A three-dimensional fatigue crack growth behavior prediction method considering surface strengthening effect

Abstract

The present invention discloses a three-dimensional fatigue crack growth behavior prediction method that considers the surface strengthening effect. The method comprises the following steps: performing surface strengthening on a metal material specimen, characterizing the residual stress of the strengthened metal material specimen, obtaining the gradient distribution of the residual stress inside the metal material specimen, and obtaining the three-dimensional spatial distribution function of the residual stress inside the specimen through numerical fitting. Based on the performance parameters of the metal material, model parameters for fatigue crack growth calculation are obtained through uniaxial tension calculation and fatigue strength calculation. According to the three-dimensional spatial distribution function of the residual stress, the performance parameters and model parameters of the metal material are combined with the residual stress, and a three-dimensional fatigue fracture phase field calculation model is used to predict the fatigue crack growth behavior of the metal material. Considering the influence of the surface strengthening treatment on the residual stress distribution inside the material, a three-dimensional fatigue fracture phase field model considering the gradient structure is established, which is beneficial for improving the prediction of the effect of surface treatment on the fatigue performance of the material.

Description

Technical Field

[0001] The present invention relates to structural fatigue life prediction, and in particular to a three-dimensional fatigue crack growth behavior prediction method considering surface strengthening effect. Background Art

[0002] The fatigue crack growth behavior of metallic structures is a crucial factor in assessing the load-bearing capacity of components. Given fixed design and manufacturing processes, surface treatment is an effective means of improving a material's fatigue fracture resistance. Surface treatment techniques include, but are not limited to, shot peening, laser shock peening, and mechanical grinding and polishing. Surface treatment can promote grain refinement, dislocation accumulation, and the formation of residual compressive stresses at great depths on the material surface, which are the primary causes of significant changes in the mechanical properties of the treated material.

[0003] In the existing technology, Miao et al. "On the potential applications of a 3D randomfinite element model for the simulation of shot peening" used a three-dimensional random shot peening finite element model to study the residual stress and plastic strain distribution in the material after multi-shot impact. Guo Xiaojun et al. "Numerical simulation of laser shock peening of high-temperature alloys and prediction of their fatigue life" proposed a three-dimensional multi-scale simulation method for laser shock peening, studied the residual stress distribution of Inconel718 after laser shock peening, and predicted the effect of laser shock peening on fatigue life based on experiments. Aswegen and Polese "Experimental and analytical investigation of the effects of laser shock peening processing strategy on fatigue crack growth inthin 2024aluminum alloy panels" performed laser shock peening on 2024 aluminum alloy, compared the experimental results with the model predictions, and studied the fatigue crack growth of the material before and after surface treatment. Currently, the gradient stress introduced by surface treatment is mainly simulated by laser shock using the commercial software ABAQUS. The residual stress results are affected by the laser size and shock time in the simulation and cannot be directly linked to the residual stress distribution obtained in the experiment. As a result, the results of fatigue life prediction and crack growth are not accurate enough. Summary of the Invention

[0004] Purpose of the invention: In view of the above shortcomings, the present invention provides a three-dimensional fatigue crack growth behavior prediction method taking into account the surface strengthening effect to improve the prediction accuracy.

[0005] Technical solution: To solve the above problems, the present invention adopts a three-dimensional fatigue crack growth behavior prediction method considering the surface strengthening effect, which includes the following steps:

[0006] Step 1: Perform surface strengthening on the metal material specimen, characterize the residual stress of the strengthened metal material specimen, obtain the gradient distribution of the residual stress inside the metal material specimen, and obtain the three-dimensional spatial distribution function of the residual stress inside the specimen through numerical fitting;

[0007] Step 2: Based on the performance parameters of the metal material, obtain the model parameters for fatigue crack growth calculation through uniaxial tension calculation and fatigue strength calculation;

[0008] Step 3: Based on the three-dimensional spatial distribution function of the residual stress, the residual stress is converted into intrinsic strain using the initial stress, the performance parameters and model parameters of the metal material are combined with the residual stress, and a fatigue fracture phase field calculation model is used to predict the fatigue crack growth behavior of the metal material.

[0009] Furthermore, the surface strengthening of the metal material specimen includes shot peening and laser shock strengthening. The gradient distribution of the residual stress inside the strengthened metal material specimen is measured using an X-ray diffraction stress meter. The three-dimensional spatial distribution function of the residual stress inside the specimen is obtained by nonlinear fitting using the least squares method, and the fitting function form includes a polynomial form, a power exponential form, and a trigonometric function form. The performance parameters of the metal material include Young's modulus, Poisson's ratio, strength limit, critical fracture toughness and fatigue strength of the metal material; these parameters are obtained by performing uniaxial tension, fracture toughness measurement and fatigue limit test on the metal material specimen. Model parameters include interface width, energy threshold and exponential parameter.

[0010] Furthermore, the three-dimensional fatigue fracture phase field calculation model includes the stress balance equation

[0011]

[0012] in, is the gradient operator, σ is the stress tensor. Fracture order parameter evolution equation:

[0013]

[0014] Among them, c(x,y,z) is the fracture phase field order parameter, η is the dynamic coefficient, l is the interface width, G c is the critical fracture energy density, t is the time, is the fracture energy density decay function, It is the history variable driving crack growth and depends on the maximum value of tensile elastic strain energy in all current time steps.

[0015] The fracture energy density attenuation function is:

[0016]

[0017] in, is the accumulated tensile elastic strain energy during fatigue loading, α T is the energy threshold, and κ is the exponential parameter.

[0018] The present invention also adopts a computer device, including a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein the processor implements the steps of the above method when executing the computer program.

[0019] The present invention also provides a computer-readable storage medium having a computer program stored thereon, wherein the computer program implements the steps of the above method when executed by a processor.

[0020] Beneficial effects: Compared with the existing technology, the significant advantage of the present invention is that it takes into account the influence of surface strengthening treatment on the residual stress distribution inside the material, and establishes a fatigue fracture phase field model with a three-dimensional spatial distribution gradient structure considering the residual stress, which is beneficial to improving the prediction of the effect of surface treatment on the fatigue performance of the material. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 It is a schematic diagram of the process of the present invention.

[0022] Figure 2 Schematic diagram of the structure of the fatigue crack extension specimen in the present invention.

[0023] Figure 3 Schematic diagram of laser shock peening of a sample in the present invention.

[0024] Figure 4 This is a comparison diagram of the measurement and fitting of the residual stress distribution of laser shock strengthening in the present invention.

[0025] Figure 5 This is the fatigue crack growth curve of the material before and after laser shock in the present invention. DETAILED DESCRIPTION

[0026] like Figure 1 As shown, a three-dimensional fatigue crack growth behavior prediction method considering the surface strengthening effect in this embodiment includes the following steps:

[0027] Step 1: Perform uniaxial tension, fracture toughness test and fatigue limit test on the metal material to obtain the material's mechanical properties such as Young's modulus, Poisson's ratio, ultimate strength, and critical fracture toughness and fatigue strength.

[0028] Uniaxial tension, fracture toughness, fatigue strength, and fatigue crack growth tests were performed on TC17 titanium alloy, and the material parameters were determined as shown in Table 1.

[0029] Table 1: Mechanical properties of TC17 titanium alloy

[0030] Parameter name Parameter value Young's modulus 112GPa Poisson's ratio 0.301 fracture toughness <![CDATA[2.15×10 4 N / m]]> R=0.1 fatigue limit 510MPa

[0031] Step 2: Perform surface strengthening (shot peening or laser shock strengthening) on ​​the metal material specimen, characterize the residual stress of the strengthened specimen, obtain the gradient distribution of the residual stress inside the specimen, and obtain the three-dimensional spatial distribution function of the residual stress inside the specimen through numerical fitting.

[0032] Laser shock peening was performed on TC17 titanium alloy compact tensile specimens. The dimensions of the compact tensile specimens are as follows: Figure 2 , its characteristic length W is 50mm and thickness is 5mm. The laser shock position is as follows Figure 3 , double-sided impact is applied to the sample surface. The residual stress in the material is measured using an X-ray diffraction stress tester, and the residual stress is distributed gradiently along the impact direction. The least squares method is used for nonlinear fitting, see Figure 4 As shown in the figure, after double-sided laser shock processing, the residual stress is symmetrically distributed along the symmetry axis in the depth direction. In the impact area with a depth of 0-2.5 mm, the fitting function of the residual stress along the laser shock direction is:

[0033]

[0034] Among them, x is the depth along the gradient direction, σ residual is the objective function of residual stress.

[0035] Step 3: Perform uniaxial tension calculation and fatigue strength calculation based on the performance parameters obtained from the test in step 1 to determine the model parameters for fatigue crack growth calculation.

[0036] The phase field fatigue fracture model mainly includes the stress equilibrium equation

[0037]

[0038] in, is the gradient operator, σ is the stress tensor. Fracture order parameter evolution equation

[0039]

[0040] Where c(x,y,z) is the fracture phase field order parameter, η is the kinetic coefficient, l is the interface width, G c is the critical fracture energy density, t is the time, is the fracture energy density decay function, As the historical variable driving crack growth, the maximum elastic energy experienced at that time step is taken, that is,

[0041]

[0042] The fracture energy decay function is in the form of

[0043]

[0044] in is the accumulated tensile elastic strain energy during fatigue loading, α T is the energy threshold value, and κ is the exponential parameter. Based on the one-dimensional uniaxial tensile fracture simulation and fatigue limit calculation of TC17 titanium alloy, the interface width, energy threshold, and exponential parameter were determined. The calibrated TC17 model parameters are shown in Table 2.

[0045] Table 2: Phase field model parameters of TC17 titanium alloy

[0046] Parameter name Parameter value Interface width 0.2mm Fatigue threshold <![CDATA[1.34375×10 7 J / m 3 ]]> Fatigue Index 0.2

[0047] Step 4: Input the three-dimensional spatial distribution function of the residual stress of the surface-strengthened specimen into the calculation model to simulate and calculate the fatigue crack growth performance of the surface-treated structure.

[0048] The mechanical parameters of TC17 titanium alloy such as Young's modulus, Poisson's ratio, ultimate strength, critical fracture toughness and fatigue strength measured in step 1, and the simulation parameters such as interface width, energy threshold and exponential parameter determined by fitting in step 3 are input into the fatigue fracture phase field calculation model. Combined with the geometry of the TC17 titanium alloy specimen and the gradient residual stress function determined in step 2, the constitutive relationship is established:

[0049] σ residual =Cε eigen

[0050] Where C is the stiffness matrix, ε eigen is the intrinsic strain tensor. The residual stress is converted into intrinsic strain using the initial stress, and the fatigue crack growth performance of the structure is obtained by finite element calculation. The crack growth curve of TC17 titanium alloy before and after laser shock strengthening is as follows: Figure 5 .

Claims

1. A three-dimensional fatigue crack growth behavior prediction method considering surface strengthening effect, characterized in that: The following steps are involved: Step 1: Perform surface strengthening on the metal material specimen, characterize the residual stress of the strengthened metal material specimen, obtain the gradient distribution of the residual stress inside the metal material specimen, and obtain the three-dimensional spatial distribution function of the residual stress inside the specimen through numerical fitting; Step 2: Based on the performance parameters of the metal material, obtain the model parameters for fatigue crack growth calculation through uniaxial tension calculation and fatigue strength calculation; Step 3: Based on the three-dimensional spatial distribution function of the residual stress, the residual stress is converted into intrinsic strain using the initial stress, and the performance parameters and model parameters of the metal material are combined with the residual stress to calculate the model based on the fatigue fracture phase field; the fatigue crack growth behavior of the metal material is predicted; the performance parameters of the metal material include Young's modulus, Poisson's ratio, ultimate strength, critical fracture toughness and fatigue strength of the metal material; the model parameters include interface width, energy threshold and exponential parameter; The fatigue fracture phase field calculation model includes the stress balance equation in, is the gradient operator, σ is the stress tensor; The fatigue fracture phase field calculation model includes the fracture order parameter evolution equation: Among them, c(x,y,z) is the fracture phase field order parameter, η is the dynamic coefficient, l is the interface width, G c is the critical fracture energy density, t is the time, is the fracture energy density decay function, It is the history variable driving crack growth and depends on the maximum value of tensile elastic strain energy in all current time steps.

2. The three-dimensional fatigue crack growth behavior prediction method according to claim 1, characterized in that: The surface strengthening of the metal material specimen includes shot peening and laser shock peening.

3. The three-dimensional fatigue crack growth behavior prediction method according to claim 1, characterized in that: The gradient distribution of residual stress inside the strengthened metal material specimens was measured using an X-ray diffraction stress measuring instrument.

4. The three-dimensional fatigue crack growth behavior prediction method according to claim 1, characterized in that: The three-dimensional spatial distribution function of the residual stress inside the specimen is obtained by nonlinear fitting using the least squares method. The fitting function forms include polynomial form, power exponential form, and trigonometric function form.

5. The three-dimensional fatigue crack growth behavior prediction method according to claim 1, characterized in that: The performance parameters of the metal material are obtained by performing uniaxial tension, fracture toughness measurement and fatigue limit test on the metal material specimen.

6. The three-dimensional fatigue crack growth behavior prediction method according to claim 1, characterized in that: The fracture energy density attenuation function is: in, is the accumulated tensile elastic strain energy during fatigue loading, α T is the energy threshold, and κ is the exponential parameter.

7. A computer device comprising a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein: When the processor executes the computer program, the steps of the method according to any one of claims 1 to 6 are implemented.

8. A computer-readable storage medium having a computer program stored thereon, characterized in that: When the computer program is executed by a processor, the steps of the method according to any one of claims 1 to 6 are implemented.

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