Method for predicting fatigue crack propagation life of laser shock peening titanium alloy lug

Abstract

The invention discloses a fatigue crack propagation life prediction method for a laser shock peening titanium alloy lug, and belongs to the technical field of fatigue crack propagation life prediction of a metal structure, and the method comprises the following steps: carrying out laser shock peening on a target lug structure, and testing to obtain a residual stress field of the target lug structure after laser shock peening; carrying out a fatigue test by adopting an identification load method, and interpreting a fracture; stress intensity factors caused by external loads and residual stress of all points of the front edge on the marking line are calculated in finite element numerical simulation software; correcting the fatigue crack growth rate model; calculating a crack propagation amount corresponding to each point of the preset crack front edge based on the fatigue crack propagation rate model, fitting to obtain a new crack front edge, and recording a corresponding cycle index; and calculating to obtain the fatigue crack propagation life. By adopting the method, accurate prediction of the three-dimensional fatigue crack propagation life under the action of the laser shock peening residual compressive stress field is realized.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of fatigue crack propagation life prediction of metal structures, in particular to a laser shock peening titanium alloy lug fatigue crack propagation life prediction method. BACKGROUND

[0002] Fatigue is one of the important factors leading to the failure of metal structures. As an advanced high-energy beam surface modification anti-fatigue manufacturing technology, laser shock peening can induce a high-amplitude and large-depth residual compressive stress field on the surface of the structure, effectively inhibit the initiation and propagation of fatigue cracks, and further improve the fatigue and damage tolerance performance of the component. However, the fatigue crack propagation analysis methods commonly used in the engineering field do not consider the residual stress field in detail, it is difficult to analyze the influence of the residual stress field on the three-dimensional fatigue crack propagation, there is a lack of methods for capturing and quantitatively analyzing the influence of the residual stress field on the fatigue crack propagation process, and the fatigue crack propagation life of the laser shock peened metal structure cannot be accurately evaluated. SUMMARY

[0003] The purpose of the application is to provide a laser shock peening titanium alloy lug fatigue crack propagation life prediction method, which solves the problem of low precision of the existing analysis method in predicting the three-dimensional fatigue crack propagation life under the action of the residual compressive stress field.

[0004] To achieve the above-mentioned purpose, the application provides a laser shock peening titanium alloy lug fatigue crack propagation life prediction method, which comprises the following steps: S1, laser shock peening is performed on the target lug structure, and the residual stress field of the target lug structure after laser shock peening is tested and obtained; S2, a fatigue crack propagation test is carried out by using the marking load method, and the fracture is interpreted, a plurality of point positions are taken on each marking line on the fracture, the coordinates of the point positions are recorded, and the propagation distances of the point positions between the marking lines are calculated; S3, a structure model is established in a finite element numerical simulation software, an external load is applied, and a residual stress field is introduced, and the stress intensity factors of the front edges of each marking line caused by the external load of each point are calculated respectively and the stress intensity factors caused by the residual stress . S4, according to the , of each marking line and the propagation distances between the marking lines, the residual stress influence factor in the fatigue crack propagation rate model is calibrated; S5, the fatigue crack propagation process is simulated by using the finite element numerical simulation, and the and , and the crack propagation amount of each point of the preset crack front is calculated based on a fatigue crack propagation rate model, and a new crack front is fitted based on the positions of the expanded points, and the corresponding cycle number is recorded; S6, repeating step S5 until the new crack front is greater than the fracture toughness, and the fatigue crack propagation life is calculated by accumulating and summing all cycle numbers before termination.

[0005] Preferably, in step S1, the X-ray method is used to test the residual stress field, and the residual stress field includes the planar residual stress distribution and the residual stress distribution in the depth direction of the laser shock peening region.

[0006] Preferably, in step S2, there are at least two marking lines on the fracture, and the corresponding marking line can be obtained by inversely fitting the point positions on the marking line.

[0007] Preferably, in step S3, the following steps are included: S31, creating a three-dimensional model in finite element numerical simulation software, meshing the three-dimensional model, and applying external load to the model; S32, importing the three-dimensional model into the finite element analysis software, introducing a crack consistent with the shape and position of the marking line as the preset crack according to the marking line information; S33, introducing the residual stress field according to the test results; S34, calculating the stress intensity factor caused by the external load of each point of the front edge on the marking line and the stress intensity factor caused by the residual stress .

[0008] Preferably, in step S4, the mathematical form of the fatigue crack propagation rate model is: ; wherein, C , m , n represent material performance parameters, represent residual stress influence factors, represent the stress intensity factor change range of each point of the crack front in a single load cycle, represent the maximum stress intensity factor of each point of the crack front in a single load cycle.

[0009] Preferably, in step S5, the calculation formula of the crack propagation amount of each point of the front edge is as follows: ; wherein, represents the cycle number corresponding to the expansion step.

[0010] Preferably, in step S6, the fatigue crack propagation life formula is as follows: ; Wherein, represents the number of cycles corresponding to the i th propagation step, n represents the total number of propagation steps, i represents the index variable.

[0011] Therefore, the present application adopts the above-mentioned laser shock peening titanium alloy ear fatigue crack propagation life prediction method, which has the following advantages: (1) In the present application, the fatigue test is carried out by using the marking load method to obtain the fatigue crack propagation condition, the stress intensity factor under the action of the residual stress field is calculated by using the finite element simulation method, the crack propagation rate model under the influence of laser shock peening is established, and the fatigue crack propagation life prediction considering the influence of laser shock peening is carried out based on this, and the precise prediction of three-dimensional fatigue crack propagation life under the action of laser shock peening residual compressive stress field is realized.

[0012] The technical solutions of the present application will be further described in detail below through the drawings and examples. DETAILED DESCRIPTION

[0013] Figure 1 It is a flowchart of the laser shock peening titanium alloy ear fatigue crack propagation life prediction method of the present application; Figure 2 It is a schematic diagram of the fracture after the fatigue crack propagation test of the embodiment of the present application by using the marking load method; Figure 3 It is a schematic diagram of the residual stress field introduced in the finite element numerical simulation software of the embodiment of the present application; Figure 4 It is a schematic diagram of the crack front corresponding to the marking line of the embodiment of the present application; and ; Figure 5 It is a schematic diagram of the residual stress influence factor of the embodiment of the present application along the crack front; Figure 6 It is a schematic diagram of each crack front in the simulation fatigue crack propagation process of the embodiment of the present application. DETAILED DESCRIPTION

[0014] In order to make the purposes, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be described clearly and completely below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are some but not all of the embodiments of the present application. The components of the embodiments of the present application described and shown in the drawings herein can be arranged and designed in various different configurations. The specific model specifications need to be selected and determined according to the actual specifications of the device, and the specific selection calculation method adopts the existing technology in the art, so it will not be described in detail.

[0015] Embodiments As Figure 1 shown, the present application provides a laser shock peening titanium alloy lug fatigue crack propagation life prediction method, comprising the following steps: S1, design and process lug test pieces of TA15 (inner diameter 24 mm, outer diameter 60 mm, thickness 15 mm), pre-prepare initial cracks on the lug hole edge by wire cutting method, and then perform laser shock peening on the 10 mm*12 mm area on the surface and hole wall centering on the lug hole edge notch, and test the residual stress field introduced after the strengthening by X-ray method; Specifically, the residual stress at 0, 0.25, 0.5, 1, 1.5, 2, 4, 6, 8, 10, 11, 12, 14 mm in the notch direction is tested on each depth plane, so as to obtain the residual stress variation on the plane, and the residual stress on the 0.25, 0.5, 1, 1.5, 2, 2.5 mm depth planes is tested in depth, so as to obtain the residual stress variation in depth.

[0016] S2, carry out fatigue crack propagation test by using the marking load method, and interpret the fracture, take a plurality of point positions on each marking line on the fracture (there should be at least two clear marking lines on the fracture, and the number of point positions on the marking line should be determined according to the complexity of the shape of the marking line, so as to ensure that the corresponding marking line can be inversely fitted by the point positions on the marking line), record the coordinates of the point positions, calculate the propagation distance of each point position between the marking lines, which specifically includes the following steps: S21, set appropriate marking load parameters according to experience and previous test, and in this embodiment, the preferred marking load method test scheme after preliminary test is: reference load: 8 kN-80 kN, stress ratio R =0.1, cycle number is 30000 times; marking load: 56 kN-80 kN, stress ratio R =0.7, cycle number is 30000 times; S22, as Figure 2As shown, the fracture is interpreted, and there are two clear marking lines on the fracture; the marking line closer to the pre-crack is named marking line 1, and the other is named marking line 2; for each marking line, the leading edge length is normalized, and six points at positions 0, 0.2, 0.4, 0.8, 0.8, and 1 on the line are taken, with the pre-crack vertex as the zero coordinate, and the point coordinates are recorded; S23, according to the point coordinates on the marking line, the extension distance corresponding to each point on the two marking lines is calculated: ; Among them, and represent the coordinate values of each point on marking line 1, and represent the coordinate values of each point on marking line 2.

[0017] S3, a structure model is established in a finite element numerical simulation software, an external load is applied, a residual stress field is introduced, and the stress intensity factor of each point on the leading edge of the marking line caused by the external load is calculated and the stress intensity factor caused by the residual stress . S31, a three-dimensional model is created in a finite element numerical simulation software, the three-dimensional model is meshed, dense mesh (element size 1 mm) is divided at the pre-crack, coarse mesh (element size 2 mm) is used in the remaining area, and according to the designed marking scheme, the maximum load 80kN is applied; S32, the three-dimensional model is imported into the finite element analysis software, and according to the marking line information, a crack consistent with the shape and position of the marking line is introduced as a preset crack; S33, as shown in Figure 3 , the residual stress field is introduced according to the test results; S34, as shown in Figure 4 , the stress intensity factor of each point on the leading edge of the marking line caused by the external load is calculated and the stress intensity factor caused by the residual stress .

[0018] S4, according to the , of each marking line and the extension distance between each marking line, the residual stress influence factor in the fatigue crack propagation rate model is calibrated; The mathematical form of the fatigue crack propagation rate model is: ; Among them, C , m , nrepresenting the material performance parameter, which can be obtained by the past accumulated data or fatigue crack propagation test fitting, , , , representing the residual stress influence factor, representing the range of stress intensity factor of each point on the crack front within a single load cycle, representing the maximum stress intensity factor borne by each point on the crack front within a single load cycle; it is assumed that the stress intensity factor between the two mark lines corresponding to the crack is linearly distributed with the distance, which is actually the distance a and the function of the influence parameter , so the above formula can be rewritten as: ; wherein, and are the crack lengths corresponding to each point on the two adjacent mark lines respectively; Accordingly, the residual stress influence factor corresponding to each point can be calculated as shown in Figure 5 , and further fitting is performed to obtain the residual stress influence factor distribution on the crack front corresponding to each mark line; for this embodiment, when the normalized distance of the front is between 0-0.8, d a cubic function can be used for fitting, and the fitting result is: ; When the normalized distance of the front is between 0.8-1 and tends to be stable, the average value 0.935 can be taken. Therefore, the multi-segment type laser shock peening metal crack propagation analytical description model can be expressed as: d ; .

[0019] S5, numerical simulation of the fatigue crack propagation process is carried out, the defect in the shape of the mark line 1 is inserted as the initial defect to carry out fatigue crack propagation life prediction, and the and of each point on the preset crack front are obtained, the crack propagation amount of each point on the preset crack front is calculated based on the fatigue crack propagation rate model, and a new crack front is obtained by using a cubic polynomial fitting, and the corresponding cycle number is recorded; The calculation formula of the crack propagation amount of each point on the front is as follows: ; wherein, ​​This represents the number of iterations corresponding to the expansion step. The expansion direction is the normal direction of each point. After expansion, each point can be fitted using multiple polynomials, spline curves, etc., to obtain a new crack front.

[0020] S6, such as Figure 6 As shown, repeat step S5 until the new crack tip is... The process terminates when the crack strength exceeds the fracture toughness. The fatigue crack propagation life is calculated by summing up all the cycles before termination. The formula for calculating fatigue crack propagation life is as follows: ; in, Representing the i The number of iterations corresponding to each expansion step n Represents the total number of expansion steps. i Represents an index variable.

[0021] Comparing the shapes of the crack propagation fronts obtained from simulation with those obtained from the analysis of the test port, the two are quite consistent. Further comparing the fatigue crack propagation lifetimes obtained from experiments and simulations, for this embodiment, calculated from the first marker line until fracture, the experimental fatigue crack propagation lifetime is 51,771 cycles, while the simulated fatigue crack propagation lifetime is 66,500 cycles, with a relative error of 28.45%, which is much smaller than the general accuracy (2 times dispersion band) of structural fatigue crack propagation lifetime prediction methods. This indicates that this method can achieve good prediction results for the ear structure affected by laser shock reinforcement.

[0022] Therefore, this invention employs the aforementioned method for predicting the fatigue crack propagation life of laser-shock-strengthened titanium alloy lugs. It uses the marked load method to conduct fatigue tests to obtain fatigue crack propagation information, calculates the stress intensity factor under residual stress field using finite element simulation, establishes a crack propagation rate model under the influence of laser shock strengthening, and, based on this, performs fatigue crack propagation life prediction considering the influence of laser shock strengthening. This solves the problem of low accuracy in predicting three-dimensional fatigue crack propagation life under residual compressive stress field using existing analysis methods, and achieves accurate prediction of three-dimensional fatigue crack propagation life under laser shock strengthening residual compressive stress field.

[0023] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A method for predicting the fatigue crack propagation life of laser-shock-strengthened titanium alloy earpieces, characterized in that: Includes the following steps: S1. The target earpiece structure is subjected to laser shock strengthening, and the residual stress field of the target earpiece structure after laser shock strengthening is obtained by testing. S2. The fatigue crack propagation test was carried out using the marking load method, and the fracture surface was interpreted. Several points were taken on each marking line on the fracture surface, the coordinates of the points were recorded, and the propagation distance between each point on each marking line was calculated. S3. Establish a structural model in finite element numerical simulation software, apply external loads, introduce residual stress fields, and calculate the stress intensity factor caused by the external load at each point on the leading edge of the marking line, using each marking line as a preset crack. Stress intensity factor caused by residual stress ; S4. According to each marking line , The residual stress influence factor in the fatigue crack propagation rate model is calibrated by measuring the extension distance between each marker line; S5. Perform finite element numerical simulation of the fatigue crack propagation process to obtain the values ​​at each point of the preset crack tip. and The crack propagation rate model is used to calculate the crack propagation amount at each point of the preset crack front, and the new crack front is obtained by fitting the position of each point after propagation, and the corresponding number of cycles is recorded. S6. Repeat step S5 until a new crack tip appears. The process terminates when the crack strength exceeds the fracture toughness. The fatigue crack propagation life is calculated by summing up all the cycles before termination.

2. The method for predicting the fatigue crack propagation of laser-shock-strengthened titanium alloy earpieces according to claim 1, characterized in that: In step S1, the residual stress field is tested using the X-ray method. The residual stress field includes the planar residual stress distribution and the residual stress distribution in the depth direction of the laser shock strengthening region.

3. The method for predicting the fatigue crack propagation of laser-shock-strengthened titanium alloy earpieces according to claim 2, characterized in that: In step S2, there are at least two marking lines on the fracture surface, ensuring that the corresponding marking lines can be obtained by backfitting the points on the marking lines.

4. The method for predicting the fatigue crack propagation of laser-shock-strengthened titanium alloy earpieces according to claim 3, characterized in that: Step S3 specifically includes the following steps: S31. Create a three-dimensional model in the finite element numerical simulation software, mesh the three-dimensional model, and apply external loads to the model; S32. Import the three-dimensional model into the finite element analysis software, and introduce cracks with the same shape and position as the marker lines as preset cracks based on the marker line information. S33. Introduce a residual stress field based on the test results; S34. Calculate the stress intensity factor caused by external load at each point on the leading edge of the marking line using the finite element numerical analysis method. Stress intensity factor caused by residual stress .

5. The method for predicting the fatigue crack propagation of laser-shock-strengthened titanium alloy earpieces according to claim 4, characterized in that: In step S4, the mathematical form of the fatigue crack propagation rate model is: ； in, C , m , n Representative material performance parameters, Represents the residual stress influence factor. This represents the range of stress intensity factor variation at various points along the crack tip within a single load cycle. It represents the maximum stress intensity factor experienced by each point at the crack tip within a single load cycle.

6. The method for predicting the fatigue crack propagation of laser-shock-strengthened titanium alloy earpieces according to claim 5, characterized in that: In step S5, the calculation formula for the crack propagation amount corresponding to each point on the leading edge is as follows: ； in, This represents the number of iterations corresponding to this expansion step.

7. The method for predicting the fatigue crack propagation of laser-shock-strengthened titanium alloy earpieces according to claim 6, characterized in that: In step S6, the formula for calculating fatigue crack propagation life is as follows: ； in, Representing the i The number of iterations corresponding to each expansion step n Represents the total number of expansion steps. i Represents an index variable.

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