Method for evaluating fatigue initiation life of test piece containing precracks

Abstract

The invention relates to a precrack-containing test piece fatigue initiation life evaluation method, which comprises the following steps: firstly, establishing a detail finite element model based on the structure information of a precrack-containing test piece, then calculating by using the detail finite element model to obtain a stress concentration coefficient at a crack tip, and calculating the fatigue initiation life of the precrack-containing test piece according to the stress concentration coefficient at the crack tip. And calculating a detail fatigue rated value of the test piece by combining the material and configuration correlation coefficients, and finally obtaining the crack initiation life of the test piece containing the precracks according to a fatigue load spectrum borne by the test piece. Therefore, through the method, the fatigue initiation life of the test piece containing the precrack can be accurately calculated, the correctness of an airplane strength damage tolerance test result is improved, and the flight safety of an airplane is further improved.

Description

Technical Field

[0001] The invention belongs to the technical field of aircraft control, and in particular relates to a fatigue initiation life assessment method for a test piece containing prefabricated cracks. Background Art

[0002] At present, in the design of aircraft strength damage tolerance test, corresponding test pieces are made for testing and evaluation. However, traditional test evaluation methods cannot calculate and predict the crack initiation life of test pieces with prefabricated cracks, and cannot be applied to test pieces with prefabricated cracks, that is, they cannot accurately calculate the fatigue initiation life of test pieces with prefabricated cracks, which seriously affects the test results of aircraft strength damage tolerance test. Summary of the invention

[0003] In order to solve the above problems existing in the prior art, the present invention provides a method for evaluating the fatigue initiation life of a test piece containing prefabricated cracks.

[0004] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0005] The fatigue initiation life assessment method of a test piece containing prefabricated cracks described in the present invention first establishes a detailed finite element model based on the structural information of the test piece containing prefabricated cracks, then uses the detailed finite element model to calculate the stress concentration factor at the crack tip, and calculates the detailed fatigue rating of the test piece through the stress concentration factor at the crack tip combined with the material and configuration correlation coefficients, and finally obtains the crack initiation life of the test piece containing prefabricated cracks based on the fatigue load spectrum borne by the test piece.

[0006] Furthermore, the fatigue initiation life assessment method of the test piece containing prefabricated cracks includes:

[0007] S1. Establish a detailed finite element model: Use the structural digital model of the test piece with prefabricated cracks to establish a detailed finite element model, which can reflect the local detailed stress;

[0008] S2. Calculate the stress concentration factor at the crack tip, including:

[0009] Add unit loads and fixed constraints at both ends of the detailed finite element model;

[0010] Using finite element software, static linear calculations are performed to obtain the unit stress at the crack tip;

[0011] According to the stress level at the crack tip and the stress level at the basic section, the stress concentration factor Kt at the crack tip is determined;

[0012] S3. Calculate the fatigue rating of the test piece details containing pre-cracks, including:

[0013] The test specimen with prefabricated cracks is treated as a tensile structure without fasteners to calculate the detailed fatigue rating DFR, and the detailed fatigue rating reference value DFR is determined based on the stress concentration factor Kt at the crack tip. base ;

[0014] Combining the material coefficient K, roughness coefficient F and component fatigue rating coefficient Rc, according to the formula DFR = DFR base ×K×F×Rc, the detailed fatigue rating DFR of the test piece is calculated;

[0015] S4. Prediction of crack initiation life of test piece: According to fatigue reliability analysis, the detailed fatigue rating DFR of the test piece at 95% reliability is converted to DFR at 50% reliability. R , and then predict the crack initiation life of the test piece based on the fatigue load spectrum of the test piece with prefabricated cracks.

[0016] Further, in step S1, the detail finite element model is a detail finite element two-dimensional model or a detail finite element three-dimensional model.

[0017] Further, in step S2, the stress concentration coefficient Kt at the crack tip is determined according to the stress level at the crack tip and the stress level at the basic cross section. Specifically, according to the stress Smax at the crack tip and the stress S at the basic cross section, the basic cross section is taken at half the distance between the crack tip and the loading / supporting section, and the stress concentration coefficient Kt at the crack tip is determined to be Smax / S.

[0018] Further, in step S4, the detailed fatigue rating DFR of the test piece at 95% reliability is converted to DFR at 50% reliability by the following formula: R ;

[0019]

[0020] In the formula, α is the shape parameter, B r =1 / lg(Sr), R S is the reliability of the detailed fatigue rating, and ln() is the logarithmic function.

[0021] Further, in step S4, the fatigue load spectrum is a constant amplitude load spectrum.

[0022] Further, when obtaining the unit stress at the crack tip in step S2, the maximum stress level is taken.

[0023] Beneficial effects of the present invention:

[0024] The present invention can accurately calculate the fatigue initiation life of the test piece containing prefabricated cracks through the above technical scheme, thereby improving the correctness of the aircraft strength damage tolerance test results and further improving the safety of aircraft flight. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 It is a schematic flow chart of the fatigue initiation life assessment method of a test piece containing prefabricated cracks according to the present invention;

[0026] Figure 2 1 is a schematic structural diagram of a test piece containing prefabricated cracks in an embodiment of a method for evaluating fatigue initiation life of a test piece containing prefabricated cracks according to the present invention;

[0027] Figure 3 It is a schematic diagram of the result of calculation of a detailed finite element model in an embodiment of the fatigue initiation life assessment method of a test piece containing prefabricated cracks according to the present invention;

[0028] Figure 4 It is a stress concentration factor curve of a test piece containing prefabricated cracks in an embodiment of the fatigue initiation life assessment method of a test piece containing prefabricated cracks according to the present invention. DETAILED DESCRIPTION

[0029] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0030] The fatigue initiation life assessment method of a test piece with prefabricated cracks described in the present invention first establishes a detailed finite element model based on the structural information of the test piece with prefabricated cracks, then uses the detailed finite element model to calculate the stress concentration factor at the crack tip, and calculates the detailed fatigue rating of the test piece through the stress concentration factor at the crack tip combined with the material and configuration correlation coefficients, and finally obtains the crack initiation life of the test piece with prefabricated cracks based on the fatigue load spectrum borne by the test piece. Specifically, Figure 1 As shown, the following steps are included:

[0031] Step S1. Establish a detailed finite element model: Use the structural digital model of the test piece with prefabricated cracks (see Figure 2 A detailed finite element model is established, and the detailed finite element model can reflect the local detailed stress, so the detail unit at the crack tip is as small as possible, especially at the crack tip. Specifically, the detailed finite element model can be a detailed finite element two-dimensional model, or a detailed finite element three-dimensional model. When the structural features of the test piece containing the prefabricated crack are complex, it is preferred to establish a detailed finite element three-dimensional model.

[0032] Step S2. Calculate the stress concentration factor at the crack tip, including:

[0033] S2-1. Add unit loads and fixed constraints at both ends of the detailed finite element model, preferably taking into account the test support size, which is determined based on the specific test;

[0034] S2-2. Use finite element software (such as Abaqus) to perform static linear calculations to obtain the unit stress at the crack tip. Preferably, the maximum stress level is taken as the unit stress at the crack tip, which represents the highest stress level of the entire test piece and is the value corresponding to SMAX;

[0035] S2-3. According to the stress level at the crack tip and the stress level at the basic cross section, determine the stress concentration factor Kt at the crack tip. Specifically, according to the stress Smax at the crack tip and the stress S at the basic cross section (see Figure 3 ,yes Figure 2 Schematic diagram of the calculation results of the established detailed finite element model), the basic section is taken at half the distance between the crack tip and the loading / supporting section, and the stress concentration factor Kt=Smax / S at the crack tip is determined;

[0036] Step S3. Calculate the fatigue rating of the test piece details containing prefabricated cracks, including:

[0037] S3-1. Calculate the detailed fatigue rating (DFRbase) of the specimen with prefabricated cracks as a tension structure without fasteners (DFRbase is the detailed fatigue rating base value, which is one of the parameters for calculating DFR). Determine DFR based on the stress concentration factor Kt at the crack tip. base , that is, according to the calculated stress concentration factor Kt at the crack tip on the stress concentration factor curve (such as Figure 4 ,yes Figure 2 The corresponding DFR can be obtained by querying the stress concentration factor curve of the test piece with prefabricated cracks base ;

[0038] S3-2. Combine the material coefficient K, roughness coefficient F and component fatigue rating coefficient Rc, according to the formula DFR = DFR base ×K×F×Rc, the detailed fatigue rating DFR of the test piece is calculated;

[0039] Step S4. Predict the crack initiation life of the test piece: According to the fatigue reliability analysis, convert the detailed fatigue rating DFR of the test piece at 95% reliability to DFR at 50% reliability. R , the following formula can be used for calculation and conversion: In the formula, α is the shape parameter, B r =1 / lg(Sr), R Sis the reliability of the detailed fatigue rating, ln() is the logarithmic function;

[0040]

[0041] Then, the crack initiation life of the test piece is predicted according to the fatigue load spectrum to which the test piece containing the prefabricated crack is subjected; wherein the fatigue load spectrum is preferably an equal amplitude load spectrum, which has fewer influencing factors and a more accurate prediction result for the crack initiation life.

[0042] It can be seen that, through the fatigue initiation life assessment method of the test piece containing prefabricated cracks described in the present invention, the fatigue initiation life of the test piece containing prefabricated cracks can be accurately calculated, thereby improving the accuracy of the aircraft strength damage tolerance test results and further improving the safety of aircraft flight.

[0043] The above is a preferred embodiment of the present invention. It should be pointed out that a person skilled in the art can make several improvements and modifications without departing from the principle of the present invention. These improvements and modifications are also considered to be within the scope of protection of the present invention.

Claims

1. A fatigue initiation life assessment method for a test piece with prefabricated cracks. It is characterized in that Firstly, based on the structural information of the specimen with prefabricated cracks, a detailed finite element model is established. Then, the stress concentration factor at the crack tip is calculated using the detailed finite element model. The detailed fatigue rating of the specimen is calculated based on the stress concentration factor at the crack tip combined with the material and configuration correlation coefficients. Finally, the crack initiation life of the specimen with prefabricated cracks is obtained according to the fatigue load spectrum borne by the specimen.

2. The method according to claim 1, It is characterized in that include: S1. Establish a detailed finite element model: Use the structural digital model of the test piece with prefabricated cracks to establish a detailed finite element model, which can reflect the local detailed stress; S2. Calculate the stress concentration factor at the crack tip, including: Add unit loads and fixed constraints at both ends of the detailed finite element model; Using finite element software, static linear calculations are performed to obtain the unit stress at the crack tip; According to the stress level at the crack tip and the stress level at the basic section, the stress concentration factor Kt at the crack tip is determined; S3. Calculate the fatigue rating of the test piece details containing pre-cracks, including: The test specimen with prefabricated cracks is treated as a tension structure without fasteners to calculate the detailed fatigue rating DFR. The steps are as follows: According to the stress concentration factor Kt at the crack tip, the detailed fatigue rating reference value DFR is determined by querying the stress concentration factor curve. base ; Combined with the material coefficient K, roughness coefficient F and component fatigue rating coefficient Rc, according to the formula DFR = DFR base ×K×F×Rc, the detailed fatigue rating DFR of the test piece is calculated; S4. Prediction of crack initiation life of test piece: According to fatigue reliability analysis, the detailed fatigue rating DFR of the test piece at 95% reliability is converted to DFR at 50% reliability. R , and then predict the crack initiation life of the test piece based on the fatigue load spectrum of the test piece with prefabricated cracks.

3. The method according to claim 2, It is characterized in that In step S1, the detail finite element model is a detail finite element two-dimensional model or a detail finite element three-dimensional model.

4. The method according to claim 2, It is characterized in that In step S2, the stress concentration factor Kt at the crack tip is determined according to the stress level at the crack tip and the stress level at the basic section. Specifically, according to the stress Smax at the crack tip and the stress S at the basic section, the basic section is taken at half the distance between the crack tip and the loading / supporting section, and the stress concentration factor Kt at the crack tip is determined to be Smax / S.

5. The method according to any one of claims 2 to 4, It is characterized in that In step S4, the detailed fatigue rating DFR of the test piece at 95% reliability is converted to DFR at 50% reliability by the following formula: R ; In the formula, α is the shape parameter, B r =1 / lg(Sr), R s is the reliability of the detailed fatigue rating, and ln() is the logarithmic function.

6. The method according to claim 5, It is characterized in that In step S4, the fatigue load spectrum is a constant amplitude load spectrum.

7. The method according to any one of claims 2 to 4 and 6, It is characterized in that When obtaining the element stress at the crack tip in step S2, the maximum stress level is taken.

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