Method for analyzing strength and service life of FBO working condition of aero-engine

Abstract

The invention discloses a method for analyzing the strength and the service life of an aero-engine fan blade in a flying-off working condition, which comprises the following steps of: performing static strength evaluation at an impact stage and a rotating speed reduction stage by adopting a finite element model, and judging whether a static strength requirement is met or not; carrying out residual stress analysis by utilizing the sub-model; and performing fatigue analysis in a windmill stage. According to the method, the load characteristics and analysis requirements of the whole process of the fan blade flying-off event are fully considered, the load screening efficiency and accuracy are improved, the problem that analysis of the influence of an initial event on the fatigue life is difficult to consider is solved, the problem that residual stress calculation based on elastic-plastic analysis cannot be carried out through an inertia release method is solved, and the reliability is improved. And the characteristics of unbalanced load are fully considered, and meanwhile, the conservative property and convenience of an analysis result are ensured.

Description

Technical Field

[0001] The present invention relates to the field of aviation, and particularly to a method for analyzing the strength and life of an aero-engine fan blade during a fan blade out (FBO) condition. Background Art

[0002] The windmilling state of an engine refers to the state in which the engine rotor continues to rotate after the engine stops in the air (i.e., under non-operating conditions). The reason for the continuous rotation may be the windmilling or mechanical effect, or a combination of both. The mechanical effect means that the main rotating system of the engine still maintains rotation after the engine stops in the air, and the windmilling effect is due to the continuous rotation caused by the aircraft continuing to fly forward and the airflow continuously blowing onto the engine fan blades. The unbalanced load generated during the continuous rotation of the engine rotor is called the windmilling load.

[0003] One of the reasons for the windmilling state of an engine is the fan blade out (FBO). FBO refers to the process in which the fan blade flies off due to reasons such as foreign object impact and fatigue failure. The fly-off will generate a large unbalanced load.

[0004] Before the fan blade out causes the engine to enter the windmilling state, the entire fly-off process has to go through the blade shedding stage and the speed reduction stage first. The stage where the blade shedding generates impact is instantaneous, and the speed reduction time is also very short.

[0005] After the engine enters the windmilling state (at this time, a single engine of the aircraft fails and stops, but the aircraft is still in flight), according to the requirement of continuous rotation in the airworthiness clause 33.74 of the aero-engine, the aircraft needs to make a diversion landing. During the diversion flight and landing process, each component of the engine in the windmilling state needs to meet the strength and life requirements to ensure the safe diversion flight of the aircraft without causing harmful consequences. Therefore, the windmilling state has to go through four stages: initial descent, cruise, final descent, and landing. As Figure 1 shown, each stage has a given altitude, flight speed, and load frequency, and the rotation speed and duration are different.

[0006] Among all the reasons that cause the windmilling state, the windmilling load is the highest after the fan blade out. During the design stage, the analysis requirements for the continuous rotation process after the fan blade out event include:

[0007] (1) Meeting the static strength requirement after the fan blade out failure;

[0008] (2) Considering the impact of the initial event of continuous rotation on the components during the analysis;

[0009] (3) The result of the fatigue analysis should show that the cumulative fatigue damage is less than or equal to the fatigue damage of component failure, so as to prove that the component will not suffer fatigue failure during the continuous rotation period.

[0010] At present, the strength assessment under FBO impact mainly focuses on static strength analysis. There is a lack of mature analysis methods for the strength and life assessment during the continuous rotation condition after FBO failure.

[0011] The main problems are as follows:

[0012] · The load data calculated by FBO based on transient dynamics is large, and there is a lack of effective key load condition screening methods for the whole process analysis of FBO.

[0013] · During the continuous rotation stage in the windmill state, the influence of initial events on fatigue life is not reasonably considered.

[0014] · Using fatigue analysis based on rain flow processing fails to consider the particularity of windmill load conditions and has low calculation efficiency.

[0015] These factors restrict the design and analysis of key components on the engine force transmission path after fan blade detachment events, such as load-bearing frames, bearings, and installation systems.

[0016] Therefore, there is an urgent need for a method for screening key load conditions of unbalanced loads, strength and life assessment in the windmill state of an aero-engine after a fan blade detachment event, to provide effective support for the strength and life analysis of the structure. Summary of the Invention

[0017] The summary of the invention is provided to introduce some concepts that will be further described in the following detailed description in a simplified form. The summary of the invention is not intended to identify the key features or essential features of the claimed subject matter; nor is it intended to be used to determine or limit the scope of the claimed subject matter.

[0018] The present invention mainly aims at the strength and life analysis of components such as load-bearing frames during the design stage of an aero-engine after an FBO event, including static strength analysis during the blade detachment impact and speed reduction stages and fatigue analysis during the windmill state stage.

[0019] The method for strength and life analysis of an aero-engine fan blade detachment condition of the present invention includes: using a finite element model to screen key condition loads during the impact and speed reduction stages according to the minimum equivalent stress strength reserve coefficient and considering the influence of the temperature field, performing key condition stress analysis on the load-bearing frame using the inertia release method, and making a failure determination according to the static strength criterion to judge whether the static strength requirement is met; under the condition of meeting the static strength requirement, using a sub-model to perform residual stress analysis; and performing fatigue analysis during the windmill stage.

[0020] Among them, for the cold-end load-bearing frame, the unit load method is used to determine the key load conditions. The equivalent stress under the actual node load is obtained by linearly superposing the node stresses under the unit load and the force and moment loads in different directions on each installation edge of the load-bearing frame. For the hot-end load-bearing frame, the coarse mesh finite element method is used to determine the key load conditions. The coarse mesh uses two-dimensional shell elements and the model is simplified.

[0021] Among them, the stress analysis of the load-bearing frame under key conditions includes: based on the transient dynamics calculation of the blade shedding failure process, obtaining the force and moment loads in different directions on each installation edge of the load-bearing frame, applying the loads and temperatures under all load conditions, using the inertia release method to perform stress analysis and calculation on the load-bearing frame, calculating the strength reserve coefficients of all nodes of the finite element model according to the equivalent stress, the first principal stress and the third principal stress obtained from the calculation, and performing static strength analysis using the key load condition with the smallest equivalent stress reserve coefficient selected by screening to determine the key load condition.

[0022] Among them, the failure determination according to the static strength criterion includes: determining the assessment position according to the finite element stress analysis result. If the equivalent stress calculated by linear elasticity at the assessment position does not exceed the material tensile limit, the static strength meets the design requirements.

[0023] Among them, the residual stress analysis using the sub-model includes: based on the result of static strength assessment, if the maximum effective stress at the assessment position exceeds the yield strength, determining the assessment position as the dangerous position, taking the corresponding feature of the dangerous position in the finite element model as the sub-model, and the sub-model is cut from the overall part model; and performing elastic-plastic stress analysis and unloading on the key load condition based on the tensile stress-strain curve of the material to obtain the residual stress at the dangerous position.

[0024] Among them, the fatigue analysis in the windmill stage includes: screening the key conditions in the windmill stage, including screening the key conditions of each stage of the windmill section according to the principle of the smallest strength reserve coefficient of the first principal stress and the third principal stress; using the selected key conditions as the fatigue cycle calculation conditions to perform stress analysis of each stage of the key conditions; and performing fatigue analysis on the initial descent stage, cruise stage, final descent stage and landing stage, superimposing the 6 stress components of the residual stress, determining the stress cycle corresponding to the key load condition of each stage at the dangerous position, and using the R=-1 fatigue life curve to calculate the allowable fatigue life.

[0025] The method of the present invention can realize the screening of key load conditions in the whole-process strength and life analysis of the FBO event of aero-engines, the analysis of the influence of the initial event in the continuous rotation stage after FBO on the fatigue life (static strength analysis and residual stress analysis in the blade shedding and speed reduction stages), and the fatigue life analysis under the windmill load condition.

[0026] These and other features and advantages will become apparent by reading the following detailed description and referring to the associated drawings. It should be understood that the foregoing general description and the following detailed description are illustrative only and do not limit the various aspects claimed. Description of the Drawings

[0027] The present invention will be described in more detail below with reference to specific embodiments shown in the drawings.

[0028] Figure 1 Four stages included in the windmill state are schematically shown;

[0029] Figure 2 The unbalanced loads in the four stages of the windmill state are schematically shown;

[0030] Figure 3 A flowchart of the method for strength and life analysis of the present invention for the FBO condition of an aeroengine is shown. Detailed Description of the Embodiments

[0031] The present invention will be described in more detail below with reference to specific embodiments shown in the drawings. By reading the following detailed description of the specific embodiments, various advantages and benefits of the present invention will become clear to those of ordinary skill in the art. However, it should be understood that the present invention can be implemented in various forms and should not be limited by the embodiments set forth herein. The following embodiments are provided to enable a more thorough understanding of the present invention. Unless otherwise specified, the technical terms or scientific terms used in this application should have the ordinary meaning understood by those skilled in the art to which this application belongs.

[0032] The present invention provides a method for strength and life analysis of load-bearing frame and other parts after an FBO event during the design stage of an aeroengine, mainly including static strength analysis, residual stress analysis, and fatigue analysis in the windmill stage during the blade shedding and speed reduction stages.

[0033] Figure 2 The unbalanced loads in the four stages of initial descent, cruise, final descent, and landing of the windmill state are schematically shown. The present invention needs to analyze the whole process of the FBO event to screen key load conditions.

[0034] The following is combined with Figure 3 to specifically explain the method of the present invention:

[0035] The method of the present invention includes step 310 of static strength analysis in the impact and speed reduction stages, step 320 of residual stress analysis, step 330 of fatigue analysis in the windmill stage, and step 340 of design modification.

[0036] First, in step 310, a finite element model is used for static strength analysis in the impact and speed reduction stages:

[0037] Since the blade shedding impact and the rotational speed drop are very short in time, these two stages are combined for processing, and a static strength assessment is carried out. Failure determination is carried out according to the static strength criterion, including: determining the assessment position based on the finite element stress analysis result. If the equivalent stress calculated linearly at the assessment position does not exceed the material tensile limit, the static strength meets the design requirements.

[0038] Step 310 further includes three sub-steps.

[0039] In step 311, first, the key load conditions in the impact and rotational speed drop stages are screened.

[0040] The key load conditions are screened according to the minimum equivalent stress strength reserve coefficient, fully considering the load characteristics of the FBO event and the influence of the temperature field:

[0041] · For the cold-end load-bearing frame, the unit load method is used to determine the key load conditions. The equivalent stress under the actual node load is obtained by linearly superposing the node stresses under the unit load and the force and moment loads in different directions on each mounting edge of the load-bearing frame.

[0042] · For the hot-end load-bearing frame, the coarse mesh finite element method is used to determine the key load conditions. The coarse mesh uses two-dimensional shell elements and the geometric solid model is appropriately simplified. The simplification means include deleting features such as chamfers, fillets, and small holes outside the key area, and performing finite element coarse mesh division on the simplified solid model.

[0043] Next, in step 312, the stress analysis of the key conditions is carried out.

[0044] Based on the transient dynamics calculation of the blade shedding failure process, the force and moment loads in different directions on each mounting edge of the engine load-bearing frame can be obtained. Apply the loads and temperatures under all load conditions, and use the inertial release method to carry out stress analysis and calculation on the load-bearing frame. Calculate the strength reserve coefficient of all nodes of the finite element model according to the equivalent stress, the first principal stress, and the third principal stress obtained from the calculation. The static strength analysis is carried out using the key load condition with the smallest equivalent stress reserve coefficient screened, and the key load condition is determined.

[0045] If in step 313, failure determination is carried out according to the static strength criterion to judge whether the static strength requirement is met. If not, this method directly proceeds to step 340 to modify the design. If so, this method enters step 320.

[0046] Secondly, in step 320, the residual stress analysis of the dangerous position is carried out using the sub-model:

[0047] Based on the results of static strength analysis, if the maximum effective stress at the assessment location exceeds the yield strength, the assessment location is a dangerous location. Take the corresponding feature of the dangerous location in the finite element model as the submodel, which is cut from the overall part model. Conduct elastic-plastic stress analysis and unloading for the key load conditions based on the tensile stress-strain curve of the material to obtain the residual stress at the dangerous location.

[0048] During the calculation, apply the overall model displacement to the installation edge and the cross-section of the submodel. Convert the inertial force automatically applied by the inertial release method into translational acceleration and angular acceleration, and apply them to the submodel in the form of loads. At the same time, consider the material nonlinearity for elastic-plastic calculation. In this way, the problem that the inertial release method cannot calculate the residual stress based on elastic-plastic analysis is solved.

[0049] Next, this method enters step 330 to conduct fatigue analysis in the windmill stage. Step 330 consists of four sub-steps. Since the windmill stage is divided into four stages: initial descent, cruise, final descent, and landing, the following steps are also carried out for each of these four stages.

[0050] First, conduct key condition screening for the windmill stage in step 331. In the windmill stage, the key conditions of the windmill profile for each stage are screened according to the principle of the minimum strength reserve coefficient of the first principal stress and the third principal stress.

[0051] Then, in step 332, take the screened key conditions as the fatigue cycle calculation conditions and conduct stress analysis for the key conditions of each stage.

[0052] Next, in step 333, conduct fatigue analysis for each stage, and calculate the allowable fatigue life by superimposing the residual stress obtained from the elastic-plastic analysis of the key conditions in the impact and speed reduction stages.

[0053] According to the stress analysis results of the previous step, determine the stress cycles corresponding to the key load conditions at the dangerous location for each stage. When determining the stress cycle, consider the residual stress obtained from the elastic-plastic analysis in step 320 mentioned above, and superimpose the six stress components of the residual stress (σ x 、σ y 、σ z 、τ xy 、τ yz 、τ xz ) to consider the influence of the FBO impact on the fatigue damage analysis of the continuous rotation stage by using the residual stress. In this way, fully consider the influence of the initial event on the fatigue life of the continuous rotation stage.

[0054] The windmill load is mainly generated by the rotor unbalance response, and the fatigue life is calculated using the R=-1 fatigue life curve. The number of cycles is calculated by multiplying the rotational speed corresponding to the moment when the first principal stress reserve factor is the smallest in each stage by the duration of each stage. The fatigue damages of all stages are superimposed to obtain the total fatigue damage D of the windmill stage. w , as shown in Equation (1) below, and the failure judgment is carried out according to the life criterion.

[0055]

[0056] Among them, D w is the total fatigue damage of the windmill stage, ω i is the rotational speed corresponding to the moment of the maximum first principal stress in each stage, t i is the duration of each stage, and N f,i is the allowable fatigue life of each stage.

[0057] The total fatigue damage is obtained by superimposing the fatigue damages of each stage. In this way, the characteristics of the unbalanced load are fully considered, and at the same time, the conservativeness and convenience of the analysis results are ensured.

[0058] In addition, when performing fatigue analysis, for the hot-end casing, the temperature field has a great influence on fatigue damage, and the temperature field under continuous rotation conditions is applied during the analysis of the initial descent stage of the windmill section.

[0059] Next, in step 334, a judgment is made on whether the life requirement is met. If it is yes, the method ends; if it is no, it proceeds to step 340 to modify the design.

[0060] The method for analyzing the strength and life under the windmill load after FBO of the present invention is also applicable to the analysis of other types of windmill loads. In addition, the analysis method is also applicable to parts on other main force transmission paths of the engine, such as bearings and mounting systems, except for the load-bearing frame.

[0061] The key load condition screening method for different parts and different stages after the FBO event proposed by the present invention fully considers the load characteristics and analysis requirements of the entire process of the FBO event, and improves the efficiency and accuracy of load screening.

[0062] The method for calculating fatigue damage and superimposing the residual stress of the previous stage proposed by the present invention solves the problem that it is difficult to consider the influence of the initial event on the fatigue life analysis.

[0063] The method for converting inertial force into acceleration and applying it to the sub-model and using material nonlinearity to analyze residual stress proposed by the present invention solves the problem that the inertial release method cannot calculate the residual stress based on elastoplastic analysis.

[0064] In each stage of the windmill section proposed by the present invention, the first principal stress and the third principal stress are used to screen key load conditions, and the stress ratio R=-1 fatigue life curve is used to calculate the fatigue life, fully considering the characteristics of unbalanced loads, while ensuring the conservativeness and convenience of the analysis results.

[0065] The above embodiments are only used to illustrate the technical solutions of the present application, rather than limiting them; although the present application has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements on some or all of the technical features; and these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present application, and they should all be covered by the scope of the claims and the description of the present application.

Claims

1. A method for strength and life analysis of aero-engine fan blades under the condition of blade-off including: Using a finite element model to conduct static strength assessment during the impact and speed reduction stages to determine whether the static strength requirements are met; Under the condition of meeting the static strength requirements, using a sub-model to conduct residual stress analysis; and Conducting fatigue analysis during the windmilling stage.

2. The method according to claim 1, wherein, the static strength assessment includes: Screening the key working condition loads during the impact and speed reduction stages according to the minimum equivalent stress strength reserve coefficient; Conducting key working condition stress analysis on the load-bearing frame using the inertia relief method; and Making a failure determination according to the static strength criterion to determine whether the static strength requirements are met.

3. The method according to claim 2, wherein, the screening of the key working condition loads during the impact and speed reduction stages takes into account the influence of the temperature field.

4. The method according to claim 3, wherein, taking into account the influence of the temperature field includes: For the cold-end load-bearing frame, using the unit load method to determine the key load conditions, and obtaining the equivalent stress under the actual node load by linearly superposing the node stresses under the unit load and the force and moment loads in different directions on each mounting edge of the load-bearing frame; and For the hot-end load-bearing frame, using the coarse mesh finite element method to determine the key load conditions, and the coarse mesh uses two-dimensional shell elements and simplifies the model.

5. The method according to claim 2, wherein, the key working condition stress analysis on the load-bearing frame includes: Based on the transient dynamics calculation of the blade-off failure process, obtaining the force and moment loads in different directions on each mounting edge of the load-bearing frame, applying the loads and temperatures under all load conditions, conducting stress analysis and calculation on the load-bearing frame using the inertia relief method, calculating the strength reserve coefficient of all nodes of the finite element model according to the calculated equivalent stress, the first principal stress and the third principal stress, and conducting static strength analysis using the key load condition with the smallest selected equivalent stress reserve coefficient to determine the key load condition.

6. The method according to claim 2, wherein, the failure determination according to the static strength criterion includes: determining the assessment position according to the finite element stress analysis result, and if the equivalent stress calculated by linear elastic calculation at the assessment position does not exceed the material tensile limit, the static strength meets the design requirements.

7. The method according to claim 1, wherein, the residual stress analysis using the sub-model includes: Based on the result of the static strength assessment, if the maximum effective stress at the assessment position exceeds the yield strength, determining the assessment position as the dangerous position, and taking the corresponding feature at the dangerous position in the finite element model as the sub-model, and the sub-model is cut from the overall part model; and Conducting elastic-plastic stress analysis and unloading on the key load conditions based on the tensile stress-strain curve of the material to obtain the residual stress at the dangerous position.

8. The method according to claim 1, wherein, the fatigue analysis during the windmilling stage includes: Conduct key working condition screening for the windmill stage, including screening the key working conditions of the windmill section in each stage according to the principle of the minimum strength reserve coefficient of the first principal stress and the third principal stress for each stage of the windmill section; Take the screened key working conditions as the fatigue cycle calculation working conditions, and conduct stress analysis on the key working conditions of each stage; and Conduct fatigue analysis on each stage, superimpose the residual stress mentioned above, and use the R=-1 fatigue life curve to calculate the allowable fatigue life, wherein each stage includes the initial descent stage, the cruise stage, the final descent stage, and the landing stage.

9. The method according to claim 8, characterized in that The fatigue analysis includes superimposing the six stress components of the residual stress (σ x , σ y , σ z , τ xy , τ yz , τ xz ) to determine the stress cycles corresponding to the key load conditions at each stage of the dangerous location.

10. The method according to claim 8, characterized in that The fatigue analysis includes obtaining the total fatigue damage D of the windmill stage by superimposing the fatigue damages of all stages according to the following formula w :[[]]END]] where ω i is the rotational speed corresponding to the maximum first principal stress moment in each stage, t i is the duration of each stage, N f,i is the allowable fatigue life of each stage.

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