Fatigue crack growth rate test method based on crack length validity judgment

Abstract

The invention discloses a fatigue crack growth rate test method based on crack length validity judgment, and belongs to the technical field of fatigue crack growth rate tests. The method comprises the following steps: measuring the lengths of cracks in the front-left, front-right, rear-left and rear-right directions in the test process and at the end of the test by adopting a visual method, recording the fatigue cycle times at the moment and the crack lengths measured by an automatic measurement method, and judging the validity of crack length data measured by adopting the visual method; if the data are invalid, converting the crack length data tested by the automatic measurement method between the two measurement points when the data are invalid and the previous data are valid into corresponding actual crack lengths in four directions, and then judging the validity of the data again; and marking the invalid data points to the # imgabs0 # curve. According to the method, the advantages of a visual method and an automatic measurement method are combined, and the validity judgment of crack length data is realized through linear transformation.

Description

Technical Field

[0001] The present invention relates to the technical field of detecting the fatigue crack growth rate of metal materials, and particularly relates to a fatigue crack growth rate test method based on the judgment of crack length validity. Background Art

[0002] As a lightweight, high-strength, and excellent corrosion-resistant material, aluminum alloy is widely used in the fields of aerospace, rail transit, ships, automobiles, etc. Fatigue failure is the main way of failure of engineering structural components. Statistical data shows that 50% - 90% of the failures of mechanical parts are fatigue failures, and in some fields, it can reach 80% - 90%. For fatigue performance prediction, the fatigue crack growth rate test method is usually adopted.

[0003] In the fatigue crack growth rate test, it is necessary to measure the fatigue crack growth length in real time. The compliance method and the DC potential method are widely adopted by each laboratory due to their simple operation, high degree of automation, and wide application range. In the fatigue crack growth rate test, there are regulations on crack symmetry and validity. GB / T 6398-2017 "Metallic materials - Fatigue testing - Fatigue crack growth method" stipulates that the conditions for invalid experimental data include that the difference in crack lengths between the front and rear surfaces exceeds 0.25B and the difference in the front ends of symmetric cracks of M(T) specimens (center crack tensile specimens) exceeds 0.05W. However, when using the compliance method or the DC potential method to automatically measure the crack length, only one crack length value can be measured, and the actual lengths of the front, rear, left, and right cracks cannot be given separately. Therefore, the symmetry of the real-time crack length cannot be calculated, and the validity of the measured crack length value during the test cannot be judged either. This may lead to inaccurate test results of the fatigue crack growth rate test and affect the fatigue performance prediction results. Summary of the Invention

[0004] To solve the above technical problems, the present invention provides a fatigue crack growth rate test method based on the judgment of crack length validity.

[0005] The technical solution adopted by the present invention is as follows: A fatigue crack growth rate test method based on the judgment of crack length validity, comprising the steps of: (1) Processing a fatigue crack growth rate test specimen according to the test requirements and standard regulations, and installing the specimen on a fatigue crack growth rate testing machine; (2) Applying a cyclic load to prefabricate a fatigue crack, and using the visual method to measure the crack lengths of the prefabricated fatigue crack in the four directions of front left, front right, rear left, and rear right and record them as , , , , and at the same time, record the crack length ; (3) Start the fatigue crack growth rate testing machine for testing. Use an automated measurement method to measure the crack length in real time, and use the visual inspection method to measure the crack lengths at the front left, front right, rear left, and rear right positions of the crack during and at the end of the test, and record them as , , , , and record the fatigue cycle count at this time as well as the crack length measured by the automated measurement method ; (4) According to the judgment basis specified in the standard, judge the validity of the crack length data of the cracks tested by the visual inspection method in the four positions in steps (2) and (3); if all the data are valid, it means that all the crack length data obtained by the automated measurement method in this test are valid, and step (5) is not performed at this time; if the crack length data at the end of the test or from the jth test pause to the end of the test is invalid, step (5) is continued at this time; (5) When the data judged in step (4) is invalid, convert the crack length data measured by the automated measurement method between the two measurement points of the invalid data and the previous valid data into the actual crack lengths in the corresponding four positions, and then judge the validity of the actual crack length data of the cracks tested by the automated measurement method after conversion, that is, judge the validity of the crack length measured by the corresponding automated measurement method; (6) Perform data processing on the obtained specimen fatigue cycle count and the crack length data measured by the automated measurement method to obtain the fatigue crack growth curve.

[0006] Further, the fatigue crack growth rate test specimen in step (1) is an M(T) specimen or a C(T) specimen.

[0007] Further, the automated measurement method in steps (2)-(5) includes the compliance method or the potentiometric method.

[0008] Further, when using the visual inspection method to measure the crack length during the test in step (3), it is necessary to pause the test at least twice to measure the crack lengths at the front left, front right, rear left, and rear right positions of the crack.

[0009] Further, if the crack length data at the end of the test or from the j-th test suspension to the end of the test in step (4) is invalid, it indicates that there is partial or total invalidity in the crack length data measured by the automated measurement method between the two measurement points where the data is invalid and the previous valid data in this test. It is necessary to use step (5) to determine the validity of the crack length data measured by the automated measurement method between the two measurement points where the data is invalid and the previous valid data.

[0010] Further, the criteria for determining the validity of the crack length in steps (4) and (5) are as follows: For M(T) specimens, the judgment basis is: ; ; ; For C(T) specimens, the judgment basis is: ; Among them, is the specimen thickness, is the specimen width, , , , are the actual crack lengths at the four positions of front left, front right, rear left, and rear right measured by visual inspection in steps (4) and (5) or the actual crack lengths at the corresponding four positions of front left, front right, rear left, and rear right converted from the crack lengths measured by the automated measurement method.

[0011] Further, step (5) is specifically as follows: (51) Combine the crack lengths at the four positions measured by visual inspection in steps (2) and (3) with the corresponding crack lengths measured by the automated measurement method to form crack length data pairs for different positions, and perform linear fitting on the crack length data pairs at the time of data invalidation and the data pairs at the previous valid data time to obtain linear regression equations for crack lengths in different positions; according to the fitted equations, take the crack lengths at the four positions of front left, front right, rear left, and rear right measured by visual inspection in the crack length data pairs at the time of data invalidation and the data pairs at the previous valid data time as the ordinates respectively, and the corresponding crack lengths measured by the automated measurement method as the abscissas, and obtain the linear regression equations for crack lengths at the positions of front left, front right, rear left, and rear right through linear fitting, that is: ; Among them, takes values of 1, 2, 3, and 4, representing the four positions of front left, front right, rear left, and rear right respectively; , is the fitting coefficient of different orientations; The crack length tested by the automated measurement method is The actual crack lengths at the front left, front right, rear left, and rear right positions of the crack length tested by the automated measurement method; (52) The crack length data tested by the automated measurement method between the two measurement points when the data is invalid and when the previous data is valid is taken as The values ​​are respectively substituted into the linear regression equation established in step (51), and the calculated The values ​​are the actual crack lengths of the crack lengths tested by the automated measurement method at the front left, front right, rear left, and rear right positions; (53) The validity of the actual crack lengths of the cracks at the four directions of front left, front right, rear left and rear right tested by the automated measurement method after conversion in step (52) is judged, thereby judging the validity of the corresponding crack lengths tested by the automated measurement method.

[0012] Furthermore, the data obtained in step (6) The curve is marked with invalid data points determined by steps (4) and (5).

[0013] Furthermore, if the obtained in step (6) If all data points on the curve are valid, all data on the curve are used to analyze the fatigue performance of the specimen;

[0014] If the obtained in step (6) If some of the data points on the curve are valid, the invalid data points will be eliminated when analyzing the fatigue performance of the specimen;

[0015] If the value obtained in step (6) If all data points on the curve are invalid, the test needs to be repeated.

[0016] The beneficial effects of the present invention are: The present invention provides a fatigue crack growth rate test method based on crack length validity judgment. The method combines the advantages of a visual method and an automated measurement method. Through linear transformation, the crack length data measured by the automated measurement method are respectively transformed into crack lengths at four positions of the crack surface, namely, front left, front right, rear left, and rear right. Symmetry judgment is performed on the surface crack length data obtained after the transformation, thereby solving the problem that it is impossible to judge whether the crack length meets the standard symmetry requirements when testing the crack length using only the automated measurement method. Moreover, the method only requires two shutdowns without multiple shutdowns for measurement, thereby avoiding the accuracy of the fatigue crack growth rate test results affected by repeated shutdowns. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the drawings in the following description are some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0018] Figure 1 is the flowchart of the present invention; Figure 2 is a schematic diagram of the M(T) specimen in the embodiment of the present invention; Figure 3 After fatigue crack growth of the M(T) specimen in the embodiment of the present invention along Figure 2 is a cross-sectional view in the A-A direction; Figure 4 is the linear relationship of crack lengths in different orientations obtained by fitting in the embodiment of the present invention. Among them, (a), (b), (c), and (d) are the linear relationships of crack lengths in the front left, front right, rear left, and rear right orientations respectively. Detailed implementation manners

[0019] The present invention provides a fatigue crack growth rate test method based on crack length effectiveness judgment. To make the purpose, technical solution and effect of the present invention clearer and more definite, the present invention is further described in detail below. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0020] Referring to Figure 1 , the present invention provides a fatigue crack growth rate test method based on crack length effectiveness judgment, including the steps of: (1) Process a fatigue crack growth rate test specimen according to the test requirements and standard regulations. Specifically, an M(T) specimen or a C(T) specimen can be selected according to the test requirements, and the specimen is installed on a fatigue crack growth rate test machine; (2) Apply a cyclic load on the fatigue crack growth rate test machine to prefabricate a fatigue crack. Use the visual method to measure the crack lengths of the prefabricated fatigue crack in the front left, front right, rear left, and rear right orientations and record them as , , , , and at the same time record the crack length measured by the automated measurement method at this time. Specifically, the flexibility method or the potential method can be used for automated measurement; (3) Start the fatigue crack growth rate testing machine for the test. Use an automated measurement method to measure the crack length in real time and record the number of fatigue cycles. Also, use the visual method to measure the crack lengths at the front left, front right, rear left, and rear right positions during and at the end of the test, and record them as , , , , , and record the number of fatigue cycles at this time and the crack length measured by the automated measurement method ; and in this step, when using the visual method to measure the crack length during the test, it is necessary to pause the test at least twice to measure the crack lengths in four directions; (4) According to the judgment basis specified in the standard, judge the validity of the crack length data in four directions of the cracks tested by the visual method in steps (2) and (3); if all the data are valid, it means that all the crack length data obtained by the automated measurement method in this test are valid, and step (5) is not performed at this time; if the crack length data at the end of the test or from the j - th test pause to the end of the test is invalid, it means that there are some or all invalid crack length data measured by the automated measurement method between the two measurement points when the data is invalid and the previous data is valid in this test, and it is necessary to use step (5) to judge the validity of the crack length data measured by the automated measurement method between the two measurement points when the data is invalid and the previous data is valid; (5) Convert the crack length data between the crack lengths measured by the automated measurement method when the data is judged to be invalid and the previous data is valid in step (4) into the corresponding actual crack lengths in four directions, and then judge the validity of the data of the actual crack lengths in four directions of the cracks measured by the automated measurement method after conversion, that is, judge the validity of the corresponding crack lengths measured by the automated measurement method; specifically: (51) Combine the crack lengths in four directions measured by the visual method in steps (2) and (3) with the corresponding crack lengths measured by the automated measurement method according to different directions to form crack length data pairs, and perform linear fitting on the crack length data pairs at the time of data invalidation and the data pairs at the previous data - valid time to obtain the crack length linear regression equations in different directions; according to the fitted equations, take the crack lengths in the front left, front right, rear left, and rear right directions measured by the visual method in the crack length data pairs at the time of data invalidation and the data pairs at the previous data - valid time as the ordinates respectively, and the corresponding crack lengths measured by the automated measurement method as the abscissas, and obtain the crack length linear regression equations in the front left, front right, rear left, and rear right directions through linear fitting, that is: ; where, The values are 1, 2, 3, and 4, representing the four directions of front left, front right, rear left, and rear right respectively; , are the fitting coefficients for different directions; is the crack length tested by the automated measurement method, is the actual crack length of the crack length tested by the automated measurement method in the front left, front right, rear left, and rear right directions; (52) Take the crack length data tested by the automated measurement method between the two measurement points when the data is invalid and the previous time when the data was valid as values are respectively substituted into the linear regression equation established in step (51), and the calculated values are respectively the actual crack lengths of the crack length tested by the automated measurement method in the front left, front right, rear left, and rear right directions; (53) Judge the validity of the actual crack lengths of the crack tested by the automated measurement method after conversion in step (52) in the four directions of front left, front right, rear left, and rear right, so as to judge the validity of the corresponding crack length tested by the automated measurement method; (6) Perform data processing on the obtained specimen fatigue cycle times and crack length data tested by the automated measurement method to obtain a fatigue crack growth curve, and the obtained curve is marked with invalid data points judged in steps (4) and (5).

[0021] In addition, when judging the validity of the crack length data in the present invention, the judgment basis used is: For the M(T) specimen, its judgment basis is: ; ; ; For the C(T) specimen, its judgment basis is: ; Among them, is the specimen thickness, is the specimen width, , , , are respectively the actual crack lengths in the four directions of front left, front right, rear left, and rear right measured by the visual method in steps (4) and (5) or the actual crack lengths in the corresponding four directions of front left, front right, rear left, and rear right converted from the crack length tested by the automated measurement method.

[0022] Through the test method of the present invention, it is possible to judge the validity of the crack length data obtained by automatic measurement, and obtain a curve marked with data invalid points, which provides a reference for the fatigue crack performance analysis of the specimen. If all the data points on the curve are valid, then all the data on this curve are used to analyze the fatigue performance of the specimen; if some of the data points on the curve are valid, then when analyzing the fatigue performance of the specimen, the invalid data points are excluded; if all the data points on the curve are invalid, then the test needs to be carried out again.

[0023] Specifically, in the embodiment of the present invention, as Figure 2 shown, 7050-T7651 aluminum alloy plates are taken to prepare M(T) specimens. The thickness B of the specimen is 5 mm, the width W of the specimen is 160 mm, and the crack length of the machined notch is 16 mm, and the total crack length is 32 mm.

[0024] According to the standard of GB / T6398-2017 "Metallic materials - Fatigue testing - Fatigue crack growth method", the fatigue crack growth rate of the above specimens is tested. The testing machine uses an MTS370.10 type electro-hydraulic servo fatigue testing machine, with a maximum load of 100 kN, a loading frequency of 10 Hz, and is clamped using a fixture with multiple groups of bolts. During the test, the compliance method is used to measure the fatigue crack length of the specimen in real time, and the corresponding number of fatigue cycles is recorded simultaneously; in addition, during the fatigue test, the experiment is paused and the crack lengths on both sides of the central hole are measured by the visual method, that is, the actual crack lengths in the four directions of left, front right, rear left, and rear right before the test, as Figure 3 shown. In addition, for the specimen thickness B = 5 mm and specimen width W = 160 mm of this embodiment, the basis for judging the validity of the obtained crack length data is: ; ; .

[0025] In this embodiment, in order to verify the linear relationship between the crack lengths measured by the visual method in the four directions and the crack length data pairs composed of the crack lengths measured by the corresponding automatic measurement method in different directions, the test is paused 4 times during the test and the visual method is used for testing. The test results are shown in Table 1. The crack length data pairs composed of different directions in Table 1 basically satisfy the linear relationship, and the data validity of the test results is analyzed according to the judgment basis.

[0026] Table 1 Crack length test results

[0027] As can be seen from the data analysis results in Table 1 above, the crack length data measured in the first to fourth pause tests are all valid, and only the crack length data measured at the end of the test is invalid. At this time, the crack length data between the fourth pause test and the end of the test may be partially valid. Therefore, it is necessary to supplement the analysis of whether the crack length data between these two measurement points, namely the fourth pause test and the end of the test, is valid.

[0028] The crack length data of the four directions measured by the visual method in Table 1 above and the corresponding crack length data measured by the automated measurement method are combined into crack length data pairs according to different directions. Since the crack length data pairs formed by different directions basically satisfy the linear relationship, the crack length data pairs at the end of the test and the data pairs at the fourth pause test are used for linear fitting to obtain the linear regression equations of crack length in different directions, as Figure 4 shown. Then, the crack length data measured by the compliance method between the fourth pause test and the end of the test are substituted into the above linear regression equations of crack length in different directions, and the crack length measured by the compliance method between these two measurement points is converted into the actual crack length measured by the corresponding automated measurement method in the front left, front right, rear left, and rear right directions, as shown in Table 2, and the validity is judged.

[0029] Table 2

[0030] By analyzing the data in Table 2, in addition to the invalid data at the end of the test, the data with crack lengths of 58.22 mm and 58.72 mm measured by the compliance method are also invalid.

[0031] Based on the above judgment of data validity, the 3 invalid data points obtained by calculation and analysis are marked on the fatigue crack growth curve. When analyzing the fatigue performance of the specimen, the invalid data points can be excluded for analysis.

[0032] It should be noted that the parts not described in the present invention can be implemented by adopting or referring to the existing technologies.

[0033] Of course, the above description is not a limitation of the present invention, and the present invention is not limited to the above examples. Changes, modifications, additions, or substitutions made by those skilled in the art within the essence of the present invention should also fall within the protection scope of the present invention.

Claims

1. A fatigue crack growth rate test method based on crack length validity judgment, characterized in that: Includes steps: (1) Process the fatigue crack growth rate test specimen according to the test requirements and standard provisions, and install the specimen on the fatigue crack growth rate testing machine; (2) Apply cyclic load to prefabricate fatigue cracks, and use the visual method to measure the crack lengths of the prefabrication fatigue cracks at the front left, front right, rear left, and rear right directions and record them as , , , , and record the crack length measured by the automated measurement method at this time ; (3) Start the fatigue crack growth rate tester to conduct the test, use the automated measurement method to test the crack length in real time, and use the visual method to measure the crack lengths in the front left, front right, rear left, and rear right directions during and at the end of the test and record them as , , , , and record the number of fatigue cycles at this time and the crack length measured by the automated measurement method ; (4) According to the judgment basis specified in the standard, the validity of the crack length data of the cracks in the four directions tested by the visual method in step (2) and step (3) is judged; if the data are all valid, it means that all the crack length data obtained by the automated measurement method in this test are valid, and step (5) is not performed at this time; if the crack length data at the end of the test or from the jth suspension of the test to the end of the test is invalid, then continue with step (5); (5) converting the crack length data of the automated measurement method between the two measurement points when the data determined in step (4) is invalid and when the previous data is valid into the corresponding actual crack lengths in the four directions, and then judging the validity of the converted data of the actual crack lengths of the cracks in the four directions tested by the automated measurement method, that is, judging the validity of the corresponding crack lengths tested by the automated measurement method; (6) Process the obtained data of fatigue cycle number of the sample and crack length tested by the automated measurement method to obtain fatigue crack extension curve.

2. A fatigue crack growth rate test method based on crack length validity judgment according to claim 1, characterized in that: The fatigue crack growth rate test specimen in step (1) is an M(T) specimen or a C(T) specimen.

3. A fatigue crack growth rate test method based on crack length validity judgment according to claim 1, characterized in that: The automated measurement method in steps (2)-(5) includes a compliance method or a potential method.

4. A fatigue crack growth rate test method based on crack length validity judgment according to claim 1, characterized in that: When the crack length is measured visually in the step (3) during the test, the test needs to be paused at least twice to measure the crack lengths at the four positions of front left, front right, rear left, and rear right.

5. A fatigue crack growth rate test method based on crack length validity judgment according to claim 1, characterized in that: In the step (4), if the crack length data at the end of the test or from the jth suspension of the test to the end of the test is invalid, it means that the crack length data tested by the automated measurement method between the two measurement points when the data is invalid and the previous data is valid in this test is partially or completely invalid, and it is necessary to use step (5) to judge the validity of the crack length data tested by the automated measurement method between the two measurement points when the data is invalid and the previous data is valid.

6. A fatigue crack growth rate test method based on crack length validity judgment according to claim 1, characterized in that: The basis for determining the validity of the crack length in step (4) and step (5) is: For M(T) samples, the judgment basis is: ； ； ； For C (T) samples, the judgment basis is: ； in, is the sample thickness, is the specimen width, , , , They are respectively the actual crack lengths at the front left, front right, rear left and rear right four directions measured by visual method in step (4) and step (5) or the actual crack lengths at the corresponding front left, front right, rear left and rear right four directions converted from the crack lengths tested by the automated measurement method.

7. A fatigue crack growth rate test method based on crack length validity judgment according to claim 1, characterized in that: The step (5) is specifically as follows: (51) The crack lengths at the four directions measured by the visual method in steps (2) and (3) and the crack lengths tested by the corresponding automated measurement method are combined into crack length data pairs according to different directions, and the crack length data pairs when the data is invalid are linearly fitted with the data pairs when the previous data is valid to obtain linear regression equations for the crack lengths at different directions; according to the fitted equation, the crack lengths at the four directions of front left, front right, rear left and rear right measured by the visual method in the crack length data pairs when the data is invalid and the data pairs when the previous data is valid are respectively used as the ordinates, and the crack lengths tested by the corresponding automated measurement method are used as the abscissas, and the crack length linear regression equations at the four directions of front left, front right, rear left and rear right are obtained by linear fitting, namely: ； in, The values ​​are 1, 2, 3, and 4, representing the front left, front right, back left, and back right directions respectively; , is the fitting coefficient of different orientations; The crack length tested by the automated measurement method is The actual crack lengths at the front left, front right, rear left, and rear right positions of the crack length tested by the automated measurement method; (52) The crack length data tested by the automated measurement method between the two measurement points when the data is invalid and when the previous data is valid is taken as The values ​​are respectively substituted into the linear regression equation established in step (51), and the calculated The values ​​are the actual crack lengths of the crack lengths tested by the automated measurement method at the front left, front right, rear left, and rear right positions; (53) The validity of the actual crack lengths of the cracks at the four directions of front left, front right, rear left and rear right tested by the automated measurement method after conversion in step (52) is judged, thereby judging the validity of the corresponding crack lengths tested by the automated measurement method.

8. A fatigue crack growth rate test method based on crack length validity judgment according to claim 1, characterized in that: The data obtained in step (6) The curve is marked with invalid data points determined by steps (4) and (5).

9. A fatigue crack growth rate test method based on crack length validity judgment according to claim 6, characterized in that: If the value obtained in step (6) If all data points on the curve are valid, all data on the curve are used to analyze the fatigue performance of the specimen; If the value obtained in step (6) If some of the data points on the curve are valid, the invalid data points will be eliminated when analyzing the fatigue performance of the specimen; If the value obtained in step (6) If all data points on the curve are invalid, the test needs to be repeated.

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