A method for calculating multi-axial composite fatigue life of low-pressure turbine shaft of high-temperature alloy

Abstract

This application pertains to step S1: calculating the mean normal stress σ on the critical surface based on the working load of the high-temperature alloy low-pressure turbine shaft. n,m Calculate the normal stress amplitude σ on the critical surface. n,a The shear stress amplitude τ on the critical surface a and its ultimate strength σ b Step S2: Select fatigue life N; Step S3: Based on the selected fatigue life N, select the σ corresponding to fatigue life N on the median life S-N curve with R = -1. max As σ N On the median life τ-N curve with R=-1, select the τ corresponding to the fatigue life N. max As τ N Substitute the above parameters into the preset formula. If the result is true, take the fatigue life N at this time as the final result; otherwise, return to step S2. The fatigue life N is obtained to achieve accurate prediction of the composite fatigue life of high temperature alloy low pressure turbine shaft under biaxial tension and compression, bending and torsion combination, tension and torsion combination and tension-bending-torsion combination.

Description

Technical Field

[0001] This application belongs to the field of fatigue life calculation, and specifically relates to a method for calculating the multi-axis composite fatigue life of a high-temperature alloy low-pressure turbine shaft. Background Technology

[0002] The low-pressure turbine shaft is a key component of an aero-engine, connecting the fan unit and the low-pressure turbine unit. Its main function is to transmit the torque generated by the high-temperature, high-pressure combustion gas in the low-pressure turbine rotor to the fan unit, thereby driving the fan to continuously compress air for efficient combustion. Modern aero-engines typically have a dual-rotor structure (see simplified structural layout diagram). Figure 1 The low-pressure turbine shaft needs to pass through the core engine and connect to the fan unit. The length of the low-pressure turbine shaft typically ranges from 1.5m to 2m, and a certain clearance must be maintained between it and the core engine in the radial direction to prevent rubbing. Therefore, its outer diameter is usually less than Ф120mm. Furthermore, due to the weight constraints of aero-engines, the low-pressure turbine shaft is usually designed as a hollow structure with a wall thickness of 5mm to 8mm. In terms of materials, low-pressure turbine shafts in low-bypass turbofan engines operate at higher temperatures and transmit lower torque, so they are typically made of structural steel or high-temperature alloys; low-pressure turbine shafts in high-bypass turbofan engines operate at lower temperatures and transmit higher torque, so they are typically made of ultra-high-strength steel.

[0003] The low-pressure turbine shaft primarily bears torque and axial force during operation. During maneuvering flight, it also experiences overload and bending moments generated by gyroscopic torque. Therefore, the low-pressure turbine shaft is subjected to multiaxial stress states, including biaxial tension / compression, combined bending / torsion, combined tension / torsion, and combined tension / bending / torsion, making it prone to complex fatigue failure under multiaxial stress. To avoid multiaxial complex fatigue failure of the low-pressure turbine shaft in aero-engines, the design typically employs a multiaxial complex fatigue life assessment method based on the Goodman diagram to calculate the composite fatigue life of the low-pressure turbine shaft. The calculation method is described in [link to calculation method]. Figure 2 and formula (1).

[0004] Existing methods for calculating the multi-axis composite fatigue life of low-pressure turbine shafts based on the Goodman diagram method:

[0005] n=(1-σ m / σ b )×σ -1 / σ a . ...

[0006] In the formula:

[0007] n—fatigue strength reserve coefficient;

[0008] σ m — Steady-state stress, MPa;

[0009] σ a —Alternating stress, MPa;

[0010] σ b —Ultimate strength, MPa;

[0011] σ -1 — Fatigue limit, MPa.

[0012] Its main drawbacks are: the existing formula (1) for calculating the multiaxial composite fatigue life of low-pressure turbine shaft based on the Goodman diagram method refers to the high-cycle fatigue life assessment method for blades. The scope of application of this method is that the steady-state stress and vibration stress are in the same direction. The multiaxial stress states of low-pressure turbine shaft include biaxial tension-compression, bending-torsion combination, tension-torsion combination and tension-bending-torsion combination. When tension-compression combination and tension-bending combination are used, the steady-state stress and vibration stress are in the same direction. When bending-torsion combination and tension-torsion combination are used, the steady-state stress and alternating stress are in orthogonal directions. The existing calculation formula (1) cannot accurately predict the composite fatigue life of low-pressure turbine shaft under bending-torsion and tension-torsion combination. There have been cases where the calculation results of bending-torsion composite fatigue life met the design requirements, but the low-pressure turbine shaft suffered fatigue failure in the test. This shows that the calculation results of low-pressure turbine shaft composite life under bending-torsion and tension-torsion combination using the calculation formula (1) have a large deviation from the actual results. The existing technical solution cannot guarantee that bending-torsion and tension-torsion combination composite fatigue failure will not occur during the operation of low-pressure turbine shaft, thus affecting the reliability of aero-engines. Summary of the Invention

[0013] To address the aforementioned problems, this application provides a method for calculating the multi-axis composite fatigue life of a high-temperature alloy low-pressure turbine shaft, comprising:

[0014] Step S1: Calculate the mean normal stress σ on the critical surface based on the working load of the high-temperature alloy low-pressure turbine shaft. n,m Calculate the normal stress amplitude σ on the critical surface. n,a The shear stress amplitude τ on the critical surface a and its ultimate strength σ b ;

[0015] Step S2: Select fatigue life N;

[0016] Step S3: Based on the selected fatigue life N, select the σ corresponding to fatigue life N on the median fatigue life SN curve with R = -1. max As σ N On the median life τ-N curve with R=-1, select the τ corresponding to the fatigue life N. max As τ N ;

[0017] Step S4: When the formula If the result is true, the fatigue life N at this point is taken as the final result; otherwise, return to step S2.

[0018] in,

[0019]

[0020] σ -1 The tensile fatigue limit is expressed in MPa.

[0021] τ -1 The shear fatigue limit is expressed in MPa.

[0022] β ST These are material constants;

[0023] iAcr is the number of plane groups;

[0024] χ Acr A constant characterizing the sensitivity of a material to iAcr.

[0025] Preferably, χ Acr The value range is 0.1 to 1.

[0026] Preferably, when the material of the low-pressure turbine shaft is a high-temperature alloy and there is no high-low cycle composite, χ Acr =0.7, which represents the value of χ during high-low cycle combined fatigue. Acr =0.3.

[0027] Preferably, β ST The value range is 1 to 2.

[0028] Preferably, when the material of the low-pressure turbine shaft is a high-temperature alloy, β ST Take 2.

[0029] The advantages of this application include: This application provides parameter value suggestions for low-pressure turbine shafts made of high-temperature alloy materials, enabling accurate prediction of the composite fatigue life of high-temperature alloy low-pressure turbine shafts under biaxial tension-compression, bending-torsion combination, tension-torsion combination, and tension-bending-torsion combination conditions, reducing the risk of multiaxial composite fatigue failure of high-temperature alloy low-pressure turbine shafts during operation, and improving the safety and reliability of aero-engines. Attached Figure Description

[0030] Figure 1 This is a simplified diagram of the overall layout of an aero engine;

[0031] Figure 2 Goodman relationship diagram. Detailed Implementation

[0032] To make the technical solution and advantages of this application clearer, the technical solution of this application will be described in a clearer and more complete manner below with reference to the accompanying drawings. It should be understood that the specific embodiments described herein are only some embodiments of this application, and are only used to explain this application, not to limit this application. It should be noted that, for ease of description, only the parts related to this application are shown in the accompanying drawings. Other related parts can be referred to the general design. In the absence of conflict, the embodiments and technical features in the embodiments of this application can be combined with each other to obtain new embodiments.

[0033] Furthermore, unless otherwise defined, the technical or scientific terms used in this application description shall have the ordinary meaning understood by one of ordinary skill in the art to which this application pertains. The terms "upper," "lower," "left," "right," "center," "vertical," "horizontal," "inner," and "outer," etc., used in this application description to indicate relative direction or positional relationship are used only to indicate relative orientation or positional relationship, and do not imply that the device or component must have a specific orientation, or be constructed and operated in a specific orientation. When the absolute position of the described object changes, its relative positional relationship may also change accordingly, and therefore should not be construed as a limitation on this application. The terms "first," "second," "third," and similar terms used in this application description are used only for descriptive purposes to distinguish different components, and should not be construed as indicating or implying relative importance. The terms "a," "one," or "the," etc., used in this application description should not be construed as an absolute limitation on quantity, but should be construed as indicating the existence of at least one. The terms "including," "comprising," etc., used in this application description mean that the element or object preceding the word covers the element or object listed after the word and its equivalents, without excluding other elements or objects.

[0034] Furthermore, it should be noted that, unless otherwise explicitly specified and limited, terms such as “installation,” “connection,” and “linkage” used in the description of this application should be interpreted broadly. For example, a connection can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium; or it can be a connection within two components. Those skilled in the art can understand its specific meaning in this application according to the specific circumstances.

[0035] The purpose of this invention is to propose a multiaxial high-low cycle composite fatigue life calculation method based on the maximum shear stress amplitude critical surface, solving the problem that the existing calculation method (1) cannot accurately evaluate the composite fatigue life of low-pressure turbine shafts under bending-torsion and tension-torsion combinations. The multiaxial high-low cycle composite fatigue life calculation method based on the maximum shear stress amplitude critical surface is given by formulas (2) to (4). This method was used to estimate the life of high-temperature alloy low-pressure turbine shafts under tension-torsion composite fatigue, and the results were compared with experimental results. The comparison results are shown in Table 1. As can be seen from the comparison results, the calculated life of the low-pressure turbine shaft under tension-torsion composite fatigue is within two times the error band, indicating that this method can calculate the composite fatigue life of high-temperature alloy low-pressure turbine shafts under orthogonal stress states well. The calculation accuracy can meet the engineering design requirements, thereby improving the design reliability of high-temperature alloy low-pressure turbine shafts.

[0036]

[0037]

[0038]

[0039] In the formula:

[0040] σ n,m —Mean normal stress on the critical surface, MPa;

[0041] σ n,a —Amplitude of normal stress on the critical surface, MPa;

[0042] τ a —Shear stress amplitude on the critical surface, MPa;

[0043] σ N —σ corresponding to lifetime N on the median lifetime SN curve with R = -1 max MPa;

[0044] τ N —The τ corresponding to lifetime N on the median lifetime τ-N curve when R=-1 max MPa;

[0045] σ b —Ultimate strength, MPa;

[0046] σ -1 —Tension fatigue limit, MPa;

[0047] τ -1 —Shear fatigue limit, MPa;

[0048] β ST —Material constant, usually between 1 and 2, with 2 for high-temperature alloys;

[0049] iAcr — Number of plane groups;

[0050] χ Acr —A constant characterizing the sensitivity of a material to iAcr, typically between 0.1 and 1. For high-temperature alloys, without high-low cycle composites, χ Acr =0.7, in high-low cycle combined fatigue, χ Acr =0.3.

[0051] Table 1 Comparison of Calculation and Experimental Results of Multi-Axis Composite Life of High-Temperature Alloy Low-Pressure Turbine Shaft

[0052]

[0053] 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 calculating the multi-axis composite fatigue life of a high-temperature alloy low-pressure turbine shaft, characterized in that, include: Step S1: Calculate the mean normal stress on the critical surface based on the working load of the high-temperature alloy low-pressure turbine shaft. Calculate the normal stress amplitude on the critical surface. Shear stress amplitude on the critical surface and its ultimate strength ; Step S2: Select fatigue life N; Step S3: Based on the selected fatigue life N, select the fatigue life corresponding to N on the median fatigue life SN curve with R=-1. As ; median lifetime at R=-1 Select the fatigue life N corresponding to the curve As ; Step S4: When the formula If the result is true, the fatigue life N at this point is taken as the final result; otherwise, return to step S2. in, ， ； The tensile fatigue limit is expressed in MPa. The shear fatigue limit is expressed in MPa. These are material constants; The number of plane groups; Characterization of materials A constant representing the degree of sensitivity.

2. The method for calculating the multi-axis composite fatigue life of a high-temperature alloy low-pressure turbine shaft as described in claim 1, characterized in that, When there is no high-low cycle composite, =0.7, during high-low cycle combined fatigue, =0.

3.

3. The method for calculating the multi-axis composite fatigue life of a high-temperature alloy low-pressure turbine shaft as described in claim 1, characterized in that, Take 2.

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