Investigation On The Effects Of Mean Strain On Low-cycle Fatigue Behavior Of Aluminum Alloy7050-T7451

Aluminum alloy7050-T7451is one of7000high-strength aluminum alloys,which have been widely used in the field of aeronautics.With the development of technical aeronautics,it is of increasingly high demanding for the aviation material,therefore the low-cycle fatigue of aviation material has become a significant issue.Flight consists of sliding off,upclimbing,cruising,dropping and sliding and in each stage it has been subjected considerably large fatigue load.At the same time there is large mean strain in the aviation material,which is of significance to research on the effect of mean strain on low-cycle fatigue behavior of aviation material.In order to research on the effects of mean strain on the low-cycle fatigue behavior of Aluminum alloy7050-T7451, strain-controlled tests were performed on MTS810by multi-specimen method.Strain ratios0.06,-0.06and0.5were chosen with six constant strain amplitude2%,1.5%,1%,0.8%,0.6%and0.4%,under degree of confidence95%.Each test had obtained the specimens’fatigue life,maximum stress and strain,minimum stress and strain, young’s modulus,plastic strain and cyclic hysteresis energy and so on.The tests showed that the cyclic stress response in low-cycle fatigue of aluminum alloy7050-T7451has something to do with mean strain that it behaved cyclic softening under symmetric straining while it behaved different cyclic softening or hardening under asymmetric straining.With the analysis of variation of the mean strain,mean stress relaxation occurred under asymmetric straining.The rate of mean stress relaxation increased with the increasing of strain amplitude and the phenomenon was not obvious when the strain amplitude was low,such as0.4%.The law of mean stress relaxation was described by Maxwell model well. With the analysis of cyclic hysteresis loop and the calculation of Bauschinger strain and inelastic strain,this material is proved to be one with Masing behavior,so the cyclic hysteresis energy could be calculated by parameters of straining fatigue.With fitting the datum of maximun strain(strain amplitude) and fatigue life,we obtained the equations describing the εa-Nf(εmax-Nf) curves and p-εa-Nf (p-εmaxNf) curves with different survival rate under different strain ratios. The curved surface Nf=0.001131/[(0.251εa/0.251-εm)-0.00207]2.6973describing the relationship among mean strain,stress amplitude and fatigue life were established.This material had been found to show a billinear phenomenon in the inelastic line when the data was fit by Manson-Coffin relationship. It was also found that fatigue strength coefficient σ’f and fatigue ductile coefficient ε’f had increased while fatigue strength exponent b and fatigue ductile exponent c had decreased with the existense of mean stain.The low-cycle fatigue life was predicted by SWT models accurately under different strain ratios.At last SEM(Scanning Electron Microscope)observation of specimen fatigue fracture of aluminum alloy7050-T7451was used to indicate the different fracture characteristics of different level of strains with different strain ratios.It was found that the low-cycle cracks were initiated on the surface of specimen. With the increasing of strain ratio and strain amplitude it appeared multi-fatigue crack initiations and the characteristic of brittle fracture.