A method for predicting the ultimate pressure resistance of combustion chamber casing

Abstract

The present invention belongs to the field of structural strength assessment of combustion chamber casings of aircraft engines, and discloses a method for predicting the ultimate pressure resistance of combustion chamber casings. First, a smooth round bar specimen and a serial notch specimen are designed to carry out unidirectional tensile tests, and the true stress-true strain curve of the combustion chamber casing material and the load-displacement curve of the serial notch specimen are obtained; then, the elastic-plastic constitutive model parameters are fitted based on the material true stress-true strain curve and set as the initial value, and the elastic-plastic constitutive model parameters of the combustion chamber casing material are obtained by inversion based on the large deformation nonlinear finite element analysis of the serial notch axisymmetric model; finally, a combustion chamber casing finite element model is established based on the material elastic-plastic constitutive model obtained by inversion, and the large deformation finite element analysis is started to use the arc length method to calculate the curve of the change of the internal pressure value of the combustion chamber casing with the radial deformation, and the ultimate pressure resistance value of the combustion chamber casing is determined. The present invention can reduce the test cost while ensuring the accuracy of the inversion optimization.

Description

Technical Field

[0001] The invention relates to a method for predicting the ultimate pressure resistance of a combustion chamber casing, in particular to a method for predicting the ultimate pressure resistance of a combustion chamber casing based on large deformation nonlinear finite elements, and belongs to the field of structural strength assessment of a combustion chamber casing of an aero-engine. Background Art

[0002] The combustion chamber casing is a core pressure-resistant component of an aircraft engine, and contains high-temperature, high-pressure, and high-speed combustion gases that are pressurized and burned by the compressor. If the combustion chamber casing ruptures under extreme conditions, the high-pressure combustion gases inside will inevitably leak, which will cause a huge change in the temperature, oil-gas balance, and pressure inside the combustion chamber, thereby affecting the engine thrust. In severe cases, the combustion chamber may explode, directly threatening the flight safety of the aircraft engine and causing a catastrophic accident. Therefore, in order to avoid the shortcomings of the ultimate pressure resistance design and pressure resistance test of the new material casing, it is necessary to use the ultimate pressure resistance prediction method of the aircraft engine combustion chamber casing based on the material mechanical behavior model.

[0003] The traditional allowable stress design method is based on the ultimate strength of the material as the allowable application, and is a method commonly used in the early stages of engine design. This method makes it difficult to consider unevenly distributed stresses and is too conservative, which is not conducive to the local weight reduction design of the casing. Therefore, in modern times, the plastic failure criterion has gradually become a structural design criterion that relaxes the overly strict restrictions on allowable stresses while strictly ensuring the load-bearing safety of the casing. Elastic-plastic analysis and prediction of ultimate pressure resistance are two important contents of the plastic failure criterion for the casing. In order to establish an accurate elastoplastic constitutive model of the casing material, a large number of tensile tests on smooth parts and notched parts are required. However, the currently available tensile data for notched parts is often a load-displacement curve, which is not cost-effective. The prediction methods for the ultimate pressure resistance of the casing are mostly based on the equivalent strain ε. eq Reaching the true ultimate strain ε of the material f (material elongation at break) is used as the criterion for the failure of the casing. At this time, the internal pressure value is the ultimate pressure resistance value of the casing. Although this method is based on the real strain response phenomenon of the material, the real ultimate strain ε of the material f It cannot represent the ultimate pressure resistance of the casing, because the casing has undergone a large plastic deformation before it ruptures, and the instability of the structure has caused an unstable decrease in the internal pressure load. The existing literature "Yang Mei, Xu Fanfan, Ning Baojun, et al. Safety prediction method for rupture of combustion chamber casing of aircraft engine [J]. Aviation Science and Technology, 2018 (5): 18-24." The Neuber method, engineering elastic-plastic method and true elastic-plastic method were used to perform numerical simulation analysis on the stress and strain of the combustion chamber casing under the internal pressure load, but the ultimate strain rupture failure criterion was still used. Therefore, no more effective method has been seen in the currently published literature and patents.

[0004] Therefore, the present invention proposes a method for predicting the ultimate pressure resistance capacity of a combustion chamber casing based on large deformation nonlinear finite element analysis, adopts serial notch specimens to increase the available data for inversion and construction of a more accurate material elastoplastic constitutive model, and adopts the casing internal pressure instability criterion as the criterion for the ultimate pressure resistance capacity of the casing in large deformation nonlinear finite element analysis. Summary of the invention

[0005] The purpose of the present invention is to provide a method for predicting the ultimate pressure resistance capacity of a combustion chamber casing, so as to solve the problem of insufficient prediction accuracy of the ultimate pressure resistance capacity of a combustion chamber casing.

[0006] To achieve the above object, the technical solution adopted by the present invention is:

[0007] A method for predicting the ultimate pressure resistance of a combustion chamber casing adopts the casing internal pressure instability criterion as the criterion for the ultimate pressure resistance of the casing in large deformation nonlinear finite element analysis; a serial notched specimen is used to increase the available data range for inversion and construction of a more accurate material elastoplastic constitutive model;

[0008] The casing internal pressure instability criterion is: calculating and obtaining a curve of the combustion chamber casing internal pressure value changing with radial deformation, the highest stress value point on the curve is the instability point of the casing internal pressure, and taking it as the ultimate withstand pressure value of the combustion chamber casing;

[0009] The method of increasing available data by using a serial notch specimen refers to designing a serial notch round bar specimen, opening a plurality of annular notches of different shapes or sizes at equal intervals within a gauge length section, and obtaining multiple sets of load-displacement curves under the same load condition.

[0010] The steps of the method for predicting the ultimate pressure resistance of the combustion chamber casing are as follows:

[0011] The first step is to obtain the true stress-true strain curve of the combustion chamber casing material and the load-displacement curve of the series notch specimen;

[0012] The second step is to obtain the parameters of the elastoplastic constitutive model of the combustion chamber casing material based on the large deformation nonlinear finite element analysis inversion of the tandem notch axisymmetric model;

[0013] The third step is to establish a finite element model of the combustion chamber casing, set material parameters, apply loads and constraints including internal pressure loads, axial loads and axial constraints or axial symmetric constraints, start large deformation finite element analysis, use the arc length method to calculate the deformation response of the casing, extract the variation curve of the internal pressure value of the combustion chamber casing with radial deformation, and determine the ultimate pressure resistance value of the combustion chamber casing.

[0014] In the step 1, the true stress-true strain curve of the combustion chamber casing material is extracted by a strain-controlled uniaxial tensile test of a smooth round bar specimen, and multiple sets of load-displacement curves are obtained by a stress-controlled uniaxial tensile test of a series-notched round bar specimen.

[0015] Among them, the serial notched round bar specimen has two or three annular notches of different shapes or sizes at equal intervals within the gauge section. The loads of the serial notches are the same, and the displacements of different notches are measured simultaneously by different extensometers across the notches, or by using the digital image correlation method.

[0016] In the step 2, the inversion process is a constitutive model parameter optimization process with the series notch load-displacement curve as the optimization target, that is, firstly, the elastic-plastic constitutive model parameters are fitted through the material true stress-true strain curve as the initial value of the inversion optimization, and then the series notch load-displacement curve is used as the optimization target to optimize the constitutive model parameters through simulation.

[0017] In step 2, when calculating the load-displacement curve of the series notch, the axisymmetric finite element model of the series notch is used, the large deformation finite element analysis is started, and the arc length method is used as the iterative control method of the nonlinear analysis.

[0018] In the step three, the applied internal pressure load value should be greater than the estimated ultimate pressure resistance capacity. The internal pressure load of the combustion chamber casing is applied using a time-load process step. The internal pressure value is the applied internal pressure load value multiplied by the time value. The radial deformation value of the casing is the sum of the radial displacements of two symmetrical points on the casing barrel in the time-load process step. The ultimate pressure resistance value of the combustion chamber casing is the highest point of the internal pressure value versus radial deformation curve obtained by large deformation nonlinear finite element analysis.

[0019] In step 3, the value of the internal pressure load applied is more than twice the estimated ultimate pressure resistance capacity.

[0020] Preferably, the annular notch is in the shape of a V-shaped notch, a U-shaped notch, a C-shaped notch or an oblique notch.

[0021] Preferably, the simulation software adopts ANSYS software, and the optimization software adopts isight.

[0022] Preferably, when ANSYS software is used to calculate the load-displacement curve of the series notch, the axisymmetric two-dimensional finite element model of the series notch is used because the axisymmetric model converts the 3D problem into a 2D problem, which can improve the efficiency of inversion optimization.

[0023] Advantages and effects of the present invention:

[0024] The present invention proposes to use series notched round rod specimens to load-displacement invert the parameters of the elastoplastic constitutive model, determine the ultimate pressure resistance of the combustion chamber casing based on large deformation nonlinear finite element analysis, and achieve the goal of obtaining multiple sets of target data while using fewer specimens. This can reduce the test cost while ensuring the accuracy of the inversion optimization, and provide a method for predicting the ultimate pressure resistance of the combustion chamber casing based on the global plastic instability characteristics of the structure.

[0025] Specifically:

[0026] 1. During static loading, structural instability occurs before material damage (cracks). The ultimate pressure failure of the combustion chamber casing is due to structural instability rather than structural rupture. Therefore, the fracture criterion based on equivalent strain in the prior art cannot predict the overall plastic instability of the casing.

[0027] The casing internal pressure instability criterion used in the method of the present invention can greatly improve the prediction accuracy of combustion chamber casing failure. Compared with the prior art that uses the true limit strain of the material as the criterion for casing rupture failure, the instability load value point reached by the casing before the actual fracture can be obtained, thereby accurately predicting the casing failure before the casing ruptures, avoiding service accidents of casing components caused by inaccurate instability criteria.

[0028] 2. The method of the present invention adopts a round rod specimen with serial notches, and designs a new specimen with multiple serial notches on the same specimen. Under the same load condition, multiple sets of load-displacement data can be obtained through a specimen test, and the material elastic-plastic constitutive model is inversely optimized based on the obtained data, thereby greatly improving the breadth and accuracy of the material stress-strain range described by the material constitutive model. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 Technical flow chart of the present invention;

[0030] Figure 2 Smooth and notched round bar specimens and extensometer locations;

[0031] Figure 3 Results of constitutive model of materials for casing;

[0032] Figure 4 Curve of internal pressure changing with radial displacement. DETAILED DESCRIPTION

[0033] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0034] The present invention is further explained below in conjunction with the accompanying drawings. A method for predicting the ultimate pressure resistance of a combustion chamber casing based on large deformation nonlinear finite element, the technical flow chart is as follows Figure 1 , including the following steps:

[0035] The first step is to obtain the true stress-true strain curve of the combustion chamber casing material and the load-displacement curve of the series notched specimen. Figure 2 (a)) is a strain-controlled uniaxial tensile test to obtain the true stress-true strain curve of the combustion chamber casing material. By connecting notched round bar specimens (such as Figure 2 (b)) A plurality of load-displacement curves are obtained by a stress-controlled uniaxial tensile test. The serial notched round bar specimen is provided with three annular notches of different shapes or sizes, such as a V-notch, at equal intervals. The true stress-true strain curve of the material is obtained from the engineering stress-strain curve.

[0036] The true stress is:

[0037] σ t =σ(1+ε)

[0038] The true strain becomes:

[0039] ε t =ln(1+ε)

[0040] Among them, σ is the engineering stress and ε is the engineering strain.

[0041] The load of the series notches is the same, and the displacements of the three notches are measured simultaneously by different extensometers across the notches. In this embodiment, four extensometers are used to measure the load-displacement curve of the notched round bar specimen during the stretching process, as shown in FIG. Figure 2 (b) as shown.

[0042] The second step is to obtain the elastic-plastic constitutive model parameters of the combustion chamber casing material based on the large deformation nonlinear finite element analysis inversion of the tandem notch axisymmetric model. The inversion process is a constitutive model parameter optimization process with the tandem notch load-displacement curve as the optimization target, that is, firstly, the elastic-plastic constitutive model parameters are fitted through the material true stress-true strain curve as the initial value of the inversion optimization.

[0043] In this embodiment, the combustion chamber casing material model is based on the Von-Mises yield criterion:

[0044]

[0045] The flow law is the associative flow law:

[0046]

[0047] Mixing Swift and Voce models:

[0048] Swift Model:

[0049] Voce Model:

[0050] Mixed Swift and Voce Model:

[0051] The casing material used in this embodiment is a titanium-aluminum intermetallic compound material. The model results used to describe its effective plastic strain and true stress are as follows: Figure 3 As shown in the figure, it can be seen that the mixed model adopted by the present invention represented by the solid line is closer to the actual test data.

[0052] Then, the load-displacement curve of the series notch is taken as the optimization target, and the constitutive model parameters are optimized by means of joint simulation of ANSYS software and isight optimization software. When the load-displacement curve of the series notch is calculated by ANSYS software, the axisymmetric two-dimensional finite element model of the series notch is used, because the axisymmetric model converts the 3D problem into a 2D problem, which can improve the efficiency of inversion optimization. Therefore, large deformation finite element analysis is started, and the arc length method is used as the iterative control method of nonlinear analysis. The series notch obtains multiple sets of load-displacement curves at the same time, providing sufficient available data for inversion to obtain accurate material elastic-plastic constitutive model parameters.

[0053] The third step is to establish a finite element model of the combustion chamber casing, set material parameters, apply loads and constraints including internal pressure loads, axial loads and axial constraints or axial symmetric constraints, start large deformation finite element analysis, use the arc length method to calculate the deformation response of the casing, extract the curve of the internal pressure value of the combustion chamber casing changing with radial deformation, and determine the ultimate pressure resistance value of the combustion chamber casing. The applied internal pressure load value should be greater than the estimated ultimate pressure resistance capacity, and it is recommended to be twice the estimated ultimate pressure resistance capacity. The internal pressure value of the combustion chamber casing is the "applied internal pressure load value" multiplied by the "time value", and the radial deformation is the sum of the radial displacements of the two symmetrical points of the casing barrel within the same "time value". The ultimate pressure resistance value of the combustion chamber casing is the highest point of the internal pressure value versus radial deformation curve obtained by large deformation nonlinear finite element analysis. Figure 4As shown in , in order to be closer to the shape of the actual casing, this embodiment uses a simple open-hole cylinder to simulate the barrel structure of the casing. The variation curve of the internal pressure value with the radial displacement obtained by the large deformation nonlinear arc length method finite element calculation is as follows: Figure 4 As shown, it can be seen that the internal pressure value first increases and then decreases with the radial displacement, and there is a load instability point. The pressure corresponding to this point is the ultimate pressure resistance capacity of the model.

[0054] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A method for predicting the ultimate pressure resistance of a combustion chamber casing, characterized in that: The casing internal pressure instability criterion is used as the criterion for the ultimate pressure resistance of the casing in large deformation nonlinear finite element analysis. The serial notched specimens are used to increase the available data range for inversion and construction of the elastic-plastic constitutive model of the material. Casing internal pressure instability criterion: Calculate and obtain the variation curve of the combustion chamber casing internal pressure value with radial deformation. The highest stress value point on the curve is the instability point of the casing internal pressure, which is taken as the ultimate withstand pressure value of the combustion chamber casing. Serial notched specimens increase available data: Design serial notched round bar specimens, with several annular notches of different shapes or sizes equally spaced in the gauge length section, and obtain multiple sets of load-displacement curves under the same load condition; The method comprises the following steps: The first step is to obtain the true stress-true strain curve of the combustion chamber casing material and the load-displacement curve of the series notch specimen; The second step is to obtain the parameters of the elastoplastic constitutive model of the combustion chamber casing material based on the large deformation nonlinear finite element analysis inversion of the tandem notch axisymmetric model; The third step is to establish a finite element model of the combustion chamber casing, set material parameters, apply loads and constraints including internal pressure loads, axial loads and axial constraints or axial symmetric constraints, start large deformation finite element analysis, use the arc length method to calculate the deformation response of the casing, extract the variation curve of the internal pressure value of the combustion chamber casing with radial deformation, and determine the ultimate pressure resistance value of the combustion chamber casing.

2. The method for predicting the ultimate pressure resistance of a combustion chamber casing according to claim 1, characterized in that: In the step 1, the true stress-true strain curve of the combustion chamber casing material is extracted by a strain-controlled uniaxial tensile test of a smooth round rod specimen, and multiple sets of load-displacement curves are obtained by a stress-controlled uniaxial tensile test of a series-notched round rod specimen; wherein the series-notched round rod specimen has two or three annular notches of different shapes or sizes at equal intervals within the gauge section, the loads of the series notches are the same, and the displacements of different notches are measured simultaneously by different extensometers spanning the notches, or are measured simultaneously by a digital image correlation method.

3. The method for predicting the ultimate pressure resistance of a combustion chamber casing according to claim 2, characterized in that: The annular notch is in the shape of a V-shaped notch, a U-shaped notch, a C-shaped notch or an oblique notch.

4. The method for predicting the ultimate pressure resistance of a combustion chamber casing according to claim 1, characterized in that: In the step 2, the inversion process is a constitutive model parameter optimization process with the series notch load-displacement curve as the optimization target, that is, firstly, the elastic-plastic constitutive model parameters are fitted through the material true stress-true strain curve as the initial value of the inversion optimization, and then the series notch load-displacement curve is used as the optimization target to optimize the constitutive model parameters through simulation.

5. The method for predicting the ultimate pressure resistance of a combustion chamber casing according to claim 4, characterized in that: When calculating the load-displacement curve of the series notch, the axisymmetric finite element model of the series notch is adopted, the large deformation finite element analysis is turned on, and the arc length method is used as the iterative control method of the nonlinear analysis.

6. The method for predicting the ultimate pressure resistance of a combustion chamber casing according to claim 4, characterized in that: The simulation software used is ANSYS and the optimization software used isight.

7. The method for predicting the ultimate pressure resistance of a combustion chamber casing according to claim 1, characterized in that: In the step three, the applied internal pressure load value should be greater than the estimated ultimate pressure resistance capacity. The internal pressure load of the combustion chamber casing is applied using a time-load process step. The internal pressure value is the applied internal pressure load value multiplied by the time value. The radial deformation value of the casing is the sum of the radial displacements of two symmetrical points on the casing barrel in the time-load process step. The ultimate pressure resistance value of the combustion chamber casing is the highest point of the internal pressure value versus radial deformation curve obtained by large deformation nonlinear finite element analysis.

8. The method for predicting the ultimate pressure resistance of a combustion chamber casing according to claim 7, characterized in that: The internal pressure load value applied is more than twice the estimated ultimate pressure resistance capacity.

Images