Mechanical Behavior And Life Prediction Of Tensile And Torsional Composite Fatigue Of Al-Li Alloy (2198-T8)

Al-Li alloy is an advanced lightweight structural material which is frequently used in nowadays aviation manufacturing industry.2198 Al-Li alloy is predicted to be widely used in the manufacture of aircraft skin and other structural parts.Therefore,it is very important to analyze the multi parameter variable fatigue failure of the material.In view of the influence of loading conditions on composite tensile-torsional fatigue,the fatigue constant of 2198-T8 Al-Li alloy was determined by uniaxial fatigue test.The results of proportional and non-proportional loading tests show that the tensile-torsional fatigue life increases with the decrease of equivalent stress amplitude.When the equivalent stress amplitude is constant,the fatigue life of 2198-T8 Al-Li alloy first increases and then decreases with the increase of phase difference,and the fatigue life of 45° phase difference is the highest.Under high stress amplitude,the material softens continuously in axial and torsional direction,while under low stress amplitude,hardening and softening occur alternately.Under proportional loading condition,scratch,wear and fatigue strip morphology co-exist in the crack propagation zone;under non proportional loading condition,the fatigue fracture micromorphology zone is not obvious,and the fatigue strip is worn.According to the error analysis,the modified Findley model and the modified SWT model proposed in this paper provide better prediction of fatigue life under proportional and non-proportional loading paths respectively,with all predicted data within a factor of two.Based on the study of crack growth behavior of 2198-T8 Al-Li alloy,the tensile-torsional fatigue crack growth test was carried out.Combined with digital image correlation technics,strain field on the surface of specimen was analyzed.The results show that the fatigue life increases with the decrease of equivalent stress in crack growth stage,and the fatigue life increases with the decrease of stress amplitude ratio.By fitting Paris formula,the material constants of the composite fatigue crack growth performance are obtained.The maximum normal stress plane has the best prediction results on crack growth direction under different stress amplitude ratios.The results show that with the increase of stress amplitude ratio,the axial strain becomes larger and the torsional strain becomes smaller,and the torsional strain concentration lags behind the axial strain concentration,the corresponding fatigue crack propagation zone changes from the pure torsion wear morphology to the coexistence of wear morphology and fatigue strip morphology,and finally to the fatigue strip morphology.The crack growth mode changes from mode II to I + II,and finally to mode I,which corresponds to the change of stress state.Aiming at the tensile-torsional fatigue failure behavior of 2198-T8 Al-Li alloy notched specimen,the multi-variable notched specimen tests were carried out.The results show that the fatigue life of specimens decreases with the increase of notch size,the number of notches and the equivalent force amplitude.Combined with DIC analysis,the axial and torsional strain concentration appears in the stage of 80% to 90% fatigue life,and cracks appear on the surface of the specimen at 95% fatigue life;the morphology of fatigue source and crack propagation zone can be found by SEM observation of strain concentration area.According to the error analysis,the modified SWT model which applied to notched specimen provide better prediction of fatigue life of notched specimen tests,with all predicted data within a factor of two.