Segmented prediction method for creep stress and low cycle fatigue damage of turbine blade

Abstract

The application provides a turbine blade creep stress and low cycle fatigue damage segmented prediction method, comprising the following steps: step one, finite element modeling analysis is performed on the blade according to the structural characteristics of the laminate, film hole and impact hole of the cooled turbine blade; step two, the loading and unloading time of each type of cycle is determined; step three, a creep stress relaxation model is established; step four, creep stress analysis is performed in sequence through short-time loading, short-time unloading, short-time loading, loading and unloading, short-time unloading and unloading; step five, the change law of the peak stress with time and the change law of the valley value stress with time in the valley value stress during the unloading process are fitted respectively; step six, the peak stress during the unloading process and the valley value stress during the unloading process are segmented and segmented cycle stress is obtained; step seven, segmented low cycle fatigue life analysis is performed; step eight, segmented time is obtained according to the change law in step five; and step nine, total damage is calculated.

Description

Technical Field

[0001] This invention relates to the field of aero-engine technology, specifically to a method for segmented prediction of creep stress and low-cycle fatigue damage in turbine blades. Background Technology

[0002] As the thrust-to-weight ratio of aero engines increases, the turbine inlet temperature becomes increasingly higher, and the local temperature of the blade even approaches the long-term service temperature of the material. The increase in temperature not only gradually increases the temperature load that the turbine blades need to withstand, but also reduces the material properties of the turbine blades, seriously affecting the low-cycle fatigue life of the turbine blades.

[0003] High thermal stress is often present at locations with low low-cycle fatigue life on turbine blades. This thermal stress undergoes significant stress relaxation during the load-bearing process. Considering creep stress relaxation can significantly reduce the impact of thermal stress on low-cycle fatigue life. Existing creep analysis methods only consider the effect of creep on deformation and cannot account for the effect of creep on stress. Using peak and valley stresses without creep effects for low-cycle fatigue life calculations would be overly conservative.

[0004] The creep stress of turbine blades gradually decreases with increasing holding time. Creep occurs throughout the engine's entire life cycle. If the creep stress after a certain operating time is directly used for low-cycle fatigue life analysis, the resulting low-cycle fatigue life will be overly risky. Therefore, it is crucial to consider the creep stress throughout the engine's entire life cycle. Summary of the Invention

[0005] In view of this, embodiments of this specification provide a segmented prediction method for creep stress and low-cycle fatigue damage of turbine blades, so as to improve the prediction accuracy of low-cycle fatigue life of turbine blades at high temperatures.

[0006] The technical solution of this invention is: a segmented prediction method for creep stress and low-cycle fatigue damage of turbine blades, comprising: Step 1, performing finite element modeling analysis on the blade based on the structural characteristics of the laminate, film cooling pores, and impact pores of the cooled turbine blade; Step 2, determining the loading and holding time T under each type of cycle using a method of uniformly distributing working time. j,i and unloading and retention time T j,i+1Step 3: Establish a creep stress relaxation model; Step 4: Perform creep stress analysis sequentially using short-time loading, short-time unloading, short-time loading, load holding, short-time unloading, and unloading holding; Step 5: Fit the variation law of peak stress with time during peak load holding and the variation law of valley stress with time during valley load holding; Step 6: Segment the peak stress and valley stress during load holding and obtain the segmented cyclic stress; Step 7: Perform segmented low-cycle fatigue life analysis. If the difference in low-cycle fatigue life between adjacent segments is less than or equal to 5 times, proceed to Step 8; if the difference in low-cycle fatigue life between adjacent segments is greater than 5 times, repeat Step 6; Step 8: Obtain the segment time based on the variation law in Step 5; Step 9: Based on the segmented low-cycle fatigue life in Step 7, the segmented time in Step 8, and the required number of cycles Nz... j,i, k is used to calculate the total damage.

[0007] Furthermore, the loading and holding time T in step two... j,i and unloading and retention time T j,i+1 They are respectively:

[0008] Where, N j Assign the number of loops under loop type j to each operating state i of the engine, T i The loading time T for each working state i i+1 The unloading time for each working state i.

[0009] Furthermore, the creep stress relaxation model in step three is as follows: in, Where is the creep strain rate, C1, C2, and C3 are defined constants, K is the temperature, σ is the stress, and e is the base of the natural logarithm.

[0010] Furthermore, step five specifically involves: fitting the peak stress σ during the peak load preservation process using a fourth-power function. j,i,max The relationship with the peak load duration T is specifically σ j,i,max (T)=a1T 4 +b1T 3 +c1T 2 +d1T+e1 T≤T j,i The valley stress σ during the valley load preservation process was fitted using a fourth power function. j,i,min The relationship between σ and the valley load retention time T is specifically σ j,i,min (T)=a2T 4 +b2T 3 +c2T 2 +d2T+e2 T≤T j,i+1Where a1, b1, c1, d1, e1, a2, b2, c2, d2, and e2 are coefficients obtained by fitting using the least squares method.

[0011] Furthermore, step six includes: based on the initial stress σ j,i,max (0) and holding time T j,i The stress σ after j,i,max (T j,i The stress is divided into n equal segments, where n can be determined based on the initial stress σ. j,i,max (0) and σ j,i,max (T j,i The difference between n and n is iterated, and n≥10.

[0012] Furthermore, step six also includes: obtaining the peak stress σ of the k-th segment with any number of segments. j,i,max,k Valley stress σ j,i,min,k ; Where k is an integer, and 1≤k≤n.

[0013] Furthermore, step seven specifically involves:

[0014] Based on the peak stress σ of the kth segment j,i,max,k Valley stress σ j,i,min,k To obtain the stress amplitude σ of the kth segment j,i,a,k and mean stress σ j,i,m,k The stress amplitude σ of the k-th segment is expressed using the Goodman curve. j,i,a,k and mean stress σ j,i,m,k Stress amplitude σ converted to a stress ratio of -1 j,i,-1,k ; through the stress amplitude σ at a stress ratio of -1 j,i,-1,k The stress-life curve analysis yields the low-cycle fatigue life N for the k-th segment. j,i,k .

[0015] Furthermore, step nine specifically involves: according to and Calculate total damage D j,i .

[0016] Compared with the prior art, the beneficial effects that can be achieved by at least one of the above-mentioned technical solutions adopted in the embodiments of this specification include: providing a method for predicting the low-cycle fatigue life of turbine blades with creep relaxation, which can improve the accuracy of predicting the low-cycle fatigue life of turbine blades at high temperatures and has strong engineering applicability. Attached Figure Description

[0017] To more clearly illustrate the technical solutions of the embodiments of this application, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0018] Figure 1 This is a flowchart illustrating an embodiment of the present invention;

[0019] Figure 2 This is a flowchart of creep stress analysis;

[0020] Figure 3 This is a schematic diagram illustrating the variation of stress over time substeps;

[0021] Figure 4 This is a comparison diagram of the effects of the present invention and the prior art. Detailed Implementation

[0022] The embodiments of this application will now be described in detail with reference to the accompanying drawings.

[0023] The following specific examples illustrate the implementation of this application. Those skilled in the art can easily understand other advantages and effects of this application from the content disclosed in this specification. Obviously, the described embodiments are only a part of the embodiments of this application, and not all of them. This application can also be implemented or applied through other different specific embodiments, and the details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of this application. It should be noted that, in the absence of conflict, the following embodiments and features in the embodiments can be combined with each other. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0024] like Figure 1 As shown in the figure, this invention provides a method for segmented prediction of creep stress and low-cycle fatigue damage in turbine blades, specifically including the following steps:

[0025] Step 1: Structural Modeling. Finite element analysis is performed on the turbine blades, considering the structural characteristics of the laminates, film cooling holes, and impact holes. Due to the large number of film cooling holes on the turbine blade wall, to reduce the model size, only the film cooling holes in the high-temperature and high nominal stress regions are selected for modeling, or a sub-model is used for film cooling hole modeling. The film cooling hole model should include adjacent film cooling holes to account for the superposition effect of stress concentration.

[0026] Step 2: Simplify the load holding time. Assign N loops to loop type j based on each engine operating state i. j and loading time Ti The loading and holding time T under each type of cycle is determined by using a method of uniformly distributing working time. j,i Similarly, the number of loops N assigned to loop type j for each simplified working state i+1 is determined. j and unloading time T i+1 The unloading and holding time T under each type of cycle is determined by using a method of evenly distributing working time. j,i+1 .

[0027]

[0028]

[0029] Step 3: Establishing the Creep Stress Relaxation Model. When performing stress analysis, both the elastoplastic stress-strain model and the creep stress relaxation model need to be input simultaneously. If a single-crystal material is used, single-crystal anisotropy also needs to be considered. The elastoplastic stress-strain model and single-crystal anisotropy are generally obtained directly from material data sheets. This invention focuses on the creep stress relaxation model. Creep after the instantaneous response and its development over time can generally be divided into three stages: initial accelerated creep (strain rate gradually decreases), steady creep (strain rate is constant), and accelerated creep (strain rate gradually increases). Turbine blades typically experience the steady creep stage during their lifespan. In engineering, to improve efficiency and simplify the analysis process, a constitutive model for the steady creep stage is generally used for creep analysis. This invention uses the Norton model to fit the constitutive model for the steady creep stage. Where... Let σ be the creep strain rate, K be the temperature, e be the base of the natural logarithm, and C1, C2, and C3 be defined constants.

[0030]

[0031] Step 4: Creep Stress Analysis. To obtain the peak and trough stresses before and after creep, the creep stress analysis process is as follows: short-time loading → short-time unloading → short-time loading → load holding → short-time unloading → unloading holding (see...). Figure 2 The short-time loading and short-time unloading load step time is 0.001 hours. The loading hold time and unloading hold time are obtained according to step 2. The loading hold and unloading hold load step size is adjusted according to the hold time. The initial step size is not less than 10. Output the result of each substep.

[0032] Step 5: Creep stress curve fitting. Based on the stress analyzed in Step 4 for each sub-step, fit the variation of peak stress with time during peak load holding and the variation of valley stress with time during valley load holding (see...). Figure 3 The peak stress σ during the peak load preservation process was fitted using a fourth power function. j,i,maxThe relationship between the peak load holding time T and the valley stress σ during the valley load holding process is fitted using a fourth power function. j,i,min The relationship between the valley load holding time T and the value. In the formula, a1, b1, c1, d1, e1, a2, b2, c2, d2, and e2 are coefficients obtained by least squares fitting.

[0033] σ j,i,max (T)=a1T 4 +b1T 3 +c1T 2 +d1T+e1 T≤T j,i ………(4)

[0034] σ j,i,min (T)=a2T 4 +b2T 3 +c2T 2 +d2T+e2 T≤T j,i+1 ………(5)

[0035] Step 6: Segment the load-bearing stress. Based on the initial stress σ j,i,max (0) and holding time T j,i The stress σ after j,i,max (T j,i The stress is divided into n (n≥10) equal segments, where n can be determined based on the initial stress σ. j,i,max (0) and σ j,i,max (T j,i The difference is iterated.

[0036] Segmented cyclic stress acquisition. Based on the number of segments n, obtain the peak stress σ of the k-th segment (1≤k≤n, k is an integer) of any number of segments. j,i,max,k Valley stress σ j,i,min,k .

[0037]

[0038]

[0039] Step 7: Segmented low-cycle fatigue life analysis. Based on the peak stress σ of the k-th segment (1≤k≤n, k is an integer) obtained in Step 7. j,i,max,k Valley stress σ j,i,min,k To obtain the stress amplitude σ of the kth segment j,i,a,k and mean stress σ j,i,m,k The stress amplitude σ is expressed using the Goodman curve. j,i,a,k and mean stress σ j,i,m,k Stress amplitude σ converted to a stress ratio of -1 j,i,-1,k Through stress amplitude σ j,i,-1,k The stress-life curve analysis yields the low-cycle fatigue life N for the k-th segment.j,i,k If the low-cycle fatigue life of adjacent segments differs by less than 5 times, proceed to step 8; otherwise, return to step 6 to further subdivide the number of stress-holding segments.

[0040]

[0041]

[0042] Step 8: Obtain data in segments. Based on the peak stress σ j,i,max,k And formula (2), σ is calculated using the inverse function. j,i,max,k The corresponding time T j,i,k .

[0043] Step 9: Total Damage Calculation. The design loop number Nz corresponding to the operating state i assigned to loop type j is obtained from the engine operating profile. j,i According to the time T of the kth segment j,i,k Total loading and holding time T j,i The proportion of Nz will be calculated in the same proportion. j,i Assigned to segment k, the design cycle number Nz for segment k is calculated. j,i,k Based on the low-cycle fatigue life N of the kth segment obtained in step 8. j,i,k And the required number of cycles Nz j,i,k Calculate the total damage D when working state i is assigned to loop type j. j,i .

[0044]

[0045]

[0046] The invention was validated through low-cycle fatigue tests on simulated turbine blade features. A comparison of the theoretical and measured lifespans of the proposed method with traditional methods is shown below. Figure 4 As can be seen, the theoretical predicted lifetime obtained by the method of the present invention differs from the actual lifetime by less than 2 times, and the accuracy of the method of the present invention is about 5 times higher than that of the traditional design method.

[0047] The above description is merely a specific embodiment of this application, but the scope of protection of this application is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in this application should be included within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

Claims

1. A method for segmented prediction of creep stress and low cycle fatigue damage of turbine blades, characterized by, The method comprises the following steps: Step one, finite element modeling analysis is performed on the blade according to the structure characteristics of the laminate, film hole and impingement hole of the cooled turbine blade; Step two, using the method of uniform distribution of working time to determine the loading and unloading time of each type of cycle and unloading time ; Step three, a creep stress relaxation model is established; Step four, creep stress analysis is performed in sequence in the short-time loading, short-time unloading, short-time loading, loading preservation, short-time unloading and unloading preservation; Step five, the variation law of the peak stress with time in the peak value preservation process and the variation law of the valley value stress with time in the valley value preservation process are fitted respectively; Step six, the peak value stress in the preservation process and the valley value stress in the preservation process are segmented and the segmented cyclic stress is obtained; Step seven, segmented low cycle fatigue life analysis is performed, when the low cycle fatigue life of adjacent segments differs by less than or equal to 5 times, step eight is performed; when the low cycle fatigue life of adjacent segments differs by more than 5 times, step six is repeated; Step eight, the segmented time is obtained according to the variation law in step five; Step nine, calculate total damage from the segmented low cycle fatigue life of step seven, the segmented time of step eight, and the required number of cycles . The loading and unloading holding time in the step two and the unloading holding time are respectively: ; where N j is the number of cycles allocated to class j cycles for each operating state i of the engine, T i is the loaded operating time for each operating state i, and T i+1 is the unloaded operating time for each operating state i. The creep stress relaxation model of the step three is: wherein, is the creep strain rate, C1, C2, C3 are defined constants, K is the temperature, σ is the stress, and e is the base of the natural logarithm.

2. The method of claim 1, wherein the method is characterized by: The step five is specifically: The peak stress in the peak value holding process is fitted by a 4th power function and the peak value holding time T, specifically ; The valley stress in the valley holding process is fitted by a 4th power function and the valley holding time T, specifically ; Wherein, a1, b1, c1, d1, e1, a2, b2, c2, d2, e2 are coefficients obtained by least square fitting.

3. The method of claim 2, wherein the method is characterized by: The step six comprises: According to the initial stress and the holding time after stress , the stress is equally divided into n segments, where n can be iterated according to the difference between the initial stress and , and n≥10.

4. The method of claim 3, wherein the method is characterized by: The step six further comprises: Acquiring peak stress of any segment and valley stress of segment and valley stress of segment ; ； ； wherein k is an integer, and .

5. The method of claim 4, wherein the method is characterized by: The step seven is specifically: According to the peak stress of the first segment and the valley stress of the second segment , the stress amplitude and the average stress of the first segment are obtained Using Goodman curves to... stress amplitude of segment and average stress Stress amplitude converted to a stress ratio of -1 ; By stress amplitude at stress ratio of -1 and stress-life curve analysis Low cycle fatigue life of paragraph .

6. The method of claim 5, wherein the method is characterized by: The step nine is specifically: According to and the total damage is calculated.

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