Thermal structure dangerous point strain spectrum conversion and life prediction method based on easy-to-measure part

Abstract

The invention discloses a thermal structure dangerous point strain spectrum conversion and life prediction method based on an easy-to-measure part, virtual stress calculation is performed according to a strain concentration factor, a proposed strain indirect measurement method of a hot end component dangerous point is also combined with a Chboche constitutive model, and further, a stress spectrum and a strain spectrum of the dangerous point are converted. In order to verify the effectiveness of the proposed method, nonlinear finite element simulation analysis is adopted to simulate multi-axis thermal mechanical loading under test conditions, and the method can accurately estimate the stress-strain state of the dangerous point of the hot end structure. Meanwhile, under the condition of considering the influence of fatigue damage, creep damage and creep-fatigue interaction damage, the comparison error of a predicted life result and a test result is dispersed within a twice band, and only a small part of the comparison error is outside a three-time band. Therefore, the method has important engineering significance on structural health assessment, structural optimization design and accurate life prediction of the hot end component.

Description

Technical Field

[0001] The present invention relates to the technical field of strength assessment of hot end components such as aircraft engines, gas turbines, and hypersonic aircraft, and in particular to a method for strain spectrum conversion and life prediction of dangerous points of a thermal structure based on easily measured locations, which is used for strain measurement and life prediction of dangerous points of a hot end structure under multi-axis thermal engine cycle loads. Background Art

[0002] Accurately measuring the stress and strain at the dangerous points of nonlinear structures is of great significance for structural optimization analysis, vibration reduction and isolation design, structural health assessment and structural fatigue strength design. For in-service engineering components, including key components in various types of aircraft engines, gas turbines, hypersonic aircraft, etc., there are a large number of gaps, and defects will inevitably occur during the actual processing and use process. These defects or gaps are the dangerous points of the structure. The existence of dangerous points may cause stress concentration, thereby increasing the risk of structural failure. Therefore, it is particularly important to accurately measure the stress and strain at these dangerous points. It is very difficult to achieve strain measurement at dangerous points, and in some cases it is even impossible to achieve. Therefore, in order to ensure the safe and reliable operation of the structure, it is necessary to perform fatigue strength analysis on the dangerous points of the structure to extend the service life of the structure and ensure its safety, so as to reduce unnecessary economic losses.

[0003] In order to ensure the accuracy of life prediction of actual structural parts and the accurate life extension of major equipment, a strain spectrum conversion technology for structural dangerous points is urgently needed to accurately obtain the stress-strain history of dangerous points and lay the foundation for life prediction. Summary of the invention

[0004] The present invention aims to propose a strain spectrum conversion and life prediction method for dangerous points of thermal structures based on easily measured parts, so as to meet the requirements of some structural parts whose stress-strain state at dangerous points is difficult to measure under harsh service environments, and the stress-strain state at dangerous points can be approximately calculated. At the same time, this method can also characterize the influence of longitudinal tree-shaped structural parts on fatigue life under multi-axial constant amplitude thermal mechanical loading, mainly the influence of fatigue damage, creep damage and creep-fatigue interaction damage. The implementation steps of this method are:

[0005] Step 1): First, the strain concentration factor is calculated. When performing structural analysis of variable temperature loading, the strain concentration factor changes due to temperature changes, which is particularly important for the strain spectrum conversion technology of dangerous points in thermal structures. Therefore, the strain concentration factors at different temperatures are first obtained through finite element simulation analysis, and then fitted into functions related to temperature to characterize the impact of temperature changes. The strain concentration factors K in different directions are ij The expression is as follows:

[0006] Kij =h(T)*k ij (normal temperature)

[0007] Where h(T) is a function of temperature T, k ij (normal temperature) (i, j = 1, 2) are the strain concentration factors at normal temperature in different directions. 11 is the axial strain concentration factor, K 22 is the lateral strain concentration factor, K 12 is the torsional strain concentration factor.

[0008] Step 2): First, simulate the test state to obtain the stress-strain of the easily measured parts and the temperature T of the dangerous point emp Process, where stress in the easily measured part and strain The expression is:

[0009]

[0010] Among them, the matrix is the axial stress component of the easily measured part, is the torsional stress component of the easily measured part is the axial strain component of the easily measured part, Torsional strain component of the easily measured part is the lateral strain component of the easily measured part, is the vertical strain component of the easily measured part.

[0011] Furthermore, based on the strain at the easily measurable part The virtual strain history of the dangerous point is calculated by considering the strain concentration factor, and then converted into the virtual stress at the dangerous point. course.

[0012] The calculation formula of virtual strain in three different directions of the dangerous point is:

[0013]

[0014] in, is the axial strain of the easily measured part, is the torsional strain at the easily measured part, K 11 is the axial strain concentration factor, K 22 is the lateral strain concentration factor, K 12 is the torsional strain concentration factor.

[0015] Virtual stress at dangerous point The expression is:

[0016]

[0017] Among them, the matrix is Virtual axial stress component at the dangerous point, is the virtual torsional stress component at the dangerous point is the virtual lateral stress component at the dangerous point.

[0018] Among them, the virtual stress can be calculated according to the virtual strain through the generalized Hooke's law, and its expression is as follows:

[0019]

[0020] Wherein, E(T) is Young's modulus at temperature T, v(T) is Poisson's ratio at temperature T, and G(T) is shear modulus at temperature T.

[0021] Then, the obtained virtual stress Processing into increments form;

[0022] Step 3): Determine whether the material has yielded and determine the virtual stress by calculating the yield criterion Whether the stage is elastic or inelastic, the yield criterion is expressed as:

[0023] f=J(σ ij -x ij )-Rk

[0024] Among them, σ ij represents stress, x ij represents the back stress, J(σ ij -x ij ) means (σ ij -x ij ), R represents the drag stress, and k represents the initial yield stress;

[0025] If f < 0, it is in the elastic stage, so the true stress at the dangerous point is also in the elastic stage, and then the calculation continues through step 4);

[0026] If f ≥ 0, it is in the inelastic stage, so the true stress at the dangerous point is also in the inelastic stage, and then the calculation continues through step 5);

[0027] Step 4): Increase the virtual stress Directly assign the real stress increment to the dangerous point Right now:

[0028]

[0029] Then, the real stress increment at the dangerous point can be Calculate the true strain increment at the critical point by generalizing Hooke's law

[0030]

[0031] Among them, E(T) represents the Young's modulus of the material, v(T) represents the Poisson's ratio of the material, and tr is expressed as The trace of ij is the Kronecker symbol;

[0032] Go to step 7) and continue calculating;

[0033] Step 5): Based on virtual stress increment The true stress increment at the dangerous point is calculated using the following expression: Right now:

[0034]

[0035] in, represents the true secant modulus under different temperature changes, represents the virtual secant modulus under different temperature changes;

[0036] True Secant Modulus The expression is as follows:

[0037]

[0038] in, It represents the true inelastic modulus under different temperature changes, and its expression is as follows:

[0039]

[0040] Where K′(T) represents the cyclic hardening coefficient at different temperatures, n′(T) represents the cyclic hardening exponent at different temperatures, Indicates the real equivalent stress at the dangerous point;

[0041] Since there will be non-proportional cyclic strain hardening effect under multi-axial thermomechanical loading, a non-proportional additional damage coefficient r is proposed, and the cyclic hardening coefficient K′(T) is replaced by the equivalent cyclic hardening coefficient K′ q (T), which is defined as follows:

[0042] K′ q (T) = r·K′(T)

[0043] Where r is the non-proportional additional damage coefficient, which can be calculated by the following formula:

[0044]

[0045] Among them, F is the rotation factor and η is the additional damage factor. They can be calculated by the following formulas:

[0046]

[0047] in, is the maximum shear strain range calculated based on the virtual multiaxial elastic strain time history, is the shear strain range on the material plane that is 45° to the virtual maximum shear strain range plane.

[0048]

[0049] in, is the maximum axial stress in the time history of imaginary multiaxial elastic stress, E(T) is the Young's modulus at temperature T, is the maximum axial strain range calculated based on the virtual multiaxial elastic strain time history, is the maximum shear stress in the time history of imaginary multiaxial elastic stress.

[0050] Virtual secant modulus The expression is as follows:

[0051]

[0052] in, It represents the true inelastic modulus under different temperature changes, and its expression is as follows:

[0053]

[0054] Where K″(T) represents the structural cyclic hardening coefficient at different temperatures, n″(T) represents the structural cyclic hardening exponent at different temperatures, represents virtual equivalent stress;

[0055] Then, the real stress increment at the critical point is Combining Hooke's law and flow laws to calculate the true strain increment

[0056]

[0057] Where Δp represents the cumulative inelastic strain increment, Indicates the true strain deviator at the dangerous point, x ij ′ represents the back stress deviator, J(σ ij -x ij ) means (σ ij -x ij )’s vonMises equivalent stress;

[0058] The expression of the cumulative inelastic strain increment Δp is:

[0059]

[0060] Among them, Z(T) and n(T) represent the parameters describing the viscoplastic behavior of the material, and Δt is the time increment;

[0061] Step 6): Back stress deviator increment Δx ij The calculation expression of ′ is:

[0062] Δx ij ′=Δx′ ij，1 +Δx′ ij,2

[0063]

[0064] Where m represents the number of stages, x′ ij,m represents the back stress deviator at stage m, Δx′ ij,m represents the back stress deviator increment in the mth stage, a m represents the stable value of the back stress deviator at stage m, C m Indicates the speed at which the back stress deviator reaches a stable value in the mth stage, Δε ij,in It represents the inelastic strain increment;

[0065] Inelastic strain increment Δε ij,in The expression is:

[0066]

[0067] Among them, σ′ ij represents the stress deviator, x ij ′ represents the back stress deviator;

[0068] Then, the drag stress increment ΔR is calculated, which is expressed as:

[0069] ΔR=b(QR)Δp

[0070] Where ΔR represents the drag stress increment, Q represents the stable value of the drag stress R, and b represents the speed at which the drag stress R reaches the stable value Q;

[0071] Then, according to the accumulated back stress deviator increment Δx ij Calculate the back stress deviator x ij ′, calculate the drag stress R according to the accumulated drag stress increment ΔR;

[0072] Step 7): Increase the real stress increment of the dangerous point Superposition calculation to obtain the real stress of the dangerous point The real strain increment of the dangerous point Superimpose calculation to obtain the real strain of the dangerous point

[0073] Subsequently, the values ​​of various parameters are updated, and the real stress and strain of the dangerous point are recorded;

[0074] Step 8): Determine whether there is any data;

[0075] If there is still data, it means the calculation is not completed, and steps 2) to 8) are recalculated;

[0076] If there is no subsequent data, it means the calculation is completed.

[0077] Through the above steps 1) to step 8), the strain-time history of the easily measured parts can be obtained only through strain gauges. Through the proposed indirect measurement approximate estimation method, the strain spectrum conversion technology of the dangerous points of the hot end components can be completed. The next step is to consider the calculation of fatigue damage, creep damage and creep-fatigue interaction damage, and then complete the life prediction.

[0078] Step 9): Calculation of fatigue damage;

[0079] The first step is to determine the critical surface of the dangerous point and find its maximum normal strain amplitude The maximum shear strain plane is taken as the pure mechanical fatigue damage critical surface. At the same time, the normal strain amplitude value between the two maximum shear strain amplitude reversal points on the critical surface should be determined as ε n * Then, the pure fatigue damage is calculated by the tensile unified multiaxial fatigue damage model:

[0080]

[0081] Among them, N pf is the pure fatigue life, σ′ f , b, ε′ f , c is the fatigue constant of the relevant material, E is the elastic modulus of the material at room temperature, D pf is the pure fatigue damage per cycle.

[0082] Step 10): Calculation of creep damage;

[0083] The axial stress-time history and temperature-time history of the dangerous point in a cycle are obtained and divided into appropriate parts by subdivision method. Then the creep stress in each interval is determined. When the axial stress value in the interval is positive, the Mises equivalent stress of the tensile-torsion stress in the interval is used as the creep stress. When the axial stress value in the interval is negative, the creep stress in the interval is taken as 0. The specific expression is as follows:

[0084]

[0085] Among them, σ ci is the creep stress at the critical point, σ i is the axial stress at the dangerous point on interval i, τ i is the shear stress at the dangerous point on interval i.

[0086] Then, the creep damage in each interval is calculated according to the creep endurance equation of the material, and the creep damage in one cycle is accumulated.

[0087] Step 11): Calculation of creep-fatigue interaction damage;

[0088] First, the cycle equivalent temperature is determined by taking the highest and lowest temperatures in the high temperature part (>500°C) of a cycle and taking their average value as the equivalent temperature of the cycle.

[0089] Secondly, the creep-fatigue interaction coefficient B T To determine the creep-fatigue interaction coefficient corresponding to a specific temperature, the high-temperature uniaxial fatigue data is fitted, and then the coefficients at different temperatures are fitted to characterize the effect of temperature on the creep-fatigue interaction coefficient. The creep-fatigue interaction coefficient is mainly determined by the following expression:

[0090]

[0091] Step 12): Calculation of total damage;

[0092] By adding up the above three types of damage, we can get the damage in one cycle, and taking the inverse of the total damage can give the life value.

[0093] D T =D P +D C +D PC

[0094]

[0095] Among them, D T is the total damage, N fT The final predicted lifespan.

[0096] Compared with the prior art, the present invention provides a strain spectrum conversion and life prediction method for dangerous points of thermal structures based on easy-to-measure parts, namely: first, the strain concentration factors at different temperatures are fitted to characterize the influence of temperature changes, and secondly, the Neuber method and the incremental method are combined to evaluate the stress-strain state of the dangerous point, and at the same time, the viscoplastic constitutive model is combined to obtain the true stress-strain state of the dangerous point. After obtaining the stress-strain history of the dangerous point, the total damage is accumulated by considering the influence of fatigue damage, creep damage and creep-fatigue interaction damage, and finally the predicted life is obtained. The physical meaning of the present invention is clear, and the calculation process is simple and fast. By comparing the true stress-strain of the dangerous point obtained by this method with the finite element simulation data, it is found that the method proposed by the present invention can approximately calculate the stress-strain state of the dangerous point, which has great engineering significance for accurately predicting the life of structural parts in actual engineering. At the same time, after comparing the predicted life and the test life, the errors are all within the 2-fold band, and only a few are outside the 3-fold band. BRIEF DESCRIPTION OF THE DRAWINGS

[0097] Figure 1 is the stress history and temperature history loaded.

[0098] Figure 2 It is the strain history of the easily measured part.

[0099] Figure 3 This is a comparison chart of the strain calculation results and finite element analysis results of dangerous points in high-temperature structures.

[0100] Figure 4 This is a comparison chart of the stress calculation results and finite element analysis results of dangerous points in high-temperature structures.

[0101] Figure 5 The figure is a flow chart of the strain spectrum conversion technology in the method of the present invention.

[0102] Figure 6 This is a comparison chart between predicted life and test life.

[0103] Figure 7 Schematic diagram of a test piece according to an embodiment of the present invention. DETAILED DESCRIPTION

[0104] The specific implementation modes of the present invention are described with reference to the accompanying drawings.

[0105] The present invention further illustrates the present invention using multi-axis thermomechanical nonlinear finite element analysis data of a longitudinal tree-shaped structural member made of GH4169 material. The stress-time history and temperature-time history loaded on the fir-tree-shaped structural member are shown in FIG. Figure 1 shown.

[0106] A method for strain spectrum conversion and life prediction of dangerous points of thermal structures based on easily measured parts, the specific implementation method is as follows:

[0107] Step 1): First, the strain concentration factor is fitted, and its expression is as follows:

[0108] K ij =h(T)*k ij (normal temperature)

[0109] Where h(T) is a function of temperature T, k ij (normal temperature) (i,j=1,2) is the strain concentration factor at normal temperature in different directions.

[0110] Step 2): Obtain the stress-time, strain-time of the easily measured parts and the temperature-time history of the dangerous points. The stress-time history and strain-time history are as follows: Figure 2 As shown, the stress in the easily measured part is and strain The expression is:

[0111]

[0112] Among them, the matrix is the axial stress component of the easily measured part, is the torsional stress component of the easily measured part is the axial strain component of the easily measured part, Torsional strain component of the easily measured part is the lateral strain component of the easily measured part, is the vertical strain component of the easily measured part.

[0113] Furthermore, based on the stress at the easily measured location Process, to calculate the virtual stress at the critical point course.

[0114] The calculation formula of virtual stress in three different directions of the dangerous point is:

[0115]

[0116] in, is the axial strain of the easily measured part, is the torsional strain at the easily measured part, K 11 is the axial strain concentration factor, K 22 is the lateral strain concentration factor, K 12 is the torsional strain concentration factor.

[0117] Among them, the virtual stress can be calculated according to the virtual strain through the generalized Hooke's law, and its expression is as follows:

[0118]

[0119] Wherein, E(T) is Young's modulus at temperature T, v(T) is Poisson's ratio at temperature T, and G(T) is shear modulus at temperature T.

[0120] Then, the obtained virtual stress Processing into increments form;

[0121] Step 3): Determine virtual stress by yield criterion calculation Is the stage in which you are in elastic or inelastic?

[0122] f=J(σ ij -x ij )-Rk

[0123] Among them, σ ij represents stress, x ij represents the back stress, J(σ ij -x ij ) means (σ ij -x ij ), R represents the drag stress, and k represents the initial yield stress;

[0124] If f < 0, it is in the elastic stage, so the true stress at the dangerous point is also in the elastic stage, and then the calculation continues through step 4);

[0125] If f ≥ 0, it is in the inelastic stage, so the real stress at the dangerous point is It is also in the inelastic stage, and then the calculation continues through step 5);

[0126] Step 4): Increase the virtual stress Directly assign the real stress increment to the dangerous point Right now:

[0127]

[0128] Then, the real stress increment at the dangerous point can be Calculate the true strain increment at the critical point by generalizing Hooke's law

[0129]

[0130] Among them, E(T) represents the Young's modulus of the material, v(T) represents the Poisson's ratio of the material, and tr is expressed as The trace of ij is the Kronecker symbol;

[0131] Go to step 7) and continue calculating;

[0132] Step 5): Based on virtual stress increment The true stress increment at the dangerous point is calculated using the following expression: Right now:

[0133]

[0134] in, represents the true secant modulus under different temperature changes, represents the virtual secant modulus under different temperature changes;

[0135] True Secant Modulus The expression is as follows:

[0136]

[0137] in, It represents the true inelastic modulus under different temperature changes, and its expression is as follows:

[0138]

[0139] Where K′(T) represents the cyclic hardening coefficient at different temperatures, n′(T) represents the cyclic hardening exponent at different temperatures, Indicates the real equivalent stress at the dangerous point;

[0140] Since there will be non-proportional cyclic strain hardening effect under multi-axial thermomechanical loading, a non-proportional additional damage coefficient r is proposed, and the cyclic hardening coefficient K′(T) is replaced by the equivalent cyclic hardening coefficient K′ q (T), which is defined as follows:

[0141] K′ q (T) = r·K′(T)

[0142] Where r is the non-proportional additional damage coefficient, which can be calculated by the following formula:

[0143]

[0144] Among them, F is the rotation factor and η is the additional damage factor. They can be calculated by the following formulas:

[0145]

[0146] in, is the maximum shear strain range calculated based on the virtual multiaxial elastic strain time history, is the shear strain range on the material plane that is 45° to the virtual maximum shear strain range plane.

[0147]

[0148] in, is the maximum axial stress in the time history of imaginary multiaxial elastic stress, E(T) is the Young's modulus at temperature T, is the maximum axial strain range calculated based on the virtual multiaxial elastic strain time history, is the maximum shear stress in the time history of imaginary multiaxial elastic stress.

[0149] Virtual secant modulus E t e The expression of (T) is as follows:

[0150]

[0151] in, It represents the true inelastic modulus under different temperature changes, and its expression is as follows:

[0152]

[0153] Where K″(T) represents the structural cyclic hardening coefficient at different temperatures, n″(T) represents the structural cyclic hardening exponent at different temperatures, represents virtual equivalent stress;

[0154] Then, the real stress increment at the critical point is Combining Hooke's law and flow laws to calculate the true strain increment

[0155]

[0156] Where Δp represents the cumulative inelastic strain increment, Indicates the true strain deviator at the dangerous point, x ij ′ represents the back stress deviator, J(σ ij -x ij ) means (σ ij -x ij )’s vonMises equivalent stress;

[0157] The expression of the cumulative inelastic strain increment Δp is:

[0158]

[0159] Where Z(T) and n(T) represent the parameters of the viscoplastic behavior of the material, and Δt is the time increment;

[0160] Step 6): Back stress deviator increment Δx ij The calculation expression of ′ is:

[0161] Δx ij ′=Δx′ ij，1 +Δx′ ij,2

[0162]

[0163] Where m represents the number of stages, x′ ij,m represents the back stress deviator at stage m, Δx′ ij,m represents the back stress deviator increment in the mth stage, a m represents the stable value of the back stress deviator at stage m, C m Indicates the speed at which the back stress deviator reaches a stable value in the mth stage, Δε ij,in It represents the inelastic strain increment;

[0164] Inelastic strain increment Δε ij,in The expression is:

[0165]

[0166] Among them, σ′ ij represents the stress deviator, x ij ′ represents the back stress deviator;

[0167] Then, the drag stress increment ΔR is calculated, which is expressed as:

[0168] ΔR=b(QR)Δp

[0169] Where ΔR represents the drag stress increment, Q represents the stable value of the drag stress R, and b represents the speed at which the drag stress R reaches the stable value Q;

[0170] Then, according to the accumulated back stress deviator increment Δx ij Calculate the back stress deviator x ij ′, calculate the drag stress R according to the accumulated drag stress increment ΔR;

[0171] Step 7): Increase the real stress increment of the dangerous point Superposition calculation to obtain the real stress of the dangerous point The real strain increment of the dangerous point Superimpose calculation to obtain the real strain of the dangerous point

[0172] Subsequently, the values ​​of various parameters are updated, and the real stress and strain of the dangerous point are recorded;

[0173] Step 8): Determine whether there is any data;

[0174] If there is still data, it means the calculation is not completed, and steps 2) to 8) are recalculated;

[0175] If there is no subsequent data, it means the calculation is completed.

[0176] Step 9): Calculation of fatigue damage;

[0177] The first step is to determine the critical surface of the dangerous point and find its maximum normal strain amplitude The maximum shear strain plane is taken as the pure mechanical fatigue damage critical surface. At the same time, the normal strain amplitude between the two maximum shear strain amplitude reversal points on the critical surface should be determined as Then, the pure fatigue damage is calculated by the tensile unified multiaxial fatigue damage model:

[0178]

[0179] Among them, N pf is the pure fatigue life, σ′ f , b, ε′ f , c is the fatigue constant of the relevant material, E is the elastic modulus of the material at room temperature, D pf is the pure fatigue damage per cycle.

[0180] Step 10): Calculation of creep damage;

[0181] The axial stress-time history and temperature-time history of the dangerous point in a cycle are obtained and divided into appropriate parts by subdivision method. Then the creep stress in each interval is determined. When the axial stress value in the interval is positive, the Mises equivalent stress of the tensile-torsion stress in the interval is used as the creep stress. When the axial stress value in the interval is negative, the creep stress in the interval is taken as 0. The specific expression is as follows:

[0182]

[0183] Among them, σ ci is the creep stress at the critical point, σ i is the axial stress at the dangerous point on interval i, τ i is the shear stress at the dangerous point on interval i.

[0184] Then, the creep damage in each interval is calculated according to the creep endurance equation of the material, and the creep damage in one cycle is accumulated.

[0185] Step 11): Calculation of creep-fatigue interaction damage;

[0186] First, the cycle equivalent temperature is determined by taking the highest and lowest temperatures in the high temperature part (>500°C) of a cycle and taking their average value as the equivalent temperature of the cycle.

[0187] Secondly, the creep-fatigue interaction coefficient B T To determine the creep-fatigue interaction coefficient corresponding to a specific temperature, the high-temperature uniaxial fatigue data is fitted, and then the coefficients at different temperatures are fitted to characterize the effect of temperature on the creep-fatigue interaction coefficient. The creep-fatigue interaction coefficient is mainly determined by the following expression:

[0188]

[0189] Step 12): Calculation of total damage;

[0190] By adding up the above three types of damage, we can get the damage in one cycle, and taking the inverse of the total damage can give the life value.

[0191] D T =D P +D C +D PC

[0192]

[0193] Among them, D T is the total damage, N fT The final predicted lifespan.

[0194] In order to fully verify the accuracy and reliability of the method proposed in the present invention, the strain-time history of the dangerous point of the hot end component obtained by the present invention is compared with the strain-time history results obtained by finite element nonlinear simulation. The comparison results are as follows: Figure 3 and Figure 4 As shown, through Figure 3 and Figure 4 The analysis shows that the method proposed in the present invention has good consistency with the results of nonlinear finite element simulation, which shows that the proposed method is feasible when converting the strain spectrum of dangerous points of hot end components. Figure 6 From the comparison between the predicted life and the test life shown in the figure, it can be seen that the comparison error between the predicted life results of the GH4169 longitudinal tree-type structural components calculated under this method and the test results is scattered within the 2-fold band, and only a few are outside the 3-fold band. Therefore, it is shown that this method is feasible and has important significance for practical applications.

[0195] The present invention discloses a method for strain spectrum conversion and life prediction of dangerous points of thermal structures based on easy-to-measure parts. In order to solve the problem that it is very difficult to measure strain at dangerous points in a high-temperature environment, the present invention adopts the calculation of virtual stress based on strain concentration factor. In addition, the proposed strain indirect measurement method of dangerous points of hot end components is also combined with Chaboche constitutive model, and then the stress spectrum and strain spectrum of dangerous points are converted. In order to verify the effectiveness of the proposed method, nonlinear finite element simulation analysis is used to simulate multi-axis thermal mechanical loading under test conditions, and it is found that the proposed method can accurately estimate the stress-strain state of dangerous points of hot end structures. At the same time, considering the influence of fatigue damage, creep damage and creep-fatigue interaction damage, the comparison error between the predicted life results and the test results is dispersed within the 2-fold band, and only a few are outside the 3-fold band, indicating that the method is acceptable and feasible. Therefore, the proposal of this method has important engineering significance for structural health assessment, structural optimization design and accurate life prediction of hot end components.

Claims

1. A method for strain spectrum conversion and life prediction of dangerous points of thermal structures based on easily measured parts, characterized by: The steps are as follows, Step 1): First, the strain concentration factor is calculated. The strain concentration factor at different temperatures is obtained by finite element simulation analysis, and then fitted into the relevant temperature function to characterize the influence of temperature change; the strain concentration factor K in different directions ij The expression is as follows: K ij =h(T)*k ij (normal temperature) Where h(T) is a function of temperature T, k ij (normal temperature) is the strain concentration factor at normal temperature in different directions; K 11 is the axial strain concentration factor, K 22 is the lateral strain concentration factor, K 12 is the torsional strain concentration factor; i, j = 1, 2; Step 2): First, simulate the test state to obtain the stress-strain of the easily measured parts and the temperature T of the dangerous point emp Process, where stress in the easily measured part and strain The expression is: Among them, the matrix is the axial stress component of the easily measured part, is the torsional stress component of the easily measured part is the axial strain component of the easily measured part, Torsional strain component of the easily measured part is the lateral strain component of the easily measured part, is the vertical strain component of the easily measured part; Furthermore, based on the strain at the easily measurable part The virtual strain history of the dangerous point is calculated by considering the strain concentration factor, and then converted into the virtual stress at the dangerous point. course; The calculation formula of virtual strain in three different directions of the dangerous point is: in, is the axial strain of the easily measured part, is the torsional strain at the easily measured part, K 11 is the axial strain concentration factor, K 22 is the lateral strain concentration factor, K 12 is the torsional strain concentration factor; Virtual stress at dangerous point The expression is: Among them, the matrix is Virtual axial stress component at the dangerous point, is the virtual torsional stress component of the dangerous point is the virtual lateral stress component at the dangerous point; Among them, the virtual stress is calculated according to the virtual strain through the generalized Hooke's law, and its expression is as follows: Where, E(T) is Young's modulus at temperature T, v(T) is Poisson's ratio at temperature T, and G(T) is shear modulus at temperature T; Then, the obtained virtual stress Processing into increments form; Step 3): Determine whether the material has yielded and determine the virtual stress by calculating the yield criterion Whether the stage is elastic or inelastic, the yield criterion is expressed as: f=J(σ ij -x ij )-Rk Among them, σ ij represents stress, x ij represents the back stress, J(σ ij -x ij ) means (σ ij -x ij ), R represents the drag stress, and k represents the initial yield stress; If f < 0, it is in the elastic stage, so the true stress at the dangerous point is also in the elastic stage, and then the calculation continues through step 4); If f ≥ 0, it is in the inelastic stage, so the true stress at the dangerous point is also in the inelastic stage, and then the calculation continues through step 5); Step 4): Increase the virtual stress Directly assign the real stress increment to the dangerous point Right now: According to the real stress increment of the dangerous point Calculate the true strain increment at the critical point by generalizing Hooke's law Among them, E(T) represents the Young's modulus of the material, v(T) represents the Poisson's ratio of the material, and tr is expressed as The trace of ij is the Kronecker symbol; Go to step 7) and continue calculating; Step 5): Based on virtual stress increment The true stress increment at the dangerous point is calculated using the following expression: Right now: in, represents the true secant modulus under different temperature changes, represents the virtual secant modulus under different temperature changes; True Secant Modulus The expression is as follows: in, It represents the true inelastic modulus under different temperature changes, and its expression is as follows: Where K′(T) represents the cyclic hardening coefficient at different temperatures, n′(T) represents the cyclic hardening exponent at different temperatures, Indicates the real equivalent stress at the dangerous point; The non-proportional additional damage coefficient r is proposed, and the cyclic hardening coefficient K′(T) is replaced by the equivalent cyclic hardening coefficient K q ′(T), which is defined as follows: K q ′(T)=r·K′(T) Where r is the non-proportional additional damage coefficient, which is calculated by the following formula: Where F is the rotation factor and η is the additional damage factor, which are calculated by the following formulas: in, is the maximum shear strain range calculated based on the virtual multiaxial elastic strain time history, is the shear strain range on the material plane that is 45° to the virtual maximum shear strain range plane; in, is the maximum axial stress in the time history of imaginary multiaxial elastic stress, E(T) is the Young's modulus at temperature T, is the maximum axial strain range calculated based on the virtual multiaxial elastic strain time history, is the maximum shear stress in the time history of imaginary multiaxial elastic stress; Virtual secant modulus The expression is as follows: in, It represents the true inelastic modulus under different temperature changes, and its expression is as follows: Where K″(T) represents the structural cyclic hardening coefficient at different temperatures, n″(T) represents the structural cyclic hardening exponent at different temperatures, represents virtual equivalent stress; Then, the real stress increment at the critical point is Combining Hooke's law and flow laws to calculate the true strain increment Where Δp represents the cumulative inelastic strain increment, Indicates the true strain deviator at the dangerous point, x ij ′ represents the back stress deviator, J(σ ij -x ij ) means (σ ij -x ij )’s vonMises equivalent stress; The expression of the cumulative inelastic strain increment Δp is: Among them, Z(T) and n(T) represent the parameters describing the viscoplastic behavior of the material, and Δt is the time increment; Step 6): Back stress deviator increment Δx ij The calculation expression of ′ is: Δx ij ′=Δx ij ′ ，1 +Δx ij ′ ,2 Among them, m represents the number of stages, x ij ' ,m represents the back stress deviator at stage m, Δx ij ' ,m represents the back stress deviator increment in the mth stage, a m represents the stable value of the back stress deviator at stage m, C m Indicates the speed at which the back stress deviator reaches a stable value in the mth stage, Δε ij,in It represents the inelastic strain increment; Inelastic strain increment Δε ij,in The expression is: Among them, σ i ' j represents the stress deviator, x ij ′ represents the back stress deviator; Then, the drag stress increment ΔR is calculated, which is expressed as: ΔR=b(QR)Δp Where ΔR represents the drag stress increment, Q represents the stable value of the drag stress R, and b represents the speed at which the drag stress R reaches the stable value Q; Then, according to the accumulated back stress deviator increment Δx ij Calculate the back stress deviator x ij ′, calculate the drag stress R according to the accumulated drag stress increment ΔR; Step 7): Increase the real stress increment of the dangerous point Superposition calculation to obtain the real stress of the dangerous point The real strain increment of the dangerous point Superimpose calculation to obtain the real strain of the dangerous point Subsequently, the values ​​of various parameters are updated, and the real stress and strain of the dangerous point are recorded; Step 8): Determine whether there is any data; If there is still data, it means the calculation is not completed, and steps 2) to 8) are recalculated; If,there is no subsequent data, it means the calculation is completed; Through the above steps 1) to 8), the strain-time history of the easily measured parts needs to be obtained through strain gauges, and the strain spectrum conversion of the dangerous points of the hot end components is completed through the proposed indirect measurement approximate estimation method, and the calculation of fatigue damage, creep damage and creep-fatigue interaction damage is considered to complete the life prediction; Step 9): Calculation of fatigue damage; The first step is to determine the critical surface of the dangerous point and find its maximum normal strain amplitude The maximum shear strain plane is taken as the pure mechanical fatigue damage critical surface. At the same time, the normal strain amplitude between the two maximum shear strain amplitude reversal points on the critical surface should be determined as Then, the pure fatigue damage is calculated by the tensile unified multiaxial fatigue damage model: Among them, N pf is the pure fatigue life, σ′ f , b, ε′ f , c is the fatigue constant of the relevant material, E is the elastic modulus of the material at room temperature, D pf is the pure fatigue damage per cycle; Step 10): Calculation of creep damage; The axial stress-time history and temperature-time history of the dangerous point in a cycle are obtained and divided into appropriate parts by the subdivision method; then the creep stress in each interval is determined. When the axial stress value in the interval is positive, the Mises equivalent stress of the tensile-torsion stress in the interval is used as the creep stress; when the axial stress value in the interval is negative, the creep stress in the interval is taken as 0; the specific expression is as follows: Among them, σ ci is the creep stress at the critical point, σ i is the axial stress at the dangerous point on interval i, τ i is the shear stress at the dangerous point on interval i; Then the creep damage in each interval is calculated according to the creep endurance equation of the material, and then the creep damage in one cycle is accumulated; Step 11): Calculation of creep-fatigue interaction damage; First, the cycle equivalent temperature is determined by taking the highest and lowest temperatures in the high temperature part of a cycle > 500°C, and taking their average value as the equivalent temperature of the cycle; Secondly, the creep-fatigue interaction coefficient B T The high temperature uniaxial fatigue data is used to fit the creep-fatigue interaction coefficient corresponding to a specific temperature, and then the coefficients at different temperatures are fitted to characterize the effect of temperature on the creep-fatigue interaction coefficient; the creep-fatigue interaction coefficient is determined by the following expression: Step 12): Calculation of total damage; The three types of damage are accumulated to obtain the damage in one cycle, and the inverse of the total damage is taken to obtain the life value; D T =D P +D C +D PC Among them, D T is the total damage, N fT The final predicted lifespan.

2. The method for strain spectrum conversion and life prediction of thermal structure dangerous points based on easy-to-measure locations according to claim 1 is characterized by: Step 1) considers the influence of temperature change and proposes an expression for the strain concentration factor under variable temperature conditions; Step 5) proposes a non-proportional additional damage coefficient to consider the influence of non-proportional additional hardening; Steps 1) to 8) propose a strain conversion technology to convert the strain history of the easily measurable part into the stress-strain history of the dangerous point.

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