Low Cycle Fatigue Behavior And Uniaxial Ratcheting Behavior Of Al 7050-T6

Aluminum alloys are more advantageous in aerospace,marine and high-speed transportation because of their lower density than other metals?such as iron,copper,etc.?,strong corrosion resistance,easy processing,high specific strength,and strong fatigue resistance.The field is widely used.However,aluminum alloy engineering components are subject to a variety of loads during service,particularly due to asymmetric stress cyclic loading.This produces a ratcheting effect,leading to premature failure of the component.Therefore,in the design process of aerospace,ship,etc.,the impact of ratcheting strain must be considered.In this paper,the fatigue behavior of 7050-T6 aluminum alloy is discussed,and the failure mechanism of low cycle strain fatigue and ratcheting effect is discussed.The research results mainly include:By observing the metallographic structure of the 7050-T6 commercial aluminum alloy,the results show that there are black granular materials with different sizes and numbers in the 7050-T6 commercial aluminum alloy.These black particulate materials are referred to as the second phase Mg Zn₂,and the second phase is generally distributed in the grain boundaries or in the crystals.These second phases accumulate at the grain boundaries,hindering the movement of dislocations.Secondly,the uniaxial static tensile test of 7050-T6 commercial aluminum alloy was carried out,and the basic mechanical properties of 7050-T6 were obtained.Static tensile results at different strain rates indicate that the material is a rate-independent material.Low-cycle strain fatigue test was conducted on 7050-T6 commercial aluminum alloy.The results show that the cyclic properties of the material at room temperature show cyclic softening characteristics.The cycle strength coefficient,cycle strength index,fatigue strength coefficient and fatigue strength index were obtained by fitting the Hollomon formula and the Manson-Coffin formula,respectively.Fatigue life prediction was performed using three different low cycle fatigue life prediction formulas.The prediction results show that the three-parameter power function model is more accurate than the Manson-Coffin and tensile hysteresis energy model prediction when predicting the low cycle fatigue life of 7050-T6 commercial aluminum alloy.The asymmetric stress cycle experiment of 7050-t6 commercial aluminum alloy was designed,and the effects of mean stress,stress amplitude,stress rate and stress ratio on ratchet strain were discussed respectively.The results show that at room temperature,the ratcheting strain increases with the increase of the average stress and stress amplitude.The fatigue life decreases as the average stress and stress amplitude increase.The effect of stress rate on ratchet strain is not significant because the material is a rate-independent material;however,an increase in the stress rate leads to an increase in fatigue life.When the stress ratio changes from-0.8 to-0.2,the ratchet strain decreases first and then increases.Finally,a fatigue life prediction model with high average stress and low stress amplitude is proposed,which has better life prediction accuracy.The failure analysis of low-cycle strain fatigue and fatigue fracture of uniaxial ratcheting test specimen were carried out.The results show that the crack origin of low-cycle strain fatigue originates from the fact that the micro-cracks are formed through continuous expansion,which eventually leads to the destruction of the material.The failure of the specimen in the asymmetric stress cycle test originates from the specimen part,because during the asymmetric stress cycle,plastic strain accumulates,resulting in a large number of secondary cracks and holes inside the fatigue fracture.