CN102967512B - Low cycle fatigue life prediction method based on asymmetrical cyclic stress control load - Google Patents

Abstract

The invention discloses a low cycle fatigue life prediction method for metal material and based on asymmetrical cyclic stress control load. The method comprises the following steps: (1) acquiring a low cycle fatigue life of a metal material through fatigue experiments of asymmetrical cyclic stress control; (2) according to working conditions of the fatigue experiments and fatigue properties of the material, determining a function relationship of fatigue strength coefficient sigma' f, fatigue strength exponent b, and the peak stress and tensile limit of the material; and (3) establishing a prediction model of low cycle fatigue life of the metal material under asymmetric cyclic stress control conditions, and predicting fatigue life of the material. The method provided by the invention can quickly predict the fatigue life of the metal material under asymmetric cyclic stress load conditions, and provides a theoretical reference for the reliable design and evaluation of parts.

Description

Low Cycle Fatigue Life Prediction method based on asymmetry circulation stress controlled loading

Technical field

The present invention relates to the Low Cycle Fatigue Life Prediction method of metal material under asymmetric stresses pulsating stress service condition.

Background technology

Fatigue failure is one of main failure forms of various material/parts, and the damage of 80% above parts is caused by fatigue break according to statistics.The mechanical property of metal material especially fatigue behaviour enjoys people to pay close attention to, and in recent years, the research of metal material fatigue properties has become focus.The phenomenon that material/parts produce plastic strains accumulation under pulsating stress controlled loading is ratcheting.Although the amount of plastic deformation that material/parts produce in each CYCLIC LOADING may be less, but the carrying out along with circulation, the amount of plastic deformation of accumulation but can not be ignored, it may directly cause that material/parts innerly produces that larger stress is concentrated, cavity and defects i.e.cracks, thereby cause unforeseen fracture failure, seriously reduced the fatigue behaviour of material/parts.Therefore, guarantee reliability, permanance and security in this type component use procedure, must in fatigue design and safety evaluation, consider ratcheting and the impact on fatigue behaviour thereof of material/parts, the existence of ratcheting has proposed severe challenge to many Parts reliability designs.

Be material/part reliability design of bearing cyclic loading and an important indicator of assessing fatigue lifetime, the load of bearing in actual military service process due to most parts is that asymmetrical period changes, and under asymmetric cyclic loading, the fatigue life prediction problem of material/parts is the important problem in fatigue study always.Existing document shows, traditional Basquin model, Cofflin-Mason model and energy theory model can Accurate Prediction material/part the fatigue lifetime under symmetric strained controlled loading condition.Wherein, because Basquin model has the prediction that the advantages such as form is simple, material constant is few are widely used in material fatigue life.The citation form of Basquin model is as follows:

σ _a＝σ′ _f(2N _f) ^b

Wherein, σ ' _ffor fatigue strength coefficient, b is fatigue strength coefficient.This model emphasis has been considered stress amplitude σ _awith N fatigue lifetime _fbetween relation, and Basquin model is mainly applicable to the estimation of fatigue life under symmetrical cycle loading environment.When material military service is during in asymmetric stresses state of cyclic operation condition, because mean stress is very remarkable on the impact of fatigue lifetime, while making fatigue lifetime of Basquin model prediction material, there is larger error.

For the low-cycle fatigue life of Accurate Prediction material under asymmetric Cyclic Load, some scholars have carried out corresponding correction on the basis of Basquin model, its basic ideas are to think that mean stress existence only affects the fatigue strength coefficient in Basquin model, and on not impact of fatigue strength exponent.Therefore, by revising the stress amplitude σ in Basquin model left side _apredict the low-cycle fatigue life under asymmetric Cyclic Load.Yet experimental result shows: under asymmetric ringing, the existence of mean stress not only affects the fatigue strength coefficient in Basquin model, and can affect its fatigue strength exponent, even more remarkable on the impact of fatigue strength coefficient.This is because material, under asymmetry circulation stress load, can produce ratcheting on the one hand, brings additional damage, the fatigure failure of accelerated material; On the other hand, fatigue damage is constantly accumulated, and fatigue failure is inevitable.If do not take into full account the impact of asymmetrical loading operating mode on fatigue strength coefficient and fatigue strength exponent, can cause predicting the outcome and actual conditions between there is very large error, and this error has dispersiveness, directly caused life appraisal result unreliable.In order to assess reliably the low cycle fatigue property of metal material, be necessary to propose a kind of energy fast, the method for convenient, the low-cycle fatigue life of Accurate Prediction metal material under asymmetry circulation stress controlled loading condition.

Summary of the invention:

The object of the present invention is to provide a kind of predict material non-to the Low Cycle Fatigue Life Prediction method under pulsating stress controlled loading, having solved at present can not the low-cycle fatigue life estimation problem of Accurate Prediction material under asymmetry circulation stress effect.

For achieving the above object, the technical solution used in the present invention is: a kind of method of predicting the low-cycle fatigue life of material under asymmetry circulation stress loads, and the step of its method is:

Step 1: at a series of peak stress σ _p(σ _s≤ σ _p< σ _u) and stress ratio R (0.4≤R≤0.4) under, carry out the low-cycle fatigue experiment of asymmetry circulation stress loading, obtain the low-cycle fatigue life of metal material, wherein σ _sand σ _ube respectively yield limit and the tension limit of material, can obtain by uniaxial tensile test or relevant material property handbook;

Step 2: under constant peak stress condition, taken the logarithm and can obtain formula (2) in the Basquin model both sides shown in formula (1).

σ _a＝σ _p(1-R)/2＝σ′ _f(2N _f) ^b (1)

ln[(1-R)/2]＝In[σ′ _f/σ _p]+bln(2N _f) (2)

Wherein, σ _afor stress amplitude, σ _pfor peak stress, σ ' _ffor fatigue strength coefficient, b is fatigue strength exponent, N _flow-cycle fatigue life.According to torture test condition and experimental result, can obtain 1n (2N _f) and ln[(1-R)/2] between graph of a relation, experimental data is carried out to regretional analysis, obtain the fatigue strength coefficient σ ' under different loading conditions _fwith fatigue strength exponent b value.By data fitting method, determine fatigue strength coefficient σ ' _f, fatigue strength exponent b and peak stress, the material tension limit funtcional relationship be:

$\left\{\begin{array}{c}{\mathrm{\&sigma;}}_f^{\&prime;}={\mathrm{\&sigma;}}_{f0}^{\&prime;}+\mathrm{mexp}[-n({\mathrm{\&sigma;}}_p/{\mathrm{\&sigma;}}_u)]\\ b=b_0\exp [-c({\mathrm{\&sigma;}}_p/{\mathrm{\&sigma;}}_u)]\end{array}\right.---\left(3\right)$

Wherein, σ ' _f0, m and n are material constant, σ ' _f0for not considering the fatigue strength limit of mean stress, σ _ufor the tension limit of material, m and n are that fatigue strength coefficient is to variable σ _p/ σ _usensitivity, b ₀for initial fatigue strength exponent, c reflection material is to σ _p/ σ _usensitivity.Wherein, material constant σ ' _f0, m and n are based on σ ' _f-(σ _p/ σ _u) relation curve adopts non-linear fitting method to determine, material constant b ₀with c be based on b-σ _p/ σ _urelation curve adopts non-linear fitting method to determine;

Step 3: by the fatigue strength coefficient σ ' obtaining in step 2 _fbring in formula (1) with the calculating formula of fatigue strength exponent b, obtain the Low Cycle Fatigue Life Prediction model of metal material under asymmetry circulation stress loading environment as follows:

$\left\{\begin{array}{c}{\mathrm{\&sigma;}}_p(1-R)/2={\mathrm{\&sigma;}}_f^{\&prime;}{\left(2N_f\right)}^b\\ {\mathrm{\&sigma;}}_f^{\&prime;}={\mathrm{\&sigma;}}_{f0}^{\&prime;}+\mathrm{mexp}[-n({\mathrm{\&sigma;}}_p/{\mathrm{\&sigma;}}_u)]\\ b=b_0\exp [-c({\mathrm{\&sigma;}}_p/{\mathrm{\&sigma;}}_u)]\end{array}\right.---\left(4\right)$

Life-span fatigue lifetime under different loading environments is predicted, and evaluated its predicated error size.

The present invention tests by the low-cycle fatigue of metal material, determines fatigue strength coefficient σ ' _fwith the calculating formula of fatigue strength exponent b, set up the Low Cycle Fatigue Life Prediction method of metal material under asymmetry circulation stress loading environment.

Beneficial effect

The present invention adopts above scheme, has the following advantages: this invention has taken into full account asymmetric CYCLIC LOADING duty parameter to fatigue strength coefficient σ ' _fwith the impact of fatigue strength exponent b, obtain the Low Cycle Fatigue Life Prediction method of metal material under asymmetry circulation stress loading environment.Utilize the method can predict rapidly the fatigue lifetime of metal material under asymmetry circulation stress loading environment, for reliability design and the assessment of parts provides theoretical reference.The method is more convenient in engineering application, applicable.

Accompanying drawing explanation

Fig. 1 ln (2N _f) and ln[(1-R)/2] graph of a relation

Fig. 2 σ ' _fwith σ _p/ σ _ugraph of a relation

Fig. 3 b and σ _p/ σ _ugraph of a relation

The comparison diagram of Fig. 4 experimental result and predicted value

Embodiment

Below in conjunction with the drawings and specific embodiments, the present invention is described in detail.

The present invention is a kind of low-cycle fatigue life method of metal material under asymmetry circulation stress controlled condition of predicting, the Low Cycle Fatigue Life Prediction of AZ91 magnesium alloy materials of take is below example, introduce in detail the implementation detail of the life-span prediction method the present invention relates to, its method comprises:

Step 1: the low-cycle fatigue that (rolling direction sampling) carried out controlling based on asymmetry circulation stress to AZ91 magnesium alloy materials is tested, and obtains the step of its low-cycle fatigue life;

CYCLIC LOADING duty parameter comprises peak stress and stress ratio, wherein the scope of peak stress is 124.0MPa～262.6MPa, the scope of stress ratio is-0.4～0.4, choose peak stress 140MPa, 160MPa, 180MPa, 200MPa, 220MPa, stress ratio-0.4 ,-0.2,0,0.2,0.4, the yield limit of material is 124.0MPa, and the tension limit is 262.6MPa.

Step 2: according to the fatigue behaviour of the working condition of fatigue experiment and material, obtain ln (2N _f) and ln[(1-R)/2] graph of a relation, by the method for data fitting, determine fatigue strength coefficient σ ' _f, fatigue strength exponent b and σ _p/ σ _uthe step of funtcional relationship;

According to experimental data, can obtain ln[(1-R)/2] and In (2N _f) between relation, as shown in Figure 1.From figure, can find ln[(1-R)/2] and In (2N _f) present good linear relationship.Adopt the method for data fitting, experimental data is carried out to regretional analysis, obtain the fatigue strength coefficient σ ' under different loading conditions _fwith fatigue strength exponent b value, as shown in table 1.According to the σ ' in table 1 _fwith b value, can obtain σ ' _fwith (σ _p/ σ _u), b and σ _p/ σ _urelation curve, σ wherein _pfor peak stress, σ _ufor the material tension limit.Then, adopt non-linear fitting method can obtain σ ' _f0, m, n, b ₀with the numerical value of 5 material constants such as c, thereby determine σ ' _f, b and σ _p/ σ _ufuntcional relationship be: σ ' _f=483.78+7.25exp[6.35 (σ _p/ σ _u)], b=-0.09638exp[1.60518 (σ _p/ σ _u)], as shown in Figures 2 and 3.

Fatigue strength coefficient under the different peak stress of table 1 (σ ' _f) and fatigue strength exponent (b)

Step 3: based on Basquin model, set up the forecast model of metal material low-cycle fatigue life under asymmetry circulation stress controlled condition, and predict the step of its fatigue lifetime;

By the σ ' obtaining _f, b and σ _p/ σ _ufunctional relation bring in Basquin model, can obtain the Low Cycle Fatigue Life Prediction model that loads based on asymmetry circulation stress as follows: $\left\{\begin{array}{c}\mathrm{\&Delta;}{\mathrm{\&sigma;}}_a=\frac{{\mathrm{\&sigma;}}_p(1-R)}{2}={\mathrm{\&sigma;}}_f^{\&prime;}{\left(2N_f\right)}^b\\ {\mathrm{\&sigma;}}_f^{\&prime;}=483.78+7.25\exp \left[6.35({\mathrm{\&sigma;}}_p/{\mathrm{\&sigma;}}_u)\right]\\ b=-0.09638\exp \left[1.60518({\mathrm{\&sigma;}}_p/{\mathrm{\&sigma;}}_u)\right]\end{array}\right..$ Figure 4 shows that the experimental result and the comparison predicting the outcome of AZ91 magnesium alloy materials material low-cycle fatigue life under asymmetry circulation stress loads.Result shows that method of the present invention can predict the low-cycle fatigue life of AZ91 magnesium alloy materials under asymmetry circulation stress controlled loading condition exactly.By reference to the accompanying drawings example of the present invention is described above; but the present invention is not limited to above-mentioned concrete embodiment, above-mentioned embodiment is only exemplary, is not circumscribed; any innovation and creation that are no more than the claims in the present invention, all within protection of the present invention.

Claims (5)

1. the metal material Low Cycle Fatigue Life Prediction method based on asymmetry circulation stress controlled loading, is characterized in that predicting rapidly the fatigue lifetime of metal material under asymmetry circulation stress loading environment, and the step of the method comprises:

Step 1: at σ _s≤ σ _p< σ _uunder the condition of-0.4≤R≤0.4, carry out the low-cycle fatigue experiment of asymmetry circulation stress loading, obtain the low-cycle fatigue life of material, wherein σ _pfor peak stress, σ _sfor the yield limit of material, σ _ufor the tension limit of material, R is stress ratio;

Step 2: according to the low-cycle fatigue life N of the working condition of fatigue experiment and material _f, obtain ln (2N _f) and ln[(1-R)/2] graph of a relation, and determine fatigue strength coefficient σ ' by the method for nonlinear data fitting _f, fatigue strength exponent b and σ _p/ σ _ufuntcional relationship be: σ ' _f=σ ' _f0+ m exp[-n (σ _p/ σ _u)], b=b ₀exp[-c (σ _p/ σ _u)], σ wherein _pfor peak stress, σ _ufor the material tension limit, σ ' _fbe respectively fatigue strength coefficient and the fatigue strength exponent of material, σ ' with b _f0, m and n are material constant, σ ' _f0for not considering the fatigue limit of mean stress, m and n are that material is to σ _p/ σ _usensitivity, b ₀with c be material constant, b ₀for initial fatigue strength exponent, c is that material is to σ _p/ σ _usensitivity;

Step 3: based on Basquin model, set up the forecast model of metal material low-cycle fatigue life under asymmetry circulation stress controlled condition, and predict its fatigue lifetime, under described asymmetry circulation stress controlled condition, the forecast model of metal material low-cycle fatigue life is: $\left\{\begin{array}{c}{\mathrm{\&sigma;}}_p(1-R)/2={\mathrm{\&sigma;}}_f^{\&prime;}{\left(2N_f\right)}^b\\ {\mathrm{\&sigma;}}_f^{\&prime;}={\mathrm{\&sigma;}}_{f0}^{\&prime;}+\mathrm{mexp}[-n({\mathrm{\&sigma;}}_p/{\mathrm{\&sigma;}}_u)]\\ b=b_0\exp [-c({\mathrm{\&sigma;}}_p/{\mathrm{\&sigma;}}_u)]\end{array}\right..$

2. the method for claim 1, is characterized in that: the fatigue experiment duty parameter described in step 1 comprises peak stress σ _pwith stress ratio R, peak stress σ _pscope be σ _s≤ σ _p< σ _u, σ wherein _sand σ _ube respectively yield limit and the tension limit of material, can obtain by uniaxial tensile test or relevant material property handbook, the scope of stress ratio R is-0.4≤R≤0.4.

3. the method for claim 1, is characterized in that: in step 2, adopt the method for data fitting, experimental data is carried out to regretional analysis, obtain the fatigue strength coefficient σ ' under different loading conditions _fwith fatigue strength exponent b value.

4. the method for claim 1, is characterized in that: the material constant σ ' described in step 2 _f0, m and n are based on σ ' _f-(σ _p/ σ _u) relation curve adopts non-linear fitting method to determine.

5. the method for claim 1, is characterized in that: the material constant b described in step 2 ₀with c be based on b-σ _p/ σ _urelation curve adopts non-linear fitting method to determine.