Determination method of fatigue stiffness degradation law based on digital image correlation technology

Abstract

The present invention relates to a method for determining fatigue stiffness degradation law based on digital image correlation technology, and belongs to the field of experimental testing technology. The purpose of the present invention is to solve the problem of errors in measuring stiffness degradation using digital image correlation (DIC) technology directly during fatigue testing. It includes the following steps: Step 1: Surface treatment of the specimen, making speckles on the surface of the specimen; Step 2: Loading, including the following steps: Step 2.1: Quasi-static loading of the specimen, while collecting the surface image of the specimen during static loading; obtaining the stiffness of the quasi-statically loaded specimen; Step 2.2: Fatigue loading of the specimen; Step 2.3: Judging "yes" or "no", "yes" means ending the loading, "no" means proceeding to step 2. The present invention does not require the use of a high-performance DIC camera, and only requires a DIC camera of ordinary configuration to obtain the true stiffness degradation law of the material.

Description

Technical Field

[0001] The invention relates to a method for determining fatigue test stiffness degradation, and belongs to the technical field of experimental testing. Background Art

[0002] The fatigue stiffness degradation law of materials is a key indicator for evaluating the fatigue performance of materials. This law is usually obtained through fatigue testing, which includes measuring the system stiffness of the test loading system or the true stiffness of the material. The measurement of system stiffness is based on the relationship between the displacement and load data of the loading system, but this method has obvious limitations. For tests equipped with test fixtures, the system displacement data may not accurately reflect the actual deformation of the material, so that the obtained stiffness degradation law cannot truly reflect the stiffness degradation characteristics of the material. The test method using the true stiffness of the material requires obtaining the strain data of the material during the fatigue test. These data can be obtained by extensometers, strain gauges or DIC technology. Although extensometers can measure the average strain between gauge segments, their use is limited in tests with non-uniform strain fields. In addition, when testing composite flat plates, extensometers are prone to slippage, which further limits their application effect under complex strain conditions. Although strain gauges can provide accurate data, their adhesives have insufficient fatigue resistance, which makes strain gauges prone to premature failure in long-term fatigue tests. As a non-contact measurement method, DIC technology can easily obtain the full-field strain of the specimen. However, when it is directly applied in fatigue testing, the shooting frequency of the DIC equipment is extremely high, which needs to be much higher than the fatigue test frequency to accurately obtain the stiffness degradation law. In addition, the specimen is always in a shaking state during fatigue testing, which will also affect the strain measurement results using DIC technology. Moreover, direct testing using DIC technology relies on a large number of photos to calculate the strain field, which will generate a large amount of data in long-term fatigue tests, bringing great challenges to data processing.

[0003] Therefore, it is urgent to propose a method for determining the fatigue stiffness degradation law based on digital image correlation technology to solve the above technical problems. Summary of the invention

[0004] The purpose of the present invention is to solve the problem that when DIC is used directly to measure stiffness degradation during fatigue testing, the specimen and the loading system are always in a shaking state during the fatigue test, which leads to some additional errors in the strain measurement based on DIC technology. In addition, in order to obtain a complete stiffness degradation curve during the fatigue process, there are also higher requirements for the shooting frequency of the DIC camera, so the conventionally configured DIC camera cannot meet the requirements. In order to overcome the above-mentioned technical defects, the purpose of the present invention is to provide a method for determining the fatigue stiffness degradation law based on digital image correlation technology. A brief overview of the present invention is given below to provide a basic understanding of certain aspects of the present invention. It should be understood that this overview is not an exhaustive overview of the present invention. It is not intended to determine the key or important parts of the present invention, nor is it intended to limit the scope of the present invention.

[0005] The technical solution of the present invention:

[0006] The method for determining fatigue stiffness degradation law based on digital image correlation technology includes the following steps:

[0007] Step 1: Surface treatment of the specimen, making speckles on the surface of the specimen;

[0008] Step 2: Loading;

[0009] Step 2.1: Perform quasi-static loading on the specimen and collect the surface image of the specimen during the static loading process; obtain the stiffness of the specimen under quasi-static loading;

[0010] Step 2.2: Fatigue loading of the specimen;

[0011] Step 2.3: Judge "yes" or "no". If "yes", end the loading; if "no", proceed to step 2.

[0012] Preferably: step one comprises the following steps:

[0013] Step 1.1: Primer spraying: Make primer on the surface of the specimen. The primer is matte white and has uniform thickness.

[0014] Step 1.2: Spraying speckles: Spray matte black paint on the surface of the specimen so that the matte black paint can be scattered on the primer on the surface of the specimen to form matte black speckles.

[0015] Preferably: in step 2, a device for determining the fatigue test stiffness degradation law is used, and the device for determining the fatigue test stiffness degradation law includes a test loading system, a single-chip microcomputer and a DIC measuring device. The test loading system is connected to the DIC measuring device through the single-chip microcomputer. The test loading system is used to perform quasi-static loading and fatigue loading on the specimen. The DIC measuring device is used to receive signals from the test loading system and calculate the specimen stiffness value during the quasi-static loading process.

[0016] Preferably: the DIC measuring equipment comprises a DIC system computer and a DIC camera, the DIC system computer is electrically connected to the DIC camera, the DIC camera is used to collect information, the testing machine adopts a tensile testing machine, the controller of the testing machine is electrically connected to the testing machine computer, the level channel output of the testing machine is set on the testing machine computer, a high and low level channel is used to connect the testing machine controller and a single-chip microcomputer, the single-chip microcomputer is electrically connected to the DIC system computer of the DIC measuring equipment, and a python program is arranged on the DIC system computer to monitor the level signal of the corresponding channel of the single-chip microcomputer in real time.

[0017] Preferably: in step 2.1, the two ends of the specimen are clamped by the fixture of the testing machine to perform quasi-static loading; when the tensile force begins to be applied, the single-chip microcomputer receives a high-level signal from the testing machine, triggers the DIC camera to start collecting pictures, and the tensile force on the specimen increases uniformly during the quasi-static loading process. When it increases to the set load f, the high-level signal of the testing machine is converted into a low-level signal, and the low-level signal turns off the DIC camera to stop collecting pictures; the pictures are stored in the DIC system computer, and the picture storage folder name is numbered from 0. The picture storage folder name is numbered +1 when the DIC camera is triggered to collect pictures next time;

[0018] When collecting images during the quasi-static loading process, the python program calls the DIC calculation software to start calculating the strain value of the effective area of ​​the specimen during this quasi-static loading, and then calculates the specimen stiffness obtained by this quasi-static loading.

[0019] Preferably: in step 2.2, the testing machine continues to perform fatigue loading on the specimen; the tensile force on the specimen is increased from the set load f to the peak load F, the tensile peak load F of the specimen is first reduced to the preset load, and then increased to the peak load F, wherein the peak load F first decreases to the preset load and then increases to the peak load F is a single fatigue process, and the single fatigue process during the fatigue loading process is n times.

[0020] Preferably: in step 2.3, when the specimen is fatigue damaged (such as cracks appear) or reaches a predetermined number of fatigue loading times, the output is "yes", and if the output is "no", the judgment starts from step 2.1 again.

[0021] Preferably: in step 2.1, the stiffness of the specimen obtained by a single quasi-static loading is calculated, and the stiffness is used as the stiffness at the i*nth cycle, and the calculation formula is as follows:

[0022] K=σ / ε,σ=F / (w·t)

[0023] Where K is the stiffness, σ is calculated as follows, ε is the strain, F is the peak load of the testing machine, and w and t are the width and thickness of the specimen, respectively.

[0024] Preferably: in step 2.2, f=0.5F, and the preset load is 0-0.1F.

[0025] Preferably, the method further comprises step three: summarizing the stiffness degradation law: plotting an XY coordinate regularity diagram of the specimen stiffness in step 2.1 according to the fatigue times, thereby obtaining the degradation law.

[0026] The present invention has the following beneficial effects:

[0027] The present invention combines DIC technology with quasi-static and fatigue alternating loading in the determination of fatigue stiffness degradation law, making the acquisition of fatigue stiffness degradation data more convenient and efficient, greatly reducing the difficulty and workload of data processing.

[0028] The quasi-static loading process of the present invention has no jitter or little jitter, which is conducive to the lens capturing the surface state of the specimen, not only reducing the requirements for the photographing frequency of the DIC camera, but also avoiding the introduction of additional variables. Only a DIC camera with ordinary configuration is needed to obtain the real stiffness degradation law of the material, thereby achieving the remarkable effect of reducing costs and increasing efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 It is a flow chart of the method for determining fatigue stiffness degradation law based on digital image correlation technology;

[0030] Figure 2 It is the principle diagram of the device for determining the stiffness degradation law of fatigue test;

[0031] Figure 3 This is the colored speckle effect diagram of the specimen;

[0032] Figure 4 It is the stiffness degradation law diagram of the specimen. DETAILED DESCRIPTION

[0033] In order to make the purpose, technical scheme and advantages of the present invention clearer, the present invention is described below by the specific embodiments shown in the accompanying drawings. However, it should be understood that these descriptions are only exemplary and are not intended to limit the scope of the present invention. In addition, in the following description, the description of well-known structures and technologies is omitted to avoid unnecessary confusion of the concept of the present invention.

[0034] Specific implementation method 1: Combination Figure 1-3 The present embodiment is described. The fatigue stiffness degradation law determination method based on digital image correlation technology of the present embodiment comprises the following steps:

[0035] Step 1: Surface treatment of the specimen, making speckles on the surface of the specimen;

[0036] Step 2: Loading, including the following steps:

[0037] Step 2.1: Perform quasi-static loading on the specimen, and use a lens device to collect the surface image of the specimen during the static loading process; call the DIC measurement device to obtain the stiffness of the quasi-statically loaded specimen;

[0038] Step 2.2: Fatigue loading is performed on the specimen; the present invention uses DIC technology to determine the stiffness degradation law of fatigue test, and transforms the DIC strain measurement in fatigue test into the DIC strain measurement in quasi-static loading process by alternating quasi-static loading and fatigue loading, which greatly reduces the configuration requirements for DIC photographing equipment, facilitates the collection of pictures and controls costs, and automatically processes DIC data after each quasi-static photograph through a python program, so that the stiffness of the corresponding number of cycles can be displayed in real time during the test, which greatly facilitates the acquisition of the stiffness degradation law of the material in fatigue test, and the test results are accurate;

[0039] Step 2.3: After fatigue loading, judge "yes" or "no". If "yes", end the loading; if "no", repeat step 2.

[0040] Specific implementation method 2: Combination Figure 1-3 This embodiment is described. In this embodiment, the fatigue stiffness degradation law determination method based on digital image correlation technology includes the following steps:

[0041] Step 1.1: Primer spraying: Primer is applied on the surface of the specimen. The primer is matte white and has a uniform thickness. The primer evenly covers the strain test area. The surface strain test area is the area where the lens device collects images.

[0042] Step 1.2: Spraying speckles: Spray matte black paint on the surface of the specimen from a distance, so that the matte black paint can be scattered on the primer on the surface of the specimen to form matte black speckles, and finally make the specimen into a specimen with speckles on the surface.

[0043] Specific implementation method three: Combination Figure 1-3 The present embodiment is described. The present embodiment is a method for determining the fatigue stiffness degradation law based on digital image correlation technology. In step 2, a device for determining the stiffness degradation law of a fatigue test is used. The device for determining the stiffness degradation law of a fatigue test includes a test loading system, a single-chip microcomputer and a DIC measuring device. The test loading system is connected to the DIC measuring device via the single-chip microcomputer. The test loading system is used for performing quasi-static loading and fatigue loading on the specimen. The DIC measuring device is used for receiving signals from the test loading system and calculating the true stiffness value of the specimen during the quasi-static loading process.

[0044] Specific implementation method four: Combination Figure 1-3 The present embodiment is described. The fatigue stiffness degradation law determination method based on digital image correlation technology of the present embodiment, the DIC measuring equipment comprises a DIC system computer and a DIC camera, the DIC system computer is electrically connected to the DIC camera, the lens of the DIC camera is aimed at the surface strain test area for collecting information, the testing machine is a tensile testing machine, the controller of the testing machine is electrically connected to the testing machine computer, the level channel output of the testing machine is set on the testing machine computer, the testing machine controller and a single-chip microcomputer are connected using a high and low level channel, that is, the controller of the testing machine is electrically connected to the single-chip microcomputer through a digital output module, the single-chip microcomputer is electrically connected to the DIC system computer of the DIC measuring equipment, and a python program is arranged on the DIC system computer to monitor the level signal of the corresponding channel of the single-chip microcomputer in real time;

[0045] The fatigue test loading adopts a step-by-step loading strategy, alternating between quasi-static loading and fatigue loading to complete the entire fatigue test process;

[0046] At the beginning of the test, the load control is used to maintain the test machine at zero load state, and the corresponding channel level signal is converted to a high level signal to trigger the camera to collect data;

[0047] Use the load loading mode to start the quasi-static loading program. The stop load of the quasi-static loading program is half of the fatigue peak load. When the quasi-static loading program ends, the high-level signal is converted to a low-level signal and the camera stops collecting data.

[0048] The computer is controlled by the python program to automatically calculate the strain field of the effective area of ​​the specimen through the DIC software based on the first photo taken each time, and then the stress value is calculated by half of the fatigue peak load and the limited cross-sectional area of ​​the specimen. The real stiffness value of the material in this quasi-static loading process is obtained by combining the strain and stress data, and the stiffness data and the corresponding cycle value are displayed. The cycle value is calculated by (quasi-static loading times - 1) * n, and n is the number of single fatigue cycles; the above can be called quasi-static loading;

[0049] Then the fatigue loading program starts, and the fatigue loading is stopped after n fatigue cycles;

[0050] The quasi-static loading step process is repeatedly executed until the specimen is fatigue-damaged or the total number of fatigue loading cycles reaches a preset value.

[0051] Specific implementation method five: Combination Figure 1-3The present embodiment is described. The fatigue stiffness degradation law determination method based on digital image correlation technology of the present embodiment, in step 2.1, the two ends of the long side of the specimen are clamped by the fixture of the testing machine to perform quasi-static loading; when the tensile force begins to be applied, the single chip microcomputer receives the high-level signal from the testing machine, and triggers the DIC camera through the DIC system computer to start collecting pictures. The tensile force on the specimen increases uniformly during the quasi-static loading process. When it increases to the set load f, the high-level signal of the testing machine is converted into a low-level signal, and the low-level signal turns off the DIC camera to stop collecting pictures; the pictures are stored in the DIC system computer, and the picture storage folder name is numbered i, i=0,1,2,3... All pictures collected by the DIC camera during a single quasi-static loading process are stored in a picture storage folder, and the picture storage folder name is numbered from 0. The picture storage folder name is numbered +1 when the DIC camera is triggered to collect pictures next time; the present invention combines DIC technology with quasi-static loading, which is convenient to obtain, improves effective data, and reduces the difficulty and workload of data processing;

[0052] When collecting pictures during the quasi-static loading process, the python program calls the DIC calculation software to start calculating the strain value of the effective area of ​​the specimen during this quasi-static loading, and then calculates the specimen stiffness obtained by this quasi-static loading; that is, the DIC system computer is controlled by the python program to automatically calculate the strain field of the strain test area on the surface of the specimen through the DIC software based on the first photo taken during each quasi-static loading process, and then the stress value is calculated by half of the fatigue peak load and the finite cross-sectional area of ​​the specimen. The real stiffness value of the material in this quasi-static loading process is obtained by combining the strain and stress data, and the stiffness data and the corresponding cycle value are displayed, where the cycle value is calculated by (quasi-static loading times - 1) * n, where n is the number of cycles of a single fatigue process; there is no jitter or little jitter during the quasi-static loading process, which is beneficial for the lens to capture the surface state of the specimen, which not only reduces the requirements for the frequency of taking pictures by the DIC camera, but also avoids the introduction of additional variables, which is convenient for data processing. Only a DIC camera with ordinary configuration is required to obtain the real stiffness degradation law of the material, and the stiffness corresponding to a specific number of cycles can be displayed in real time during the test process, reducing costs and increasing efficiency.

[0053] Specific implementation method six: Combination Figure 1-3The present embodiment is described. The fatigue stiffness degradation law determination method based on digital image correlation technology of the present embodiment, in step 2.2, when the load is increased to the set load f, the testing machine continues to perform fatigue loading on the specimen; the tensile force on the specimen is increased from the set load f to the peak load F, the tensile peak load F of the specimen is first reduced to the preset load, and then increased to the peak load F, wherein the peak load F is first reduced to the preset load and then increased to the peak load F for a single fatigue process, and the single fatigue process in the fatigue loading process is n times, n = 0, 1, 2, 3 ...;

[0054] Specific implementation method seven: Combination Figure 1-3 To explain this embodiment, the fatigue stiffness degradation law determination method based on digital image correlation (DIC) technology of this embodiment, in step 2.3, when the specimen is fatigue damaged (such as cracks appear) or reaches a predetermined number of fatigue loading times, the output is "yes", and if the output is "no", it starts from step 2.1, goes through step 2.2, and then re-judges, and the predetermined number of fatigue loading times is set by the testing machine computer.

[0055] Specific implementation method eight: Combination Figure 1-3 The present embodiment is described. The fatigue stiffness degradation law determination method based on digital image correlation technology of the present embodiment, in step 2.1, when the quasi-static loading stops, the test loading system converts the high-level signal into a low-level signal, the python program detects the low-level signal, turns off the DIC camera and stops collecting, and at the same time the python program calls the DIC calculation software to start calculating the strain value of the effective area of ​​the specimen during this quasi-static loading, and then calculates the specimen stiffness obtained by this quasi-static loading, and the stiffness is used as the stiffness at the i*nth cycle, and the calculation formula is as follows:

[0056] K=σ / ε,σ=F / (w·t)

[0057] Where K is the stiffness, σ is the stress, ε is the strain, F is the testing machine load, and w and t are the specimen width and thickness, respectively.

[0058] Specific implementation method nine: Combination Figure 1-3 This embodiment is described. In the method for determining fatigue stiffness degradation law based on digital image correlation technology of this embodiment, in step 2.2, f=0.5F, and the preset load is 0-0.1F.

[0059] Specific implementation method ten: Combination Figure 1-4 To explain this embodiment, the method for determining the fatigue stiffness degradation law based on digital image correlation technology in this embodiment also includes step three: summarizing the stiffness degradation law: plotting an XY coordinate regularity diagram of the specimen stiffness in step 2.1 according to the number of fatigue times (quasi-static loading or fatigue loading times), thereby obtaining the degradation law.

[0060] Embodiment 1:

[0061] Taking the fatigue test of a T800 carbon fiber 6K plain weave composite laminate specimen as an example, the method for determining the stiffness degradation law of the material in the fatigue test specifically includes:

[0062] Step A: Figure 3 As shown, matte white spray paint was used to make the primer on the surface of the specimen, and matte black spray paint was used to make the speckle;

[0063] Step B: Figure 2 As shown, the digital output module of the test machine controller is connected to the single-chip microcomputer, the single-chip microcomputer is connected to the computer of the DIC system, and a python program is arranged on the computer of the DIC system to monitor the change of the level signal;

[0064] Step C: Test loading and signal transmission process Figure 1 As shown, the loading system starts quasi-static loading and sends a high-level signal to the microcontroller at the same time; the Python program on the DIC system computer detects the high-level signal and starts camera acquisition;

[0065] Step D: Figure 1 As shown, the quasi-static loading stops, and the loading system converts the high-level signal to a low-level signal; the Python program detects the low-level signal and turns off the camera acquisition system. At the same time, the Python program calls the DIC calculation software to start calculating the strain value of the effective area of ​​the specimen during this quasi-static loading, and then calculates the specimen stiffness obtained by this quasi-static loading. The calculation formula is as follows. This stiffness is used as the stiffness at the i*nth cycle;

[0066] K=σ / ε,σ=F / (w·t)

[0067] Where K is stiffness, σ is stress, ε is strain, F is the testing machine load, w and t are the specimen width and thickness respectively;

[0068] The number of fatigue cycles for each fatigue loading is set to 500, and steps C and D are repeated until the specimen is fatigue-damaged or the total number of fatigue cycles reaches the set number;

[0069] The stiffness degradation curve of the specimen in the fatigue test is as follows: Figure 4 shown.

[0070] It should be noted that in the above embodiments, as long as the technical solutions are not contradictory, they can be arranged and combined, and those skilled in the art can exhaust all possibilities based on the mathematical knowledge of arrangement and combination. Therefore, the present invention will no longer describe the technical solutions after arrangement and combination one by one, but it should be understood that the technical solutions after arrangement and combination have been disclosed by the present invention.

[0071] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for determining fatigue stiffness degradation law based on digital image correlation technology, characterized in that: The following steps are involved: Step 1: Surface treatment of the specimen, making speckles on the surface of the specimen; Step 2: Loading, including the following steps: Step 2.1: Perform quasi-static loading on the specimen and collect the surface image of the specimen during the static loading process; obtain the stiffness of the specimen under quasi-static loading; In step 2.1, the two ends of the specimen are clamped by the fixture of the testing machine for quasi-static loading. When the tensile force begins to be applied, the single-chip microcomputer receives a high-level signal from the testing machine, triggering the DIC camera to start collecting pictures. During the quasi-static loading process, the tensile force on the specimen increases uniformly. When it increases to the set load f, the high-level signal of the testing machine is converted into a low-level signal, and the low-level signal turns off the DIC camera to stop collecting pictures. The pictures are stored in the DIC system computer, and the picture storage folder name is numbered from 0. The picture storage folder name is numbered +1 when the DIC camera is triggered to collect pictures next time. When collecting images during the quasi-static loading process, the python program calls the DIC calculation software to start calculating the strain value of the effective area of ​​the specimen during this quasi-static loading, and then calculates the specimen stiffness obtained by this quasi-static loading; Step 2.2: Fatigue loading is performed on the specimen; DIC technology is used to determine the stiffness degradation law of fatigue test. By alternating between quasi-static loading and fatigue loading, the DIC strain measurement in fatigue test is transformed into the DIC strain measurement in quasi-static loading process, which greatly reduces the configuration requirements for DIC camera equipment, facilitates image collection and cost control, and uses the python program to automatically process DIC data after each quasi-static photo. The stiffness of the corresponding number of cycles can be displayed in real time during the test, which greatly facilitates the acquisition of the stiffness degradation law of the material in fatigue test and the test results are accurate. In step 2.2, the testing machine continues to perform fatigue loading on the specimen; the tensile force on the specimen is increased from the set load f to the peak load F, the tensile peak load F of the specimen is first reduced to the preset load, and then increased to the peak load F, wherein the peak load F is first reduced to the preset load and then increased to the peak load F for a single fatigue process, and the single fatigue process in the fatigue loading process is n times; Step 2.3: Judge "yes" or "no", if "yes", end the loading, if "no", repeat step 2; In step 2.3, if the specimen is fatigue damaged or reaches the predetermined number of fatigue loading times, the output is "yes", and if the output is "no", start from step 2.1 and re-judge; Step 3: Summarize the stiffness degradation law: Draw an XY coordinate diagram of the specimen stiffness in step 2.1 according to the number of fatigue times to obtain the degradation law.

2. The method for determining fatigue stiffness degradation law based on digital image correlation technology according to claim 1 is characterized in that: Step one includes the following steps: Step 1.1: Primer spraying: Make primer on the surface of the specimen. The primer is matte white and has uniform thickness. Step 1.2: Spraying speckles: Spray matte black paint on the surface of the specimen so that the matte black paint can be scattered on the primer on the surface of the specimen to form matte black speckles.

3. The method for determining fatigue stiffness degradation law based on digital image correlation technology according to claim 1, characterized in that: In step 2, a device for determining the fatigue test stiffness degradation law is used. The device for determining the fatigue test stiffness degradation law includes a test loading system, a single-chip microcomputer and a DIC measuring device. The test loading system is connected to the DIC measuring device through the single-chip microcomputer. The test loading system is used to perform quasi-static loading and fatigue loading on the specimen. The DIC measuring device is used to receive signals from the test loading system and calculate the specimen stiffness value during the quasi-static loading process.

4. The method for determining fatigue stiffness degradation law based on digital image correlation technology according to claim 3 is characterized in that: The DIC measuring equipment includes a DIC system computer and a DIC camera. The DIC system computer is electrically connected to the DIC camera. The DIC camera is used to collect information. The testing machine adopts a tensile testing machine. The controller of the testing machine is electrically connected to the testing machine computer. The level channel output of the testing machine is set on the testing machine computer. A high and low level channel is used to connect the testing machine controller and a single-chip microcomputer. The single-chip microcomputer is electrically connected to the DIC system computer of the DIC measuring equipment. A python program is arranged on the DIC system computer to monitor the level signal of the corresponding channel of the single-chip microcomputer in real time.

5. The method for determining fatigue stiffness degradation law based on digital image correlation technology according to claim 4 is characterized in that: In step 2.1, the stiffness of the specimen obtained by quasi-static loading is calculated and used as the stiffness at the i*nth cycle. The calculation formula is as follows: K=σ / ε,σ=F / (w·t) Where K is the stiffness, σ is the stress, ε is the strain, F is the peak load of the testing machine, and w and t are the width and thickness of the specimen, respectively.

6. The method for determining fatigue stiffness degradation law based on digital image correlation technology according to claim 5 is characterized in that: In step 2.2, f=0.5F, and the preset load is 0~0.1F.

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