Low Cycle Deformation And Fatigue Damage Of Polymer In Triaxial Stress State

It is well known that percent of fatigue failure exceed 80%. Notches at the component are often the cradles of fatigue microcrack. It is one of the cause of structure failure that components are often in the triaxial stress state because of the presence of notches. Engineer s and researchers are very concerned at the reliability of components. Researches of mechanical properties of materials, especially fatigue behaviors in triaxial stress state, are of important theory and engineering values.Though polymers are being widely used in engineering structures, the studies on their fatigue behaviors are very limited. Effects of stress triaxiality on fatigue behaviors of polymers are not investigated in opened literature including cyclic deformation, fatigue damage and fatigue lifetime.In the paper, low cycle deformation and fatigue damages of polymer were experimentally and theoretically investigated. Series of tests for circumferentially notched bars with different notch radii were tested in Intron5544 testing machine with non-contacting video extensometer. Conclusions were summarized as follows: (1) Cycle deformation in the triaxial stress state of polymer under stress-controlled and strain-controlled modeIt can be found that strain range at notch exceeds applied strain range under strain-controlled mode and stress range at notch exceeds applied stress range under stress-controlled mode because of stress triaxiality. It is defined as harden.Cyclic soften that stress range decrease with the increase of cyclic number can be found under strain-controlled mode for low cycle fatigue.Stress relaxation that mean stress decrease with the increase of cyclic number can be found under strain-controlled mode for low cycle fatigue.cyclic soften that strain range increase with the increase of cyclic number and cyclic harden that strain range decrease with the increase of cyclic number alternate under stress-controlled mode for low cycle fatigue.(2)Cyclic soften under strain-controlled mode and alternationof cyclic soften and cyclic harden under stress-controlled mode in the triaxial stress state of polymer can be explained by presented concepts,including initial cyclic stress range, initial cyclic strain range,subjoined stress range and subjoined strain range caused by stress triaxiality.Under strain-controlled mode, cyclic stress rangeΔσ0 (N) caused by initial cyclic strain rangeΔε0 expresses cyclic soften of polymer in unaxial and cyclic stress rangeΔσT(N) caused by subjoined strain rangeΔεT.(N) result in decrease of cyclic soften rate of stress range curve. It is the reason of cyclic soften for polymer in the triaxial stress state that cyclic soften rate caused by initial cyclic strain range exceed decrese of cyclic soften rate caused by subjoined strain range. Under stress-controlled mode, cyclic strain rangeΔε0(N) caused by initial cyclic stress rangeΔσ0 expresses cyclic soften of polymer in unaxial. Cyclic strain rangeΔε0(N) caused by initial cyclic stress rangeΔσ0 decrease of cyclic soften rate of strain range curve in unaxial because of the notch effect of confine deformation. It results in the cyclic harden at the cyclic stage. Cyclic strain rangeΔεΥ(N) caused by subjoined stress rangeΔσT(N) result in increase of cyclic soften rate of strain range curve in unaxial. It is the reason that alternation of cyclic soften and cyclic harden under stress-controlled mode in the triaxial stress state of polymer. The proportion of softening stage and hardening stage is dependent on stress triaxiality and stress level.(3)The effects of stress triaxiality,stress level and strain level on fatigue lifetimeFatigue lifetime is related with initial stress range, cyclic soften rate and stress relaxation rate under strain-controlled mode. Initial stress range, cyclic soften rate and stress relaxation rate decrease with the increase of stress triaxiality at same strain range.It is the reason that Fatigue lifetime increase with the increase of stress triaxiality. Initial stress range, cyclic soften rate and stress relaxation rate increase with the increase of strain range at same stress triaxiality. It is the reason that Fatigue lifetime decrease with the increase of stress range.Fatigue lifetime is related with ratcheting strain and cyclic creep under stress-controlled mode. Ratcheting strain and cyclic creep increase with the increase of stress triaxiality at same stress range. It is the reason that Fatigue lifetime decrease with the increase of stress triaxiality. Ratcheting strain and cyclic creep increase with the increase of stress range at same stress triaxiality. It is the reason that Fatigue lifetime decrease with the increase of stress range. (4) Fatigue damage variable for polymerDamage can be evaluated by stress range under strain-controlled mode and by ratcheting strain under stress-controlled mode.(5)Fatigue damage model is based on Wang-Lou's model in the triaxial stress state for polymersUnder strain-controlled mode with the mean strain, low cyclic fatigue damage consists of cyclic plastic damage and the time-dependent stress relaxation damage.Under stress-controlled mode with the mean stess, low cyclic fatigue damage consists of cyclic plastic damage and the time-dependent cyclic creep damage.Damage evolution equation, stress relaxation evolution equation and cyclic creep evolution equation are including in the model.It is derived from the model that the relations of plastic damage parameter and mean stress evolution parameter, mean strain evolution parameter are linear.(6)Comparisons between model and the experimental results are presented and agood agreements are found in cyclic late stages.The low machining accuracy of notches at components results in error between model and the experimental results in cyclic early stages because stress triaxiality influences initial values of model parameters.(7)Based on the damage mechanics criterion for ductile fracture presented by Wang tiejun and Manson-Coffin formula, a prediction model of low cycle fatigue lifetime in the triaxial stress state is developed.