A nonlinear method for predicting high- and low-cycle combined fatigue life of turbine blades

Abstract

The present invention discloses a nonlinear method for predicting the high- and low-cycle composite fatigue life of turbine blades, including: based on the nonlinear damage accumulation evolution process, introducing fatigue driving force into fatigue life prediction; considering the coupling damage generated by the interaction of high-cycle fatigue and low-cycle fatigue according to the fatigue accumulation damage theory, introducing coupling damage into the composite fatigue damage including low-cycle fatigue damage and high-cycle fatigue damage; based on the failure mechanism of high- and low-cycle composite fatigue, establishing the equivalent stress range ratio of high- and low-cycle composite fatigue, and proposing a nonlinear life prediction model under high- and low-cycle composite loads. The method proposed in the present invention can predict the fatigue life of turbine blade structures and standard specimens under high- and low-cycle composite fatigue loads, and has a high prediction accuracy, providing theoretical support for the life determination and reliability evaluation of engine turbine blades.

Description

Technical Field

[0001] The invention relates to fatigue prediction of turbine blades, and in particular to a nonlinear fatigue life prediction method of turbine blades under high and low cycle combined loads. Background Art

[0002] With the increasing requirements for high performance, high speed and high reliability of aircraft engines, the service environment of hot end components such as turbine blades has become increasingly harsh, the alternating loads they bear have become more complex, and the probability of various damages has also increased. In particular, turbine blades work under extreme conditions of high temperature, high pressure, high speed and heavy load, and their structural integrity may be threatened by a variety of damage mechanisms, including high-cycle fatigue, low-cycle fatigue, thermal fatigue, multi-axial fatigue, creep, oxidation and their combination, among which fatigue failures caused by high- and low-cycle combined fatigue (CCF) account for 38% of the total, posing a serious threat to the structural integrity of the engine. When aircraft engines are in service, due to the needs of a large number of complex flight missions and the influence of multiple uncertain factors, their fatigue life shows a large dispersion. Therefore, in order to ensure the reliability of the engine during service, it is urgent to carry out more accurate fatigue life prediction of turbine blades under high- and low-cycle combined fatigue loads.

[0003] The most commonly used life prediction method for high- and low-cycle combined fatigue in actual engineering is the linear Miner rule, but it does not take into account the coupling damage caused by the load sequence effect and the interaction between high-cycle fatigue and low-cycle fatigue. Studies have shown that the predicted life of the Miner rule under high- and low-cycle combined fatigue loads is dangerous. Compared with the Miner rule, the nonlinear fatigue cumulative damage method has higher prediction accuracy when evaluating the high- and low-cycle combined fatigue life. Therefore, it is necessary to carry out high- and low-cycle combined fatigue life prediction from the perspective of nonlinear cumulative damage to improve the prediction accuracy. Summary of the invention

[0004] In order to solve the above-mentioned defects existing in the prior art, the purpose of the present invention is to provide a life prediction method that can predict the nonlinearity of turbine blade components and test pieces under high and low cycle combined fatigue loads, which solves the problem that the currently commonly used linear damage method cannot accurately estimate the life under high and low cycle combined loads, and in the life prediction process, considers the load sequence and the coupled damage caused by high cycle fatigue and low cycle fatigue from the perspective of nonlinear cumulative damage.

[0005] The present invention is achieved through the following technical solutions.

[0006] The present invention provides a nonlinear turbine blade high-low cycle composite fatigue life prediction method, comprising the following steps:

[0007] Step 1, fatigue tests are performed on turbine blade samples under different stresses to obtain stress-life curves, and the fatigue driving force is introduced into fatigue life prediction;

[0008] Step 2, based on the nonlinear damage accumulation method of fatigue driving force, the expression of fatigue driving force under different load levels is obtained;

[0009] Step 3, introducing the fatigue driving force into the high-low cycle composite fatigue cumulative damage process of the engine turbine blade to obtain the number of composite cycle blocks;

[0010] Step 4: Based on the failure mechanism of high- and low-cycle combined fatigue and the fatigue cumulative damage theory, the coupling damage is introduced into the combined fatigue damage including high-cycle fatigue damage and low-cycle fatigue damage, and a modified high- and low-cycle combined fatigue damage expression is obtained;

[0011] Step 5, considering the coupled damage caused by the interaction of high-cycle fatigue and low-cycle fatigue, an equivalent stress range ratio of high- and low-cycle combined fatigue is introduced in combination with the maximum stress to carry out life prediction of high- and low-cycle combined fatigue.

[0012] The present invention adopts the above technical solution, which has the following beneficial effects:

[0013] The method of the present invention is based on the nonlinear damage accumulation process of fatigue driving force, emphasizes the load sequence effect, and takes into account the coupling damage between high-cycle fatigue and low-cycle fatigue. According to the established nonlinear damage accumulation model based on fatigue driving force, high- and low-cycle composite life prediction is carried out, and the fatigue life of the turbine blade can be obtained by combining the high- and low-cycle composite test load spectrum. The method proposed in the present invention not only has a simple prediction process, but also is relatively close to the test results, has a high life prediction accuracy, and can provide certain theoretical support for the life determination and reliability design of turbine blade structures. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The drawings described herein are used to provide a further understanding of the present invention, constitute a part of the present application, and do not constitute an improper limitation of the present invention. In the drawings:

[0015] Figure 1 It is the load spectrum of high and low cycle composite fatigue test;

[0016] Figure 2 It is a nonlinear damage accumulation process based on fatigue driving force;

[0017] Figure 3 Comparison between experimental results and predicted results for turbine blades;

[0018] Figure 4 Comparison of experimental results and predicted results for specimens of different turbine blade alloy materials. DETAILED DESCRIPTION

[0019] The present invention will be described in detail below in conjunction with the accompanying drawings and specific embodiments. The exemplary embodiments and descriptions of the present invention are used to explain the present invention but are not intended to limit the present invention.

[0020] The present invention is to predict the high- and low-cycle composite fatigue life of turbine blades based on the nonlinear fatigue driving force, and the specific implementation steps are as follows:

[0021] Step 1: Perform fatigue tests on turbine blade samples under different loads to obtain stress-life curves, such as Figure 1 The figure shows the load spectrum of the high-low cycle composite fatigue test. The fatigue driving force is a function related to the applied load, the number of load actions and the fatigue life, and is related to the stress-life curve (SN curve). It is expressed by a power function:

[0022] C=σN f b (1)

[0023] Among them, N f is the fatigue life corresponding to the applied load σ, and C and b are material constants fitted by experiments.

[0024] Fatigue driving force σ D It can be expressed as

[0025]

[0026] Where n is the number of times the load σ acts.

[0027] Step 2: Based on formula (2) and the damage accumulation mode of fatigue driving force, σ under the first level load is obtained: 1 The fatigue driving force σ D1 for

[0028]

[0029] Among them, σ D1 is the σ under the first level load 1 Fatigue driving force, N f1 is the load σ 1 The corresponding fatigue life, n 1 is the number of times the load acts.

[0030] Similarly, under the second level load, σ 2 Equivalent fatigue driving force for

[0031]

[0032] in, is the σ under the second level load 2Equivalent fatigue driving force, N f2 is the load σ 2 The fatigue life of is the second level load σ 2 The number of equivalent loads under .

[0033] like Figure 2 As shown in the figure, according to the nonlinear damage accumulation process of fatigue driving force, the fatigue driving force under the first level load is equal to the equivalent fatigue driving force under the second level load level, so there is

[0034]

[0035] If 2 is the second level load σ 1 Afterload level σ 2 The remaining number of load actions under the condition of fatigue load is σ, then the final fatigue driving force σ D2 At the second level of load If it reaches

[0036]

[0037] Combining equations (5) and (6), the fatigue driving force under the second level load can be obtained as

[0038]

[0039] When the fatigue driving force is equal to the fatigue critical value C,

[0040]

[0041] Among them, σ DC is the fatigue strength with a critical value of C, and

[0042] Step 3: Introduce the fatigue driving force into the high- and low-cycle combined fatigue cumulative damage process of the engine turbine blade, take the logarithm of both sides of equation (8), and transform the low-cycle fatigue load σ L As the first level load, high cycle fatigue load σ H As the second level load, we have

[0043]

[0044] Among them, under high and low cycle combined load N is the number of combined cycles under high and low cycle combined loads, N LCF and N HCF They are low cycle fatigue life and high cycle fatigue life respectively.

[0045] Furthermore, it can be obtained that the combined cycle block N under high and low cycle combined load is

[0046]

[0047] Step 4: Based on the fatigue cumulative damage theory, the high-low cycle composite fatigue damage is composed of low cycle fatigue damage D L and high cycle fatigue damage D H Composition, that is

[0048] D=D L +D H (11)

[0049] According to the failure mechanism of high and low cycle combined fatigue, the coupled damage D is considered. C The influence of composite fatigue life and its introduction into the total damage of high-low cycle composite fatigue gives

[0050]

[0051] Among them, D C is the coupling damage,

[0052] Furthermore, the damage expression considering coupling damage and load sequence under high and low cycle combined loads can be obtained as follows:

[0053]

[0054] Where α is the ratio of high to low cycle stress amplitude, σ L and σ H are the corresponding low cycle fatigue stress and high cycle fatigue stress respectively.

[0055] Step 5: Based on the maximum stress, the equivalent stress range ratio of high and low cycle combined load is obtained as

[0056]

[0057] Among them, σ max is the maximum stress under high and low cycle combined loads,

[0058] Furthermore, the equivalent stress range is α eq Introduced into the damage expression (13), the high-low cycle composite fatigue life prediction is obtained as

[0059]

[0060] In order to verify the effectiveness of the nonlinear high-low cycle combined fatigue life prediction method for turbine blades proposed in this invention, the high-low cycle combined fatigue life values ​​predicted by this method are compared with the experimental values ​​of the specimen and turbine blade components under experimental conditions. The results show that the life prediction values ​​obtained by the calculation method proposed in this invention are mostly within the 2 times life dispersion band compared with the experimental life, such as Figure 3 and Figure 4 As shown. This method takes into account the influence of load sequence on composite fatigue damage from the perspective of fatigue driving force, and takes into account the coupling damage caused by high-cycle fatigue-low-cycle fatigue interaction by introducing load interaction factors. Therefore, the method proposed in the present invention has a higher life prediction accuracy. However, the present invention is not limited to aircraft engine turbine blades under high and low cycle composite loads. The life prediction of standard specimens under high and low cycle composite fatigue loads is within the protection of the present invention.

Claims

1. A nonlinear method for predicting the high- and low-cycle combined fatigue life of turbine blades, characterized in that: The steps include: Step 1, fatigue tests are performed on turbine blade samples under different stresses to obtain stress-life curves, and the fatigue driving force is introduced into fatigue life prediction; The fatigue driving force is a function of applied stress, number of loadings and fatigue life, represented by the SN curve: C=σN f b Among them, N f is the fatigue life corresponding to the applied load σ, C and b are material constants fitted by experiments; The fatigue driving force can be expressed as: Where n is the number of times the load σ acts; Step 2, based on the nonlinear damage accumulation method of fatigue driving force, the expression of fatigue driving force under different load levels is obtained; Based on the nonlinear damage accumulation process of fatigue driving force, the expression when the fatigue driving force is a critical value is obtained: Among them, σ DC Fatigue strength when it is the critical value C, N f1 and N f2 are the fatigue lives corresponding to the first level load σ1 and the second level load σ2, respectively, and n1 and n2 are the number of load actions of the first-level load σ1 and the second-level load σ2, respectively; Step 3, introducing the fatigue driving force into the high-low cycle composite fatigue cumulative damage process of the engine turbine blade to obtain the number of composite cycle blocks; Introducing the fatigue driving force into the high-low cycle composite fatigue and taking the logarithm, we have: Among them, N LCF and N HCF are low cycle fatigue life and high cycle fatigue life, respectively, n is the number of times the load σ acts, and N is the composite load block under high and low cycle composite loads; Step 4: Based on the failure mechanism of high- and low-cycle combined fatigue and the fatigue cumulative damage theory, the coupling damage is introduced into the combined fatigue damage including high-cycle fatigue damage and low-cycle fatigue damage, and a modified high- and low-cycle combined fatigue damage expression is obtained; Step 5: Consider the coupled damage caused by the interaction between high-cycle fatigue and low-cycle fatigue, introduce an equivalent stress range ratio of high- and low-cycle combined fatigue in combination with the maximum stress, and carry out life prediction of high- and low-cycle combined fatigue; The high and low cycle composite fatigue life is: Among them, α eq It is the ratio of the equivalent stress range of high and low cycle combined fatigue.

2. The method according to claim 1, characterized in that In step 3, the composite load block under high and low cycle composite load is:

3. The method according to claim 1, characterized in that In step 4, the influence of coupled damage caused by high-cycle fatigue and low-cycle fatigue on the composite fatigue life is considered. Based on the fatigue cumulative damage theory, the high-low cycle composite fatigue damage is corrected to: Where α is the ratio of high to low cycle stress amplitudes, and σ L and σ H They are low cycle fatigue stress and high cycle fatigue stress respectively.

4. The method according to claim 1, characterized in that Step 5: The equivalent stress range ratio of high and low cycle combined loads obtained based on the maximum stress is: Among them, σ max is the maximum stress under high and low cycle combined loads, σ L and σ H They are low cycle fatigue stress and high cycle fatigue stress respectively.

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