CN115329546A - Fatigue life prediction method based on critical plane equivalent energy damage parameter - Google Patents
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Abstract
The invention discloses a fatigue life prediction method based on critical plane equivalent energy damage parameters, belonging to the technical field of multi-axial fatigue strength. Step 1: obtaining a total count inverse number; step 2: calculating an axial stress correction coefficient and a shear stress correction coefficient; and 3, step 3: calculating an axial equivalent stress correction coefficient; and 4, step 4: calculating normal stress, maximum normal stress and maximum shear stress on a critical surface; and 5: obtaining a relational expression between an axial equivalent stress correction coefficient and the fatigue failure cycle number; and 6: calculating equivalent energy damage parameters based on the critical surface; and 7: sorting equivalent energy damage parameters based on the critical surface; and 8: estimating fatigue damage per cycle or iteration; and step 9: calculating total accumulated fatigue damage; step 10: the number of load blocks required for fatigue failure is determined. The method has higher fatigue life prediction precision, does not contain empirical constants, and is convenient for engineering application.
Description
Technical Field
The invention relates to the technical field of multi-axial fatigue strength, in particular to a fatigue life prediction method based on critical plane equivalent energy damage parameters.
Background
In-service mechanical components or parts can fail due to a variety of factors, fatigue, wear and fracture are common, however, more than 80% of the failure modes are fatigue-induced. For example, in various industries and fields such as aircraft engines, gas turbines, hypersonic aircrafts, pressure vessels, nuclear power plants, metallurgical machinery, power machinery, hoisting and transportation machinery, petroleum drilling equipment, railroad bridges and the like, key parts of the parts usually bear complex single-shaft, multi-shaft-proportion, multi-shaft-non-proportion and multi-shaft-amplitude-variable interaction cyclic load effects. The classical uniaxial fatigue strength theory can not meet the design requirements of fatigue strength, service life and the like of actual engineering components, so that the fatigue boundary generally pays attention to the multiaxial fatigue research which is more practical in recent years.
The multiaxial non-proportional fatigue behavior of a material is the biggest reason why the multiaxial fatigue problem is more complex than the uniaxial fatigue problem. The principal strain/stress axis rotates during multi-axis non-proportional loading, which results in changes in the material slip system and microstructure, exhibiting a non-proportional additive cycle hardening phenomenon that is not present during either single-axis or multi-axis proportional loading. The non-proportional additive hardening phenomenon complicates the material cyclic deformation behavior and fatigue damage mechanism, and also makes fatigue life estimation under non-proportional multi-axial loading difficult. Therefore, in order to ensure the safe and reliable operation of a mechanical structure and prevent property and economic losses caused by sudden fatigue damage, the research on the fatigue life prediction method based on the critical plane equivalent energy damage parameter, which is suitable for engineering application, has important engineering application value and theoretical significance.
Disclosure of Invention
The invention aims to provide a fatigue life prediction method based on critical plane equivalent energy damage parameters, which is characterized by comprising the following steps of:
step 1: counting all the repetitions by using a Wang-Brown multi-axis circulation counting algorithm to obtain a total counting repetition number n;
and 2, step: calculating an axial stress correction coefficient and a shear stress correction coefficient;
And 4, step 4: under the working condition of uniaxial tension and compression load, calculating normal stress on a critical surface, and calculating the maximum normal stress and the maximum shear stress on the critical surface under symmetrical cyclic loading based on a Basquin-coffee-Manson equation;
and 5: obtaining an axial equivalent stress correction coefficient from the maximum normal stress and the maximum shear stress in the step 4Relation between fatigue failure cycle number;
And 6: considering the additional strengthening effect of the non-proportional load path, and calculating an equivalent energy damage parameter AESA based on a critical surface;
and 7: correction coefficient of axial equivalent stress in step 5Sorting the equivalent energy damage parameter AESA based on the critical surface in the step 6 by a relational expression between the fatigue failure cycle number and the fatigue failure cycle number to obtain a sorted equivalent energy damage parameter AESA';
and 8: estimating fatigue damage of each cycle or repetition by using the equivalent energy damage parameter AESA' finished in the step 7;
and step 9: calculating the total accumulated fatigue damage D by using Miner linear fatigue damage accumulation theory total ;
Step 10: determining the number of load blocks N required for fatigue failure block 。
The calculation formula of the axial stress correction coefficient in the step 2 is as follows:
wherein σ f ' is the axial fatigue strength coefficient, σ n,max Is the maximum normal stress on the critical plane;
the calculation formula of the shear stress correction coefficient is as follows:
wherein, tau f ' is the shear fatigue Strength coefficient, Δ τ max And/2 is the maximum shear stress amplitude at the critical plane.
The axial equivalent stress correction coefficient in the step 3The calculation formula of (2) is as follows:
the normal stress calculation formula on the critical surface in the step 4 is as follows:
wherein σ x Is a stress tensor component;
the maximum normal stress is calculated by the formula:
wherein, σ' f And b is the fatigue strength coefficient and fatigue strength index, N f Is the fatigue life;
the maximum shear stress is calculated by the formula:
the critical plane is located at an oblique cross section of 45 °.
The equivalent energy damage parameter calculation formula in the step 6 is as follows:
wherein the content of the first and second substances,is the axial equivalent strain amplitude on the critical surface, and the calculation formula is as follows:
wherein, delta gamma max /2 is the shear strain amplitude at the maximum shear plane,is the virtual positive strain course between the adjacent maximum shear strain inflection points on the critical plane, and E is the modulus of elasticity.
The calculation formula of the equivalent energy damage parameter after finishing in the step 7 is as follows:
the calculation formula of the fatigue damage in each cycle or repetition in the step 8 is as follows:
wherein, N fi The ith repeated fatigue life.
Total cumulative fatigue damage D in said step 9 total The calculation formula is as follows:
where n is the total number of cycles or iterations.
The number of load blocks N in the step 10 block The calculation formula is as follows:
wherein D is fatigue damage.
The invention has the beneficial effects that:
1. the critical surface-based equivalent energy damage parameter provided by the invention can be degraded into a form of a classic Smith-Watson-Topper equation;
2. the method can consider the additional strengthening effect of the non-proportional load path, and the fatigue life prediction precision is higher;
3. the method does not contain empirical constants, and is convenient for engineering application.
Drawings
FIG. 1 is a flowchart of a fatigue life prediction method based on critical plane equivalent energy damage parameters according to the present invention;
FIG. 2 (a) shows the fatigue life and test results of 7050-T7451 aluminum alloy determined by a fatigue life prediction method based on critical plane equivalent energy damage parameters under a multi-axial amplitude load;
fig. 2 (b) shows the fatigue life and the test result of the En15R steel test piece determined by the fatigue life prediction method based on the critical plane equivalent energy damage parameter under the multi-axial amplitude load.
Detailed Description
The invention provides a fatigue life prediction method based on critical plane equivalent energy damage parameters, and the invention is further explained by combining the drawings and specific embodiments.
Fig. 1 is a flowchart of a fatigue life prediction method based on critical plane equivalent energy damage parameters, which includes the steps of:
step 1): all iterations are counted using the Wang-Brown multi-axis cycle counting algorithm to obtain a total count iteration number n.
In the multi-axis cycle counting method of Wang-Brown, the load spectrum is rearranged by first defining the maximum von Mises equivalent strain of the entire load history as the initial reference point. Then, the equivalent relative strain of the subsequent point with respect to the reference point is calculated. Once the equivalent relative strain no longer monotonically increases, a drop occurs, and the load between the reference point and the point at which the equivalent relative strain drops is counted as a half cycle (or iteration). And defining the descending point as a new relative reference point, repeating the previous process to continue counting the latter half cycle, and finally determining all counting iterations of the whole load process. The calculation formula of von Mises equivalent strain in the Wang-Brown multiaxial cycle counting method is as follows:
wherein, v eff Is the effective Poisson's ratio, epsilon x (t)、ε y (t)、ε z (t) is the normal stress of the coordinate axis corresponding to the moment t respectively; gamma ray xy 、γ yz 、γ xz Respectively, the shear strain of the coordinate axis corresponding to the time t.
time t relative to t r Relative equivalent strain at time of dayThe calculation formula of (2) is as follows:
wherein the corresponding strain ε in formula (2) r x (t),ε r y (t),ε r z (t),The calculation expressions of (a) are respectively: epsilon r x (t)=ε x (t)-ε x (t r ),ε r y (t)=ε y (t)-ε y (t r ),ε r z (t)=ε z (t)-ε z (t r ),ε ij (t r ) Is t r The strain tensor of the time point.
Stress tensor σ ij The expression of (a) is as follows:
tensor σ of visible stress ij Contains 6 components, each of which is σ x ,σ y ,σ z ,τ xy ,τ yz ,τ xz 。
Stress tensor epsilon ij The expression of (c) is as follows:
visible stress tensor epsilon ij Comprising 6 components, each being ε x ,ε y ,ε z ,γ xy ,γ yz ,γ xz 。
Step 2): the axial stress correction factor and the shear stress correction factor can be calculated for each counting iteration by the following equations:
wherein, σ' f And τ' f Respectively axial fatigue strength coefficient and shear fatigue strength coefficient, sigma n,max And Δ τ max And/2 is the maximum normal stress and maximum shear stress amplitude on the critical plane.
Step 3): proposed axial equivalent stress correction factorThis can be calculated by the following formula:
step 4): under the working condition of uniaxial tension and compression load, the critical plane is positioned at an oblique section of 45 degrees, and the normal stress on the critical plane can be expressed as the following formula:
based on the Basquin-coffee-Manson equation, the maximum normal stress on the critical surface under the symmetrical cyclic loading can be expressed as the following formula:
wherein, σ' f And b is the fatigue strength coefficient and fatigue strength index, N f Is the fatigue life;
similarly, the maximum shear stress at the critical plane under symmetric cyclic loading can be expressed as:
and step 5): substituting equations (9) and (10) into equation (7) can obtain the axial equivalent stress correction coefficientRelation (11) with fatigue failure cycle number:
step 6): the equivalent energy damage parameter based on the critical surface is provided as follows:
wherein the content of the first and second substances,is the axial equivalent strain amplitude on the critical plane, and adopts a stretching type Shang-Wang multiaxialFatigue damage parameterThe expression of the stretching type Shang-Wang multiaxial fatigue damage parameter is as follows:
wherein, delta gamma max /2 is the shear strain amplitude at the maximum shear plane,is the virtual positive strain course between adjacent maximum shear strain inflection points on the critical plane, and E is the modulus of elasticity.
Step 7): substituting equations (11) and (13) into equation (12) may determine the following relationship:
multiplication of equal sign of equation (14) on both sidesAfter the arrangement, equation (15) can be obtained:
step 8) estimating fatigue damage of each cycle (or repetition) by adopting a notched part fatigue life prediction method based on a load control modeN fi Is the i-th repeated fatigue life calculated by the formula (15);
step 9) calculating the total accumulated fatigue damage D by utilizing Miner linear fatigue damage accumulation theory total :
Wherein n is the total number of cycles (or iterations);
step 10) determining the number N of load blocks required for fatigue failure block :
Under the proportional multi-axis cyclic load, the stretching type Shang-Wang multi-axis fatigue damage parameter can be degraded into an equivalent strain amplitudeAnd, under uniaxial tension and compression load, the tensile type Shang-Wang multiaxial fatigue damage parameter can be degraded into an axial strain amplitudeAnd, under symmetrical cyclic loading, σ n,max /σ′ f And Δ τ max /(2·τ′ f ) Can be expressed as:
then proposed axial equivalent stress correction factorThis can be calculated by the following formula:
thus, the equivalent energy damage parameters based on the critical plane are as follows:
the equal sign of the pair (24) is divided by the equal signAfter work-up the following formula can be obtained:
therefore, the critical plane-based equivalent energy damage parameter can be degenerated into the form of the classical Smith-Watson-Topper equation.
In order to verify the fatigue life prediction method based on the critical plane equivalent energy damage parameter, the obtained prediction result is compared with the fatigue test results of the En15R steel and the 7050-T7451 aluminum alloy test piece, as shown in fig. 2 (a) and 2 (b).
The experimental data are derived from the following documents:
[1]Wang CH,Brown MW.Life prediction techniques for variable amplitude multiaxial fatigue–Part I:Theories.J.Eng.Mater.Technol.–Trans.ASME 1996;118(3):367–70.
[2]Chen H,Shang DG,Tian YJ,Liu JZ.Comparison of multiaxial fatigue damage models under variable amplitude loading.J Mech Sci Technol 2012;26(11):3439–46.
test verification results show that the fatigue life prediction results are mostly within a factor of 2 times (the prediction results are between 0.5 times and 2 times of the experimental results). Therefore, the fatigue life prediction method can better predict the fatigue life under the multi-axial variable amplitude load, does not contain empirical constants, and is convenient for engineering application.
The present invention is not limited to the above embodiments, and any modifications or alterations that can be easily conceived by those skilled in the art within the technical scope of the present invention are intended to be covered by the present invention. Therefore, the protection scope of the present invention shall be subject to the protection scope of the claims.
Claims (10)
1. The fatigue life prediction method based on the critical plane equivalent energy damage parameter is characterized by comprising the following steps:
step 1: counting all the iterations by using a Wang-Brown multi-axis circulation counting algorithm to obtain a total counting iteration number n;
step 2: calculating an axial stress correction coefficient and a shear stress correction coefficient;
And 4, step 4: under the working condition of uniaxial tension and compression load, calculating normal stress on a critical surface, and calculating the maximum normal stress and the maximum shear stress on the critical surface under symmetrical cyclic loading based on a Basquin-coffee-Manson equation;
and 5: obtaining an axial equivalent stress correction coefficient from the maximum normal stress and the maximum shear stress in the step 4A relationship to the number of fatigue failure cycles;
and 6: considering the additional strengthening effect of the non-proportional load path, and calculating an equivalent energy damage parameter AESA based on the critical plane;
and 7: correction coefficient of axial equivalent stress in step 5Sorting the equivalent energy damage parameter AESA based on the critical surface in the step 6 by a relational expression between the fatigue failure cycle number and the fatigue failure cycle number to obtain a sorted equivalent energy damage parameter AESA';
and 8: estimating fatigue damage of each cycle or repetition by using the equivalent energy damage parameter AESA' finished in the step 7;
and step 9: calculating the total accumulated fatigue damage D by using Miner linear fatigue damage accumulation theory total ;
Step 10: determining the number of load blocks N required for fatigue failure block 。
2. The fatigue life prediction method based on the critical plane equivalent energy damage parameter as claimed in claim 1, wherein the calculation formula of the axial stress correction coefficient in step 2 is:
wherein, σ' f Is the axial fatigue strength coefficient, σ n,max Is the maximum normal stress on the critical plane;
the calculation formula of the shear stress correction coefficient is as follows:
wherein, tau' f Is the shear fatigue strength coefficient, Δ τ max And/2 is the maximum shear stress amplitude at the critical plane.
4. the fatigue life prediction method based on the critical plane equivalent energy damage parameter as claimed in claim 1, wherein the normal stress calculation formula on the critical plane in step 4 is as follows:
wherein σ x Is a stress tensor component;
the maximum normal stress is calculated by the formula:
wherein, σ' f And b is the fatigue strength coefficient and fatigue strength index, N f Is the fatigue life;
the maximum shear stress is calculated by the formula:
5. the fatigue life prediction method based on the critical plane equivalent energy damage parameter according to claim 1 or 4, wherein the critical plane is located at a 45 ° oblique section.
6. The method for predicting fatigue life based on critical plane equivalent energy damage parameters according to claim 1, wherein the equivalent energy damage parameter calculation formula in the step 6 is as follows:
wherein the content of the first and second substances,is the axial equivalent strain amplitude on the critical plane, and the calculation formula is as follows:
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