A method for predicting low-cycle fatigue life of laser shock strengthened powder high-temperature alloy based on crystal plasticity

Abstract

The invention discloses a method for predicting the low-cycle fatigue life of a laser shock strengthened powder high-temperature alloy based on crystal plasticity, comprising the following steps: obtaining the longitudinal and transverse residual stresses of an LSP powder high-temperature alloy test piece distributed with depth, and the work hardening field of the LSP powder high-temperature alloy test piece distributed with depth; establishing an overall model, assigning the residual stress field, the work hardening field and the corresponding material properties to the overall model, and performing finite element simulation to obtain the displacement field; calibrating the material parameters in the crystal plasticity constitutive model through a low-cycle fatigue test; establishing a crystal plasticity sub-model; assigning the residual stress field and the work hardening field introduced by LSP to the sub-model; performing finite element simulation using the sub-model, taking the displacement field of the overall model as the boundary condition of the sub-model, analyzing the low-cycle fatigue damage evolution law of the LSP powder high-temperature alloy test piece, and obtaining the fatigue indication factor; and realizing the fatigue life prediction of the LSP powder high-temperature alloy test piece.

Description

Technical Field

[0001] The invention belongs to the field of structural strength and finite element simulation, and particularly relates to a method for predicting low-cycle fatigue life of laser shock strengthened powder high-temperature alloy based on crystal plasticity. Background Art

[0002] Nickel-based powder superalloys are widely used as key materials for manufacturing rotating parts such as high-performance aircraft engine turbine disks due to their good high-temperature strength, high structural stability, low fatigue crack growth rate and excellent creep and oxidation resistance. With the continuous improvement of the thrust-to-weight ratio (power-to-weight ratio) and fuel efficiency of aircraft engines, the inlet temperature of the engine turbine is also increasing, which puts higher requirements on the strength, fatigue performance, reliability and durability of powder superalloys for turbine disks. As one of the most critical hot end components of aircraft engines, turbine disks are subjected to complex alternating mechanical loads and thermal loads during service, which leads to low-cycle fatigue failure of powder superalloys for turbine disks during service, and most of the fatigue life is consumed in the initiation and expansion stage of fatigue microcracks. The low-cycle fatigue failure of powder superalloys for turbine disks is closely related to their microstructure and surface integrity. At present, powder superalloys for turbine disks are often subjected to surface strengthening treatments such as shot peening, low-plasticity polishing, and laser shock strengthening to improve their service fatigue life. Compared with traditional shot peening and low plasticity polishing processes, LSP has the characteristics of non-contact strengthening, which can significantly reduce the roughness change caused by strong plastic deformation of the surface. The generated surface residual compressive stress layer has a large depth and good thermal stability, and can flexibly process components with complex surfaces. By using the LSP process to strengthen the powder high-temperature alloy for turbine disks to introduce deeper surface residual compressive stress, it can effectively improve its fatigue microcrack initiation and extension resistance, thereby improving the safety, reliability and durability of aircraft engines. All of the above shows that laser shock strengthening technology has great application prospects in the field of aircraft engine technology.

[0003] At present, there have been many achievements in the study of fatigue properties of various metal materials after laser shock strengthening under different conditions, but there are few patents on the low-cycle fatigue life prediction methods of materials after laser shock strengthening. The patent with application number CN202110067373.0 "Rapid prediction method and device for crack extension life of laser shock strengthened components" considers the influence of laser shock strengthening on the extension life of components, but does not consider the influence of laser shock strengthening on the fatigue life of components. The patent with application number CN202210103839.2 "A fatigue life prediction method considering the effect of laser shock strengthening" considers the influence of laser shock strengthening on the fatigue life of components, but simply simplifies the influence of residual stress on fatigue life into the average stress effect, and cannot consider the coupling effect of surface microstructure and residual stress, and cannot accurately predict the dispersion of fatigue life. Therefore, it is necessary to actively develop fatigue life prediction methods that consider the influence of residual stress and microstructure. Summary of the invention

[0004] The purpose of the present invention is to provide a method for predicting the low-cycle fatigue life of laser shock strengthened powder high-temperature alloy based on crystal plasticity. The method takes into account the influence of the residual stress field and work hardening field caused by laser shock strengthening on the low-cycle fatigue life of the powder high-temperature alloy, so as to achieve accurate prediction of the fatigue life of the structure after laser shock strengthening, and can serve and support the accurate prediction of the fatigue life of aircraft engine turbine components after laser shock strengthening treatment.

[0005] To achieve the above object, the present invention adopts the following technical solutions:

[0006] A method for predicting low cycle fatigue life of laser shock strengthened powder high temperature alloy based on crystal plasticity comprises the following steps:

[0007] (1) The longitudinal and transverse residual stresses of the laser shock strengthened powder superalloy specimens were obtained with depth distribution by X-ray diffraction and electrolytic polishing technology, and the work hardening field of the laser shock strengthened powder superalloy specimens with depth distribution was obtained by electron backscatter diffraction technology;

[0008] (2) The overall model of the laser shock strengthened powder high-temperature alloy specimen was established in ABAQUS software. The residual stress field, work hardening field and corresponding material properties were assigned to the overall model through the inverse intrinsic strain method and the USDFLD and HARDINI subroutines. Finite element simulation was performed to obtain the displacement field of the overall model.

[0009] (3) Calibrate the material parameters in the crystal plasticity constitutive model through low-cycle fatigue tests;

[0010] (4) Based on the microstructure characterization results of electron backscatter diffraction technology, the crystal plasticity sub-model was established in ABAQUS software using Dream.3D software;

[0011] (5) assigning the residual stress field and work hardening field introduced by laser shock strengthening to the crystal plasticity sub-model established in step (4) through the inverse intrinsic strain method and USDFLD and HARDINI subroutines;

[0012] (6) using the crystal plasticity sub-model to perform finite element simulation, using the displacement field of the overall model obtained in step (2) as the boundary condition of the crystal plasticity sub-model, analyzing the low-cycle fatigue damage evolution law of the laser shock strengthened powder high-temperature alloy test piece, and obtaining the fatigue indicator factor;

[0013] (7) Realize the fatigue life prediction of laser shock strengthened powder high-temperature alloy specimens.

[0014] In the step (1), the work hardening field is obtained by characterizing and analyzing the corresponding side surface or the corresponding cross section of the laser shock strengthened powder high temperature alloy test piece using electron backscatter diffraction technology.

[0015] In the step (3), the fatigue hysteresis loop of the powder high-temperature alloy is obtained by conducting corresponding low-cycle fatigue tests, and then the low-cycle fatigue hysteresis loop is fitted by a trial-and-error method to obtain the crystal plastic constitutive parameters.

[0016] In the step (4), the grain size and Euler angle of the sample surface are obtained by electron backscatter diffraction technology, and then the grain information obtained by the electron backscatter diffraction technology is imported into Dream.3d software in a ctf format file, and the grain orientation distribution information and grain size distribution are output in a txt format; then the grain orientation distribution information file and the grain size distribution parameters fitted by the lognormal distribution function are input into the Dream.3d software, and the model size and grid size are set to automatically generate a representative volume unit that conforms to the grain orientation and size distribution of the powder high-temperature alloy, and then the representative volume unit is imported into ABAQUS software to generate a crystal plasticity sub-model.

[0017] In the step (5), the residual stress introduced by laser shock strengthening is assigned to the crystal plasticity sub-model by the inverse intrinsic strain method, wherein the residual stress field obtained by the experiment is introduced into the ABAQUS finite element model, and the eigenstrain corresponding to the corresponding residual stress is obtained by the inverse intrinsic strain reconstruction method, and then introduced into the finite element model through the subroutine USDFLD, and the work hardening is introduced into the finite element model in the form of equivalent plastic strain, which is implemented by the HARDINI subroutine; the method for obtaining the eigenstrain by the known residual stress is as follows:

[0018]

[0019]

[0020] In the formula, y represents the depth, is the intrinsic strain in the x direction distributed with depth, T i (y) is the Chebyshev polynomial function, N is the order of the Chebyshev polynomial, m is the number of residual stresses measured in the experiment, σ xx (y) is the residual stress value distributed along the depth direction obtained by measuring the experiment, is the residual stress value corresponding to each order Chebyshev polynomial distributed with depth, C i is the unknown coefficient, C i Obtained by least squares method, and then by C i The eigenstrain of the corresponding depth is obtained by multiplying the Chebyshev polynomials corresponding to each depth.

[0021] In the step (6), the crystal plasticity constitutive model in step (3) is substituted into the crystal plasticity sub-model in step (5), that is, the residual stress field and work hardening field generated by laser shock hardening are introduced into the crystal plasticity sub-model, and then the sub-model boundary conditions are given through step (2), so as to obtain the simulation results considering the laser shock hardening effect, and the shear strain, partial shear stress and normal stress on the specific slip plane are extracted from the simulation results, and the cumulative shear strain energy dissipation density is calculated to form a fatigue indication factor (FIP), which is specifically in the following form:

[0022]

[0023] Where α is the slip system, N1 is the number of all slip systems activated, τ α is the shear stress, is the shear strain rate, W P is the fatigue indicator factor, where:

[0024] τ α =m *α ·τ·s *α

[0025] τ=det(F)σ

[0026] Among them, m *α is the normal direction of the slip surface, τ is the Kirchhoff stress, s *α is the slip direction, F is the total strain gradient, and σ is the Cauchy stress;

[0027]

[0028] in, is the reference shear strain rate, n is the rate sensitivity index, χα is the back stress term of the slip system, Critical shear stress.

[0029] In step (7), the fatigue life result is predicted based on the fatigue indicator factor obtained in step (6), as shown below:

[0030] W pc =W P，N +(N total -N)ΔW

[0031] Among them, W pc is the critical cumulative shear strain energy dissipation density, determined by experiment, W P，N is the cumulative shear strain energy dissipation density under a specific number of cycles, obtained through finite element simulation, N total is the full life of the test piece, N is the number of simulation cycles, and ΔW is the cumulative shear strain energy dissipation density increment;

[0032] ΔW changes only in the first five cycles and remains stable in subsequent cycles. P，N is the cumulative shear strain energy dissipation density obtained after five real loading cycles of finite element simulation, that is, W P，N＝5 ; W pc The determination of the life span of a test piece requires a set of low-cycle fatigue tests to obtain the true life span of the test piece. and simulation parameter W P，N＝5 , ΔW, and substituting into the above equation we get W pc , and assume it to be a fixed value in the same batch of test pieces; determine W pc Finally, the fatigue life of the test specimens at different load levels is predicted using the above life prediction criteria.

[0033] Beneficial effects: Compared with the prior art, the present invention has the following advantages:

[0034] (1) The present invention comprehensively considers the influence of the coupling effect of microstructure and residual stress in laser shock strengthened powder high-temperature alloy on its low-cycle fatigue life, and can well demonstrate the dispersion of fatigue life.

[0035] (2) The present invention comprehensively considers the microstructure and residual stress coupling in the laser shock strengthened powder high-temperature alloy, and can accurately predict its fatigue life by introducing a fatigue indicator factor.

[0036] (3) The present invention comprehensively considers the microstructure and residual stress coupling in the laser shock strengthened powder high-temperature alloy, and also takes residual stress relaxation into account in the life prediction, which is more in line with the actual use of the laser shock strengthened structure.

[0037] (4) The present invention comprehensively considers the microstructure and residual stress coupling in laser shock strengthened powder high-temperature alloy, and combines the sub-model technology, which can save a lot of simulation time and obtain accurate simulation results. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 This is a flow chart of a method for predicting low-cycle fatigue life of laser shock strengthened powder high-temperature alloy based on crystal plasticity;

[0039] Figure 2 It is the representative volume unit of the crystal plasticity of powder high temperature alloy;

[0040] Figure 3 is the sub-model for introducing residual stress field;

[0041] Figure 4 is the variation law of the cumulative shear strain energy dissipation density increment with the cycle number. DETAILED DESCRIPTION

[0042] The present invention will be further explained below in conjunction with the accompanying drawings.

[0043] like Figure 1 As shown, a method for predicting low cycle fatigue life of laser shock strengthened powder high temperature alloy based on crystal plasticity of the present invention comprises the following steps:

[0044] (1) The longitudinal and transverse residual stresses of the laser shock strengthened (LSP) powder superalloy specimens were obtained with depth distribution by X-ray diffraction (XRD) and electrolytic polishing technology, and the work hardening field of the laser shock strengthened (LSP) powder superalloy specimens with depth distribution was obtained by electron backscatter diffraction (EBSD);

[0045] The work hardening field is obtained by characterizing and analyzing the corresponding side surface or the corresponding cross section of the laser shock strengthened (LSP) powder high temperature alloy test piece using electron backscatter diffraction technology (EBSD).

[0046] (2) The overall model of the laser shock hardening (LSP) powder high-temperature alloy specimen was established in ABAQUS software. The residual stress field, work hardening field and corresponding material properties were assigned to the overall model through the inverse intrinsic strain method and the USDFLD and HARDINI subroutines. Finite element simulation was performed to obtain the displacement field of the overall model.

[0047] (3) Calibrate the material parameters in the crystal plasticity constitutive model through low-cycle fatigue tests;

[0048] Among them, the fatigue hysteresis loop of the powder high-temperature alloy is obtained by conducting corresponding low-cycle fatigue tests, and then the low-cycle fatigue hysteresis loop is fitted by the trial and error method to obtain the crystal plastic constitutive parameters.

[0049] (4) Based on the microstructure characterization results of electron backscatter diffraction (EBSD) technology, the crystal plasticity sub-model was established in ABAQUS software using Dream.3D software;

[0050] Among them, the grain size and Euler angle of the sample surface are obtained by electron backscatter diffraction technology (EBSD), and then the grain information obtained by electron backscatter diffraction technology (EBSD) is imported into Dream.3d software in ctf format file, and the grain orientation distribution information (ODF) and grain size distribution are output in txt format; then the ODF file and the grain size distribution parameters fitted by the lognormal distribution function are input into Dream.3d software, and the model size and grid size are set to automatically generate representative volume units that conform to the grain orientation and size distribution of the powder high-temperature alloy, and then the representative volume units are imported into ABAQUS software to generate a crystal plasticity sub-model.

[0051] (5) assigning the residual stress field and work hardening field introduced by laser shock strengthening (LSP) to the crystal plasticity sub-model established in step (4) through the inverse intrinsic strain method and USDFLD and HARDINI subroutines;

[0052] Among them, the residual stress introduced by laser shock strengthening (LSP) is assigned to the crystal plasticity sub-model through the inverse intrinsic strain method. The residual stress field obtained by the experiment is introduced into the ABAQUS finite element model. The eigenstrain corresponding to the corresponding residual stress needs to be obtained through the inverse intrinsic strain reconstruction method, and then introduced into the finite element model through the subroutine USDFLD. The work hardening is introduced into the finite element model in the form of equivalent plastic strain, which is realized through the HARDINI subroutine; the method of obtaining the eigenstrain through the known residual stress is as follows:

[0053]

[0054]

[0055] In the formula, y represents the depth, is the intrinsic strain in the x direction distributed with depth, T i (y) is the Chebyshev polynomial function, N is the order of the Chebyshev polynomial, m is the number of residual stresses measured in the experiment, σ xx (y) is the residual stress value distributed along the depth direction obtained by measuring the experiment, is the residual stress value corresponding to each order Chebyshev polynomial distributed with depth, C iis the unknown coefficient, C i Obtained by least squares method, and then by C i The eigenstrain of the corresponding depth is obtained by multiplying the Chebyshev polynomials corresponding to each depth.

[0056] (6) using the crystal plasticity sub-model to perform finite element simulation, using the displacement field of the overall model obtained in step (2) as the boundary condition of the crystal plasticity sub-model, analyzing the low-cycle fatigue damage evolution law of the laser shock strengthened (LSP) powder high-temperature alloy test piece, and obtaining the fatigue indicator factor;

[0057] The crystal plasticity constitutive model (i.e., UMAT) in step (3) is substituted into the crystal plasticity sub-model in step (5), that is, the residual stress field and work hardening field generated by laser shock strengthening (LSP) are introduced into the crystal plasticity sub-model, and then the sub-model boundary conditions are given through step (2) to obtain the simulation results considering the effect of laser shock strengthening (LSP). The shear strain, partial shear stress and normal stress on the specific slip plane are extracted from the simulation results, and the cumulative shear strain energy dissipation density is calculated to form the fatigue indication factor (FIP), which is in the following specific form:

[0058]

[0059] Where α is the slip system, N1 is the number of all slip systems activated, τ α is the shear stress, is the shear strain rate, W P is the fatigue indicator factor, where:

[0060] τ α =m *α ·τ·s *α

[0061] τ=det(F)σ

[0062] Among them, m *α is the normal direction of the slip surface, τ is the Kirchhoff stress, s *α is the slip direction, F is the total strain gradient, and σ is the Cauchy stress;

[0063]

[0064] in, is the reference shear strain rate, n is the rate sensitivity index, χ α is the back stress term of the slip system, is the critical shear stress.

[0065] (7) Achieve fatigue life prediction of laser shock strengthened (LSP) powder high temperature alloy test pieces;

[0066] The fatigue life result is predicted based on the fatigue indicator factor obtained in step (6), as shown below:

[0067] W pc =W P，N +(N total -N)ΔW

[0068] Among them, W pc is the critical cumulative shear strain energy dissipation density, determined by experiment, W P，N is the cumulative shear strain energy dissipation density under a specific number of cycles, obtained through finite element simulation, N total is the full life of the test piece, N is the number of simulation cycles, and ΔW is the cumulative shear strain energy dissipation density increment;

[0069] In the present invention, it is believed that ΔW changes only within the initial five cycles and remains stable in subsequent cycles. The specific change rule is as follows: Figure 4 As shown, W P，N is the cumulative shear strain energy dissipation density obtained after five real loading cycles of finite element simulation, that is, W P，N＝5 .W pc The determination of the life span of a test piece requires a set of low-cycle fatigue tests to obtain the true life span of the test piece. and simulation parameter W P，N＝5 , ΔW, and substituting into the above equation we get W pc , and assume that it is a fixed value in the same batch of test pieces. Determine W pc The above life prediction criteria can then be used to predict the fatigue life of test specimens at different load levels.

[0070] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A method for predicting low cycle fatigue life of laser shock strengthened powder high temperature alloy based on crystal plasticity, characterized by: The following steps are involved: (1) The longitudinal and transverse residual stresses of the laser shock strengthened powder superalloy specimens were obtained with depth distribution by X-ray diffraction and electrolytic polishing technology, and the work hardening field of the laser shock strengthened powder superalloy specimens with depth distribution was obtained by electron backscatter diffraction technology; (2) The overall model of the laser shock strengthened powder high-temperature alloy specimen was established in ABAQUS software. The residual stress field, work hardening field and corresponding material properties were assigned to the overall model through the inverse intrinsic strain method and the USDFLD and HARDINI subroutines. Finite element simulation was performed to obtain the displacement field of the overall model. (3) Calibrate the material parameters in the crystal plasticity constitutive model through low-cycle fatigue tests; (4) Based on the microstructure characterization results of electron backscatter diffraction technology, the crystal plasticity sub-model was established in ABAQUS software using Dream.3D software; (5) assigning the residual stress field and work hardening field introduced by laser shock strengthening to the crystal plasticity sub-model established in step (4) through the inverse intrinsic strain method and USDFLD and HARDINI subroutines; (6) using the crystal plasticity sub-model to perform finite element simulation, using the displacement field of the overall model obtained in step (2) as the boundary condition of the crystal plasticity sub-model, analyzing the low-cycle fatigue damage evolution law of the laser shock strengthened powder high-temperature alloy test piece, and obtaining the fatigue indicator factor; (7) Realize the fatigue life prediction of the laser shock strengthened powder high temperature alloy test piece; predict the fatigue life result based on the fatigue indicator factor obtained in step (6), as shown below: IN pc =In P,N +(N total -N)ΔW Among them, W pc is the critical cumulative shear strain energy dissipation density, determined by experiment, W P,N is the cumulative shear strain energy dissipation density under a specific number of cycles, obtained through finite element simulation, N total is the full life of the test piece, N is the number of simulation cycles, and ΔW is the cumulative shear strain energy dissipation density increment; ΔW changes only in the first five cycles and remains stable in subsequent cycles. P,N is the cumulative shear strain energy dissipation density obtained after five real loading cycles of finite element simulation, that is, W P,N＝5 ; W pc The determination of the life span of a test piece requires a set of low-cycle fatigue tests to obtain the true life span of the test piece. and simulation parameter W P,N＝5 , ΔW, and substituting into the above equation we get W pc , and assume it is a fixed value in the same batch of test pieces; determine W pc Finally, the fatigue life of the test specimens at different load levels is predicted using the above life prediction criteria.

2. The method for predicting low cycle fatigue life of laser shock strengthened powder high temperature alloy based on crystal plasticity according to claim 1 is characterized by: In the step (1), the work hardening field is obtained by characterizing and analyzing the corresponding side surface or the corresponding cross section of the laser shock strengthened powder high temperature alloy test piece using electron backscatter diffraction technology.

3. The method for predicting low cycle fatigue life of laser shock strengthened powder high temperature alloy based on crystal plasticity according to claim 1 is characterized in that: In the step (3), the fatigue hysteresis loop of the powder high-temperature alloy is obtained by conducting corresponding low-cycle fatigue tests, and then the low-cycle fatigue hysteresis loop is fitted by a trial-and-error method to obtain the crystal plastic constitutive parameters.

4. The method for predicting low cycle fatigue life of laser shock strengthened powder high temperature alloy based on crystal plasticity according to claim 1 is characterized in that: In the step (4), the grain size and Euler angle of the sample surface are obtained by electron backscatter diffraction technology, and then the grain information obtained by the electron backscatter diffraction technology is imported into Dream.3d software in a ctf format file, and the grain orientation distribution information and grain size distribution are output in a txt format; then the grain orientation distribution information file and the grain size distribution parameters fitted by the lognormal distribution function are input into the Dream.3d software, and the model size and grid size are set to automatically generate a representative volume unit that conforms to the grain orientation and size distribution of the powder high-temperature alloy, and then the representative volume unit is imported into ABAQUS software to generate a crystal plasticity sub-model.

5. The method for predicting low cycle fatigue life of laser shock strengthened powder high temperature alloy based on crystal plasticity according to claim 1, characterized in that: In the step (5), the residual stress introduced by laser shock strengthening is assigned to the crystal plasticity sub-model by the inverse intrinsic strain method, wherein the residual stress field obtained by the experiment is introduced into the ABAQUS finite element model, and the eigenstrain corresponding to the corresponding residual stress is obtained by the inverse intrinsic strain reconstruction method, and then introduced into the finite element model through the subroutine USDFLD, and the work hardening is introduced into the finite element model in the form of equivalent plastic strain, which is implemented by the HARDINI subroutine; the method for obtaining the eigenstrain by the known residual stress is as follows: In the formula, y represents the depth, is the intrinsic strain in the x direction distributed with depth, T i (y) is the Chebyshev polynomial function, N is the order of the Chebyshev polynomial, m is the number of residual stresses measured in the experiment, σ xx (y) is the residual stress value distributed along the depth direction obtained by measuring the experiment, is the residual stress value corresponding to each order Chebyshev polynomial distributed with depth, C i is the unknown coefficient, C i Obtained by least squares method, and then by C i The eigenstrain of the corresponding depth is obtained by multiplying the Chebyshev polynomials corresponding to each depth.

6. The method for predicting low cycle fatigue life of laser shock strengthened powder high temperature alloy based on crystal plasticity according to claim 1 is characterized by: In the step (6), the crystal plasticity constitutive model in step (3) is substituted into the crystal plasticity sub-model in step (5), that is, the residual stress field and work hardening field generated by laser shock strengthening are introduced into the crystal plasticity sub-model, and then the sub-model boundary conditions are given through step (2), so as to obtain the simulation results considering the laser shock strengthening effect, and the shear strain, partial shear stress and normal stress on the specific slip plane are extracted from the simulation results, and the cumulative shear strain energy dissipation density is calculated to form the fatigue indication factor FIP, which is specifically in the following form: Where α is the slip system, N1 is the number of all slip systems activated, τ α is the shear stress, is the shear strain rate, W P is the fatigue indicator factor, where: t α =m *α ·t·s *α T=det(F)σ Among them, m *α is the normal direction of the slip surface, τ is the Kirchhoff stress, s *α is the slip direction, F is the total strain gradient, and σ is the Cauchy stress; in, is the reference shear strain rate, n is the rate sensitivity index, χ α is the back stress term of the slip system, Critical shear stress.

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