A method and system for quickly determining fatigue crack growth threshold

Abstract

The present invention proposes a method and system for rapidly determining a threshold value for fatigue crack growth, including setting up a measurement environment and prefabricating an initial fatigue crack; performing a stress load-keeping operation on a sample and observing a crack; determining a load reduction ratio and a next-level load; performing a next-level load reduction; and calculating a threshold value. The present invention controls the range of fatigue crack growth through plastic dimensions, controls the range of load reduction amplitudes through a confidence interval of the material fatigue crack growth threshold value of the metal sample to be tested, and the load reduction amplitude takes the maximum value within the range. The load reduction ratio obtained by the method of the present invention exceeds 10%, so the present invention can make the stress intensity factor under the current load quickly approach the threshold value without multiple load reductions, greatly reducing unnecessary load loading time, achieving rapid load reduction, reducing test time and test costs, and improving the accuracy of the average fatigue crack growth rate corresponding to the material fatigue crack threshold value to 10mm ‑8 / cycle .

Description

Technical Field

[0001] The invention relates to the technical field of fatigue crack extension of metal materials, and in particular to a method and system for determining a fatigue crack threshold value of a metal material. Background Art

[0002] The quality of diesel engine heavy parts (mainly steel) in high-speed rail, airplanes, ships, and vehicles directly affects the operating quality of diesel engines. The working environment of diesel engines is harsh. During the working process, key parts are prone to defects such as fatigue cracks. If they continue to be used blindly, it is very easy to cause danger, resulting in casualties and property losses. However, if they are directly scrapped blindly, it will cause a huge waste of resources. Therefore, accurately predicting the life of diesel engine heavy parts and making the correct judgment on whether they need to be replaced when fatigue cracks appear in key parts is crucial to ensure the reliability of high-speed rail, airplanes, and ships for ultra-long-term use. Usually, the fatigue crack threshold value of the metal material of the key parts is measured, and then the fatigue crack threshold value of the metal material of the key parts is used to predict the service life of the key parts and decide whether they need to be replaced.

[0003] According to the national standard GB / T 6398-2000 "Method for testing fatigue crack growth rate of metallic materials", the terms involved in the present invention are defined as follows:

[0004] (1) Stress intensity factor K : The physical quantity that reflects the strength of the elastic stress field at the fatigue crack tip is called the stress intensity factor;

[0005] (2) Stress intensity factor range : the algebraic difference between the maximum and minimum stress intensity factors in a cycle;

[0006] (3) Fatigue crack growth rate : The fatigue crack extension length is , then the fatigue crack extension per stress cycle is ( mm / times), called fatigue crack growth rate, which is usually expressed as differential under limit conditions. express;

[0007] (4) Fatigue crack extension threshold :The fatigue crack growth threshold refers to the critical value of fatigue crack growth of the material under fatigue load, which refers to the crack growth rate Approaching 0 The asymptote value of Defined as the fatigue crack growth rate equal to The corresponding The fatigue crack extension threshold is a very important parameter, which directly affects the service life and safety performance of the material;

[0008] (5) Load reduction method: also known as the K reduction method, a test method for determining the fatigue crack growth threshold of a material. By continuously reducing the load, the stress intensity factor is reduced to approach the fatigue crack growth threshold of the material. ;

[0009] (6) Load : The load applied to the metal material in the i-th level load reduction test, i=0,1,2..., where It refers to the load applied to the metal material when the initial fatigue crack is preformed;

[0010] (7) Load reduction range : refers to the value of the load reduction between two adjacent levels in the load reduction test. ;

[0011] (8) Load reduction ratio : It is the force reduction percentage, which refers to the ratio of the force reduction of two adjacent levels in the load reduction test. The calculation formula is as follows:

[0012] (1)

[0013] (9) Load history effect: The phenomenon of slowing down the fatigue crack growth rate in the low-load cycle after the tensile high load is called high-load hysteresis. High-load hysteresis usually occurs in the middle and late stages of fatigue crack growth, that is, after the fatigue crack has grown to a certain extent, the crack closure rate slows down during loading and unloading, showing obvious hysteresis. The application of high load can cause the subsequent low-load cycle to Decrease, or even stop the expansion of fatigue cracks;

[0014] (10) Lifespan value: N fp It is the predicted value of the life of key parts predicted by software. N ft Refers to the test value of the life of key parts obtained through actual testing. N f Refers to the actual life of the key parts. N fp and N ft Comparison of the two can verify N f accuracy.

[0015] In the fatigue crack research of metal materials, the fatigue crack growth threshold is called Fatigue crack growth threshold in English, which is referred to as the threshold value in this application. The measurement accuracy of the threshold value will directly affect the reliability of the fatigue life prediction of key parts. When the threshold value measurement accuracy is not high, if the threshold value is underestimated, the predicted life will be too conservative, resulting in unnecessary repairs or replacements, causing a waste of resources; if the threshold value is overestimated, it will cause crack growth in the parts during actual use and fail to take timely measures, causing safety risks. Therefore, the higher the threshold value measurement accuracy, the higher the working reliability of the parts. Especially for metal materials used in high-speed railways, airplanes, ships, vehicles, etc., which have very strict requirements for the prediction of service life, they have higher requirements for the accuracy of threshold value measurement. Therefore, a method that can accurately measure the threshold value is determined. The experimental method is urgent.

[0016] There are two main methods for determining the threshold value of materials: one is the internationally accepted ASTM E647 standard. K The first is the value reduction technology, which is referred to as the Unloading Low-Amplitude method in this application, abbreviated as ULA; the second is the threshold value improvement method based on the in-situ fatigue test method, which is referred to as the in-situ observation method in this application, but both of them currently have certain defects, as follows.

[0017] Unloading - Low-amplitude load reduction method ULA is the internationally accepted ASTM E647 standard K The value reduction technology has formed a unified threshold value determination standard in major countries in the world. The latest Chinese national standard is GB / T 6398-2017 "Method of fatigue crack growth in fatigue test of metallic materials" (referred to as Document 1 in this application), but technicians in this field are more accustomed to using the old version of GB / T 6398-2000 "Method of fatigue crack growth rate test of metallic materials" (referred to as Document 2 in this application) because the provisions of Document 2 are more detailed and specific. Document 2 points out: "When the force is graded, the crack growth increment should be Greater than the previous level The corresponding plastic zone size 4~6 times of the average crack growth rate When approaching , the K reduction test is over” (see Section 8.1 of Reference 2); “The force reduction rate of each level shall not exceed 10%, and the force reduction percentage R1 can be taken as 5%~10%” (see Section 8.1.1 of Reference 2).

[0018] Chinese authorized patent CN102679931B (see paragraphs 0026-0029 of the specification) points out the defects of the ULA method: after several cycles, the fatigue specimen is removed from the fatigue testing machine and placed under a metallographic microscope to measure its length. This method has the following defects: (1) When measuring the crack extension length, it is necessary to remove the load, disassemble the specimen, and measure the crack length under a microscope; after the measurement, the specimen needs to be reinstalled and reloaded. The entire operation process is cumbersome, time-consuming and labor-intensive, and may cause changes in the clamping and alignment parameters of the CT specimen, affecting the repeatability of the test results; (2) The removal of the specimen interrupts the effect of the load, which affects the subsequent fatigue crack extension rate. has a significant impact, resulting in the measured Deviate from the true value; (3) Rely on the previous level The number of cycles required for value estimation and the inability to observe the real-time crack extension length reduce the experimental efficiency; (4) The entire operation process requires a high level of operator experience.

[0019] Although GB / T 6398-2000 does not explicitly stipulate that the load reduction ratio (force reduction rate per stage) should remain unchanged, it is well known to those skilled in the art that in actual fatigue crack tests of metal materials, the load reduction ratio is kept unchanged unless the change of the load reduction ratio is specifically mentioned. Chinese invention patent CN102607948B (see paragraphs 0034, 0050, and 0066 of the specification) discloses a method for characterizing fatigue crack growth behavior, which adopts the method of "maintaining the graded load reduction percentage at 5%" in all three embodiments. There are also those who adjust the load reduction ratio during the test for different purposes. The Chinese invention patent CN113551965B (see paragraphs 0016, 0017, and 0023 of the specification) stipulates that the force value reduction in the first stage shall not exceed 10%, and the force value reduction in the second stage shall be reduced to no more than 5%, thereby making the method for determining the fatigue crack growth threshold value more reliable; Chinese patent CN102679931B (see paragraph 0054 of the specification) points out that "in order to ensure that the changes in the collected data are within the allowable range, the load reduction amplitude of each level shall not exceed 10%. When exist When the load reduction ratio is within the range, the load reduction amplitude can be appropriately reduced to ensure that there are enough fitting data points. It can be seen that the existing technology does not control the load reduction ratio by more than 10% in the test of fatigue crack threshold value determination of metal materials.

[0020] In summary, the existing methods for determining the fatigue crack threshold of metal materials have the following problems:

[0021] (1) Influenced by China's national standards, the existing technology has given a technical bias that the load reduction ratio must be set to no more than 10% in the load reduction test to ensure normal crack expansion. Such a load reduction ratio setting requires a lot of test time and high test costs;

[0022] (2) Due to cost considerations, the current determination of fatigue crack threshold value of metal materials can only measure the fatigue crack growth rate to So far, the life prediction of key components based on the measurement accuracy of the fatigue crack threshold value of such metal materials cannot meet the prediction requirements for the life of key components in actual engineering. Summary of the invention

[0023] against The existing technology in the measurement technology has the following bias: in the load reduction test, the load reduction ratio must be set to no more than 10% to ensure normal crack expansion. The present invention proposes a method for quickly determining the fatigue crack expansion threshold value, which overcomes the bias of the existing technology: the load reduction ratio obtained by the method of the present invention is more than 10%, and during the test, a judgment is made in time whether the load reduction is needed according to the fatigue crack expansion of the metal material, and based on this, it is judged whether the load reduction ratio needs to be reduced, so that the stress intensity factor under the current load quickly approaches the threshold value, thereby greatly shortening the test time and improving the accuracy of the fatigue crack expansion threshold value to The magnitude of the load is increased, thereby improving the accuracy of the life prediction of the key parts and meeting the needs of the project. The present invention is a method of loading high-amplitude, abbreviated as LHA.

[0024] In order to achieve the above technical effects, the present invention provides a method for quickly determining a fatigue crack growth threshold value, comprising:

[0025] S1: Perform the i-th level fatigue crack growth test on the metal sample to be tested; ;

[0026] S2: placing the metal sample to be tested under a high-power optical microscope for crack observation;

[0027] S3: Calculate the load reduction ratio required for the metal sample to be tested to perform the i+1 level fatigue crack growth test, and execute S1 to S2 according to the load reduction ratio; the load reduction ratio is greater than 10%;

[0028] S4: Calculate the fatigue crack growth rate of the i+1th level;

[0029] S5: If the i+1-th level fatigue crack growth rate meets the termination condition, the fatigue crack growth test is terminated and the process proceeds to S6; otherwise, the test is continued and the process proceeds to S1; the termination condition is that the i+1-th level fatigue crack growth rate ≤ ; c ≥8

[0030] S6: Calculating the fatigue crack growth threshold value of the metal sample to be tested according to the i+1th level fatigue crack growth rate.

[0031] Furthermore, obtaining the load reduction ratio required for the metal sample to be tested to perform the i+1 level fatigue crack growth test includes:

[0032] Calculate the fatigue crack extension value of the metal sample under test during the i+1 level fatigue crack extension test the scope of Related to the size of the plastic zone of the metal sample to be tested;

[0033] Calculate the load reduction amplitude for the i+1-level fatigue crack growth test the scope of The fatigue crack growth threshold of the metal sample to be tested The confidence interval of The scope of

[0034] Take the maximum value within the range and calculate the load reduction ratio required for the i+1 level fatigue crack growth test based on it. and load .

[0035] Furthermore, the fatigue crack extension value of the metal sample to be tested during the i+1 level fatigue crack extension test is calculated. The scope includes:

[0036] control , calculated based on this Range ,in , is the i-th level load in the i-th level fatigue crack growth test P i The plastic zone size of the metal sample to be tested is .

[0037] Furthermore, the load reduction amplitude of the i+1-th level fatigue crack growth test is calculated The scope includes:

[0038] Find out the material fatigue crack growth threshold value of the metal sample to be tested The confidence interval for ,exist Under the limitation of range, the stress intensity factor range is calculated satisfy Confidence interval ,remember The interval is .

[0039] Furthermore, the With the The relationship is as follows:

[0040]

[0041] In the formula, G is the component shape factor.

[0042] Furthermore, the calculation of the fatigue crack growth rate of the i+1th level includes:

[0043] In satisfying Under the premise of the range, the metal sample to be tested is subjected to the i+1 level load reduction test;

[0044] Record the fatigue crack length of the metal sample to be tested , calculate the fatigue crack extension value ;

[0045] Calculate the i+1th level , i+1th level The formula is as follows:

[0046]

[0047] in, are the crack lengths before and after extension at the current load level, respectively.

[0048] Furthermore, calculating the fatigue crack extension threshold value of the metal sample to be tested includes:

[0049] According to the fatigue crack growth rate of all i+1 levels and the range of stress intensity factors for all i+1 levels The experimental data were fitted by linear regression method. The equation of the line is calculated from this hour value, that is, The corresponding fatigue crack extension threshold value under the accuracy; c ≥8.

[0050] The present invention also provides a system for rapidly determining a fatigue crack growth threshold value, which can implement the above-mentioned method for rapidly determining a fatigue crack growth threshold value, and the system includes a load holding module, a crack observation module, and a calculation verification module;

[0051] The load-keeping module is used to ensure that the tension of the metal sample to be tested remains unchanged after being unloaded from the fatigue machine. When unloading from the fatigue machine, the support force provided by the stress conversion device replaces the tension in the fatigue machine, and after the crack observation is completed, the tension in the fatigue machine replaces the support force provided by the stress conversion device.

[0052] The crack observation module is used for clear observation of the crack propagation process. The stress conversion device with the metal sample to be tested is placed under a high-power optical microscope for crack observation to observe the crack length of the metal sample to be tested.

[0053] The calculation verification module is used to record and calculate various data during the test, including the calculation of the next level of load reduction ratio. and the next level of load ,calculate , calculate the fatigue crack growth threshold and predicting the life of the metal samples to be tested;

[0054] Among them, in determining the next level of load reduction ratio and the next level of load When the fatigue crack extension value of the metal sample to be tested is controlled by the size of the plastic zone of the metal sample to be tested, , calculated based on this range; check the material fatigue crack extension threshold value of the metal sample to be tested The confidence interval of Under the limitation of the range, calculate the satisfy Confidence interval ; Take the maximum value in the interval to calculate and .

[0055] The present invention also discloses a server, comprising a memory, a processor and a computer program stored in the memory and executable on the processor, wherein the server is characterized in that the processor implements the steps of the above method when executing the computer program.

[0056] The present invention also discloses a computer-readable storage medium, wherein the computer-readable storage medium stores a computer program, and is characterized in that: when the computer program is executed by a processor, the steps of the above method are implemented.

[0057] Compared with the prior art, the present invention has the following beneficial effects:

[0058] (1) The present invention overcomes the prejudice that a relatively low load reduction ratio (≤10%) must be set in the prior art load reduction test to ensure normal crack expansion, and proposes a new threshold value determination method. The present invention controls the range of fatigue crack expansion by plastic size, controls the range of load reduction amplitude by the confidence interval of the fatigue crack expansion threshold value of the material of the metal sample to be tested, and takes the maximum value within the range. The load reduction ratio obtained by the method of the present invention exceeds 10%, which can greatly reduce the measurement time.

[0059] (2) The present invention improves the load reduction ratio, breaking through the limitations of test time and cost. Corresponding accuracy Upgrade to Based on the determination accuracy of the fatigue crack threshold value of this metal material, the life of key parts is predicted, which can meet the prediction requirements for the life of key parts in actual engineering.

[0060] (3) The present invention improves the accuracy of life prediction of key components, which will significantly improve the fatigue analysis level of key components in my country, significantly reduce the test cost of key components, thereby reducing the cost of engineering projects, meeting actual engineering needs, and has extremely high engineering practical value and significance, and has made a significant contribution to the field of fatigue reliability analysis. BRIEF DESCRIPTION OF THE DRAWINGS

[0061] Figure 1 It is a load shedding flow chart of the LHA of the present invention.

[0062] Figure 2 In the embodiment, the ULA and LHA methods are Relationship diagram.

[0063] Figure 3 In the embodiment, the ULA and LHA methods are Relationship diagram.

[0064] Figure 4 Schematic diagram of crack observation under the ULA and LHA methods in the embodiment.

[0065] Figure 5 In the embodiment, the ULA and LHA methods are Relationship diagram.

[0066] Figure 6 The double logarithmic coordinate axis of the ULA and LHA methods in the whole load reduction process in the embodiment Relationship diagram.

[0067] Figure 7 is the near-threshold area under the double logarithmic coordinate axis of the LHA method in the embodiment Relationship diagram.

[0068] Figure 8 The near-threshold area under the double logarithmic coordinate axis of the ULA method in the embodiment Relationship diagram.

[0069] Fig. 9 Comparison between the predicted crankshaft life and the true life under four working conditions in the embodiment. DETAILED DESCRIPTION

[0070] The present invention will be further described below in conjunction with examples and accompanying drawings. It should be understood that the examples are only used to illustrate the present invention and are not intended to limit the scope of the present invention. Without departing from the spirit and scope of the present invention, variations and advantages that those skilled in the art can expect are all included in the present invention, and are protected by the appended claims and equivalents.

[0071] It is understood that the term "one" should be understood as "at least one" or "one or more", that is, in one embodiment, the number of an element can be one, while in another embodiment, the number of the element can be multiple, and the term "one" cannot be understood as a limitation on the number. In the present invention, except for the names and terms that have been clearly defined by the inventor, other names and terms are common names in the art.

[0072] In one embodiment of the present invention, Figure 1 As shown, a method for quickly determining a fatigue crack growth threshold is provided, the method comprising:

[0073] S1: Perform the i-th level fatigue crack growth test on the metal sample to be tested; ;

[0074] S2: placing the metal sample to be tested under a high-power optical microscope for crack observation;

[0075] S3: Calculate the load reduction ratio required for the metal sample to be tested to perform the i+1 level fatigue crack growth test, and execute S1 to S2 according to the load reduction ratio; the load reduction ratio is greater than 10%;

[0076] S4: Calculate the fatigue crack growth rate of the i+1th level;

[0077] S5: If the i+1-th level fatigue crack growth rate meets the termination condition, the fatigue crack growth test is terminated and the process proceeds to S6; otherwise, the test is continued and the process proceeds to S1; the termination condition is that the i+1-th level fatigue crack growth rate ≤ ; c ≥8;

[0078] S6: Calculating the fatigue crack growth threshold value of the metal sample to be tested according to the i+1th level fatigue crack growth rate.

[0079] In another embodiment of the present invention, it also includes setting up a measurement environment and prefabricating an initial fatigue crack.

[0080] The metal sample to be tested is placed in the exact center of the stress conversion device, and the fatigue machine is used to clamp the stress conversion device containing the metal sample to be tested, so as to complete the construction of the measurement environment and preform the initial fatigue crack; at this time, the stress conversion device does not apply a load to the metal sample to be tested, and the fatigue machine applies a load to the metal sample to be tested through the stress conversion device, so that fatigue cracks are generated in the metal sample to be tested;

[0081] For the i-th level of load reduction, the load currently applied to the metal sample to be tested is recorded as P i After the fatigue crack of the metal sample is extended by N cycles, the support force provided by the stress conversion device to the metal sample to be tested gradually replaces the tensile force in the fatigue machine; the stress conversion device with the metal sample is used to observe the crack under a high-power optical microscope, and the observed crack length is recorded as ; After the crack observation is completed, the fatigue machine is used to clamp the stress conversion device, and the tensile force in the fatigue machine gradually replaces the support force provided by the stress conversion device to the metal sample to be tested; i=1,2,3...; the initial fatigue crack length is recorded as , the initial load is denoted as P 0 .

[0082] In another embodiment of the present invention, the load reduction ratio of the i+1th level is determined and i+1th level load

[0083] The fatigue crack extension value Control it to be greater than 2~3 times the size of the plastic zone, and calculate it accordingly Range , check the measured material The confidence interval is , considering In the range of satisfy Confidence interval ,thereby The interval is [0, c 2 ], in order to Fall into this range as soon as possible, , thus calculating and the load at the i+1th load reduction .

[0084] In another embodiment of the present invention, the i+1th level of load shedding is performed, and the calculation include:

[0085] Perform the i+1 level load reduction on the metal sample to be tested. Under this premise, calculate the average If the termination condition is met, the test ends and the statistics data; if the termination condition is not met, the load will continue to be reduced; the termination condition refers to the average Less than or equal to ;in c ≥8.

[0086] In another embodiment of the present invention, performing an i-level fatigue crack growth test on a metal sample to be tested comprises:

[0087] (1) For the i-th level of load reduction, at load P i In the next cycle, the fatigue crack of the metal sample to be tested is extended by N cycles. The current load applied to the metal sample is recorded as P i When the fatigue crack of the metal sample to be tested is extended, the stress conversion device does not apply tension to the metal sample to be tested. The fatigue machine indirectly applies tension to the metal sample to be tested through the stress conversion device, causing fatigue cracks to be generated in the metal sample to be tested, and the fatigue crack of the metal sample to be tested is extended for N cycles.

[0088] (2) The support force provided by the stress conversion device to the metal sample to be tested gradually replaces the tension in the fatigue machine. The tension of the fatigue machine on the stress conversion device is unloaded at a uniform speed F T,i ,pull F T,i It is also the tensile force indirectly applied by the fatigue machine to the metal sample to be tested. While unloading the tensile force of the fatigue machine on the stress conversion device, the stress conversion device is gradually driven to provide support force to the metal sample to be tested. F N,i ; Constantly detect the strain value of the metal sample to be tested along the tensile direction, and achieve it by maintaining the constant strain value F N,i right F T,i An exact replacement for F N,i and F T,i The corresponding load is P i ; Maintain the support force of the stress conversion device on the metal sample to be tested, and remove the stress conversion device with the metal sample to be tested from the fatigue machine. σ The crack remains fully open and can be clearly observed.

[0089] (3) Observe the cracks of the metal sample to be tested. Place the stress conversion device with the metal sample to be tested flat under a high-power optical microscope for observation, and record the observed crack length of the metal sample to be tested as , calculate the fatigue crack extension value .

[0090] (4) Re-fix the stress conversion device to the chuck of the fatigue machine. After the crack observation is completed, the stress conversion device is unloaded at a uniform speed to provide support to the metal sample to be tested. F N,i , and at the same time drive the fatigue machine to pull the stress conversion device (indirectly the metal sample to be tested) F T,i , constantly detect the strain value of the metal sample to be tested along the direction of tension, and achieve it by maintaining the constant strain value F T,i right F N,i An accurate replacement for .

[0091] In yet another embodiment of the present invention, calculating the load reduction ratio required for the i+1-th level fatigue crack growth test includes:

[0092] The technical concept of this part is: based on the existing i-th level load reduction that has actually occurred, calculate the load that needs to be applied to the metal sample to be tested in the i+1th level (i+1) load reduction Then we first control the i+1 level fatigue crack extension value based on the theory. Theoretical value range; then find out the fatigue crack extension threshold value of the material of the metal sample to be tested Confidence interval, in Within the theoretical value range, calculate the value that can make The load reduction range that meets the confidence interval scope; in order to Fall into this confidence interval as quickly as possible. Take the maximum value within the range to determine The value of can be calculated according to the formula mentioned above to calculate the load applied to the metal sample to be tested in the i+1th level load reduction. And the load reduction ratio of the i+1th level Theoretical value.

[0093] (1) Calculate the fatigue crack extension value In the process of load reduction, in order to avoid the influence of load history effect and ensure the normal expansion of cracks in the metal sample to be tested, ,control , calculated based on this The theoretical value range is , , is the i-th load shedding P i The plastic zone size under the load reduction is a known number, The calculation formula is as follows:

[0094]

[0095] In the formula, is the material yield strength; for plane stress state, , for the plane strain state, ; The maximum load of this level The corresponding stress intensity factor K is is a known quantity. Note that It is a theoretical value calculated based on theoretical calculations, not a value calculated based on actual observed fatigue cracks.

[0096] (2) Calculate In order to reduce the load Fast Approach , check the fatigue crack growth threshold value of the material of the metal sample to be tested The confidence interval is ,exist Under the premise of theoretical value range, calculate the satisfy Confidence interval ,remember The interval is .

[0097] (3) Calculate the load reduction ratio and To make Fall into the confidence interval as quickly as possible, , based on which the load at the i+1th load reduction is calculated , The theoretical value and , The calculation formula is as follows:

[0098]

[0099] in, , B is the sample thickness, and W is the sample width.

[0100] The initial fatigue crack length and initial load are denoted as and ; If it is the first load reduction, that is When the national standard method is used to calculate , that is, the crack growth rate under the initial load should be Above.

[0101] It should be noted that the stress intensity factor range is and load reduction There are also the following relationships:

[0102]

[0103] In the formula, G is the component shape factor, is the fatigue crack growth threshold Confidence interval of .

[0104] because , ,in, is the material yield strength; for plane stress state, , for the plane strain state, ; for The maximum stress intensity factor Calculate scope After that, Minimum value of Substitution You can get and The maximum value of the load is calculated based on this value. . Research shows that metal Usually in the micron range, much smaller than 0.25 mm, so Greater than 10%.

[0105] pass Knowable , and the load reduction ratio , so the load reduction ratio ,because Greater than 10%, so the load reduction ratio Also greater than 10%.

[0106] In another embodiment of the present invention, the next level of load reduction is performed and the calculation include:

[0107] (1) The metal sample to be tested is subjected to the i+1 level load reduction, and the fatigue machine exerts a tensile force on the metal sample to be tested. for The corresponding pulling force;

[0108] (2) Record the crack image and fatigue crack length of the metal sample to be tested , calculate the fatigue crack extension value The actual value of

[0109] (3) Calculate the level , grasp the crack propagation situation, , , , They are before and after the current load level expansion The value of

[0110] (4) Lower expansion After that, the i+1th level load reduction can be carried out. , in satisfying Under this premise, calculate the average After that, the cycle of this level can be stopped and the average value of this level can be calculated. and Corresponding For comparison, if the average Still greater than , then continue to reduce the load; if the average Less than or equal to , the test ends and the statistics data; c ≥8.

[0111] In yet another embodiment of the present invention, the fatigue crack growth threshold is calculated Including: According to the various levels and The experimental data were fitted by linear regression method. The equation of the line is calculated from this hour value, that is, The corresponding accuracy ;based on Combined with software to measure the life of materials N f Make predictions; c ≥8.

[0112] In order to explore the influence of threshold value determination accuracy on the prediction accuracy of key parts life and verify the accuracy of the load-holding-high amplitude load reduction method LHA of the present invention, this embodiment takes 42CrMo steel diesel engine crankshaft as an example to carry out experimental research, and its main mechanical performance indicators are as follows: yield strength σ ys The tensile strength is 940MPa σ b 1080MPa, elongation after fracture σ h14%. 16 MT specimens were made from the specimens, combined with the size of the stress conversion device. In order to eliminate the influence of the material structure on the crack extension factor near the threshold area, this paper selected two specimens with the closest phase structure morphology and distribution, numbered them as specimen 1 and specimen 2, and respectively used the prior art unloading-low amplitude load reduction method ULA and the present invention's load holding-high amplitude load reduction method LHA to test the two specimens. Table 1 and Table 2 are the basic data of 42CrMo steel.

[0113]

[0114]

[0115] Based on ULA and LHA The tests were conducted using an SDS100 fatigue machine and the load was determined using a stress ratio R =0.1, frequency h =10Hz sine wave, the test environment is room temperature atmosphere. Among them, ULA uses fatigue machine chuck to directly fix the two ends of sample 1; while LHA uses stress conversion device to avoid unloading-induced crack closure. Before the test, the two ends of the stress conversion device are fixed with fatigue machine chuck, and then the sample is fixed to the center of the stress conversion device by customized bolts. During the test, the cyclic load will be transferred from the fatigue machine clamp to sample 2 through the stress conversion device.

[0116] Based on ULA, a threshold value test was carried out on sample 1. Since the experimental time of this method is too long, in order to reduce the experimental and time costs, the threshold value was measured based on ULA. The test is stopped when the corresponding rate Recorded as Based on LHA, the threshold value test was carried out on sample 2. Since this method reduces the cost limit compared with ULA, in order to improve The accuracy of measurement and life prediction is The test will be stopped when the LHA The corresponding Recorded as , The corresponding Recorded as .

[0117] Prediction of life of key parts N fp The accuracy reflects Therefore, this embodiment first obtains the measurement accuracy in two ways. The crankshaft life was studied under four common working conditions by combining Abaqus and Franc3d software. Nf Prediction simulation study, the simulation configuration is shown in Table 3. In order to improve the reliability of simulation, Paris model, based on which the life prediction values ​​of key parts under two methods are obtained N fp The second step is to verify N fp Accuracy, fatigue tests were carried out on 10 crankshafts under the same 4 common working conditions as the simulation, and the crankshaft test value N under each working condition was calculated. ft The mean value analysis is used to reduce the error. N fp The prediction error analysis is used to reversely verify the two methods. The measurement accuracy is then used to evaluate the effectiveness of LHA.

[0118]

[0119] The following provides the key results of fatigue crack testing under ULA and LHA, which respectively explore the load reduction configuration, crack growth behavior, The result is a difference; and through the crankshaft N f Prediction simulation and experimental verification The accuracy of the results was evaluated to assess the reliability of the LHA.

[0120]

[0121] Figure 2 is the load P and fatigue crack length of each level of load reduction of ULA and LHA in this embodiment From the corresponding graph, it can be seen that the load reduction amplitude of the LHA of the present invention applied to the metal sample to be tested is significantly higher than that of the ULA. achieve , while the existing ULA method requires 9 load shedding To achieve The LHA method of the present invention has been subjected to five load sheddings. You can achieve , which cannot be achieved by existing technology ULA measurement accuracy.

[0122] By further comparison, Table 4 shows the data of the load reduction ratios at each level during the load reduction process. It can be seen from the data that the load reduction ratios at each level of the ULA method in the prior art are all less than 10%, while the load reduction ratios at each level of the LHA method of the present invention are all more than 10%, and as the load reduction proceeds, the load reduction ratios gradually decrease. This is because the fatigue cracks of the metal sample to be tested are getting longer and longer, the crack growth rate gradually approaches 0, and the load applied to the metal sample to be tested is gradually reduced. It is getting closer and closer to the threshold value, so the load reduction ratio needs to be reduced accordingly.

[0123] Comparison of test time. The test load reduction configuration directly determines the test time. As mentioned above, LHA can reduce the number of tests by increasing the load reduction ratio. n (Corresponding to the experimental time), since the load reduction ratio of the present invention is 10% higher than that of the prior art, the present invention shortens the experimental time. Figure 3 For ULA and LHA The corresponding graph with the experimental time t shows that Same measurement The time used by the LHA of the present invention is t LHA (49h) is approximately the time taken by the existing ULA technology t ULA The experimental time of the present invention is much shorter than that of the prior art, which breaks through the limitation of experimental cost and can also Accuracy increased to Magnitude, Measured When the LHA of the present invention is used for 72 hours, although it is higher than of t LHA (49h), but still far below of t ULA (336h), proving that the method of the present invention not only greatly reduces the test time / test cost, but also improves the measurement accuracy by an order of magnitude.

[0124] However, the improvement The accuracy needs to be further reduced, which means Δσ eff The value will further decrease from the original low level, which will lead to P 5 The crack propagation of the lower level is more susceptible to the load history effect than the first four levels, and takes longer time. As shown in Table 4, in order to prevent the crack from stopping prematurely, LHA repeatedly calculated and tested R s4 Reduced to 20%; at the same time, in order to shorten Δt 5 , LHA to P 5 Down The minute changes in the Figure 3 As shown, LHA successfully The accuracy is improved by an order of magnitude, and now t LHA (72h) still better than Measured to of t ULA In summary, LHA can significantly improve the test efficiency and is suitable for conducting Down Determination research.

[0125] Comparison of fatigue crack growth paths. Accurate determination is crucial, as Figure 4 As shown, to reduce the load process N / N total As the horizontal axis, the crack observation of ULA and LHA at different stages is intuitively compared. It can be found that the crack observation of the two is not the same. Although the crack under ULA can be clearly observed at the beginning of load reduction, in the process of continued load reduction, a thinner and blurred part appears in the front part, which makes it difficult to determine whether it is a crack; on the contrary, the crack under LHA is fully open with obvious boundaries, and its entire evolution can basically be clearly monitored. This is because the unloading of ULA triggers the crack closure effect, resulting in the contact between the upper and lower sides of the crack and the partial closure of the crack tip; while LHA is subjected to stress σ Maintain σ c1 Therefore, for the sake of caution, only the crack length that can be clearly observed is recorded as , and use it for subsequent Calculating.

[0126] In order to compare the degree of crack closure, the unclear and difficult to observe length is recorded as , by comparison The length of the crack shows that the degree of crack closure of ULA is different at different stages. This is due to the stress on the sample at the initial stage of load reduction. σ Higher, the crack closure effect is weaker, and the crack is still in an open state. ; and with σ As the load decreases, the crack closure effect gradually increases, and the cracks begin to close. When the load is reduced to the later stage, the crack closure effect is significant, resulting in The length increases dramatically, making it difficult to ensure crack observation accuracy. N / N total=90%, the LHA crack tip showed a slight closure phenomenon, which was caused by environmental factors.

[0127]

[0128] and Comparison of situations. Figure 5 Shows Relationship diagram, Figure 6 It is the whole process of load shedding Relationship diagram, Figure 7 It is the near-threshold area under the double logarithmic coordinate axis of the LHA method Relationship diagram, Figure 8 It is the near-threshold area under the double logarithmic coordinate axis of the ULA method Relationship diagram, Figure 7 and Figure 8 The corresponding fitting formulas are marked in the figures. is a material property that is independent of the crack size and sample geometry. To further improve the reliability of verification, as shown in Table 5, the 42CrMo steel measured by different methods is The test results were compared. Fig. 9 The predicted values ​​of crankshaft life under four working conditions are compared with the true values ​​of life. It can be clearly seen that compared with the LHA of the present invention, the crack growth rate curve of ULA is obviously shifted to the right as a whole. The main reason for this phenomenon is that the crack closure leads to The observation length is short, which makes The calculated result is too small; secondly, the crack closure effect will also lead to Δσ eff Reduce and N The consumption increases, which will aggravate the deviation phenomenon. Further observation shows that as the load decreases, the degree of curve deviation gradually increases, which is caused by the gradual strengthening of the crack closure effect. It is also worth noting that yes ( c ≥8) K Therefore, as shown in Table 5, the ULA shift to the right will also cause it to Measured at Above LHA , the difference can reach 15.69%; in addition, under the same LHA, Corresponding Also higher than , the difference can be as high as 21.21%.

[0129] Accuracy comparison. The change of value has an impact on the life prediction value N of the crankshaft, a key component fpThe value has a significant impact, so this embodiment is based on 3 different Actual crankshaft life N f The predicted simulation and the experimental value of crankshaft life N ft For comparison, verify the LHA measurement accuracy. Fig. 9 By drawing error bands, we can visually compare the different N fp Prediction effect, obviously, LHA (10 -8 )of N fp The prediction effect is the best. It proves that LHA can avoid unloading-induced crack closure and improve Accuracy and effectively improve The measurement accuracy.

[0130]

[0131] The cost advantage of this invention is compared with the existing method. LHA can accurately control the load at each level. Therefore, there is no need to limit its value like ULA. P i+1 Down More than 2~3 times This can effectively avoid the influence of load history effect, thus greatly shortening Δt. a To accurately calculate the level Δ K , so check Δ K th The confidence interval is based on the premise of avoiding the influence of load history effect, so that Δ K The purpose is to quickly fall into this range and design the maximum load reduction amplitude ΔP and R s , reduce the number of load shedding n , LHA can make the initial load P 0 Rapidly reduce to Δ K th Corresponding P. Therefore, compared with the national standard unloading-low amplitude load reduction method ULA, the LHA method of the present invention avoids the crack closure effect induced by unloading, eliminates the error caused by it, and thus significantly shortens the test time, thereby greatly reducing the cost, and accordingly the threshold value determination accuracy can be improved by an order of magnitude. Compared with the in-situ observation method based on the in-situ fatigue test method, the test equipment and test cost of the LHA method of the present invention are extremely low; there is no restriction on various test loading conditions, and it can be applied to various test scenarios, which is more in line with the needs of actual working conditions. Table 6 is a comparison of the test and time costs of the present invention and the above two different methods, all based on the rate at The measurement cost is calculated based on the threshold value of the test. The total cost of the existing ULA is 48,000 yuan, while the total cost of the LHA of the present invention is 57,600 yuan, which is less than 1 / 8 of the cost of ULA. The method of the present invention significantly shortens the test time and thus greatly reduces the total cost.

[0132] The present invention overcomes the prejudice that a lower load reduction ratio must be set in the prior art load reduction test to ensure normal crack expansion, and proposes a new threshold value test method, using the fatigue crack expansion threshold value Δ K th The confidence interval of the load reduction amplitude range of the level is calculated, and the load reduction amplitude is taken as the larger value within the load reduction amplitude range, and the load reduction ratio of the level is calculated on this basis. The load reduction ratio of each level obtained by this method exceeds 10%. During the test, a judgment is made in time whether the load reduction is needed according to the fatigue crack propagation of the metal material, so that the stress intensity factor under the current load quickly approaches the threshold value, and there is no need to reduce the load multiple times, which greatly reduces unnecessary load loading time, realizes rapid load reduction, and reduces test time and test cost. The present invention improves the load reduction ratio, breaks through the limitations of test time and cost, and can be used to K th Corresponding accuracy Upgrade to Based on the determination accuracy of the fatigue crack threshold value of this metal material, the life of key parts is predicted, which can meet the prediction requirements for the life of key parts in actual engineering.

[0133] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit the same. Although the present invention has been described in detail with reference to the aforementioned embodiments, a person skilled in the art should understand that the technical solutions described in the aforementioned embodiments may still be modified, or some of the technical features may be replaced by equivalents. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the technical solutions of the embodiments of the present invention.

Claims

1. A method for rapidly determining a fatigue crack growth threshold, characterized in that: The method comprises: S1: Perform the i-th level fatigue crack growth test on the metal sample to be tested; i=0, 1, 2, 3...; S2: placing the metal sample to be tested under a high-power optical microscope for crack observation; S3: Calculate the load reduction ratio required for the metal sample to be tested to perform the i+1 level fatigue crack growth test, and execute S1 to S2 according to the load reduction ratio; the load reduction ratio is greater than 10%; wherein obtaining the load reduction ratio required for the metal sample to be tested to perform the i+1 level fatigue crack growth test includes: Calculate the fatigue crack extension value Δa of the metal sample to be tested during the i+1 level fatigue crack extension test i+1 The range of Δa i+1 Related to the size of the plastic zone of the metal sample to be tested; Calculate the load reduction amplitude ΔP of the i+1-level fatigue crack growth test i+1 The range of ΔP i+1 The fatigue crack growth threshold value ΔK of the material of the metal sample to be tested th The confidence interval of Δa i+1 The scope of ΔP i+1 Take the maximum value within the range and calculate the load reduction ratio R required for the i+1 level fatigue crack growth test based on it. s,i+1 and load P i+1 ; S4: Calculate the fatigue crack growth rate of the i+1th level; S5: If the i+1-th level fatigue crack growth rate meets the termination condition, the fatigue crack growth test is terminated and the process proceeds to S6; otherwise, the test is continued and the process proceeds to S1; the termination condition is that the i+1-th level fatigue crack growth rate is ≤10 -c mm / cycle; said c≥8; S6: Calculating the fatigue crack growth threshold value of the metal sample to be tested according to the i+1th level fatigue crack growth rate.

2. The method according to claim 1, characterized in that Calculate the fatigue crack extension value Δa of the metal sample to be tested during the i+1 level fatigue crack extension test i+1 The scope includes: Control Δa i+1 >(2~3)r y_i , based on which Δa is calculated i+1 The range is (c1, +∞), where c1 = (2 to 3)r y_i , r y_i is the i-th level load P in the i-th level fatigue crack growth test i The plastic zone size r of the metal sample to be tested under y .

3. The method according to claim 1, characterized in that Calculate the load reduction amplitude ΔP of the i+1-level fatigue crack growth test i+1 The scope includes: Find out the material fatigue crack extension threshold value ΔK of the metal sample to be tested th The confidence interval is [x,y], in Δa i+1 Under the limitation of the range, the stress intensity factor range ΔK is calculated. i+1 Satisfy ΔK th Confidence interval for ΔP i+1 , denoted by ΔP i+1 The interval is [0,c2].

4. The method according to claim 3, characterized in that The ΔK i+1 With the ΔP i+1 The relationship is as follows: Where G is the component shape factor.

5. The method according to claim 1, characterized in that The calculation of the fatigue crack growth rate at level i+1 includes: In satisfying Δa i+1 Under the premise of the range, the metal sample to be tested is subjected to the i+1 level load reduction test; Record the fatigue crack length a of the metal sample to be tested i+1 , calculate the fatigue crack extension value Δa i+1 ; Calculate the da / dN of the i+1th level. The formula for the da / dN of the i+1th level is as follows: Among them, a i 、a i+1 are the crack lengths before and after extension at the current load level, respectively.

6. The method according to claim 1, characterized in that Calculating the fatigue crack growth threshold value of the metal sample to be tested includes: According to the da / dN of all i+1-level fatigue crack growth rates and the range of stress intensity factors ΔK of all i+1-level i+1 The test data was fitted with the da / dN-ΔK linear equation by linear regression method, and da / dN=10 was calculated. -c The ΔK value at mm / cycle is the fatigue crack extension threshold corresponding to the da / dN accuracy; and c≥8.

7. A system for rapidly determining fatigue crack growth threshold, characterized in that: The method according to any one of claims 1 to 6 can be implemented, wherein the system comprises a load holding module, a crack observation module and a calculation verification module; The load-keeping module is used to ensure that the tension of the metal sample to be tested remains unchanged after being unloaded from the fatigue machine, and the support force provided by the stress conversion device is used to replace the tension in the fatigue machine when it is unloaded from the fatigue machine, and the tension in the fatigue machine replaces the support force provided by the stress conversion device after the crack observation is completed; The crack observation module is used for clear observation of the crack propagation process. The stress conversion device with the metal sample to be tested is placed under a high-power optical microscope to observe the crack and observe the crack length of the metal sample to be tested. The calculation verification module is used to record and calculate various data during the test, including calculating the next level of load reduction ratio R s,i+1 and the next level load P i+1 , calculate da / dN, calculate fatigue crack growth threshold ΔK th And predict the life of the metal sample to be tested; Among them, in determining the R s,i+1 and the P i+1 When the fatigue crack extension value Δa of the metal sample to be tested is calculated by the plastic zone size of the metal sample to be tested i+1 In the range of Δa i+1 The range and the fatigue crack extension threshold value ΔK of the material of the metal sample to be tested th Under the common limitation of the confidence interval, the load reduction amplitude ΔP is calculated i+1 Range; ΔP i+1 Take the maximum value within the interval to calculate the R s,i+1 and the P i+1 .

8. A server comprising a memory, a processor, and a computer program stored in the memory and executable on the processor, characterized in that: When the processor executes the computer program, the method according to any one of claims 1 to 6 is implemented.

9. A computer-readable storage medium storing a computer program, characterized in that: When the computer program is executed by a processor, the method according to any one of claims 1 to 6 is implemented.

Images