RELIABILITY OF THERMOMECHANICAL FATIGUE IN POLYMER MATRIX COMPOSITES

A methodology to compute probabilistic fatigue life of polymer matrix laminated composites has been developed and demonstrated. Matrix degra dation effects due to long term environmental exposure and mechanical/ thermal cyclic loads are accounted for in the simulation process. A un ified time-temperature-stress dependent multi-factor interaction relat ionship developed at NASA Lewis Research center has been used to model the degradation/aging of material properties due to cyclic loads. The fast probability integration method is used to compute probabilistic distribution of response. Sensitivity of fatigue Life reliability to u ncertainties in the primitive random variables e.g. constituent proper ties, fiber volume ratio, void volume ratio, ply thickness, etc. are c omputed and their significance in the reliability based design for max imum Life is discussed. Effect of variation in the thermal cyclic load s on the fatigue reliability for a graphite/epoxy (0/+/-45/90)degrees( s) laminate with ply thickness 0.127 mm, with respect to impending fai lure modes has been studied. The results show that, at low mechanical cyclic loads and low thermal cyclic amplitudes, fatigue life for 0.999 reliability is most sensitive to matrix compressive strength, matrix modulus, thermal expansion coefficient, and ply thickness. Whereas at high mechanical cyclic loads and high thermal cyclic amplitudes, fatig ue Life at 0.999 reliability is more sensitive to the shear strength o f matrix, longitudinal fiber modulus, matrix modulus, and ply thicknes s.