A segmented test method for fatigue crack growth rate

Abstract

The present invention discloses a fatigue crack growth rate test method for segmented testing, and belongs to the technical field of fatigue crack growth rate testing. The method comprises the following steps: (1) determining the test requirements, including the sample width, crack length extension range, and measurement range; (2) calculating the initial crack length at the sample width required by the test according to the standard sample preparation requirements, and if the initial crack length is greater than the minimum crack length extension range required by the test, dividing the test into several segments according to the crack length extension range; (3) determining the sample width, initial crack length, final crack length, loading load, and measurement range of each test sample; (4) conducting the test according to the divided segments, merging the segmented test data, and obtaining the da / dN‑curve. The test method of the present invention can solve the problem that the test conditions required by the test cannot be completed by one sample under the limitation of the relationship between the sample size and the crack length required by the standard.

Description

Technical Field

[0001] The invention relates to the technical field of fatigue crack growth rate detection of metal materials, and in particular to a fatigue crack growth rate test method of segmented testing. Background Art

[0002] With the development of aircraft structure design technology and the emergence of damage tolerance design concepts, the formation and growth of metal fatigue cracks have received great attention. Metal fatigue cracks need to go through a steady-state growth period to expand to sudden failure under cyclic loads, and the crack growth during this period is large enough. If the fatigue crack growth behavior characteristics of the material at this stage can be effectively mastered and controlled, the accuracy of aircraft life estimation will be significantly improved. Therefore, studying the fatigue crack growth rate of cracked structures under conditions such as applied stress, crack length, and component geometry, as well as the influence of factors such as average stress, test frequency, and environmental conditions on the crack growth rate mechanism, has important engineering application needs and scientific theoretical guidance value. At present, in important engineering fields such as aviation, the fatigue crack growth performance of metal materials has become one of the key performance indicators for design, material selection, and life estimation.

[0003] The fatigue crack growth rate of metal materials under steady-state conditions is often expressed by the Paris formula, that is, , which is often used in the damage tolerance design of materials. The commonly used standards for the fatigue crack growth rate test methods of metal materials include ASTM E647, GB / T6398-2017, HB5216-82, etc. The test method is relatively complex and belongs to the complex test of the mechanical properties of metal materials. The difficulty of this test lies mainly in the measurement of the fatigue crack length of the specimen and the subsequent data processing. In general, a specimen is used, and an initial notch length a is first machined into the specimen. n Then, a fatigue crack of about 1 mm is prefabricated on the notch length, and then a fatigue crack growth test is carried out at a fixed load and stress ratio with a fixed test frequency. During the test, the load, fatigue crack length, number of cycles and other information are recorded. Finally, the data is processed to obtain the fitting formula of the Paris formula and the fatigue crack growth da / dN- under the condition of steady-state crack growth. curve.

[0004] At present, there are three kinds of specimens commonly used in fatigue crack growth rate tests: center crack tensile specimens, compact tensile specimens, and single-edge notched three-point bending specimens; there are three methods for measuring crack length: visual method, flexibility method, and potential method, among which the flexibility method is more commonly used. However, there are restrictions on the crack length of specimens tested by the current material flexibility method, which also limits its scope of application. There are prerequisites for using the flexibility method to test the crack length of specimens. For center crack tensile specimens, 2a is required. n≥0.2W, for compact tensile specimens, a n ≥0.2W, where a n is the initial crack length, and W is the specimen width. However, some product specifications, when giving the fatigue crack growth rate test conditions of materials, do not specify the initial length, final length, specimen size, There are requirements for the minimum and maximum values ​​of the center crack test specimen. Under these conditions, it is actually impossible to use the flexibility method test. For example, for the center crack tensile specimen, the specimen width is required to be 160mm, the center crack length is extended from 4mm to 55mm, and it is required to be able to measure from Expand to The corresponding fatigue crack growth rate, if according to 2a n ≥0.2W, in order to obtain accurate crack length using the flexibility method, 2a n It should be at least 32mm. It is impossible to test the fatigue crack growth rate from 4mm to 32mm. If the test is started according to the initial crack of 32mm, from Expand to The corresponding fatigue crack growth rate, when reaching When the crack length is too long, the entire specimen has already become unstable and fractured, and it is impossible to measure the fatigue crack growth rate of the specimen in the later stage of expansion. Summary of the invention

[0005] In order to solve the above technical problems, the present invention provides a fatigue crack growth rate test method of segmented testing.

[0006] The technical solution adopted by the present invention is:

[0007] The present invention provides a segmented test fatigue crack growth rate test method, which uses a compliance method to perform a fatigue crack growth rate test on a central crack tensile specimen, comprising the steps of:

[0008] (1) Determine the fatigue crack growth rate test requirements of the specimen to be tested, including the specimen width W0 and the crack length extension range 2a 0min ~2a 0max , Measuring range ~ ;

[0009] (2) According to the sample preparation requirements for the center crack tensile specimen in the flexibility test method, calculate the minimum initial crack length 2a0 of the center crack tensile specimen at the specimen width W0 in step (1), and analyze whether the initial crack length 2a0 is greater than the minimum crack length extension range 2a required by the test in step (1). 0min , if 2a0>2a0min , then it is divided into n sections for testing according to the crack length extension range requirements;

[0010] (3) When testing in sections, it is necessary to determine the number of sections and the specimen width, initial crack length, final crack length, loading load, and The measurement range, and the specimen width and initial crack length of each test specimen meet the sample preparation requirements for the center crack tensile specimen in the flexibility test method;

[0011] (4) Carry out the test in the divided sections. After the section test is completed, merge the section test data to obtain the crack length extension range and da / dN under measuring range Curve, that is, fatigue crack growth rate curve.

[0012] Furthermore, in the step (2), when the width of the sample is W0, the initial crack length of the center crack tensile sample is 2a0=0.2W0.

[0013] Furthermore, the specific steps of segmenting step (3) are as follows:

[0014] (31) According to the crack length extension range required by step (1), the minimum value of the crack length extension range is 2a 0min The initial crack length of the first test specimen is 2a. 1min , and the crack extension range of the specimen is set to (0.2~0.8)W1, then the final crack length of the first test specimen is 2a 1max ≤0.8W1, and according to the initial crack length 2a of the first test specimen 1min The calculated width of the first test specimen is W1=10a. 1min ;

[0015] (32) If the final crack length of the first test specimen is 2a 1max ≥0.2 W0, then set the initial crack length of the second test specimen to 2a 2min =2a 1max , W2= W0; if 2a 1max <0.2 W0, set the initial crack length of the second test specimen to 2a 2min =2a 1max , W2=10a 2min , and the crack extension range of the specimen is set to (0.2~0.8)W2, then the final crack length of the second test specimen is 2a 2max ≤0.8W2;

[0016] (33) Determine the final crack length 2a of the second test specimen2max Is it more than 2a? 0max , if 2a 2max ≥ 2 years 0max , the test is completed, otherwise proceed to step (34);

[0017] (34) If 2a 2max <2a 0max , then continue to add the nth test specimen, and the method for determining the initial crack length, final crack length and specimen width of the added nth test specimen is the same as step (32) until the crack length extension range required by the test is completed, that is, 2a nmax ≥ 2 years 0max .

[0018] Furthermore, the loading load of each test specimen in step (3) is and scope ~ The method for determining is:

[0019] The nth test sample for:

[0020] ;

[0021] Loading load of the nth test specimen for:

[0022] ;

[0023] The nth test sample can obtain for:

[0024] ;

[0025] in, is the coefficient, ranging from 0.8 to 1.0, , , n is the serial number, indicating the nth test sample, W n is the width of the nth test specimen, 2a nmin is the initial crack length of the nth test specimen, 2a nmax is the final crack length of the nth test specimen, and B is the specimen thickness.

[0026] Furthermore, when judging whether the test is completed in step (3), in addition to meeting the crack length extension range requirements, it is also necessary to simultaneously judge whether the ∆K measurement range meets the requirements, that is, when the last section of the test specimen meets 2a nmax ≥ 2 years 0max At the same time, , the test is completed if , then take a new test sample and increase the load of the nth and / or n-1th test samples during the test until , the test is completed.

[0027] Furthermore, the step (4) is specifically as follows:

[0028] (41) Merge the segmented test data of n test samples to form -da / dN data pairs, the data pairs are divided into Sort in ascending order and get da / dN- curve;

[0029] (42) -da / dN data pairs are taken to the base 10 logarithm to form log( )-log(da / dN) data pairs, and according to log( )-log(da / dN) data were linearly fitted to obtain log( ), log(da / dN) related fitting formula, and transform the fitting formula into Paris formula, namely:

[0030] ;

[0031] Among them, da / dN is the crack extension per unit cycle, that is, the crack growth rate; C and m are constants related to the material. is the range of stress intensity factor.

[0032] The beneficial effects of the present invention are:

[0033] The present invention provides a fatigue crack growth rate test method for segmented testing, which divides the crack length expansion interval required by the test into The measurement range is divided into several sections for testing, and the relationship between the specimen size and crack length of each test specimen meets the standard requirements. Then the test data of each section are combined to obtain a complete fatigue crack growth rate curve that meets the test requirements. This can solve the problem that the test conditions required by the test cannot be completed by one specimen under the limitation of the relationship between the specimen size and crack length required by the standard. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.

[0035] Figure 1 is a flow chart of the present invention;

[0036] Figure 2 da / dN- obtained in Example 1 of the present invention curve;

[0037] Figure 3 da / dN- obtained in Example 2 of the present invention curve. DETAILED DESCRIPTION

[0038] The present invention provides a fatigue crack growth rate test method for segmented testing. In order to make the purpose, technical solution and effect of the present invention clearer and more specific, 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.

[0039] Reference Figure 1 The present invention provides a fatigue crack growth rate test method for segmented testing, which mainly uses the compliance method to perform a fatigue crack growth rate test on a central crack tensile specimen, comprising the steps of:

[0040] (1) Determine the fatigue crack growth rate test requirements of the specimen to be tested, including the specimen width W0 and the crack length extension range 2a 0min ~2a 0max , Measuring range ~ ;

[0041] (2) According to the sample preparation requirements for the center crack tensile specimen in the flexibility test method, calculate the minimum initial crack length 2a0=0.2W0 of the center crack tensile specimen at the specimen width W0 in step (1), and analyze whether the initial crack length 2a0 is greater than the minimum crack length extension range 2a0 required by the test in step (1). 0min , if 2a0>2a 0min , then it is divided into n sections for testing according to the crack length extension range requirements;

[0042] (3) When testing in sections, it is necessary to determine the number of sections and the specimen width, initial crack length, final crack length, loading load, and The measurement range, and the specimen width and initial crack length of each test specimen meet the sample preparation requirements for the center crack tensile specimen in the flexibility test method;

[0043] (4) Carry out the test in the divided sections. After the section test is completed, merge the section test data to obtain the crack length extension range and da / dN under measuring range Curve, that is, fatigue crack growth rate curve.

[0044] In a preferred embodiment of the present invention, the specific steps of segmenting the above step (3) are:

[0045] (31) According to the crack length extension range required by step (1), the minimum value of the crack length extension range is 2a 0min The initial crack length of the first test specimen is 2a. 1min , and the crack extension range of the specimen is set to (0.2~0.8)W1, then the final crack length of the first test specimen is 2a 1max ≤0.8W1, and according to the initial crack length 2a of the first test specimen 1min The calculated width of the first test specimen is W1=10a. 1min ;

[0046] (32) If the final crack length of the first test specimen is 2a 1max ≥0.2 W0, then set the initial crack length of the second test specimen to 2a 2min =2a 1max , W2= W0; if 2a 1max <0.2 W0, set the initial crack length of the second test specimen to 2a 2min =2a 1max , W2=10a 2min , and the crack extension range of the specimen is set to (0.2~0.8)W2, then the final crack length of the second test specimen is 2a 2max ≤0.8W2;

[0047] (33) Determine the final crack length 2a of the second test specimen 2max Is it more than 2a? 0max , if 2a 2max ≥ 2 years 0max , and at the same time , the test is completed; if 2a 2max ≥ 2 years 0max ,but , then take a new second test specimen or the first and second test specimens and increase the load to test until , the test is completed; if 2a 2max <2a 0max , then continue to add test samples to proceed to step (34);

[0048] (34) If 2a 2max <2a 0max, then continue to add the nth test specimen, and the method for determining the initial crack length, final crack length and specimen width of the added nth test specimen is the same as step (32) until the crack length extension range and the required crack length extension range of the test are completed. Range, i.e. 2a nmax ≥ 2 years 0max , and at the same time satisfy , the test is completed; if 2a nmax ≥ 2 years 0max ,but , then take a new test sample and increase the load of the nth and / or n-1th test samples during the test until , the test is completed;

[0049] In addition, the above-mentioned nth test sample for:

[0050] ;

[0051] Loading load of the nth test specimen for:

[0052] ;

[0053] The nth test sample can obtain for:

[0054] ;

[0055] in, is the coefficient, ranging from 0.8 to 1.0, , , n is the serial number, indicating the nth test sample, W n is the width of the nth test specimen, 2a nmin is the initial crack length of the nth test specimen, 2a nmax is the final crack length of the nth test specimen, and B is the specimen thickness.

[0056] In a preferred embodiment of the present invention, the above step (4) is specifically:

[0057] (41) Merge the segmented test data of n test samples to form -da / dN data pairs, the data pairs are divided into Sort in ascending order and get da / dN- curve;

[0058] (42) -da / dN data pairs are taken to the base 10 logarithm to form log( )-log(da / dN) data pairs, and according to log( )-log(da / dN) data were linearly fitted to obtain log( ), log(da / dN) related fitting formula, and transform the fitting formula into Paris formula, namely:

[0059] ;

[0060] Among them, da / dN is the crack extension per unit cycle, that is, the crack growth rate; C and m are constants related to the material. is the range of stress intensity factor.

[0061] Example 1

[0062] The fatigue crack growth rate test was carried out on a sample using the above test method. The test requirements were: sample width 160mm, thickness 5mm, center crack length extended from 4mm to 55mm, and it was required to be able to measure from Expand to The corresponding fatigue crack growth rate.

[0063] When the specimen width is 160 mm, according to the sample preparation requirements for the center crack tensile specimen in the flexibility test method, the initial crack length is at least 32 mm, which cannot meet the requirement that the center crack length is extended from 4 mm to 55 mm. Therefore, this embodiment uses the fatigue crack growth rate test method of the segmented test to conduct the test.

[0064] This embodiment is divided into three sections for testing. The three test samples are three samples of the same brand, specification, state, sampling position and sampling direction, and the sample thickness is 5mm as required by the test. According to the sample preparation requirements for the center crack tensile sample in the flexibility test method, the sample width W of each test sample is set n , initial crack length 2a nmin , Final crack length 2a nmax , Load and scope ~ As shown in Table 1 below.

[0065] Table 1 Test parameters of each segment test specimen

[0066]

[0067] According to the above segmentation, three test samples are taken and tested, and the test results are combined to obtain da / dN- Curves, such as Figure 2 shown.

[0068] And for the above Figure 2 The data is fitted and the fitting relationship is obtained as follows:

[0069] .

[0070] Under the sample preparation requirements of the flexibility test method for the center crack tensile specimen, this embodiment cannot use a specimen to test the fatigue crack growth rate when the center crack length extends from 4mm to 55mm. Therefore, this embodiment uses the segmented test method proposed by the present invention to perform segmented tests and obtain the test parameters of each segmented test specimen in Table 1, that is, not only the specimen width, initial crack length, final crack length and loading load of each segmented test specimen are determined, but also the fatigue crack growth rate that can be obtained in each segment is determined. Theoretical range. Tests were conducted according to the test parameters of each segment test specimen in Table 1, and the three segment test results were combined to obtain the following Figure 2 The da / dN shown Curve, from Figure 2 It can be seen that by testing according to the test parameters in Table 1, continuous da / dN- Curve, and the actual test results The range also meets the test requirements. That is, this embodiment obtains the test requirements of the central crack length from 4mm to 55mm through three test specimens, and from Expand to The corresponding fatigue crack growth rate.

[0071] Example 2

[0072] The test requirements set in this embodiment are: the length of the central crack is extended from 10mm to 55mm, The test range is ~ 31 , the sample thickness is 5mm.

[0073] According to the sample preparation requirements for the center crack tensile specimen of the flexibility test method, when the initial crack length is 10mm, the specimen width is ≤50mm. The specimen width is 50mm. Using one specimen cannot complete the fatigue crack growth rate test required by the test.

[0074] In this embodiment, the test is divided into two sections, and the two test samples are two samples of the same brand, specification, state, sampling position and sampling direction, and the thickness of the samples is 5mm as required by the test. The test parameters of each segment test sample are shown in Table 2 below.

[0075] Table 2 Test parameters of each segment test specimen

[0076]

[0077] This embodiment uses a 50mm sample for testing, and the crack length is extended from 10mm to 20mm, and a da / dN- curve; another 100mm specimen was used, and the crack length was extended from 20mm to 55mm to obtain another da / dN- The test results of the two samples are combined to obtain a complete da / dN- Curves, such as Figure 3 As shown, the above data are fitted to obtain the fitting relationship:

[0078] .

[0079] Under the sample preparation requirements of the flexibility test method for the center crack tensile specimen, this embodiment cannot use a specimen to test the fatigue crack growth rate when the center crack length extends from 10 mm to 55 mm. Therefore, this embodiment uses the segmented test method proposed by the present invention to perform segmented tests and obtain the test parameters of each segmented test specimen in Table 2, that is, not only the specimen width, initial crack length, final crack length and loading load of each segmented test specimen are determined, but also the fatigue crack growth rate that can be obtained in each segment is determined. Theoretical range. Tests were conducted according to the test parameters of each segment test specimen in Table 2, and the test results of the two segments were combined to obtain the following Figure 3 The da / dN shown Curve, from Figure 3 It can be seen that by testing according to the test parameters in Table 2, continuous da / dN- Curve, and the actual test results The range also meets the test requirements. That is, this embodiment obtains the test requirements of the central crack length from 10mm to 55mm through two test specimens, and From 12 Expand to The corresponding fatigue crack growth rate.

[0080] It should be noted that some parts not described in the present invention can be implemented by adopting or drawing on existing technologies.

[0081] 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 technicians in this technical field within the essential 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 for segmented testing, characterized in that: Includes steps: (1) Determine the fatigue crack growth rate test requirements of the specimen to be tested, including the specimen width W0 and the crack length extension range 2a 0min ~2a 0max , Measuring range ~ ; (2) According to the sample preparation requirements for the center crack tensile specimen in the flexibility test method, calculate the minimum initial crack length 2a0 of the center crack tensile specimen at the specimen width W0 in step (1), and analyze whether the initial crack length 2a0 is greater than the minimum crack length extension range 2a required by the test in step (1). 0min , if 2a0>2a 0min , then it is divided into n sections for testing according to the crack length extension range requirements; (3) When testing in sections, it is necessary to determine the number of sections and the specimen width, initial crack length, final crack length, loading load, and The measurement range, and the specimen width and initial crack length of each test specimen meet the sample preparation requirements for the center crack tensile specimen in the flexibility test method; (4) Carry out the test in the divided sections. After the section test is completed, merge the section test data to obtain the crack length extension range and da / dN under measuring range Curve, that is, fatigue crack growth rate curve.

2. A fatigue crack growth rate test method of segmented testing according to claim 1, characterized in that: In the step (2), when the width of the sample is W0, the initial crack length of the center crack tensile sample is 2a0=0.2W0.

3. The fatigue crack growth rate test method of the segmented test according to claim 1 is characterized in that: The specific steps of segmenting step (3) are as follows: (31) According to the crack length extension range required by step (1), the minimum value of the crack length extension range is 2a 0min The initial crack length of the first test specimen is 2a. 1min , and the crack extension range of the specimen is set to (0.2~0.8)W1, then the final crack length of the first test specimen is 2a 1max ≤0.8W1, and according to the initial crack length 2a of the first test specimen 1min The calculated width of the first test specimen is W1=10a. 1min ; (32) If the final crack length of the first test specimen is 2a 1max ≥0.2 W0, then set the initial crack length of the second test specimen to 2a 2min =2a 1max , W2= W0; if 2a 1max <0.2 W0, set the initial crack length of the second test specimen to 2a 2min =2a 1max , W2=10a 2min , and the crack extension range of the specimen is set to (0.2~0.8)W2, then the final crack length of the second test specimen is 2a 2max ≤0.8W2; (33) Determine the final crack length 2a of the second test specimen 2max Is it more than 2a? 0max , if 2a 2max ≥ 2 years 0max , the test is completed, otherwise proceed to step (34); (34) If 2a 2max <2a 0max , then continue to add the nth test specimen, and the method for determining the initial crack length, final crack length and specimen width of the added nth test specimen is the same as step (32) until the crack length extension range required by the test is completed, that is, 2a nmax ≥ 2 years 0max .

4. A fatigue crack growth rate test method of segmented testing according to claim 3, characterized in that: The loading load of each test specimen in step (3) and scope ~ The method to determine is: The nth test sample for: ； Loading load of the nth test specimen for: ； The nth test sample can obtain for: ； in, is the coefficient, ranging from 0.8 to 1.0, , , n is the serial number, indicating the nth test sample, W n is the width of the nth test specimen, 2a nmin is the initial crack length of the nth test specimen, 2a nmax is the final crack length of the nth test specimen, and B is the specimen thickness.

5. A fatigue crack growth rate test method of segmented testing according to claim 4, characterized in that: When judging whether the test is completed in step (3), in addition to meeting the crack length extension range requirements, it is also necessary to simultaneously judge Whether the measurement range meets the requirements, that is, when the last section of the test sample meets 2a nmax ≥ 2 years 0max When , the test is completed if , then take a new test sample and increase the load of the nth and / or n-1th test samples during the test until , the test is completed.

6. The fatigue crack growth rate test method of the segmented test according to claim 3 is characterized in that: The step (4) is specifically as follows: (41) Merge the segmented test data of n test samples to form -da / dN data pairs, the data pairs are divided into Sort in ascending order and get da / dN- curve; (42) -da / dN data pairs are taken to the base 10 logarithm to form log( )-log(da / dN) data pairs, and according to log( )-log(da / dN) data were linearly fitted to obtain log( ), log(da / dN) related fitting formula, and transform the fitting formula into Paris formula, namely: ； Among them, da / dN is the crack extension per unit cycle, that is, the crack growth rate; C and m are constants related to the material. is the range of stress intensity factor.

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