A method for acquiring a nonlinearly corrected weld fatigue parameter

Abstract

The present application relates to a kind of nonlinear correction weld fatigue parameter acquisition method, the method includes the following steps: S01, by designing two kinds of boundary stress state sample of bending ratio r=0 and r=1 is carried out fatigue test, respectively obtains the load-life data under two kinds of stress states;S02, by strength test respectively obtains the load-displacement curve corresponding to two kinds of stress states, wherein, load-displacement curve is nonlinear;S03, the rigidity of stress state linear section is calculated according to the load-displacement curve of strength test, obtains the linear load-displacement curve based on the rigidity.The present application combines strength tensile test, and the principle of equal work is used to modify the nonlinear load of fatigue test process to linear load, to improve the precision of fitting weld fatigue parameter, while the final output is still S-N curve, can be conveniently applied in mainstream commercial fatigue analysis software, carries out engineering application.

Description

Technical Field

[0001] This invention belongs to the field of weld fatigue calculation technology, specifically relating to a nonlinear correction method for obtaining weld fatigue parameters. Background Technology

[0002] As a crucial means of connection in automobiles, welds are prone to serious safety issues due to fatigue cracking. Therefore, it is essential to strictly control weld fatigue quality during the automobile design phase. CAE analysis, as a vital tool for weld quality control, primarily requires obtaining accurate parameter inputs.

[0003] The current mainstream method for obtaining fatigue parameters of automotive welds is the structural stress method. This method assumes that the loads in fatigue tests are all linear and does not consider the nonlinear changes in loads during low-cycle fatigue loading. Low-cycle fatigue is often the area of ​​interest in structural durability analysis. The actual structural stress S is obtained by directly multiplying the load from the fatigue test with the structural stress calculated by the finite element method. Therefore, the obtained SN curve is more conservative than the actual stress.

[0004] Chinese patent document CN106354898A discloses a technology entitled "A method for calculating the fatigue life of welds based on total strain energy density". This technology proposes to consider elasticity and plasticity using the energy density method and to use the total strain energy density-life curve instead of the SN curve. However, this technology requires back-calculation of parameters through finite element simulation of cyclic loading process, which is complicated. Moreover, existing commercial software cannot directly calculate the life from strain energy density, resulting in low calculation efficiency and great difficulty in engineering application. Summary of the Invention

[0005] The purpose of this invention is to provide a nonlinear correction method for obtaining weld fatigue parameters, which solves the technical problem that existing methods for obtaining weld fatigue parameters do not take into account the plastic deformation during the test process, have insufficient accuracy, and are complicated to implement, making them difficult to apply in engineering.

[0006] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is: a method for obtaining weld fatigue parameters with nonlinear correction, comprising the following steps:

[0007] S01, fatigue tests were conducted on specimens with two boundary stress states, r=0 and r=1, to obtain load-life data under the two stress states respectively.

[0008] S02, load-displacement curves corresponding to two stress states are obtained through strength testing, where the load-displacement curves are nonlinear;

[0009] S03, calculate the stiffness of the linear segment of the stress state based on the load-displacement curve of the strength test, and obtain the linear load-displacement curve based on the stiffness.

[0010] S04. Based on the principle of equal work done, and using linear load-displacement curves and nonlinear load-displacement curves, the corrected load-life data is obtained.

[0011] S05, combined with the finite element model, the structural stress of the weld element is extracted, and the SN curve of the weld under two boundary states is obtained by fitting the corrected load-life data.

[0012] Preferably,

[0013] In S01, the steps for designing specimens with two bending ratios are as follows:

[0014] S011, Establish the finite element analysis model of the sample;

[0015] S012, Apply unit load;

[0016] S013, calculate the element with the greatest damage;

[0017] S014, extract the nodal forces and moments of the element, calculate the membrane stress and bending stress respectively, and then calculate the bending ratio r of the element.

[0018] Preferably,

[0019] In step S04, the work done under linear load and the work done under nonlinear load are made equal, and the corrected load-life data are obtained by the following steps:

[0020] S041, based on the linear segment of the load-displacement curve from the strength test, select three different points on the linear segment to calculate the stiffness, take the average value, and then calculate the stiffness K based on the linear segment. L The linear load F is calculated. L ;

[0021] S042, Fit the entire nonlinear load-displacement curve of the strength test based on the Voce hardening criterion;

[0022] S043, Calculation of work W based on linear load-displacement formula L ;

[0023] S044, Calculation of work W based on nonlinear load-displacement formula NL ;

[0024] S045, by equalizing the linear work and the nonlinear work, we obtain different nonlinear loading loads F. NL The corresponding linear load F L .

[0025] Preferably,

[0026] In S041, the stiffness K of the linear segment L Calculate using the following formula:

[0027]

[0028] Where, ΔF i (i = 1, 2, 3) represents the load difference between two points on the linear segment, ΔX i (i = 1, 2, 3) represents the displacement difference between the two points;

[0029] Linear load F L The calculation formula is as follows:

[0030] F L =K L X L

[0031] Among them, X L This represents the displacement corresponding to a linear load.

[0032] Preferably,

[0033] In S042, the following formula is used for fitting:

[0034]

[0035] Among them, F NL For the load of the strength test, X NL The displacement is the load corresponding to the strength test, and a, b, c, and d are coefficients.

[0036] Preferably,

[0037] In S043, work W is done L The calculation formula is:

[0038]

[0039] Preferably,

[0040] In S044, work W is done NL The calculation formula is:

[0041] W NL =∫F NL dx

[0042] Preferably,

[0043] In S045, different nonlinear loading loads F are obtained according to the following formula. NL The corresponding linear load F L :

[0044]

[0045] Among them, X NL The displacement is given by the nonlinear load-displacement curve, from which the corrected F can be obtained. L -N data.

[0046] Preferably,

[0047] In S05, the corrected F L Multiply by the structural stress to obtain the SN curve.

[0048] By adopting the above technical solution, the beneficial technical effects that this invention can achieve are as follows: This invention proposes a method for obtaining weld fatigue parameters that considers the elastoplasticity of fatigue test loads and performs nonlinear correction on them. Combined with tensile strength tests, the nonlinear loads in the fatigue test process are corrected to linear loads using the principle of equal work done, thereby improving the accuracy of the fitted weld fatigue parameters. At the same time, the final output of this method is still the SN curve, which can be easily applied in mainstream commercial fatigue analysis software for engineering applications. Attached Figure Description

[0049] Figure 1 This is a flowchart of the present invention;

[0050] Figure 2 A schematic diagram of a specimen with a bending ratio r = 0 and the loading process;

[0051] Figure 3 This is a schematic diagram of a method for calculating the stiffness of a linear segment;

[0052] Figure 4 A schematic diagram illustrating the principle of correcting a nonlinear load to a linear load based on the principle of equal work done;

[0053] Figure 5 It is a linear load obtained by correcting the fatigue test load based on the nonlinear load data from the strength test;

[0054] Figure 6 This is a comparison of the SN curves before and after the correction. Detailed Implementation

[0055] The invention will now be further described with reference to the accompanying drawings.

[0056] like Figure 1 As shown, this invention proposes a method for obtaining weld fatigue parameters with nonlinear correction, including the following steps:

[0057] S01, fatigue tests were conducted on specimens under boundary stress states with two bending ratios, r=0 and r=1, to obtain load-life data for each stress state. The steps for designing specimens with the two bending ratios are as follows: S011, establishing a finite element analysis model of the specimen; S012, applying a unit load; S013, calculating the element with the greatest damage; S014, extracting the nodal forces and moments of the element, calculating the membrane stress and bending stress respectively, and then calculating the bending ratio r of the element. Figure 2 As shown, taking the acquisition of fatigue parameters of a weld with a bending ratio r = 0 as an example, the specimen is first designed, and the loading direction is as shown by the arrow. The maximum damage element around the weld is calculated by applying a unit load through finite element simulation. The nodal force and moment of the element are extracted, the membrane stress and bending stress are calculated, and then the bending ratio r of the element is calculated. When r ≤ 0.1, the specimen is considered to meet the requirements.

[0058] S02, load-displacement curves corresponding to two stress states are obtained through strength testing, where the load-displacement curves are non-linear; S03, the stiffness of the linear segment of the stress state is calculated based on the load-displacement curves from the strength tests, and a linear load-displacement curve based on this stiffness is obtained. Strength tests are performed on the specimens; three specimens are selected and broken separately to obtain three load-displacement curves. The average value of the peak loads of the three curves is taken, and the curve with the peak load closest to this load is selected as the strength curve of the specimen.

[0059] S04, based on the principle of equal work done, and using both linear and nonlinear load-displacement curves, the corrected load-life data is obtained. Specifically, in S04, the work done under linear load and the work done under nonlinear load are made equal, and the corrected load-life data is obtained through the following steps:

[0060] S041, based on the linear segment of the load-displacement curve from the strength test, select three different points on the linear segment to calculate the stiffness, take the average value, and then calculate the stiffness K based on the linear segment. L The linear load F is calculated. L The stiffness K of the linear segment L Calculate using the following formula:

[0061]

[0062] Where, ΔF i (i = 1, 2, 3) represents the load difference between two points on the linear segment, ΔX i (i = 1, 2, 3) represents the displacement difference between the two points;

[0063] Linear load F L The calculation formula is as follows:

[0064] F L=K L X L

[0065] Among them, X L This represents the displacement corresponding to a linear load. The strength curve is processed first, and the stiffness is calculated, such as... Figure 3 As shown, three sets of data for the linear segment were selected for stiffness calculation, and the average stiffness of the three sets was taken as the stiffness of the linear segment of the specimen.

[0066]

[0067] In this example, K L = 93109 N / mm.

[0068] The formula for the intensity curve is: F L =93109X L

[0069] S042, the entire nonlinear load-displacement curve of the strength test is fitted based on the Voce hardening criterion. The following formula is used for fitting:

[0070]

[0071] Among them, F NL For the load of the strength test, X NL Let a, b, c, and d be the displacements corresponding to the loads in the strength test, and a, b, c, and d be coefficients. The strength curve was fitted using Matlab software, referencing the Voce hardening criterion. The fitting formula used is as follows:

[0072]

[0073] In this embodiment, a = 58670, b = 0.09625, c = -56920, and d = -2.277.

[0074] Fatigue tests were performed on the specimens. Based on the peak load of the test, fatigue tests were first conducted at 50% of the peak load. The load was adjusted according to the test life results, and finally data were obtained under 5 different load levels, so that the test life results were distributed between 1,000 and 2 million cycles. Each load level was tested 3 times to obtain the load-life data of the specimens.

[0075] S043, Calculation of work W based on linear load-displacement formula L Work done W L The calculation formula is:

[0076]

[0077] S044, Calculation of work W based on nonlinear load-displacement formulaNL Work done W NL The calculation formula is:

[0078] W NL =∫F NL dx

[0079] Work done based on nonlinear load-displacement formula:

[0080]

[0081] S045, by equalizing the linear work and the nonlinear work, we obtain different nonlinear loading loads F. NL The corresponding linear load F L The different nonlinear loading loads F are obtained according to the following formula. NL The corresponding linear load F L :

[0082]

[0083] Among them, X NL The displacement is given by the nonlinear load-displacement curve, from which the corrected F can be obtained. L -N data. According to the work-energy theorem, let linear work equal nonlinear work, i.e., W. L =W NL The different nonlinear loading loads F are derived. NL The corresponding linear load F L Based on the load from the fatigue test, find the corresponding displacement value in the strength test curve, and substitute it into the above formula to obtain the corrected load.

[0084] like Figure 5 As shown, based on the overlapping area of ​​the nonlinear curve and the linear curve, it can be determined that the load above 30000N has entered the plastic region. Therefore, the fatigue test load exceeding 30000N is corrected. The loads before and after correction are shown in the table below.

[0085]

[0086]

[0087] S05, combining the finite element model to extract the structural stress of the weld element, and fitting the corrected load-life data to obtain the SN curves of the weld under two boundary states. The corrected F... L Multiply by the structural stress to obtain the SN curve.

[0088] Based on the structural stress results, the corrected load is multiplied by the structural stress under unit load to obtain the corrected SN curve. This curve can be imported into commercial software for calculation. The corrected low-cycle load is significantly increased, such as... Figure 6 As shown, the modified SN curve has a longer life in the low-cycle region than the unmodified curve. By taking the low-cycle plastic deformation into account in the SN curve, the analysis results are more accurate.

Claims

1. A method for obtaining weld fatigue parameters with nonlinear correction, characterized in that, Includes the following steps: S01, fatigue tests were conducted on specimens under two boundary stress states with bending ratios r=0 and r=1 to obtain load-life data under the two stress states. The steps for designing specimens with the two bending ratios are as follows: S011, Establish the finite element analysis model of the sample; S012, Apply unit load; S013, calculate the element with the greatest damage; S014, extract the nodal forces and moments of the element, calculate the membrane stress and bending stress respectively, and then calculate the bending ratio r of the element; S02, load-displacement curves corresponding to two stress states are obtained through strength testing, where the load-displacement curves are nonlinear; S03, calculate the stiffness of the linear segment of the stress state based on the load-displacement curve of the strength test, and obtain the linear load-displacement curve based on the stiffness. S04, assuming the work done under linear load is equal to the work done under nonlinear load, based on the linear load-displacement curve and the nonlinear load-displacement curve, obtain the corrected load-life data, including... S041, based on the linear segment of the load-displacement curve from the strength test, select three different points on the linear segment to calculate the stiffness, take the average value, and then calculate the stiffness K based on the linear segment. L The linear load F is calculated. L ; S042, Fit the entire nonlinear load-displacement curve of the strength test based on the Voce hardening criterion; S043, Calculation of work W based on linear load-displacement formula L ; S044, Calculation of work W based on nonlinear load-displacement formula NL ; S045, by equalizing the linear work and the nonlinear work, we obtain different nonlinear loading loads F. NL The corresponding linear load F L； S05, combined with the finite element model, the structural stress of the weld element is extracted, and the SN curve of the weld under two boundary states is obtained by fitting the corrected load-life data.

2. The method according to claim 1, characterized in that, In S041, the stiffness K of the linear segment L Calculate using the following formula: in, The load difference between two points on the linear segment. This represents the displacement difference between the two points. Linear load F L The calculation formula is as follows: Among them, X L This represents the displacement corresponding to a linear load.

3. The method according to claim 2, characterized in that, In S042, the following formula is used for fitting: in, The load for strength testing, The displacement is the load corresponding to the strength test, and a, b, c, and d are coefficients.

4. The method according to claim 3, characterized in that, In S043, the work W is done L The calculation formula is: 。 5. The method according to claim 4, characterized in that, In S044, work W is done NL The calculation formula is: 。 6. The method according to claim 5, characterized in that, In S045, different nonlinear loading loads F are obtained according to the following formula. NL The corresponding linear load F L : in, The displacement is under the nonlinear load-displacement curve, from which the corrected displacement can be obtained. data.

7. The method according to claim 6, characterized in that, In S05, the corrected F L Multiply by the structural stress to obtain the SN curve.

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