CN117935997B - Dynamic measurement method for metal material fatigue crack tip plastic region - Google Patents
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Abstract
The invention provides a dynamic measurement method of a metal material fatigue crack tip plastic region, which comprises the following steps: s1: 1 piece in a group of samples is selected for da/dN test, and a mathematical relationship between r 0 and V is established; s2: a calculation program is compiled, and the change of the r 0 value is dynamically observed in real time according to the V value; s3: continuously carrying out da/dN tests on other samples, continuously monitoring the opening displacement variation V of the crack mouth of the sample in the test process, starting the program programmed in the step S2, and stopping the test when the ratio of the size of the plastic region to the crack length reaches a preset value; s4: according to the obtained test data, the effective equation of da/dN and delta K is obtained according to the Paris formula fitting form. The invention has simple operation, avoids carrying out a large amount of data calculation and evaluation after the test is finished, saves test cost and test time, improves test efficiency and provides safe support for engineering structural design.
Description
Technical Field
The invention relates to the technical field of metal material fatigue test methods, in particular to a dynamic measurement method for a metal material fatigue crack tip plastic region.
Background
In the design of damage tolerance of engineering structures, expressions of fatigue crack propagation rate da/dN and crack tip stress intensity factor range DeltaK of metal materials are often used. According to the existing test standards of GB/T6398-2017 fatigue crack propagation method for fatigue test of metal materials, ASTME647-2015 standard test method for fatigue crack propagation rate measurement and the like, series (da/dN, delta K) data can be obtained, and expressions of da/dN and delta K are usually obtained by fitting by using a Paris formula.
As a driving parameter for fatigue crack propagation, Δk is a physical quantity characterizing the stress field strength of the crack tip, and is suitable for describing the mechanical state near the crack tip that satisfies the line elasticity or small-range yield. By definition of small range yield, it is stated that the size of the plastic region of the crack tip is not more than 10% compared to the crack length, i.e. the plastic region of the crack tip cannot be excessively large throughout the fatigue crack propagation. However, in fact, in the fatigue crack growth rate da/dN test, as the crack length increases, the plastic region of the crack tip also increases so much that in the late stage of crack growth, the crack tip has generally not met the small range yield condition, which is not illustrated in the above-mentioned criteria.
And normally continuing the test until the test is finished, and fitting the obtained test data to obtain a Paris formula which brings safety risks when the engineering design is applied. The common method is that all delta K data obtained through the test are subjected to power law relation fitting directly according to a Paris formula form without screening treatment; or even if the delta K data are calculated one by one, the delta K data which do not meet the line elasticity or yield in a small range are screened out through a large amount of calculation after all the sample tests are finished. There is no mention in the literature or patent published in the prior art of the relevant preferred treatment methods.
Disclosure of Invention
In view of the above, the present invention aims to provide a dynamic measurement method for a plastic region at a fatigue crack tip of a metal material, so as to solve the problem that the expression of da/dN and Δk obtained by fitting a Paris formula in the prior art brings about a safety risk during engineering design application.
In order to achieve the above purpose, the technical scheme of the invention is realized as follows:
a dynamic measurement method for a metal material fatigue crack tip plastic region comprises the following steps:
S1: 1 piece in a group of samples is selected for fatigue crack propagation rate da/dN test, and the relation between the size r 0 of a crack tip plastic region and the opening displacement variation V of a crack nozzle of the sample is established;
S2: a calculation program is compiled, and the change of the r 0 value is dynamically observed in real time according to the macroscopic measurable V value which can be displayed in real time;
S3: continuously carrying out da/dN tests on other samples, continuously monitoring the opening displacement variation V of the crack mouth of the sample in the test process, simultaneously starting the program compiled in the step S2, dynamically monitoring the variation of the r 0 value in real time, and stopping the test when the ratio of the size of the plastic region to the crack length reaches a preset value Q;
S4: according to the obtained test data, the effective equation of da/dN and delta K is obtained according to the Paris formula fitting form.
Further, in step S1, the steps of:
S11: 1 piece in a group of samples is selected, test parameters and a data acquisition mode are set, and da/dN tests are carried out until the samples are broken, wherein the test parameters comprise load amplitude, test frequency, stress ratio, test environment and the like;
S12: calculating the size r 0 of the plastic region of the crack tip under the corresponding cycle according to the obtained test data;
S13: calculating the ratio r 0/a of the series r 0 to the crack length a;
S14: fitting the mathematical relationship between V and r 0/a according to the r 0/a values calculated in the previous steps under different cycles and the variable V value of the opening displacement of the crack mouth acquired under the corresponding cycles:
Wherein v=v max-Vmin,Vmax is the maximum displacement of the crack tip, V min is the minimum displacement of the crack tip, and V max、Vmin is acquired in the test;
S15: determining a critical value V c of the opening displacement variation of the crack mouth of the sample;
S16: based on the r 0 value calculated in step S12 and the V value calculated in step S14, a mathematical relationship of r 0 and V is established.
Further, in the fatigue crack growth rate da/dN test, the specimen takes the form of a three-point bending SEB, and in step S12, the plastic region size of the crack tip is calculated according to formula (1) while the specimen crack tip is in a plane stress state:
（1）
Wherein, r 0 is the size of the plastic area of the crack tip, and mm; k, crack tip stress intensity factor, MPa.m 0.5;Rp0.2, yield strength of material, MPa;
for the SEB samples, the expression of K is:
（2）
Wherein Y is a dimensionless shape factor, which is related to the shape of the sample; sigma-applied stress, MPa; a, crack length, mm;
The shape factor Y of the SEB sample is:
（3）
Wherein a is crack length, mm; w is the width of the sample, mm; beta=a/W is normalized crack length, dimensionless.
Further, in step S13, the fatigue crack lengths at different cycles are measured according to the compliance method, and the normalized crack length is expressed as:
（5）
Where a/W is normalized crack length, C 0、C1、C2、C3、C4、C5 is compliance coefficient, U x is dimensionless compliance, U x is related to elastic modulus, sample size, test load of the test material, expressed as follows:
（6）
wherein B is the thickness of the sample and mm; v r is the opening displacement of the crack mouth of the sample, and mm; e is the elastic modulus of the material and MPa; p is the test load, N.
Further, in step S15, according to the fitted mathematical relationship of r 0/a and V, when r 0/a is equal to a preset value Q, a critical value V c of the opening displacement amount at the crack mouth of the corresponding sample is calculated.
Further, the value range of the preset value Q is Q less than or equal to 10 percent.
Further, the value range of the preset value Q is more than or equal to 5% and less than or equal to 10%.
Further, in step S16, a mathematical relationship of r 0 and V is fitted using a least squares method, based on obtaining the series of r 0 values and V values for different cycles.
Compared with the prior art, the dynamic measurement method for the metal material fatigue crack tip plastic region has the following advantages:
(1) According to the dynamic measurement method for the metal material fatigue crack tip plastic region, the relation between the physical quantity and the crack tip plastic region size can be displayed by utilizing the macroscopic measurable quantity of the crack tip opening displacement variation which is established in advance, the test is stopped when the crack tip plastic region size does not meet the small-range yield condition, the data condition of Paris formula fitting is ensured, so that safe support is provided for engineering structural design, meanwhile, the test cost and test time are saved, and the test efficiency is improved.
(2) According to the dynamic measurement method for the metal material fatigue crack tip plastic region, a fatigue crack expansion rate da/dN test is carried out by selecting one sample at random, related test data are collected in the test to establish a mathematical relationship between r 0 and V, and then other samples can refer to the relationship between r 0 and V of a first sample to compile a calculation program; when the da/dN test software runs, a calculation program is run at the same time, so that real-time dynamic measurement and display of r 0 can be realized, the test is terminated when r 0/a reaches a preset value Q, related data are saved, data of the crack tip meeting the line elasticity and the small-range yield mechanical state are processed, the Paris formula form is used for fitting equations of da/dN and delta K meeting the requirements, the operation is simple, a large amount of data calculation and evaluation are avoided after the test is finished, the test cost and test time are saved, the test efficiency is improved, and a safe support is provided for engineering structural design.
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The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the invention. In the drawings:
FIG. 1 is a graph showing the mathematical relationship between samples V and r 0/a in example 1 of the present invention;
FIG. 2 is a graph showing the mathematical relationship between samples r 0 and V in example 1 of the present invention;
FIG. 3 is a graph showing the mathematical relationship between samples V and r 0/a in example 2 according to the present invention;
FIG. 4 is a graph showing the mathematical relationship between samples r 0 and V in example 2 of the present invention;
FIG. 5 is a graph showing the mathematical relationship between samples V and r 0/a in example 3 according to the present invention;
FIG. 6 is a graph showing the mathematical relationship between samples r 0 and V in example 3 of the present invention.
Detailed Description
In order to facilitate understanding of the technical means, objects and effects of the present invention, embodiments of the present invention will be described in detail with reference to the accompanying drawings.
It is to be noted that all terms used for directional and positional indication in the present invention, such as: "upper", "lower", "left", "right", "front", "rear", "vertical", "horizontal", "inner", "outer", "top", "low", "lateral", "longitudinal", "center", etc. are merely used to explain the relative positional relationship, connection, etc. between the components in a particular state (as shown in the drawings), and are merely for convenience of description of the present invention, and do not require that the present invention must be constructed and operated in a particular orientation, and thus should not be construed as limiting the present invention. Furthermore, the description of "first," "second," etc. in this disclosure is for descriptive purposes only and is not to be construed as indicating or implying a relative importance or implicitly indicating the number of technical features indicated.
In the description of the present invention, unless explicitly stated and limited otherwise, the terms "mounted," "connected," and "connected" are to be construed broadly, and may be, for example, fixedly connected, detachably connected, or integrally connected; may be a mechanical connection; can be directly connected or indirectly connected through an intermediate medium, and can be communication between two elements. The specific meaning of the above terms in the present invention will be understood in specific cases by those of ordinary skill in the art.
In the description of the present specification, reference to the terms "one embodiment," "some embodiments," "illustrative embodiments," "examples," "specific examples," or "some examples," etc., means that a particular feature, structure, material, or characteristic described in connection with the embodiment or example is included in at least one embodiment or example of the invention. In this specification, schematic representations of the above terms do not necessarily refer to the same embodiments or examples. Furthermore, the particular features, structures, materials, or characteristics described may be combined in any suitable manner in any one or more embodiments or examples.
In the fatigue crack growth rate da/dN test, as the crack length is continuously increased, the plastic region of the crack tip is also increasingly larger, so that the crack tip usually does not meet the small-range yield condition at the later stage of crack growth, and the Paris formula obtained by fitting the obtained test data brings safety risks when the engineering design is applied. If the problem is to be avoided, after the test is finished, the relevant data (usually thousands of pieces of data in one sample) corresponding to the size of the plastic region of the tip of the larger crack is removed through a large amount of calculation, and the workload is very large.
The invention discloses a dynamic measurement method of a metal material fatigue crack tip plastic region, which comprises the following steps:
S1: 1 piece in a group of samples is selected for fatigue crack propagation rate da/dN test, and the relation between the size r 0 of a crack tip plastic region and the opening displacement variation V of a crack nozzle of the sample is established;
S2: a calculation program is compiled, and the change of the r 0 value is dynamically observed in real time according to the macroscopic measurable V value which can be displayed in real time;
s3: continuously carrying out da/dN tests on other samples, continuously monitoring the opening displacement variation V of the crack mouth of the sample in the test process, simultaneously starting the program compiled in the step S2, dynamically monitoring the variation of the r 0 value in real time, and stopping the test when the ratio of the size of the plastic region to the crack length reaches a preset value;
S4: according to the obtained test data, the effective equation of da/dN and delta K is obtained according to the Paris formula fitting form.
Because of the great difficulty in directly measuring the size of the plastic region of the crack tip, if a mathematical relationship between a certain easily-measured physical quantity and the size of the plastic region of the crack tip can be established in advance, the purpose of real-time dynamic measurement when the size of the plastic region of the crack tip is measured can be achieved by measuring the easily-measured physical quantity, and when the ratio of the size of the plastic region to the length of the crack reaches a preset value (for example, 10%), the test is stopped, so that the subsequent complicated data processing can be avoided, and meanwhile, the test cost and time can be saved and the test efficiency can be improved because the test is finished in advance.
Based on the method, the macroscopically measurable relation between the physical quantity and the size of the crack tip plastic region, which is the crack tip opening displacement variable quantity established in advance, is creatively utilized, the test is stopped when the size of the crack tip plastic region does not meet the small-range yield condition, the data condition fitted by the Paris formula is ensured, so that the safety support is provided for engineering structural design, the test cost and the test time are saved, and the test efficiency is improved.
As a preferred example of the present application, in the fatigue crack growth rate da/dN test, the specimen takes the form of a three-point bending SEB, and the specimen crack tip is in a plane stress state, and the plastic region size of the crack tip is:
（1）
Wherein, r 0 is the size of the plastic area of the crack tip, and mm; k, crack tip stress intensity factor, MPa.m 0.5;Rp0.2, yield strength of material, MPa;
for the SEB samples, the expression of K is:
（2）
Wherein Y is a dimensionless shape factor, which is related to the shape of the sample; sigma-applied stress, MPa; a, crack length, mm;
The shape factor Y of the SEB sample is:
（3）
Wherein a is crack length, mm; w is the width of the sample, mm; beta=a/W is normalized crack length, dimensionless.
In the fatigue crack propagation rate da/dN test, the thickness of the test specimen is required to satisfy a plane stress state, the test specimen mostly adopts a three-point bending SEB form, when the crack tip of the test specimen is in the plane stress state, the plastic region size r 0 of the crack tip is calculated according to a given formula (1), the plastic region size is a parameter describing the plastic region size of the crack tip, in order to calculate the crack tip stress intensity factor K, a stress intensity factor expression (2) of the SEB test specimen is used, each parameter in the expression comprises an external stress sigma, a crack length a and a dimensionless shape factor Y, the crack tip stress intensity factor is a key mechanical parameter, the calculation of the dimensionless shape factor Y relates to the crack length a and the test specimen width W, and according to a given formula (3), the dimensionless shape factor Y is calculated, wherein beta=a/W represents the normalized crack length, and the dimensionless shape factor considers the influence of the geometric shape of the test specimen on the stress intensity factor. The method is characterized in that the size of a plastic region of a crack tip is calculated through a formula (1), a parameter quantitatively describing the size of the plastic deformation region of the crack tip can be obtained, the plastic behavior of the crack tip in the crack propagation process can be accurately understood, the stress intensity factor of the crack tip is taken as an important parameter for evaluating the stress state of the crack tip, the stress intensity factor of the crack tip can be obtained by calculating the stress intensity factor expression (2) of an SEB sample, the stress intensity factor of the crack tip can be obtained by calculating the stress intensity factor expression (2) of the SEB sample, the influence of the geometric shape of the sample on the stress intensity factor can be accurately considered, the stress field of the crack tip can be better simulated in an actual test, and a more comprehensive description method of the crack tip behavior can be obtained by combining the size of the plastic region of the crack tip with the stress intensity factor of the crack tip.
From the formulas (1) and (2), it is known that:
（4）
from the equations (3) and (4), the crack tip plastic region size r 0 is a parameter directly related to the crack length a.
In the whole da/dN test process, the fatigue crack length is measured according to the compliance technology, and the normalized crack length is expressed as:
（5）
Where a/W is a normalized crack length, C 0、C1、C2、C3、C4、C5 is a coefficient of compliance (constant), U x is dimensionless compliance, and is related to modulus of elasticity, sample size, test load, etc. of the test material, expressed as follows:
（6）
wherein B is the thickness of the sample and mm; v r is the opening displacement of the crack mouth of the sample, and mm; e is the elastic modulus of the material and MPa; p is the test load, N.
(5) Equation (6) is a principle equation for measuring crack length by a compliance method, which shows that the crack length is obtained by some measurable physical quantity through complex calculation. Meanwhile, as can be seen from the two formulas, there is a certain mathematical relationship between the crack length a and the displacement variation V at the crack tip of the sample. Therefore, there are:
（7）
Wherein F is a function symbol and has no physical meaning; v is the amount of change in the opening displacement at the crack tip of the sample, i.e., the difference v=v max-Vmin, mm between the maximum displacement V max and the minimum displacement V min.
The setting provides a tool for describing crack propagation behaviors more comprehensively and accurately by considering the plastic region size and stress intensity factor of the crack tip and combining a dimensionless shape factor, and if a relation between the plastic region size r 0 of the crack tip and the opening displacement variable V of a crack mouth of a sample can be established through a sample, V can be dynamically measured in real time through a clamp extensometer, and the relation can be applied to other samples in a real-time group, so that the dynamic measurement of the plastic region size r 0 of the crack tip is realized, and a theoretical basis is provided for the dynamic measurement method for improving the plastic region of the fatigue crack tip of the metal material.
As a preferred example of the present application, in step S1, the steps of:
S11: 1 piece in a group of samples is selected, test parameters and a data acquisition mode are set, and da/dN tests are carried out until the samples are broken, wherein the test parameters comprise load amplitude, test frequency, stress ratio, test environment and the like;
s12: according to the obtained test data and the formula (1), calculating the size r 0 of the plastic region of the crack tip under the corresponding cycle;
S13: calculating the ratio r 0/a of the series r 0 to the crack length a;
S14: fitting the mathematical relationship between V and r 0/a according to the r 0/a values calculated in the previous steps under different cycles and the variable V value of the opening displacement of the crack mouth acquired under the corresponding cycles:
Wherein v=v max-Vmin,Vmax is the maximum displacement of the crack tip, V min is the minimum displacement of the crack tip, and V max、Vmin is acquired in the test;
S15: determining a critical value V c of the opening displacement variation of the crack mouth of the sample;
S16: based on the r 0 value calculated in step S12 and the V value calculated in step S14, a mathematical relationship of r 0 and V is established.
The steps disclose a specific step of establishing a mathematical relationship between r 0 and V according to a da/dN test performed on a sample, and provide a mathematical model of the relationship between the size of a crack tip plastic region and the opening displacement of a crack mouth.
As a preferred example of the present application, in step S11, the test specimen takes the form of a three-point bending SEB in the fatigue crack growth rate da/dN test.
The SEB sample form has the effects of better simulating crack growth conditions in an actual engineering structure, providing more accurate test data and being beneficial to understanding and researching fatigue crack growth behaviors of metal materials.
As a preferred example of the present application, in step S12, when the specimen crack tip is in a plane stress state, the plastic region size of the crack tip is calculated according to formula (1):
（1）
Wherein, r 0 is the size of the plastic area of the crack tip, and mm; k, crack tip stress intensity factor, MPa.m 0.5;Rp0.2, yield strength of material, MPa;
for the SEB samples, the expression of K is:
（2）
Wherein Y is a dimensionless shape factor, which is related to the shape of the sample; sigma-applied stress, MPa; a, crack length, mm;
The shape factor Y of the SEB sample is:
（3）
Wherein a is crack length, mm; w is the width of the sample, mm; beta=a/W is normalized crack length, dimensionless.
The arrangement facilitates understanding of the mechanical state of the crack tip in the SEB sample, particularly in consideration of the planar stress state of the crack tip, provides a tool that describes the size of the plastic region of the crack tip and the stress intensity of the crack tip, and by taking into account the plastic region size and the stress intensity factor of the crack tip, in combination with the dimensionless shape factor, ensures that the tool that describes the crack propagation behavior is more comprehensive and accurate.
As a preferred example of the present application, in step S13, the fatigue crack lengths at different cycles are measured according to the compliance method, and the normalized crack length is expressed as:
（5）
Where a/W is normalized crack length, C 0、C1、C2、C3、C4、C5 is a coefficient of compliance (constant), U x is dimensionless compliance, and U x is related to modulus of elasticity, sample size, test load of the test material, expressed as follows:
（6）
wherein B is the thickness of the sample and mm; v r is the opening displacement of the crack mouth of the sample, and mm; e is the elastic modulus of the material and MPa; p is the test load, N.
The normalized crack length and the corresponding parameters introduced by the compliance method enable the measurement of the fatigue crack length to be more comprehensive, comprehensive and practical, and provide a more accurate basis for the subsequent data analysis and modeling.
As a preferred example of the present application, in step S15, a threshold value V c of the opening displacement amount at the crack tip of the corresponding specimen is calculated when r 0/a is equal to a preset value Q, based on the mathematical relationship of the fitted V and r 0/a.
Through the setting, the dynamic control of the size of the plastic area of the crack tip is realized, the size of the plastic area of the crack tip in a test is ensured to be in a reasonable range, the condition of small-range yielding is met, the workload of subsequent data processing is reduced, and thus the safety and reliability requirements of engineering design are better met.
As a preferred example of the application, the preset value Q is within the value range Q less than or equal to 10 percent. Preferably, the value range of the preset value Q is more than or equal to 5% and less than or equal to 10%.
Through the arrangement, the data obtained by the test are more in line with the condition of small-range yield, so that the test can be stopped more timely, and the test result is more accurate and reliable.
As a preferred example of the present application, in step S16, a mathematical relationship of r 0 and V is fitted using a least squares method, based on obtaining a series of r 0 values and V values for different numbers of cycles. The method is beneficial to optimizing the mathematical model, improving the fitting precision, ensuring that the mathematical model better reflects the characteristics of test data, enabling the prediction result to be more accurate, reducing the fitting error and improving the feasibility and applicability of the test method.
According to theoretical analysis, the size r 0 of the plastic region of the fatigue crack tip of the metal material and the opening displacement variation V of the crack mouth of the sample have a certain mathematical relationship, one sample is selected at will in a group of (usually 3 samples are selected) da/dN tests, related test data are collected, and the mathematical relationship of r 0 and V is established through the optimal fitting method. Since all samples are parallel samples, i.e., each sample has the same sample size and properties, the other samples can be referenced to the relationship of r 0 and V for the first sample, and a calculation program can be programmed. When the da/dN test software runs, a calculation program is run simultaneously, so that real-time dynamic measurement and display of r 0 can be realized, when r 0/a reaches 10%, the test is terminated, related data are stored, data of the line elasticity and the small-range yield mechanical state of the crack tip are processed, the Paris formula form is used for fitting equations of da/dN and delta K meeting requirements, the operation is simple, a large amount of data calculation and evaluation are avoided after the test is finished, the test cost and test time are saved, the test efficiency is improved, and a safe support is provided for engineering structural design.
DETAILED DESCRIPTION OF EMBODIMENT (S) OF INVENTION
3 SEB samples were processed using 17Cr16Ni2 stainless steel as the test material. Wherein, the 1# sample is used for establishing the mathematical relationship between V and r 0/a and between r 0 and V, and according to the mathematical relationship, the corresponding V c value when r 0/a is equal to 10% is solved. Samples # 2 and # 3 were subjected to the da/dN test according to the procedure of the present invention, and the effect of the method of the present invention was verified.
The test was terminated when the V values of the two samples reached the V c value obtained from the solution of sample # 1. The degree to which the 2# and 3# samples approach the small range yield mechanical state at the termination of the comparative analysis test.
The da/dN test is carried out on an MTS810-100kN electrohydraulic servo material tester according to GB/T6398-2017 fatigue crack propagation method for fatigue test of metallic materials. Firstly, a section of fatigue crack is prefabricated by adopting a K-reducing method, the influence of a notch on the fatigue crack expansion is eliminated, then, the constant load amplitude delta P=5.4 kN (P max=6.0kN,Pmin =0.6 kN) is adopted for control, the load ratio is 0.1, the test frequency is 10Hz, and the sine waveform is adopted. The crack length is measured by a clamp type displacement sensor with the precision of 0.001mm through a compliance technology, and the measurement principle is shown in the formula (5) and the formula (6).
The mathematical relationship graphs for samples V and r 0/a and r 0 and V are shown in FIGS. 1 and 2, respectively.
As can be seen from fig. 1, when r 0/a=10%, the corresponding crack tip opening displacement variation threshold V c =0.32 mm, and the corresponding crack tip plastic region size r 0 is 1.702mm, see fig. 2. That is, when r 0/a > 10%, the corresponding displacement variation at the crack tip also increases, but since the crack tip plastic region is too large, the small range yield state is not satisfied, and thus the test is stopped at this time.
According to the test result of the No. 1 sample, the No. 2 and No. 3 sample tests do not need to be finished. The test is ended only when the displacement variation of the crack mouth reaches a critical value V c =0.32 mm. The crack tip plastic regions of the 2# and 3# specimens were observed for meeting the conditions of small range yield to verify the acceptability of the invention.
The mathematical relationship graphs of V and r 0/a and r 0 and V for sample #2 and sample # 3 are shown in FIGS. 3-6, respectively.
As can be seen from fig. 3 and 5, when the displacement variation at the crack tip reaches the critical value V c =0.32 mm, the corresponding r 0/a is 9.72% and 10.22%, respectively, and the error of the required value 10% is 2.8% and 2.2%, respectively; as can be seen from fig. 4 and 6, the corresponding r 0 is 1.679mm and 1.777mm, respectively, and the error compared to r 0 in sample # 1 is 1.35% and 4.41%, respectively, and the above data are listed in table 1 for comparison.
Table 13 comparative data for samples
As can be seen from the data in Table 1, according to the dynamic measurement method for the plastic region of the fatigue crack tip of the metal material, 1 sample is taken out from a group of samples, according to the method, mathematical relationships of r 0 and V are established in advance, the visual display value of a displacement sensor for measuring the displacement variation of the crack mouth of the sample is observed, the data of the plastic region r 0 of the crack tip can be dynamically obtained, and whether r 0/a is equal to 10% or not is compared, and the test can be stopped when the ratio is equal to 10%. The measurement errors of r 0/a and r 0 caused by the difference of parallel samples are not more than 5%, and the method can be used for real-time dynamic monitoring of the crack tip plastic region r 0.
In addition, as the method of the invention finishes the test in advance, the subsequent unnecessary data processing trouble is obviously reduced, the test cost is saved, and the test efficiency is improved.
The foregoing description of the preferred embodiments of the invention is not intended to be limiting, but rather is intended to cover all modifications, equivalents, alternatives, and improvements that fall within the spirit and scope of the invention.
Claims (4)
1. A dynamic measurement method for a metal material fatigue crack tip plastic region is characterized by comprising the following steps:
S1: 1 piece in a group of samples is selected for fatigue crack propagation rate da/dN test, and the relation between the size r 0 of a crack tip plastic region and the opening displacement variation V of a crack nozzle of the sample is established;
wherein, in step S1, the steps of:
S11: 1 piece in a group of samples is selected, and test parameters and data acquisition modes are set until the samples are broken, wherein the test parameters comprise load amplitude, test frequency, stress ratio and test environment;
S12: calculating the size r 0 of the plastic region of the crack tip under the corresponding cycle according to the obtained test data; in the fatigue crack growth rate da/dN test, the specimen takes the form of a three-point bending SEB, and in step S12, the plastic region size of the crack tip is calculated according to formula (1) while the specimen crack tip is in a plane stress state:
（1）
Wherein, r 0 is the size of the plastic area of the crack tip, and mm; k, crack tip stress intensity factor, MPa.m 0.5;Rp0.2, yield strength of material, MPa;
for the SEB samples, the expression of K is:
（2）
Wherein Y is a dimensionless shape factor, which is related to the shape of the sample; sigma-applied stress, MPa; a, crack length, mm;
The shape factor Y of the SEB sample is:
（3）
Wherein a is crack length, mm; w is the width of the sample, mm; beta=a/W is normalized crack length, dimensionless;
S13: calculating the ratio r 0/a of the crack tip plastic region size r 0 to the crack length a: （4）
Fatigue crack lengths at different cycles are measured according to the compliance method, and normalized crack lengths are expressed as follows:
（5）
Where a/W is normalized crack length, C 0、C1、C2、C3、C4、C5 is compliance coefficient, U x is dimensionless compliance, U x is related to elastic modulus, sample size, test load of the test material, expressed as follows:
（6）
Wherein B is the thickness of the sample and mm; v r is the opening displacement of the crack mouth of the sample, and mm; e is the elastic modulus of the material and MPa; p is the test load, N;
S14: fitting the mathematical relationship between V and r 0/a according to the calculated r 0/a values under different cycles and the variation V value of the opening displacement of the crack mouth acquired under the corresponding cycles;
Wherein v=v max-Vmin,Vmax is the maximum displacement of the crack tip, V min is the minimum displacement of the crack tip, and V max、Vmin is acquired in the test;
S15: determining a critical value V c of the opening displacement variation of the crack mouth of the sample;
S16: according to the r 0 value calculated in the step S12 and the V value calculated in the step S14, according to the r 0 value and the V value of the series under different cycle times, adopting a least square method to fit the mathematical relationship of r 0 and V, and establishing the relationship between the size r 0 of the plastic region of the crack tip and the opening displacement variation V of the crack mouth of the sample;
S2: a calculation program is compiled, and the change of the r 0 value is dynamically observed in real time according to the macroscopic measurable V value which can be displayed in real time;
S3: continuously carrying out da/dN tests on other samples, continuously monitoring the opening displacement variation V of the crack mouth of the sample in the test process, simultaneously starting the program compiled in the step S2, dynamically monitoring the variation of the r 0 value in real time, and stopping the test when the ratio of the size of the plastic region to the crack length reaches a preset value Q;
S4: according to the obtained test data, the effective equation of da/dN and delta K is obtained according to the Paris formula fitting form.
2. The method according to claim 1, wherein in step S15, the threshold V c of the opening displacement of the crack tip of the corresponding specimen is calculated based on the mathematical relationship between the fitted V and r 0/a when r 0/a is equal to the preset value Q.
3. The method for dynamically measuring the plastic region of the fatigue crack tip of a metallic material according to claim 2, wherein the preset value Q is Q.ltoreq.10%.
4. The dynamic measurement method for the metal material fatigue crack tip plastic zone according to claim 2, wherein the preset value Q is within a range of 5% to 10%.
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Citations (8)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JP2012073126A (en) * | 2010-09-29 | 2012-04-12 | Hitachi Ltd | Evaluation method and device for crack evolution velocity of metallic material |
| JP2016105104A (en) * | 2016-02-03 | 2016-06-09 | 国立研究開発法人 海上・港湾・航空技術研究所 | Fatigue crack detection method |
| JP2020030074A (en) * | 2018-08-21 | 2020-02-27 | トヨタ自動車株式会社 | Fatigue life estimation method for mechanical joint |
| WO2022038840A1 (en) * | 2020-08-17 | 2022-02-24 | 株式会社日立製作所 | Crack progress evaluation device and crack progress evaluation program |
| CN114323994A (en) * | 2021-12-21 | 2022-04-12 | 中国科学院金属研究所 | Method for testing fatigue crack propagation rate of three-point bending sample in T-shaped wave load-holding test |
| CN114323940A (en) * | 2021-12-21 | 2022-04-12 | 中国科学院金属研究所 | Method for testing fatigue crack propagation rate of compact tensile sample under trapezoidal wave loading condition |
| CN116341216A (en) * | 2023-03-06 | 2023-06-27 | 洛阳船舶材料研究所(中国船舶集团有限公司第七二五研究所) | Measuring method for calculating fatigue crack initiation of metal material by using frequency attenuation |
| CN117309563A (en) * | 2023-10-31 | 2023-12-29 | 兰州理工大学 | Multi-axis high-cycle fatigue testing device and testing method for metal material |
Family Cites Families (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US10989640B2 (en) * | 2015-03-24 | 2021-04-27 | Bell Helicopter Textron Inc. | Method for defining threshold stress curves utilized in fatigue and damage tolerance analysis |
| CN114878374B (en) * | 2022-07-11 | 2022-09-20 | 天津大学 | Characterization method for short crack and long crack propagation of metal material |
-
2024
- 2024-03-25 CN CN202410340182.0A patent/CN117935997B/en active Active
Patent Citations (8)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JP2012073126A (en) * | 2010-09-29 | 2012-04-12 | Hitachi Ltd | Evaluation method and device for crack evolution velocity of metallic material |
| JP2016105104A (en) * | 2016-02-03 | 2016-06-09 | 国立研究開発法人 海上・港湾・航空技術研究所 | Fatigue crack detection method |
| JP2020030074A (en) * | 2018-08-21 | 2020-02-27 | トヨタ自動車株式会社 | Fatigue life estimation method for mechanical joint |
| WO2022038840A1 (en) * | 2020-08-17 | 2022-02-24 | 株式会社日立製作所 | Crack progress evaluation device and crack progress evaluation program |
| CN114323994A (en) * | 2021-12-21 | 2022-04-12 | 中国科学院金属研究所 | Method for testing fatigue crack propagation rate of three-point bending sample in T-shaped wave load-holding test |
| CN114323940A (en) * | 2021-12-21 | 2022-04-12 | 中国科学院金属研究所 | Method for testing fatigue crack propagation rate of compact tensile sample under trapezoidal wave loading condition |
| CN116341216A (en) * | 2023-03-06 | 2023-06-27 | 洛阳船舶材料研究所(中国船舶集团有限公司第七二五研究所) | Measuring method for calculating fatigue crack initiation of metal material by using frequency attenuation |
| CN117309563A (en) * | 2023-10-31 | 2023-12-29 | 兰州理工大学 | Multi-axis high-cycle fatigue testing device and testing method for metal material |
Non-Patent Citations (6)
| Title |
|---|
| Jingyi Zhao,et al..A Study on the Effect of Microstructure Evolution of the Aluminum Metallization Layer on Its Electrical Performance During Power Cycling.《IEEE Transactions on Power Electronics ( Volume: 34, Issue: 11, November 2019)》.2019,全文. * |
| Zhang Yajun,et al..Study on the reliability enhancement test of MEMS layered structure.《2014 Prognostics and System Health Management Conference (PHM-2014 Hunan)》.2014,全文. * |
| 不同驱动参量的金属材料疲劳裂纹扩展速率模型综述;张亚军等.;《热加工工艺》;20240310;全文 * |
| 弹性模量在用柔度法测量疲劳裂纹长度中的影响;张亚军.;《材料开发与应用》;20030415;全文 * |
| 试样厚度对船用钢疲劳裂纹扩展速率影响的试验;王艺陶;冯国庆;任慧龙;;船舶工程;20191125(11);全文 * |
| 金属材料疲劳裂纹扩展速率不同模型应用分析;张亚军等.;《材料开发与应用》;20210415;全文 * |
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