阅读说明：本技术 一种搅拌摩擦焊构件变幅疲劳寿命预测方法 (Method for predicting amplitude-variable fatigue life of friction stir welding component ) 是由 孙国芹 刘鹤 刘金峰 于 2021-06-18 设计创作，主要内容包括：一种搅拌摩擦焊构件变幅疲劳寿命预测方法,属于机械构件疲劳诊断分析技术领域。模拟变幅载荷下按加载顺序的薄弱区域预制微裂纹,通过变幅载荷确定薄弱区域变化,模拟构件变幅下裂纹扩展规律；进行不同载荷疲劳小裂纹扩展中断疲劳实验,计算小裂纹扩展速率,得到变幅下裂纹扩展规律；计算对应的修正因子M-(K)、f,在Elber裂纹闭合方法中有效应力强度因子基础上,加入修正因子M-(K)、f,得出在不同应力比下不同区域的ΔK-(eff)-da/dN裂纹扩展速率基线,拟合出材料常数C、m；确定初始裂纹尺寸a-(0)和临界裂纹尺寸a-(c)；确定当前载荷水平下薄弱区域,据此选择修正因子M-(K)、f及初始裂纹尺寸a-(0)取值。通过循环计算,得到不同加载水平下疲劳寿命。(A method for predicting the amplitude-variable fatigue life of a friction stir welding component belongs to the technical field of fatigue diagnosis and analysis of mechanical components. Simulating a weak area prefabricating micro-crack in a loading sequence under the variable amplitude load, determining the change of the weak area through the variable amplitude load, and simulating the crack expansion rule under the variable amplitude of the component; carrying out small crack propagation interruption fatigue experiments of different load fatigue, and calculating the small crack propagation rate to obtain a crack propagation rule under variable amplitude; calculating a corresponding correction factor M K F, adding a correction factor M on the basis of an effective stress intensity factor in an Elber crack closing method K F, obtaining delta K of different areas under different stress ratios eff -da/dN crack propagation rate baseline fitted with material constants C, m; determination of the initial crack size a 0 And critical crack size a c (ii) a Determining the weak area at the current load level, and selecting the correction factor M according to the weak area K F and initial crack size a 0 And (4) taking values. And obtaining the fatigue life under different loading levels through cyclic calculation.)

1. A method for predicting the amplitude-variable fatigue life of friction stir welding components based on load sequence and performance of each joint area is characterized by comprising the following steps:

step 1): simulating the change of a weak area of the friction stir welding component under different load levels by using finite element software, carrying out partition modeling on a model according to the characteristic that different areas of the friction stir welding component have different material attributes, wherein the model is mainly divided into four areas, namely a Nugget (NZ), a heat engine (TMAZ), a heat influence (HAZ) and a Base Metal (BM), the material attributes of each area are obtained by micro-tensile test data, the four areas comprise stress strain data and elastic modulus of each area of a joint, different loads are applied, and the maximum stress areas under different load levels are used as the weak areas under the load levels;

step 2): according to different weak areas of the corresponding joint at different loading levels obtained in the step 1), according to the change of the load range under the variable amplitude condition, selecting a preset micro-crack at the weak area of the friction stir welding component corresponding to one load level according to the loading sequence, only changing the size of the load level on the basis of not changing other settings, determining the change of the weak area of the component after changing the latter load level, applying the subsequent load after obtaining the preset micro-crack at the weak area in the previous step, and repeating the step;

step 3): according to the change rule and the size of the microcracks of the weak area of the friction stir welding component under the variable amplitude loading obtained in the step 2), in order to determine the crack expansion rule of the friction stir welding component under the variable amplitude condition, the microcracks are preset in the weak area corresponding to the variable amplitude load level respectively in the direction of vertical load loading by using a finite element expansion method, the elastic modulus and the poisson ratio are selected as the material parameters of each area of the friction stir welding joint, the maximum main stress parameter is set as the damage criterion, low-cycle fatigue analysis is selected, and the variable amplitude fatigue load is applied to obtain the crack expansion rule of the friction stir welding component under the variable amplitude condition;

step 4): and designing an interrupted fatigue test of the friction stir welding component according to the weak area and the crack propagation rule in the steps. Loading the friction stir welding component under different load sequences, laminating and copying the fatigue small cracks, and calculating the small crack propagation rate by adopting a cutting line method to obtain the crack propagation rules and data of the friction stir welding component in different areas under the condition of amplitude variation, wherein the formula of the cutting line method is as follows:

where Δ N is the cycle interval, Δ a is the crack length variation value, a_iIs a number of cycles of N_iCrack length of time.

Step 5): the method for calculating the effective stress intensity factor of the friction stir welding component is obtained by correcting based on the form of an Elber crack closure theory, and the Elber crack closure theory formula is as follows:

ΔK_eff＝K_max-K_op (2)

wherein: k_maxIs the maximum stress intensity factor; k_opIs a crack opening stress intensity factor; Δ K_effIs an effective stress intensity factor; y is a geometric correction factor; k is a stress intensity factor; sigma is external stress; a is the surface semiellipse depth;

based on the influence of the load sequence and the performance of each region of the member on the fatigue mechanism of the friction stir welding member, a load sequence coefficient M is provided_KAnd establishing a new effective stress intensity factor calculation method by using the correction factor f.

The calculation formula of the corrected effective stress intensity factor is as follows:

△K_eff＝K_max-M_K·f·K_op (4)

wherein: m_KIs the load order coefficient, f is the correction factor;

coefficient of load order M_KThe value of (A) is related to the load sequence and the performance of each area of the joint, the change of the load sequence and the difference of the material properties of the friction stir welding component cause the change of the crack opening stress level of different areas of the joint, thereby causing the change of the crack propagation driving force, and therefore, the load sequence coefficient M is introduced_KTo correct the stress intensity factor term under the crack opening stress level, M under different load sequences_KThe values of (A) are different;

m under high-low loading sequence_KThe formula is as follows:

low high load order M_KThe formula is as follows:

the parameters required by the above formula are defined as follows:

wherein: a is_OLCrack length under overload; r is_OLMonotonic compression of the plastic zone size due to overload; a is_ULCrack length at low load; r is_ULMonotonic tensile plastic zone size due to low load; r is_yiThe current cycle plastic zone size; sigma_yiYield stress of each region of the component; delta K_ULStress intensity factor amplitude at low load; delta K_iIs the load stress intensity factor amplitude; k'_bThe cyclic strength coefficient of the weak area corresponding to the load; k'_b+1The cyclic intensity coefficient of the adjacent area; n is a load order effect index of a corresponding region of the friction stir welding member; delta K_OLStress intensity factor amplitude during overload;

the value of the correction factor f is related to the grain size, the difference of the grain sizes of different areas of the friction stir welding component has a larger influence on the crack propagation mode, the influence of the grain size on the crack propagation rate is gradually weakened along with the increase of the crack length, therefore, a correction factor f is introduced to correct a stress intensity factor item at the crack opening stress level, and the calculation formula of the correction factor f is as follows:

wherein: d is the average grain size of the corresponding area of the friction stir welding component; a is_iCurrent crack length;

coefficient of load order M_KAnd a correction factor f for correcting the stress intensity factor term at the crack opening stress level, wherein the weld nucleus region (NZ), the thermomechanical influence region (TMAZ), the heat influence region (HAZ) and the base material region (BM) of the friction stir welding component correspond to different M respectively_KF value;

step 6) calculating corresponding correction factor M through the performance and load sequence effect of each area of the friction stir welding component_KAnd f required material parameters;

step 7): the crack propagation rate delta K of the friction stir welding component under different stress ratios is obtained by combining the constant amplitude fatigue crack propagation data under different loading levels and utilizing the corrected effective stress intensity factor method_effA da/dN baseline, which is defined by a correction factor M_KF, gathering the crack propagation rate curves of different stress ratios and fractures in different areas, and fitting to obtain a material constant C, m;

step 8): initial crack size a₀And critical crack size a_cDetermination of (1); the initial sizes of microstructures in each region (a nugget region (NZ), a thermomechanical affected region (TMAZ), a heat affected region (HAZ) and a base metal region (BM)) of the friction stir welded member are different, metallographic observation is performed on each region of the friction stir welded member, and the average size of the reinforcing particles in the corresponding region is taken as the initial crack size a corresponding to the region₀Critical crack length a_cThrough fracture toughness K_ICThe formula is as follows:

wherein K_ICIs the fracture toughness, σ, of the material_maxThe maximum external load stress.

Step 9): combining the weak areas determined in the step 1), respectively selecting the initial crack lengths a of the corresponding weak areas₀Calculating the crack opening stress sigma under the load corresponding to the initial crack size by using a crack opening stress calculation formula provided by Duquessay DL_op1，σ_op2And respectively calculating corresponding opening stress intensity factors K according to a formula (3)_op1，K_op2The crack opening stress calculation formula is as follows:

wherein sigma_opIs a splaying stress; sigma_maxIs the maximum stress in the load sequence; sigma_minIs the minimum stress in the load sequence; sigma_yIs the yield strength; a. beta is an empirical constant;

step 10): combining the weak areas determined in the step 1), respectively selecting the initial crack lengths a of the corresponding weak areas₀Determining a weak area correction factor M at the current load level_KAnd f, calculating an effective stress intensity factor delta K corresponding to the initial crack size by using the corrected effective stress intensity factor calculation formula (4)_eff1，ΔK_eff2；

Step 11): constant C, m determined by step 7) and effective stress intensity factor Δ K determined by step 5)_effRespectively calculating effective stress intensity factors delta K of two cracks by a formula_eff1,i,ΔK_eff2,iAnd calculating the crack propagation increment of the weak area at each cycle by using the Paris formula:

Δa_i＝C(ΔK_eff,i)^m (14)

the crack size at the current cycle is calculated by the following formula:

a_i＝a_i-1+Δa_i (15)

step 12): the crack growth increment Δ a per cycle was calculated by the formula (4) and the formula (14)_i,1,Δa_i,2The crack length a at the current cycle is calculated by the formula (15)_i,1,a_i,2(ii) a Through continuous cycle calculation, the current crack size a_iContinuously updated when the size of one of the cracks reaches the critical crack size a for the load_cAnd then the fatigue life of the friction stir welding component under different loading levels is obtained.

2. The method for predicting the amplitude fatigue life of a friction stir welding member based on the load order and the performance of each zone of the joint as recited in claim 1, wherein: the variable amplitude loading simulation of the friction stir welding joint in the step 2) proves that the microcracks are preset at the weak areas of the friction stir welding member corresponding to one load level according to the loading sequence, the change of the weak areas of the friction stir welding member under the variable amplitude condition can be more obviously obtained when the size of the microcracks is 200 micrometers through tests, only the size of the load level is changed on the basis of not changing other settings, and the change of the weak areas of the member along with the change of the load level after the load level is changed is found.

3. The method for predicting the amplitude fatigue life of a friction stir welding member based on the load order and the performance of each zone of the joint as recited in claim 1, wherein: and 3) simulating the crack propagation rule of the friction stir welding member under the amplitude variation condition, finding that the weak area of the friction stir welding member at the early stage of crack propagation changes along with the change of the load level, stopping changing the position of the weak area after the size of the crack reaches a certain value, continuing crack propagation, and finally locating the failure position of the sample at the weak area with the longest size of the crack.

4. The method for predicting the amplitude fatigue life of a friction stir welding member based on the load order and the performance of each zone of the joint as recited in claim 1, wherein: and 4) interrupting the fatigue test to find that a plurality of cracks appear on the surface of the joint under the condition of variable amplitude loading due to the particularity of the material properties of each area of the friction stir welding joint and a test phenomenon that the plurality of cracks simultaneously expand appears.

5. The method for predicting the amplitude fatigue life of a friction stir welding member based on the load order and the performance of each zone of the joint as recited in claim 1, wherein: the load order coefficient M of the correction factor in the step 5)_KAnd a correction factor f, which considers the influence of the performance of each area of the joint and the load order effect on the fatigue life of the friction stir welding joint, wherein the load order effect is a main factor causing the change of the crack propagation driving force, and the influence of the grain size on the crack propagation rate is weakened along with the increase of the range of the stress intensity factor of the fatigue crack tip area.

6. The method for predicting the amplitude fatigue life of a friction stir welding member based on the load order and the performance of each zone of the joint as recited in claim 1, wherein: and in the step 12), starting from the initial crack, accumulating the crack propagation increment in each cycle, and defining the occurrence of fatigue failure when the crack size reaches the critical fracture size.

Technical Field

The invention relates to a fatigue life prediction method, in particular to a friction stir welding component amplitude-variable fatigue life prediction method based on a load sequence and the performance of each joint area, and belongs to the technical field of mechanical component diagnosis and analysis.

Background

In engineering applications, main parts in the fields of construction machinery, light industry, ships, railways and the like are often subjected to periodic alternating loads. According to literature statistics, the fatigue load causes the failure of the welding component, and can reach 80-90% of the failure component. Therefore, the research on the high-cycle fatigue failure position and the service life prediction of the metal structural member under the variable amplitude condition is very necessary for the friction stir welding member.

When the life of the friction stir welding member is predicted under the condition of amplitude variation, the influence of the load sequence is not only required to be considered, but also the influence of factors such as microstructure change, fatigue performance and the like in different areas of the joint is required to be considered. The friction stir welding area of the aluminum alloy is divided into four areas, namely a weld Nugget (NZ), a heat engine (TMAZ), a heat influence (HAZ) and a Base Metal (BM), and the difference of the static mechanical property and the microstructure of each area causes the difference of crack propagation rules initiated in different areas of the joint. In 1970 Elber first proposed the concept of crack closure to describe the crack opening stress S_opAnd (4) from the perspective, providing a fatigue crack propagation prediction model based on the range of the effective stress intensity factor. Numerous closed models appear in succession, notably the AFFDL closed model, the Dugdale model and the Newman model. However, these models all have the same preconditions: the components under study must be of the same material. However, for friction stir welding members, the joint area is equal to different materials, the friction stir welding members are fractured at different positions of the joint due to the change of the load level, and through the research on the fracture positions of the friction stir welding members under constant amplitude loading of the same subject group, the failure positions of the members are close to the nugget area from the base material area along with the change of the load level from small to large, so that the model based on crack closure is not suitable for predicting the fatigue life of the friction stir welding members. Besides the influence of the material property on the fatigue life, the mutual effect of the load sequences also has an extremely deep influence on the fatigue life. The Wheeler model considers that a large overload cycle results in an overload plastic region, and then the range of the cyclic plastic region generated in the cycle does not exceed the range of the overload plastic region, so that the phenomenon of delaying crack propagation is caused. The method firstly utilizes an extended finite element method to determine the failure position of the component under the condition of amplitude variation, then considers the microscopic performance and the load order effect of different regions of the component, corrects the effective stress intensity factor in the Elber crack closing method, can accurately predict the fatigue life of the friction stir welding component by combining the Paris formula, and has safe service on the welding componentTheoretical significance and engineering application value.

Disclosure of Invention

The invention aims to provide a friction stir welding component amplitude-variable fatigue life prediction method based on a load sequence and performance of each joint area.

In order to achieve the purpose, the technical scheme adopted by the invention is a friction stir welding member fatigue life prediction method based on a load sequence and performance of each joint area, and the method comprises the following specific steps:

step 1): simulating weak area changes under different load levels of a friction stir welding component by using finite element software (e.g. ABAQUS), carrying out partition modeling on a model according to the characteristics that different areas of the friction stir welding component have different material attributes, wherein the model is mainly divided into four areas, namely a weld Nugget (NZ), a heat engine (TMAZ), a heat influence (HAZ) and a Base Material (BM), the material attributes of each area are obtained by micro-tensile test data, the four areas comprise stress-strain data and elastic modulus of each area of a joint, different loads are applied, and the stress maximum areas under different load levels are used as the weak areas under the load levels;

step 2): according to different weak areas of the corresponding joint at different loading levels obtained in the step 1), selecting a pre-set microcrack at the weak area of the friction stir welding component corresponding to one loading level according to the change of the loading range under the variable amplitude condition, testing, obtaining the change of the weak area of the friction stir welding component under the variable amplitude condition more obviously when the size of the microcrack is 200 mu m, only changing the size of the loading level on the basis of not changing other settings, determining the change of the weak area of the component after changing the latter loading level, applying subsequent loads after obtaining the pre-set microcrack at the weak area in the former step, and repeating the step;

step 3): according to the change rule and the size of the microcracks of the weak area of the friction stir welding component under the variable amplitude loading obtained in the step 2), in order to determine the crack propagation rule of the friction stir welding component under the variable amplitude condition, the microcracks are respectively preset in the weak area corresponding to the variable amplitude load level by using a finite element expansion method in the direction of vertical load loading, the elastic modulus and the poisson ratio are selected as the material parameters of each area of the friction stir welding head, the maximum main stress parameter is set as the damage criterion, low-cycle fatigue analysis is selected, and variable amplitude fatigue load is applied to obtain the crack propagation rule of the friction stir welding component under the variable amplitude condition: the weak area of the friction stir welding component at the early stage of crack propagation changes along with the change of the load level, when the size of the crack reaches a certain value, the position of the weak area stops changing, but the crack propagation is continuous, and finally the failure position of the sample is positioned at the weak area with the longest size of the crack;

step 4): and designing an interrupted fatigue test of the friction stir welding component according to the weak area and the crack propagation rule in the steps. Loading the friction stir welding component under different load sequences, laminating and copying the fatigue small cracks, and calculating the small crack propagation rate by adopting a cutting line method to obtain the crack propagation rules and data of the friction stir welding component in different areas under the condition of amplitude variation, wherein the formula of the cutting line method is as follows:

where Δ N is the cycle interval, Δ a is the crack length variation value, a_iIs a number of cycles of N_iCrack length of time.

Step 5): the method for calculating the effective stress intensity factor of the friction stir welding component is obtained by correcting based on the form of an Elber crack closure theory, and the Elber crack closure theory formula is as follows:

ΔK_eff＝K_max-K_op (2)

wherein: k_maxIs the maximum stress intensity factor; k_opIs a crack opening stress intensity factor; Δ K_effIs the effective stress intensity factor range; y is a geometric correction factor; k is a stress intensity factor; sigma is external stress; and a is the surface semielliptical depth.

Based on the influence of the load sequence and the performance of each region of the member on the fatigue mechanism of the friction stir welding member, a load sequence coefficient M is provided_KAnd establishing a new effective stress intensity factor calculation method by using the correction factor f.

The calculation formula of the corrected effective stress intensity factor is as follows:

△K_eff＝K_max-M_K·f·K_op (4)

wherein: m_KIn order to be the load order factor,_fis a correction factor.

Coefficient of load order M_KThe value of (A) is related to the load sequence and the performance of each area of the joint, the change of the load sequence and the difference of the material properties of the friction stir welding component cause the change of the crack opening stress level of different areas of the joint, thereby causing the change of the crack propagation driving force, and therefore, the load sequence coefficient M is introduced_KTo correct the stress intensity factor term under the crack opening stress level, M under different load sequences_KThe values of (A) are different;

m under high-low loading sequence_KThe formula is as follows:

low high load order M_KThe formula is as follows:

the parameters required by the above formula are defined as follows:

wherein: a is_OLCrack length under overload; r is_OLMonotonic compression of the plastic zone size due to overload; a is_ULCrack length at low load; r is_ULMonotonic tensile plastic zone size due to low load; r is_yiCurrent cycle plastic zone size. Sigma_yiYield stress of each region of the component; delta K_ULStress intensity factor amplitude at low load; delta K_iIs the load stress intensity factor amplitude; k'_bThe cyclic strength coefficient of the weak area corresponding to the load; k'_b+1The cyclic intensity coefficient of the adjacent area; n is a load order effect index of a corresponding region of the friction stir welding member; delta K_OLThe magnitude of the stress intensity factor when overloading.

Correction factor_fThe value of (2) is related to the grain size, the difference of the grain sizes of different areas of the friction stir welding component has a larger influence on the crack propagation mode, the influence of the grain size on the crack propagation rate is gradually weakened along with the increase of the crack length, therefore, a correction factor f is introduced to correct a stress intensity factor term at the crack opening stress level, and the calculation formula of the correction factor f is as follows:

wherein: d is the average grain size of the corresponding area of the friction stir welding component; a is_iCurrent crack length.

Coefficient of load order M_KAnd a correction factor f for correcting the stress intensity factor term at the crack opening stress level, wherein the weld nucleus region (NZ), the thermomechanical influence region (TMAZ), the heat influence region (HAZ) and the base material region (BM) of the friction stir welding component correspond to different M respectively_KAnd f is the same as the first threshold value.

Step 6) calculating corresponding correction factor M through the performance and load sequence effect of each area of the friction stir welding component_KAnd f desired material parameters.

Step 7): the crack propagation rate delta K of the friction stir welding component under different stress ratios is obtained by combining the constant amplitude fatigue crack propagation data under different loading levels and utilizing the corrected effective stress intensity factor method_effA da/dN baseline, which is defined by a correction factor M_KAnd f, aggregating the crack propagation rate curves of different stress ratios and fractures in different areas, and fitting to obtain the material constant C, m.

Step 8): initial crack size a₀And critical crack size a_xDetermination of (1); the initial sizes of microstructures in each region (a nugget region (NZ), a thermomechanical affected region (TMAZ), a heat affected region (HAZ) and a base metal region (BM)) of the friction stir welded member are different, metallographic observation is performed on each region of the friction stir welded member, and the average size of the reinforcing particles in the corresponding region is taken as the initial crack size a corresponding to the region₀Critical crack length a_cThrough fracture toughness K_ICThe formula is as follows:

wherein K_ICIs the fracture toughness, σ, of the material_maxThe maximum external load stress.

Step 9): combining the weak areas determined in the step 1), respectively selecting the initial crack lengths a of the corresponding weak areas₀Crack opening response proposed by Duquesnag DLCalculating crack opening stress sigma under load corresponding to two initial crack sizes by using a force calculation formula_op1、σ_op2And respectively calculating corresponding opening stress intensity factors k according to a formula (3)_op1、K_op2Wherein the crack opening stress calculation formula is as follows:

wherein sigma_opIs a splaying stress; sigma_maxIs the maximum stress in the load sequence; sigma_minIs the minimum stress in the load sequence; sigma_yIs the yield strength; alpha and beta are empirical constants, and if the material is an aluminum alloy, alpha is 0.45, and beta is 0.2.

Step 10): combining the weak areas determined in the step 1), respectively selecting the initial crack lengths a of the corresponding weak areas₀Determining a weak area correction factor M at the current load level_KAnd f, calculating an effective stress intensity factor delta k corresponding to the initial crack size by using the corrected effective stress intensity factor calculation formula (4)_eff1、ΔK_eff2。

Step 11): constant C, m determined by step 7) and effective stress intensity factor Δ K determined by step 5)_effRespectively calculating effective stress intensity factors delta K of two crack propagation processes by using a formula_eff1,i、ΔK_eff2,iAnd calculating the crack propagation increment of the weak area at each cycle by using the Paris formula:

Δa_i＝C(ΔK_eff,i)^m (14)

the crack size at the current cycle is calculated by the following formula:

a_i＝a_i-1+Δa_i (15)

step 12): the crack growth increment Δ a per cycle was calculated by the formula (4) and the formula (14)_i,1,Δa_i,2The crack length a at the current cycle is calculated by the formula (15)_i,1,a_i,2. Through continuous loop calculation, currentCrack size a_iContinuously updated when the size of one of the cracks reaches the critical crack size a for the load_cAnd then the fatigue life of the friction stir welding component under different loading levels is obtained.

The variable amplitude loading simulation of the friction stir welding joint in the step 2) proves that the microcracks are preset at the weak areas of the friction stir welding member corresponding to one load level according to the loading sequence, the change of the weak areas of the friction stir welding member under the variable amplitude condition can be more obviously obtained when the size of the microcracks is 200 micrometers through tests, only the size of the load level is changed on the basis of not changing other settings, and the change of the weak areas of the member along with the change of the load level after the latter load level is changed is found.

And 3) simulating the crack propagation rule of the friction stir welding member under the variable amplitude condition, finding that the weak area of the friction stir welding member at the early stage of crack propagation changes along with the change of the load level, stopping changing the position of the weak area when the size of the crack reaches a certain value, continuing crack propagation, and finally locating the failure position of the sample at the weak area with the longest size of the crack.

And 4) interrupting the fatigue test to find that a plurality of cracks appear on the surface of the joint under the condition of variable amplitude loading due to the particularity of the material properties of each area of the friction stir welding joint and a test phenomenon that the plurality of cracks simultaneously expand appears.

Correcting factor load order coefficient M in step 5)_KAnd a correction factor f, which considers the influence of the performance of each area of the joint and the load order effect on the fatigue life of the friction stir welding joint, wherein the load order effect is a main factor causing the change of the crack propagation driving force, and the influence of the grain size on the crack propagation rate is weakened along with the increase of the range of the stress intensity factor of the fatigue crack tip area.

And step 12), starting from the initial crack, accumulating the crack propagation increment in each cycle, and defining the occurrence of fatigue failure when the crack size reaches the critical fracture size.

The invention has the advantages that: the method combines the actual working conditions of the welding members, not only considers the influence of the performance of each area of the friction stir welding joint on the fatigue short crack propagation, but also considers the action of a load sequence on the crack propagation, and the proposed life prediction model can be applied to the actual working conditions of the welding members and has certain engineering practical significance.

Drawings

FIG. 1 is a flow chart of a method for predicting fatigue life of friction stir welding members based on load sequence and joint zone performance in accordance with the present invention.

FIG. 2 is a crack propagation cloud picture of the friction stir welding member under the variable amplitude high-low condition in the step 3).

FIG. 3 is a crack propagation cloud picture of the friction stir welding member in the step 3) under the condition of low amplitude and high amplitude.

Detailed Description

As shown in FIG. 1, the method for predicting the fatigue life of the friction stir welding member based on the load sequence and the performance of each joint region comprises the following steps:

step 1): simulating weak area changes under different load levels of a friction stir welding component by using finite element software (e.g. ABAQUS), carrying out partition modeling on a model according to the characteristics that different areas of the friction stir welding component have different material attributes, wherein the model is mainly divided into four areas, namely a weld Nugget (NZ), a heat engine (TMAZ), a heat influence (HAZ) and a Base Material (BM), the material attributes of each area are obtained by micro-tensile test data, the four areas comprise stress-strain data and elastic modulus of each area of a joint, different loads are applied, and the stress maximum areas under different load levels are used as the weak areas under the load levels;

step 2): according to different weak areas of the corresponding joint at different loading levels obtained in the step 1), selecting a pre-set microcrack at the weak area of the friction stir welding component corresponding to one loading level according to the change of the loading range under the variable amplitude condition, testing, obtaining the change of the weak area of the friction stir welding component under the variable amplitude condition more obviously when the size of the microcrack is 200 mu m, only changing the size of the loading level on the basis of not changing other settings, determining the change of the weak area of the component after changing the latter loading level, applying subsequent loads after obtaining the pre-set microcrack at the weak area in the former step, and repeating the step;

step 3): according to the change rule and the size of the microcracks of the weak area of the friction stir welding component under the variable amplitude loading obtained in the step 2), in order to determine the crack propagation rule of the friction stir welding component under the variable amplitude condition, the microcracks are preset in the weak area corresponding to the variable amplitude load level in the direction of vertical load loading by using a finite element expansion method, the elastic modulus and the poisson ratio are selected as the material parameters of each area of the friction stir welding head, the maximum main stress parameter is set as the damage criterion, low-cycle fatigue analysis is selected, and the variable amplitude fatigue load is applied to obtain the crack propagation rule of the friction stir welding component under the variable amplitude condition: the weak area of the friction stir welding member at the early stage of crack propagation changes along with the change of the load level, when the crack size reaches a certain value, the position of the weak area stops changing, but the crack propagation continues, and finally the failure position of the sample is located at the weak area with the longest crack size. In fig. 2, a is the maximum stress position under high load when the analysis step is 10, b is the maximum stress position under low load when the analysis step is 10, c is the maximum stress position under high load when the analysis step is 100, d is the maximum stress position under low load when the analysis step is 100, e is the maximum stress position when the analysis step is 200, f is the maximum stress position when the analysis step is 300, a is the maximum stress position under low load when the analysis step is 10, b is the maximum stress position under high load when the analysis step is 10, c is the maximum stress position under low load when the analysis step is 100, d is the maximum stress position under high load when the analysis step is 100, e is the maximum stress position when the analysis step is 250, and f is the maximum stress position when the analysis step is 300;

step 4): and designing an interrupted fatigue test of the friction stir welding component according to the weak area and the crack propagation rule in the steps. Loading the friction stir welding component under different load sequences, laminating and copying the fatigue small cracks, and calculating the small crack propagation rate by adopting a cutting line method to obtain the crack propagation rules and data of the friction stir welding component in different areas under the condition of amplitude variation, wherein the formula of the cutting line method is as follows:

where Δ N is the cycle interval, Δ a is the crack length variation value, a_iIs a number of cycles of N_iCrack length of time.

Step 5): the method for calculating the effective stress intensity factor of the friction stir welding component is obtained by correcting based on the form of an Elber crack closure theory, and the Elber crack closure theory formula is as follows:

ΔK_eff＝K_max-K_op (2)

wherein: k_maxIs the maximum stress intensity factor; k_opIs a crack opening stress intensity factor; Δ K_effIs an effective stress intensity factor; y is a geometric correction factor; k is a stress intensity factor; sigma is external stress; and a is the surface semielliptical depth.

Based on the influence of the load sequence and the performance of each region of the member on the fatigue mechanism of the friction stir welding member, a load sequence coefficient M is provided_KAnd establishing a new effective stress intensity factor calculation method by using the correction factor f.

The calculation formula of the corrected effective stress intensity factor is as follows:

△K_eff＝K_max-M_K·f·K_op (4)

wherein: m_KIn order to be the load order factor,_fis a correction factor.

Coefficient of load order M_KThe value of (A) is related to the load sequence and the performance of each area of the joint, the change of the load sequence and the difference of the material properties of the friction stir welding component cause the change of the crack opening stress level of different areas of the joint, thereby causing the change of the crack propagation driving force, and therefore, the load sequence coefficient M is introduced_KTo correct the crack tensionStress intensity factor term at opening stress level, M under different load sequences_KThe values of (A) are different;

high low load M_KThe formula is as follows:

low high load M_KThe formula is as follows:

the parameters required by the above formula are defined as follows:

wherein: a is_OLCrack length under overload; r is_OLMonotonic compression of the plastic zone size due to overload; a is_ULCrack length at low load; r is_ULMonotonic tensile plastic zone size due to low load; r is_yiCurrent cycle plastic zone size. Sigma_yiYield stress of each region of the component; delta K_ULStress intensity factor amplitude at low load; delta K_iIs the load stress intensity factor amplitude; k'_bThe cyclic strength coefficient of the weak area corresponding to the load; k'_b+1The cyclic intensity coefficient of the adjacent area; n is friction stir weldingComponent corresponding region load order effect index; delta K_OLThe magnitude of the stress intensity factor when overloading.

The value of the correction factor f is related to the grain size, the difference of the grain sizes of different areas of the friction stir welding component has a larger influence on the crack propagation mode, the influence of the grain size on the crack propagation rate is gradually weakened along with the increase of the crack length, therefore, a correction factor f is introduced to correct a stress intensity factor item at the crack opening stress level, and the calculation formula of the correction factor f is as follows:

wherein: d is the average grain size of the corresponding area of the friction stir welding component; a is_iCurrent crack length.

Coefficient of load order M_KAnd a correction factor f for correcting the stress intensity factor term at the crack opening stress level, wherein the weld nucleus region (NZ), the thermomechanical influence region (TMAZ), the heat influence region (HAZ) and the base material region (BM) of the friction stir welding component correspond to different M respectively_KAnd f is the same as the first threshold value.

Step 6) calculating corresponding correction factor M through the performance and load sequence effect of each area of the friction stir welding component_KAnd f desired material parameters.

Step 7): the crack propagation rate delta K of the friction stir welding component under different stress ratios is obtained by combining the constant amplitude fatigue crack propagation data under different loading levels and utilizing the corrected effective stress intensity factor method_effA da/dN baseline, which is defined by a correction factor M_KAnd f, aggregating the crack propagation rate curves of different stress ratios and fractures in different areas, and fitting to obtain the material constant C, m.

Step 8): initial crack size a₀And critical crack size a_cDetermination of (1); initial sizes of microstructures of each region (a weld nucleus region (NZ), a thermomechanical influence region (TMAZ), a heat influence region (HAZ) and a base metal region (BM)) of the friction stir welding component are different, metallographic observation is carried out on each region of the friction stir welding component, and strengthening in the corresponding region is carried outThe average size of the particles is taken as the initial crack size a corresponding to the region₀Critical crack length a_cThrough fracture toughness K_ICThe formula is as follows:

wherein K_ICIs the fracture toughness, σ, of the material_maxThe maximum external load stress.

Step 9): combining the weak areas determined in the step 1), respectively selecting the initial crack lengths a of the corresponding weak areas₀Calculating the crack opening stress sigma under the load corresponding to the initial crack size by using a crack opening stress calculation formula provided by Duquessay DL_op1，σ_op2And respectively calculating corresponding opening stress intensity factors K according to a formula (3)_op1，K_op2The crack opening stress calculation formula is as follows:

wherein sigma_opIs a splaying stress; sigma_maxIs the maximum stress in the load sequence; sigma_minIs the minimum stress in the load sequence; sigma_yIs the yield strength; alpha and beta are empirical constants, and when the material is an aluminum alloy, alpha is 0.45, and beta is 0.2.

Step 10): combining the weak areas determined in the step 1), respectively selecting the initial crack lengths a of the corresponding weak areas₀Determining a weak area correction factor M at the current load level_KAnd f, calculating an effective stress intensity factor delta K corresponding to the initial crack size by using the corrected effective stress intensity factor calculation formula (4)_eff1，ΔK_eff2。

Step 11): constant C, m determined by step 7) and effective stress intensity factor Δ K determined by step 5)_effRespectively calculating effective stress intensity factors delta K of two cracks by a formula_eff1,i,ΔK_eff2,iAll right (1)The crack propagation increment of the weak area at each cycle is calculated using the Paris equation:

Δa_i＝C(ΔK_eff,i)^m (14)

the crack size at the current cycle is calculated by the following formula:

a_i＝a_i-1+Δa_i (15)

step 12): the crack growth increment Δ a per cycle was calculated by the formula (4) and the formula (14)_i,1,Δa_i,2The crack length a at the current cycle is calculated by the formula (15)_i,1,a_i,2. Through continuous cycle calculation, the current crack size a_iContinuously updated when the size of one of the cracks reaches the critical crack size a for the load_cAnd then the fatigue life of the friction stir welding component under different loading levels is obtained.

In order to verify the accuracy of the friction stir welding member fatigue life prediction model based on the load sequence and the parameters of each region of the member, the life prediction result calculated by the method is compared with the variable amplitude fatigue test result, and the prediction result and the test result are within a factor of two, so that the proposed life prediction method can better predict the fatigue life of the friction stir welding member under the variable amplitude load.