Multiaxial Low-cycle Fatigue Failure Analysis And Life Prediction Of 7075 Aluminum Alloy

As a kind of hard aluminum alloy,7075 aluminum alloy is widely used in the aviation field.Aircraft structures are often subject to multiaxial fatigue loads,where load types and interactions,corrosive environments in service processes,and discontinuous features in the structure can have a significant impact on failure behavior.Therefore,it is important to analyze material's failure behaviors.Tension-torsion fatigue tests with different loading conditions were carried out on 7075 aluminum alloy thin-walled tubular specimens aiming at studying multiaxial fatigue failure of aviation aluminum alloy.With the decrease of equivalent stress amplitude,the multiaxial fatigue life increases.Under the same equivalent stress,the multiaxial fatigue life rises as the stress amplitude ratio augments while the tension-torsion phase has little effect on fatigue life.Under high stress amplitude,the material mainly softens in the axial and torsional directions while the hardening and softening alternately occur under the low stress amplitude.The platform area of macro fracture decreases with the increase of stress amplitude ratio.Fatigue striations and secondary cracks can be observed in the crack propagation region.Mixed mode dimples appear in the instantaneous fracture region.Based on Basquin's criteria,a modified model is proposed,and fine life prediction results are obtained,with the life prediction values in 2x scatter bands.Focus on the loading sequential effects on the multiaxial fatigue damage accumulation,12 groups of two-step loading spectra were conducted on 7075 aluminum alloy.With comparison of experimental results,it's observed that fatigue damage accumulation rate is decreased by the two-step loading interaction.Cyclic loading curves and fracture morphology analysis reveal that the loading step corresponding to lower fatigue life is dominant.Considering the stress amplitude ratio effects and loading interaction,a non-linear accumulative damage model is proposed on basis of the modified Basquin's criterion.Fine life prediction results are obtained by the proposed model with all predicted data in the two scatter bands.Considering the alternate process of corrosion on ground and fatigue in air for aircrafts.,the effects of pre-corrosion and alternate corrosion on multiaxial fatigue behavior of 7075 aluminum alloy was studied.Under the same equivalent stress amplitude,the multiaxial fatigue life decreases with the increase of pre-corrosion time while the number and size of pits on specimen's surface augment indicating that the weight of corrosion in-creases.For alternate corrosion-multiaxial fatigue tests,with the increase of unit corrosion time,fatigue life de-creases.During the tests,corrosion on the surface of specimen tends to be more serious with the augmentation of alternate stage.Based on Miner model and life data from pre-corrosion fatigue tests,a modified accumulative damage model is proposed which shows fine results within 2x scatter bands in alternate corrosion-multiaxial fatigue life prediction.Aiming at the notch effect on fatigue performance under multiaxial fatigue loading conditions,tension-torsion fatigue tests of 7075 aluminum alloy notched specimens under different equivalent stress conditions was carried out with combination of Digital Image Correlation(DIC)technology.With the increase of equivalent stress,notched multiaxial fatigue life of the 7075-T651 aluminum alloy is reduced,and the maximum axial,torsional and shear engineering strain around the notch is increased.Under different equivalent stress conditions,as the loading cycle increases,the localized strain concentration of the notch increases gradually.Based on the critical plane analyzed by DIC,the critical surface angle and the maximum positive strain are introduced to propose the modified SWT model.The life prediction values are within 2x scatter bands,which shows fine life prediction results.