CN110987676A - Full-life prediction method considering crack closure effect under random multi-axis load - Google Patents

Full-life prediction method considering crack closure effect under random multi-axis load Download PDF

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CN110987676A
CN110987676A CN201911338703.4A CN201911338703A CN110987676A CN 110987676 A CN110987676 A CN 110987676A CN 201911338703 A CN201911338703 A CN 201911338703A CN 110987676 A CN110987676 A CN 110987676A
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crack
life
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尚德广
董恩军
王海潮
李罗金
刘鹏程
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Beijing University of Technology
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N3/00—Investigating strength properties of solid materials by application of mechanical stress
    • G01N3/32—Investigating strength properties of solid materials by application of mechanical stress by applying repeated or pulsating forces
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N2203/00—Investigating strength properties of solid materials by application of mechanical stress
    • G01N2203/0058—Kind of property studied
    • G01N2203/006—Crack, flaws, fracture or rupture
    • G01N2203/0062—Crack or flaws
    • G01N2203/0066—Propagation of crack
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N2203/00—Investigating strength properties of solid materials by application of mechanical stress
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Abstract

The invention discloses a full-life prediction method considering a crack closure effect under random multi-axis load, which relates to the field of random multi-axis fatigue crack propagation and life prediction, and comprises the following steps: (1) performing Wang-Brown cycle counting on the random load to obtain a plurality of cycles, selecting a surface with the largest shear stress range as a critical surface, and calculating a driving force for expanding the crack on the critical surface; (2) calculating an equivalent stress intensity factor by using a proposed effective stress intensity factor formula; (3) calculating the crack expansion amount of each cycle by using an improved Paris formula, and obtaining the service life when the critical crack length is reached, namely the total service life; (4) the predicted life is compared with the life obtained by the test. The result shows that the method can better predict the crack propagation life under random multi-axis load.

Description

Full-life prediction method considering crack closure effect under random multi-axis load
Technical Field
The invention can be applied to the field of fatigue life prediction under random multi-axis load, and particularly relates to a fatigue life prediction method under random multi-axis load considering crack closure effect.
Background
The failure of components during use, especially the failure of some components under the repeated action of low stress, is an important field of research, which relates to the fatigue problem and is very important for predicting the service life of parts. Therefore, the method has important significance and value for the research in the field of fatigue.
In the initial research, only simple uniaxial loading is generally considered, but mechanical structural parts in engineering practice usually bear complex load action, and the establishment of a more accurate full-life prediction model has important theoretical significance in the process of treating the complex load action.
Under random multi-axis load, a crack propagation mechanism and a closing effect are considered, a unified life prediction method is provided and applied to engineering practice, and the method is very valuable.
Disclosure of Invention
The invention aims to provide a full-life prediction method considering the crack closure effect under the condition of random multi-axis load conforming to the reality and improve the accuracy of fatigue life prediction. The existing research mainly uses single-axis or multi-axis constant amplitude loading, is not in accordance with engineering practice, and does not consider how to unify the propagation of long and short cracks after combining the crack closing effect under random multi-axis loading. The invention considers the influence of the crack closing effect on the crack propagation under the random multi-axis load, well unifies the long and short cracks, and provides a more accurate life-cycle prediction method.
The invention provides a full-life prediction model based on a crack closure effect under random multi-axis load, which comprises the following steps:
step 1): the test piece is selected as a thin-wall pipe fitting, and the crack initiation and propagation of the thin-wall pipe fitting are considered to mainly occur on a plane where the shear stress amplitude is maximum and the normal stress range is large under random multi-axis load; selecting the plane as a critical plane, representing the effective driving force of crack propagation by using damage parameters on the critical plane, determining a plurality of half cycles by adopting a W-B cycle counting method due to random load, and then determining the critical plane of each half cycle;
step 2): considering the influence of the crack closing effect on the crack propagation driving force, a reasonable counting method is adopted, and after the critical plane is determined, the method is adoptedCalculating the effective stress intensity factor delta K by an effective stress intensity factor formulaeffThe formula is as follows:
Figure BDA0002331663770000021
wherein Y is a shape factor, Ue is a closing coefficient, a is a half crack length, and △ σ eq is an equivalent stress;
step 3): the baseline fitting is carried out by using test data under uniaxial loading to obtain two important constants C, m, and the calculation is carried out on the basis, and the formula of the modified Paris crack propagation curve is as follows:
Figure BDA0002331663770000022
wherein the content of the first and second substances,
Figure BDA0002331663770000023
the crack propagation rate, C, m are constants fitted under uniaxial loading;
step 4): calculating the fatigue life:
(1)a0selection of (2): since the microstructure has a significant effect on crack propagation, the microstructure feature scale is chosen here as the initial crack length, the size of which is related to the material grain size;
(2) calculation of crack propagation length: according to the formula in the step 3), deforming to obtain the crack propagation length delta a of each cycleiThe formula (c) is as follows:
Figure BDA0002331663770000031
wherein, Δ KeffC, m is an effective stress intensity factor, namely a constant simulated in the step 3), and i is a cyclic sequence number;
and then accumulating the crack propagation length according to the cycle number obtained by counting the W-B cycles in the step 1) to obtain the total crack propagation length:
Δa＝Δa1+Δa2+…+Δai
(3) and (3) life prediction: and obtaining the final length of the crack according to the total crack propagation length obtained by the accumulation calculation in the last step, wherein the service life when the critical crack length is reached is the predicted service life, and then comparing the predicted service life with the service life obtained by the test to verify the accuracy of the method.
The invention has the advantages that: a full-life prediction method considering the crack closure effect under random multi-axis load is provided. After the random multi-axis load is circularly counted, the critical surface with crack expansion is selected to calculate the effective driving force, in the process, the crack expansion mechanism and the crack closing effect are fully considered, and the method has more physical significance and very practical engineering significance.
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FIG. 1 is a schematic diagram of a stress state and a crack initiation position of a test piece of a thin-wall pipe under random multi-axis load.
FIG. 2 is a flow chart of a full-life prediction method considering crack closure effect under random multi-axis load provided by the method of the present invention.
Detailed Description
The invention is described in connection with the accompanying drawings.
The invention is further explained by fatigue test, the test is divided into two parts, one part is crack propagation test under uniaxial constant amplitude loading, the stress ratio is-1, the length value of the crack is measured by a laminating method, and the value of a constant C, m is fitted by applying Paris formula. And the other part is a random multi-axis loading test, then data are obtained, W-B counting is carried out, a critical surface is determined, and an effective stress intensity factor is calculated to serve as a driving force.
The method for predicting the total service life considering the crack closure effect under random multi-axis load comprises the following specific calculation methods:
step 1): as shown in fig. 1, the test piece is selected as a thin-wall pipe, and it is considered that crack initiation and propagation mainly occur in a plane where the shear stress amplitude is maximum and the normal stress range is large under random multi-axis load; selecting the plane as a critical plane, representing the effective driving force of crack propagation by using damage parameters on the critical plane, determining a plurality of half cycles by adopting a W-B cycle counting method due to random load, and then determining the critical plane of each half cycle;
step 2): considering the influence of the crack closing effect on the crack propagation driving force, after determining the critical plane by using a reasonable counting method, calculating by using an effective stress intensity factor formula, wherein the effective stress intensity factor delta KeffThe formula is as follows:
Figure BDA0002331663770000041
wherein Y is a shape factor, Ue is a closing coefficient, a is a half crack length, and △ σ eq is an equivalent stress;
step 3): the baseline fitting is carried out by using test data under uniaxial loading to obtain two important constants C, m, and the calculation is carried out on the basis, and the formula of the modified Paris crack propagation curve is as follows:
Figure BDA0002331663770000042
wherein the content of the first and second substances,
Figure BDA0002331663770000043
the crack propagation rate, C, m are constants fitted under uniaxial loading;
step 4): calculating the fatigue life:
(1)a0selection of (2): since the microstructure has a significant effect on crack propagation, the microstructure feature scale is chosen here as the initial crack length, which is related to the material grain size;
(2) calculation of crack propagation length: according to the formula in the step 3), deforming to obtain the crack propagation length delta a of each cycleiThe formula (c) is as follows:
Figure BDA0002331663770000051
wherein, Δ KeffC, m is a common factor of effective stress intensity, which is simulated in step 3)Counting; and then accumulating the crack propagation length according to the cycle number obtained by counting the W-B cycles in the step 1) to obtain the total crack propagation length:
Δa＝Δa1+Δa2+…+Δai
(3) and (3) life prediction: and obtaining the final length of the crack according to the accumulated calculation of the last step, wherein the service life when the critical crack length is reached is the predicted service life, and then comparing the predicted service life with the service life obtained by the test to verify the accuracy of the method.
The result shows that compared with the test value, the result of the calculated value of the total life prediction method considering the crack closing effect under the random multi-axis load provided by the invention is within three times of the error factor, and the result is better. The method considers the crack closing effect, selects the random multi-axis condition which is more in line with the actual engineering, and has important significance.

Claims (5)

1. The full-life prediction method considering the crack closure effect under random multi-axis load is characterized by comprising the following steps of: the steps are as follows,
step 1): the test piece is selected as a thin-wall pipe fitting, and the crack initiation and propagation are considered to mainly occur on a plane where the shear stress amplitude is maximum and the normal stress range is large under random multi-axis load; selecting the plane as a critical plane, representing the effective driving force of crack propagation by using damage parameters on the critical plane, determining a plurality of half cycles by adopting a W-B cycle counting method due to random load, and then determining the critical plane of each half cycle;
step 2): considering the influence of the crack closing effect on the crack propagation driving force, after determining the critical plane by using a reasonable counting method, calculating by using an effective stress intensity factor formula, wherein the effective stress intensity factor delta KeffThe formula is as follows:
Figure FDA0002331663760000011
wherein Y is a shape factor, Ue is a closure coefficient, a is a half crack length, △ σeqIs an equivalent stress;
step 3): the baseline fitting is carried out by using test data under uniaxial loading to obtain two important constants C, m, and calculation is carried out on the basis of the two important constants, wherein the formula of the modified Paris crack propagation curve is as follows:
Figure FDA0002331663760000012
wherein the content of the first and second substances,
Figure FDA0002331663760000013
the crack propagation rate, C, m are constants fitted under uniaxial loading;
step 4): and calculating the fatigue life.
2. The method of predicting full life under random multiaxial loading considering crack closure effect of claim 1, wherein: the crack propagation driving force selected in step 2) is considered to be mainly influenced by the crack closing effect, and propagation is considered to be performed on the selected critical surface based on stress.
3. The method of predicting full life under random multiaxial loading considering crack closure effect of claim 1, wherein: and 4) selecting the microstructure characteristic scale of the material as the initial crack size in the step 4), and according with the crack initiation and propagation mechanism.
4. The method of predicting full life under random multiaxial loading considering crack closure effect of claim 1, wherein: in the step 4), the influence of the crack length on the effective stress intensity factor is considered, and after different cyclic loading, the sum of the expansion amount of each crack is the total expansion amount of the crack.
5. The method of predicting full life under random multiaxial loading considering crack closure effect of claim 1, wherein: the implementation of step 4 is as follows,
(1)a0selection of (2): because of the fact thatThe microstructure has an important influence on crack propagation, wherein the structural characteristic dimension is selected as the initial crack length, and the dimension is related to the grain size of the material;
(2) calculation of crack propagation length: according to the formula in step 3), the deformation can be obtained, the crack propagation length delta a of each cycleiThe formula (c) is as follows:
Figure FDA0002331663760000021
wherein, Δ KeffC, m is a constant simulated in the step 3) as an effective stress intensity factor; and then accumulating the crack propagation length according to the cycle number obtained by counting the W-B cycles in the step 1) to obtain the total crack propagation length:
Δa＝Δa1+Δa2+…+Δai
(3) and (3) life prediction: and obtaining the final crack length according to the accumulated calculation of the last step, wherein the service life when the critical crack length is reached is the predicted service life, and comparing the predicted service life with the service life obtained by the test.
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CN113109190A (en) * 2021-03-26 2021-07-13 北京工业大学 Short crack-based life prediction method under multi-axis thermomechanical load
CN113176142A (en) * 2021-03-11 2021-07-27 合肥通用机械研究院有限公司 Method for calculating axial-radial crack stress intensity factor of outer wall of ultrahigh pressure container barrel
CN113758815A (en) * 2021-06-18 2021-12-07 北京工业大学 Method for predicting amplitude-variable fatigue life of friction stir welding component
CN114216803A (en) * 2021-09-30 2022-03-22 中国航发北京航空材料研究院 High cycle fatigue full-life prediction method for metal material

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Cited By (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN111579397A (en) * 2020-05-06 2020-08-25 北京化工大学 Fatigue life prediction method for laser additive manufacturing alloy steel component
CN113176142A (en) * 2021-03-11 2021-07-27 合肥通用机械研究院有限公司 Method for calculating axial-radial crack stress intensity factor of outer wall of ultrahigh pressure container barrel
CN113109190A (en) * 2021-03-26 2021-07-13 北京工业大学 Short crack-based life prediction method under multi-axis thermomechanical load
CN113758815A (en) * 2021-06-18 2021-12-07 北京工业大学 Method for predicting amplitude-variable fatigue life of friction stir welding component
CN113758815B (en) * 2021-06-18 2023-12-29 北京工业大学 Method for predicting luffing fatigue life of friction stir welding component
CN114216803A (en) * 2021-09-30 2022-03-22 中国航发北京航空材料研究院 High cycle fatigue full-life prediction method for metal material

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Application publication date: 20200410