Durability and damage tolerance of a polyimide chopped fiber composite subjected to thermomechanical fatigue missions and creep loadings

Although polyimide based composites have been used for many years in a wide variety of-elevated temperature applications, very little work has been do ne to examine the durability and damage behavior under more prototypical th ermomechanical fatigue (TMF) loadings. Synergistic effects resulting from s imultaneous temperature and load cycling can potentially lead to enhanced, if:not,unique., damage modes and contribute to a number of nonlinear deform ation responses. The goal of this research waste examine the effects of a T MF loading spectrum, representative of a gas turbine engine compressor appl ication, on a polyimide sheet molding compound (SMC). High performance SMCs present alternatives to prepreg forms with great potential for low cost co mponent production through less labor intensive, more easily automated manu facturing. To examine the issues involved. with TMF, a detailed experimenta l investigation was conducted to characterize the durability of a T650-35/P MR-15 SMC subjected to TMF mission cycle loadings. Fatigue. damage progress ion was tracked through macroscopic deformation and elastic stiffness: Addi tional properties, Such as the glass transition temperature (T-g) and dynam ic mechanical properties were examined. The fiber distribution orientation was characterized through a detailed quantitative image analysis. Damage to lerance was quantified on the basis of residual static tensile properties a fter a prescribed number of TMF missions. Detailed micro-structural examina tions were conducted using optical and scanning electron microscopy to char acterize the local damage. The:imposed baseline TMF missions had only a mod est impact on inducing fatigue damage with no statistically significant deg radation occurring in the measured macroscopic properties: Micro-structural damage was, however, observed subsequent to 100 h of TMF cycling which con sisted primarily of fiber debonding and transverse cracking local to predom inantly transverse fiber bundles. The TMF loadings did introduce creep rela ted effects (strain accumulation) which led to rupture in some of the more aggressive stress scenarios examined. In some cases, this creep behavior oc curred at temperatures in excess of 150 degrees C below commonly cited valu es for T-g. Thermomechanical exploratory creep tests revealed that the SMC was subject to time dependent deformation at stress/temperature thresholds of 150 MPa/230 degrees C and 170 MPa/180 degrees C.