| Fatigue failure under random loading is the main failure form of aircraft components.Therefore,accurately and effectively predicting the fatigue life of structural components under random loads is very important for the safe operation and cost reduction of aircraft equipment.It is necessary to consider the low cycle fatigue performance,fatigue damage,and the description of cyclic stress-strain response when estimating the fatigue life of the structure.In view of the above problems,there have been a large number of related research reports,but the lack of strain ratio(mean strain)on the low cycle fatigue behavior and micro mechanism of the material in depth systematic study.Moreover,the constitutive description of cyclic stress-strain responses such as cyclic softening and mean stress relaxation is relatively few.Therefore,in view of the widely used 2124-T851 aluminum alloy in aircraft structural parts,low cycle fatigue tests under four different strain ratios(R=-1,-0.06,0.06 and 0.5)and random load tests under three different intensities of the same load spectrum are carried out.The main research contents and results are as follows:The low cycle fatigue behavior and microscopic mechanism of 2124-T851 aluminum alloy under four different strain ratios were systematically studied.The effects of strain amplitude and strain ratio on low cycle fatigue behavior and micro mechanism were also investigated.The results of the study show that:First of all,the material exhibits cyclic softening characteristics at different strain ratios.Under symmetrical loading,the cyclic softening rate decreases linearly with the increase of strain amplitude.At a given strain amplitude,the cyclic softening rate of the material increases linearly with the increase of strain ratio.The cyclic strength factor K’ and cyclic hardening exponent n’ showed a linear increase with the increase of strain ratio.Secondly,during the low cycle fatigue,the amplitude and strain ratio have a significant influence on the length and density of the slip bands,the volume fraction and size of the coarse constituents,and the fracture morphology.Further studies show that the fatigue ductility and fracture toughness of materials are closely related to the strain ratio and will decrease significantly with the increase of strain ratio.The mean stress relaxation behavior of 2124-T851 aluminum alloy on different strain ratios had been investigated in this paper.The effects of cyclic deformation characteristics,initial mean stress,strain amplitude and strain ratio on mean stress relaxation are mainly investigated.It is also revealed that the mean stress relaxation rate increases with the increase of strain amplitude,and decreases with the increase of strain ratio.Based on the analysis of the experimental results,an improved model for predicting the mean stress relaxation behavior under different strain ratios is proposed on the basis of a typical mean stress relaxation model-Landgraf model.The improved model can predict the mean stress relaxation behavior of materials at different strain ratios more conveniently and accurately.The Cottrell method offers an easy way of extracting quantitative values of the friction stress and the back stress directly from hysteresis loops.The cyclic softening behavior and mean stress relaxation behavior of 2124-T851 aluminum alloy were studied from the view point of internal force.It is found that the cyclic softening characteristic of the material is the combined mechanism of friction stress and back stress.The rapid softening of materials in the initial 1/4 cycle is attributed to the rapid reduction of friction stress.The cyclic softening after initial 1/4 cycles is caused by a decrease in back stress.The mean stress relaxation behavior of the material is due to the gradual decrease of the mean back stress of the material.Based on the experimental data of internal force(frictional stress and back stress),the cyclic plastic constitutive equation of materials under strain fatigue control is studied.Firstly,the five back stress is determined based on the Chaboche model with three back stress and Ohno-Wang model with piecewise linear,and the combined hardening constitutive model was determined by the superposition isotropic hardening model.Secondly,according to the evolution data of friction stress and back stress,the parameters of the combined hardening constitutive model are determined.The cyclic plastic constitutive model reasonably describes the cyclic stress-strain response of 2124-T851 aluminum alloy.Based on the analysis of internal force evolution,the parameters in the model are more reasonable and the physical meaning is clearer.Three different strength random fatigue tests were carried out for 2124-T851 aluminum alloy double notch and center hole specimens under the same load spectrum.A transient local stress-strain transformation method(transient method)is proposed,combining a combined hardening constitutive model and a Neuber calculation method.Based on local stress-strain method,the results of the estimation of the crack initiation life of the crack under the random load with the steady-state method and the transient-method(Ramberg-Osgood,combined hardening constitutive model and Neuber formula can be transformed for local stress at the notch,respectively)are compared and analyzed.The results show that the fatigue life of the 2124-T851 aluminum alloy bilaterally notched and central hole specimens is predicted by the steady state method more or less.However,the transient method can improve the results of overestimation predicted material fatigue life. |
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