition card into the computer, use image processing software to process the digital image and obtain the cross-sectional area of the neck and the axial displacement of the sample; through the pressure sensor on the equipment, the load change signal during the stretching process is converted into a voltage change signal, which is determined by A data acquisition card collects this voltage signal and transmits it to the computer. In order to ensure that the image acquisition card and the data acquisition card acquire at the same time, the bus trigger mode is used to run the two signals synchronously. The whole process is controlled and operated only by the software system compiled by LabVIEW. 4. true stress as claimed in claim 2, true strain measuring system is characterized in that: the image acquisition part comprises initialization of equipment, the setting of acquisition channel, image acquisition and image reading four parts; The whole process of image acquisition is at first Starting from the initialization, the IMAQ Init.vi module is used. The function of the module is to configure the IMAQ file and initialize the external equipment for image acquisition; then set the channel, and the setting of the acquisition channel is realized through a property node. The channel is the image The acquisition card has been pre-set when it is installed, and the channel names of the two are corresponding to each other. The image acquisition is realized by the IMAQ Grab Setup.vi module. The function of this function is to continuously acquire the images collected from the image acquisition card; and IMAQ Init.vi Used in conjunction with the module; the image to be selected for application is read by the IMAQ Acquire.vi module; the minimum diameter capture and gauge length measurement program of the full deformation unit includes the following functional sub-modules: image acquisition, median filter, threshold segmentation , the closing operation of the binary image, boundary extraction and size measurement; the front part of the binarization sub-VI in the figure is an image acquisition module, and after the image is acquired, the binarization sub-VI performs median filtering and threshold segmentation to make the image more It is clear and convenient for boundary detection; before boundary extraction, according to the shape of the edge to be detected, use the area delineation subVI to set the search area for diameter measurement and length measurement in the image; execute Clamp Horizontal Min in the set search area command to measure the minimum distance on the boundary in the horizontal direction, that is, to track the change of the cross-sectional area of the constriction in real time, and execute the Clamp Vertical Min command in the vertical search area to scan the change of the gauge length in real time to obtain the minimum diameter and real-time gauge length. 5. true stress as claimed in claim 2, true strain measurement system, its load data processing program is: During the stretching process, the load signal is obtained from the DC voltage output of the tensile testing machine port, and the frequency of writing the load to the memory is the same as the frequency of image acquisition. The data acquisition program is written using DAQmx functions; set the function DAQmxCreat Channel to create a voltage analog input channel; in the function DAQmx Read, select AnologDBL 1 Chan 1 Samp, that is, one channel is used to read one sample, and the read The voltage value is multiplied by the voltage equivalent to get the load value. Voltage equivalent refers to the load value represented by each volt of voltage. The functions and settings of each function are: (1) DAQmx Create Channel: This function is used to create a DAQmx data acquisition channel, where the voltage analog input channel is selected, and its main parameters are: Physical channels-set the physical channel; Minimum value- the minimum input voltage value, here is set to -10V; Maximum value- the maximum voltage input value, here is set to +10V; (2) DAQmx Timming DAQmx: Select the sample clock subVI here. It can set the number of samples, the sample rate, and set the buffer if necessary. Its main parameters are: Rate-set the sampling rate of each channel; Source-set the clock signal source when sampling, you can choose the default setting; Active edge-sample at the rising edge and falling edge of the clock; Sample mode-sampling mode, select continuous sampling; Samples per channel-the number of samples per channel when limited sampling; (3) DAQmx Start Task: start the DAQmx task function; (4) DAQmx Trigger: By configuring this function, define the action when the trigger signal is received. Select start digital edge here, that is, start collecting data when a digital trigger signal is received; (5) DAQmx Read: DAQmx read data function, choose Anolog Wfm 1Chan Nsamp here, it returns the one-dimensional waveform data of analog input, including 1 channel, each channel has N samples. Its main parameters are: Task/Channel in - input task name or virtual channel name, here we set the input channel as Channel8; Number of samples per channel - execute the amount of data retrieved from each channel once; Timeout - timeout, Set the waiting time for sampling. If there is not enough data collected, it will return as much and report an error. If it is set to -1, it will wait indefinitely. Set the waiting time to 10 seconds according to the sampling frequency and number of samples. 6. true stress as claimed in claim 2, true strain measurement system is characterized in that: in the system that adopts image processing technology to measure, the intermediate result of calculation is represented by number of pixels, and only the unit on the object surface is obtained The corresponding relationship between the length (mm) and the number of pixels on the image surface can convert the unit of the measurement result into millimeter length. The calibration process is basically the same as the full deformation unit test procedure. It also collects the sample image, selects a calibration target range, filters and binarizes the original diameter image in the range, and finally measures the vertical and horizontal distances. The sub-VI measures the number of pixels occupied by the diameter of the sample, divides the diameter value (mm) of the sample by the measurement result, and obtains the actual length represented by each pixel, which is the pixel equivalent. In the process of calibration, how many pixels are required at this diameter position, and the ratio of the original diameter value to the corresponding number of pixels is the pixel equivalent. Once the pixel equivalent is obtained, it will be used as a standard value. In the subsequent calculation program, it will be used as the calculation reference quantity to accurately calculate the diameter and gauge length at any time.

Images