Test Method for Fatigue Crack Growth Rate Based on the Validity Judgment of Crack Length

Abstract

The present invention discloses a fatigue crack growth rate test method based on the judgment of crack length effectiveness, belonging to the technical field of fatigue crack growth rate test. The method includes the steps of: during the test, visually measuring 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, recording the fatigue cycle times at this time and the crack lengths measured by the automated measurement method, and judging the effectiveness of the crack length data measured by the visual method; if there is invalid data, converting the crack length data measured by the automated measurement method between the two measurement points when the data is invalid and the previous time when the data is valid into the actual crack lengths of the corresponding four positions, and then judging the data effectiveness again; marking the invalid data points on the #imgabs0# curve. This method combines the advantages of the visual method and the automated measurement method, and realizes the judgment of the effectiveness of the crack length data 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 effectiveness. 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 effectiveness. GB / T 6398-2017 "Metallic materials - Fatigue testing - Method for fatigue crack growth" stipulates that the conditions for invalid experimental data include that the difference in the crack lengths of the front and rear surfaces exceeds 0.25B and the difference in the symmetric crack fronts of the M(T) specimen (center crack tension specimen) 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 crack 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 thus the effectiveness of the measured crack length value during the test cannot be judged, which 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 effectiveness.

[0005] The technical solution adopted by the present invention is as follows:

[0006] A fatigue crack growth rate test method based on the judgment of crack length effectiveness, comprising the steps of:

[0007] (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;

[0008] (2) Applying cyclic loads to prefabricate fatigue cracks, and using the visual method to measure the crack lengths of the prefabricated fatigue cracks in the four directions of front left, front right, rear left, and rear right and recording them as 、 、 、 , and record the crack length measured by the automated measurement method at this time ;

[0009] (3) Start the fatigue crack growth rate testing machine for testing. Use the automated measurement method to measure the crack length in real time, and 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 fatigue cycle number at this time and the crack length measured by the automated measurement method ;

[0010] (4) According to the judgment basis specified in the standard, judge the validity of the crack length data at the four positions of the crack measured 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, step (5) is continued at this time;

[0011] (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 (when the previous data was valid) to the actual crack lengths at the corresponding four positions, and then judge the validity of the actual crack length data at the four positions of the crack measured by the automated measurement method after conversion, that is, judge the validity of the corresponding crack length measured by the automated measurement method;

[0012] (6) Perform data processing on the obtained fatigue cycle number of the specimen and the crack length data measured by the automated measurement method to obtain the fatigue crack growth curve.

[0013] Further, the fatigue crack growth rate test specimen in step (1) is an M(T) specimen or a C(T) specimen.

[0014] Further, the automated measurement method in steps (2)-(5) includes the compliance method or the potentiometric method.

[0015] Further, when using the visual 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.

[0016] Further, if the crack length data at the end of the test or from the j-th pause of the test to the end of the test in step (4) is invalid, it indicates that part or all of the crack length data measured by the automated measurement method between the two measurement points when the data is invalid and the previous valid data in this test is invalid. 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 when the data is invalid and the previous valid data.

[0017] Further, the criteria for determining the validity of the crack length in steps (4) and (5) are as follows:

[0018] For the M(T) specimen, the judgment basis is:

[0019] ;

[0020] ;

[0021] ;

[0022] For the C(T) specimen, the judgment basis is:

[0023] ;

[0024] 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 lengths measured by the automated measurement method.

[0025] Further, step (5) is specifically as follows:

[0026] (51) Combine the crack lengths in the four directions measured by the visual method in steps (2) and (3) with the crack lengths measured by the corresponding automated measurement method to form crack length data pairs for different directions, 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 to obtain the crack length linear regression equations for different directions; according to the fitted equations, use the crack lengths in the four directions of front left, front right, rear left, and rear right measured by the visual method in the crack length data pairs at the time of data invalidation and the data pairs at the previous valid data as the ordinates respectively, and the crack lengths measured by the corresponding automated measurement method as the abscissas, and obtain the crack length linear regression equations for the front left, front right, rear left, and rear right directions through linear fitting, that is:

[0027] ;

[0028] 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 tested by the automated measurement method;

[0029] (52) The crack length data of the automated measurement method between the two measurement points when the data is invalid and the previous data is valid is used 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 at the front left, front right, rear left, and rear right positions tested by the automated measurement method;

[0030] (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.

[0031] Furthermore, the data obtained in step (6) The curve is marked with invalid data points determined by steps (4) and (5).

[0032] 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;

[0033] If the obtained in step (6) If some of the data points on the curve are valid, invalid data points will be eliminated when analyzing the fatigue performance of the specimen;

[0034] If the obtained in step (6) If the data points on the curve are invalid, the test needs to be repeated.

[0035] The beneficial effects of the present invention are:

[0036] The present invention provides a fatigue crack growth rate test method based on the judgment of crack length validity. This method combines the advantages of the visual method and the automated measurement method. Through linear transformation, the crack length data measured by the automated measurement method are respectively transformed into the crack lengths at the four positions of the front left, front right, rear left, and rear right on the crack surface. And the symmetry of the surface crack length data obtained after transformation is judged, which solves the problem that the crack length cannot be judged whether it meets the standard symmetry requirement when using only the automated measurement method to test the crack length. Moreover, this method only needs to stop the machine twice and does not require multiple stops for measurement, avoiding the influence of repeated stops on the accuracy of the fatigue crack growth rate test results. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] In order 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 use in 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 be obtained based on these drawings.

[0038] Figure 1 is the flowchart of the present invention;

[0039] Figure 2 is a schematic diagram of the M(T) specimen in the embodiment of the present invention;

[0040] Figure 3 is a cross-sectional view of the M(T) specimen in the embodiment of the present invention along the Figure 2 A-A direction after fatigue crack growth;

[0041] Figure 4 is the linear relationship of crack lengths in different directions obtained by fitting in the embodiment of the present invention. Among them, (a), (b), (c), and (d) are the linear relationships of crack lengths at the four positions of the front left, front right, rear left, and rear right respectively. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0042] The present invention provides a fatigue crack growth rate test method based on the judgment of crack length validity. To make the purpose, technical solutions and effects of the present invention clearer and more definite, the following further details the present invention. 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.

[0043] Referring to Figure 1 , the present invention provides a fatigue crack growth rate test method based on the judgment of crack length validity, including the steps:

[0044] (1) Process fatigue crack growth rate test specimens according to test requirements and standard specifications. Specifically, M(T) specimens or C(T) specimens can be selected according to test needs, and install the specimens on a fatigue crack growth rate testing machine.

[0045] (2) Apply cyclic loads on the fatigue crack growth rate testing machine to prefabricate fatigue cracks. Use the visual method to measure the crack lengths of the prefabricated fatigue cracks 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 measured by the automated measurement method at this time. Specifically, the compliance method or the potential method can be used for automated measurement.

[0046] (3) Start the fatigue crack growth rate testing machine to conduct the test. Use the automated measurement method to continuously test the crack length and record the fatigue cycle times. And use the visual method to measure the crack lengths of the cracks in the four directions of front left, front right, rear left, and rear right during the test process and at the end of the test and record them as , , , , , and at the same time record the fatigue cycle times 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 process, it is necessary to pause the test at least twice to measure the crack lengths in the four directions.

[0047] (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 method in the four directions 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. 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.

[0048] (5) When the data judged in step (4) is invalid, convert the crack length data between the crack lengths measured by the automated measurement method when the previous data was valid into the actual crack lengths in the corresponding four directions, and then judge the data validity of the actual crack lengths in the four directions of the crack measured by the automated measurement method after conversion, that is, judge the validity of the crack length measured by the corresponding automated measurement method; specifically:

[0049] (51) Combine the crack lengths in the four directions measured by the visual method in steps (2) and (3) with the crack lengths measured by the corresponding automated measurement method to form crack length data pairs in different directions, and use the crack length data pairs when the data is invalid and the data pairs when the previous data was valid for linear fitting to obtain the crack length linear regression equations in different directions; according to the fitted equations, use the crack lengths in the four directions of the 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 was valid as the ordinate respectively, and the crack lengths measured by the corresponding automated measurement method as the abscissa, 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:

[0050] ;

[0051] Among them, takes values of 1, 2, 3, and 4, representing the four directions of the front left, front right, rear left, and rear right respectively; 、 are the fitting coefficients in different directions; is the crack length measured by the automated measurement method, is the actual crack length of the crack measured by the automated measurement method in the front left, front right, rear left, and rear right directions;

[0052] (52) Substitute the crack length data measured by the automated measurement method between the two measurement points when the data is invalid and when the previous data was valid as the values into the linear regression equations established in step (51) respectively, and the calculated values are respectively the actual crack lengths of the crack measured by the automated measurement method in the front left, front right, rear left, and rear right directions;

[0053] (53) Judge the validity of the actual crack lengths of the crack measured by the automated measurement method in the four directions of the front left, front right, rear left, and rear right after conversion in step (52), so as to judge the validity of the crack length measured by the corresponding automated measurement method;

[0054] (6) Perform data processing on the obtained specimen fatigue cycle times and the crack length data measured by the automated measurement method to obtain fatigue crack growth curve, and the obtained The curve is marked with invalid data points judged in steps (4) and (5).

[0055] In addition, when the present invention judges the validity of crack length data, the judgment basis used is:

[0056] For the M(T) specimen, the judgment basis is:

[0057] ;

[0058] ;

[0059] ;

[0060] For the C(T) specimen, the judgment basis is:

[0061] ;

[0062] Among them, is the specimen thickness, is the specimen width, , , , are respectively the actual crack lengths at the four positions of front left, front right, rear left, and rear right measured by the visual method in steps (4) and (5) or the corresponding actual crack lengths at the four positions of front left, front right, rear left, and rear right converted from the crack lengths tested by the automated measurement method.

[0063] Through the test method of the present invention, the validity of the crack length data obtained by automated measurement can be judged, and a curve marked with data invalid points can be obtained, providing a reference for the analysis of the fatigue crack performance of the specimen. If all the data points on the obtained curve are valid, the fatigue performance of the specimen is analyzed using all the data on the curve; if some of the data points on the obtained curve are valid, when analyzing the fatigue performance of the specimen, the invalid data points are excluded; if all the data points on the obtained curve are invalid, a new test needs to be carried out.

[0064] Specifically, in the embodiment of the present invention, as Figure 2 shown, an M(T) specimen is prepared from 7050-T7651 aluminum alloy plate, the specimen thickness B = 5 mm, the specimen width W = 160 mm, and the crack length of the machined notch is 16 mm, and the total crack length is 32 mm.

[0065] The fatigue crack growth rate of the above specimens was tested according to the standard of GB / T 6398-2017 "Metallic materials - Fatigue testing - Fatigue crack growth method". An MTS370.10 type electro-hydraulic servo fatigue testing machine was used, with a maximum load of 100 kN and a loading frequency of 10 Hz. A fixture with multiple groups of bolts was used for clamping. During the test, the compliance method was used to measure the fatigue crack length of the specimen in real time, and the corresponding fatigue cycle times were recorded simultaneously. In addition, during the fatigue test, the experiment was paused and the crack lengths on both sides of the central hole were measured by the visual method, that is, the actual crack lengths at the four positions of left, front right, rear left, and rear right before the test, as Figure 3 shown. In addition, for the specimen of this embodiment, the thickness B = 5 mm and the width W = 160 mm. The basis for judging the validity of the obtained crack length data is:

[0066] ;

[0067] ;

[0068] .

[0069] In this embodiment, in order to verify the linear relationship between the crack lengths at the four positions measured by the visual method and the crack length data pairs composed of the crack lengths measured by the corresponding automated measurement methods in different positions, the experiment was paused 4 times during the test and the visual method was used for testing. The test results are shown in Table 1. The crack length data pairs composed of different positions in Table 1 basically satisfy the linear relationship, and the data validity of the test results was analyzed according to the judgment basis.

[0070] Table 1 Test results of crack length

[0071]

[0072] From the data analysis results in the above Table 1, it can be seen that the crack length data measured in the first to fourth paused 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 paused 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 the fourth paused test and the end of the test is valid.

[0073] The crack lengths at the four positions measured by the visual method in the above Table 1 and the crack lengths measured by the corresponding automated measurement methods were combined into crack length data pairs according to different positions. Since the crack length data pairs composed of different positions basically satisfy the linear relationship, the crack length linear regression equations for different positions were obtained by linearly fitting the crack length data pair at the end of the test and the data pair at the fourth paused test, as Figure 4As shown. Then, substitute the crack length data measured by the compliance method between these two measurement points during the fourth pause test and at the end of the test into the linear regression equations of crack lengths in different orientations mentioned above, and convert the crack lengths measured by the compliance method between these two measurement points into the actual crack lengths measured by the corresponding automated measurement method in the front left, front right, rear left, and rear right orientations, as shown in Table 2, and conduct validity judgment.

[0074] Table 2

[0075]

[0076] By analyzing the data in Table 2, except for the data at the end of the test being invalid, the data when the crack lengths measured by the compliance method are 58.22 mm and 58.72 mm are also invalid.

[0077] Based on the above data validity judgment, mark the 3 invalid data points obtained from the calculation and analysis on the fatigue crack growth curve. Then, when analyzing the fatigue performance of the specimen, the invalid data points can be excluded for analysis.

[0078] It should be noted that the parts not described in the present invention can be implemented by adopting or referring to the existing technologies.

[0079] Of course, the above description is not a limitation to the present invention, and the present invention is not limited to the above examples. Any changes, modifications, additions, or substitutions made by those skilled in the art within the substantial scope 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 the judgment of crack length effectiveness, characterized in that Including the steps: (1) Process the fatigue crack growth rate test specimens according to the test requirements and standard specifications, and install the specimens on the fatigue crack growth rate test machine; (2)Apply cyclic loading to prefabricate fatigue cracks, and use the visual method to measure the crack lengths of the prefabricated fatigue cracks in the four directions of front left, front right, rear left, and rear right 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 testing machine for testing. Use an automated measurement method to measure the crack length in real time, and use the visual 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 , , , . At the same time, record the fatigue cycle count and 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 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 carried out 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) Convert the crack length data tested by the automated measurement method between the two measurement points when the data judged in step (4) is invalid and the previous time when the data was valid into the actual crack lengths in the corresponding four directions, and then judge the validity of the data of the actual crack lengths in the four directions of the cracks tested by the automated measurement method after conversion, that is, judge the validity of the crack lengths tested by the corresponding automated measurement method; (6) Process the fatigue cycle times of the obtained specimens and the crack length data measured by the automated measurement method to obtain the fatigue crack growth curve.

2. The fatigue crack growth rate test method based on the judgment of crack length effectiveness according to claim 1, wherein 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 the effectiveness judgment of crack length according to claim 1, characterized in that, The automated measurement method in steps (2)-(5) includes the compliance method or the potentiometric method.

4. A fatigue crack growth rate test method based on the judgment of crack length effectiveness according to claim 1, characterized in that When measuring the crack length by the visual method during the test in step (3), the test needs to be paused at least twice to measure the crack lengths in the four directions of the front left, front right, rear left, and rear right.

5. A fatigue crack growth rate test method based on the judgment of crack length effectiveness according to claim 1, characterized in that, In step (4), 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, it means that some or all of 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 in this test is 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 time when the data was valid.

6. The fatigue crack growth rate test method based on the judgment of the effectiveness of crack length according to claim 1, characterized in that, The judgment basis for judging the validity of the crack length in steps (4) and (5) 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 crack lengths in the front left, front right, rear left, and rear right directions of the crack tested by the visual method in steps (2) and (3) or the actual crack lengths in the front left, front right, rear left, and rear right directions of the crack tested by the automated measurement method after conversion in step (5).

7. A fatigue crack growth rate test method based on the judgment of crack length effectiveness according to claim 1, characterized in that, Step (5) is specifically: (51) Combine the crack lengths in the four directions measured by the visual method in steps (2) and (3) with the crack lengths tested by the corresponding automated measurement method to form crack length data pairs in different directions, and perform linear fitting on the crack length data pairs at the time of data invalidation and the data pairs at the previous time when the data was valid to obtain the crack length linear regression equations in different directions; according to the fitted equations, use the crack lengths in the four directions of the front left, front right, rear left, and rear right measured by the visual method in the crack length data pairs at the time of data invalidation and the data pairs at the previous time when the data was valid as the ordinate respectively, and the crack lengths tested by the corresponding automated measurement method as the abscissa, 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: ； Among them, takes values of 1, 2, 3, and 4, representing the four directions of front left, front right, rear left, and rear right respectively; , are fitting coefficients for different directions; is the crack length measured by the automated measurement method, is the actual crack length in the front left, front right, rear left, and rear right directions of the crack length measured by the automated measurement method; (52) Use the crack length data obtained from the automated measurement method test between the two measurement points when the data is invalid and when the previous data was valid as the values are respectively substituted into the linear regression equation established in step (51), and the values obtained are respectively 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) Judge the effectiveness of the actual crack lengths of the cracks 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 effectiveness of the crack lengths tested by the corresponding automated measurement method.

8. A fatigue crack growth rate test method based on the judgment of crack length effectiveness according to claim 1, characterized in that, The obtained in step (6) Invalid data points judged in steps (4) and (5) are marked on the curve.

9. A fatigue crack growth rate test method based on the judgment of crack length effectiveness according to claim 6, characterized in that, If the data points on the curve obtained in step (6) are all valid, then all the data on this curve are used to analyze the fatigue performance of the specimen; If the data points on the curve obtained in step (6) are partially valid, when analyzing the fatigue performance of the specimen, the invalid data points are excluded; If the data points on the curve obtained in step (6) are all invalid, the experiment needs to be conducted again.

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