ANALYSIS AND MODELING OF THE CREEP-BEHAVIOR OF THE THERMOSTABLE PMR-15 POLYIMIDE

To establish the constitutive equations for carbon/PMR-15 composites a nd to determine the contribution of the matrix to the mechanical respo nse, an analysis of the behavior of the PMR-15 polyimide alone has bee n performed under thermomechanical loading conditions corresponding to military aeroengine applications. In particular, the creep behavior o f the material has been studied for several combinations of stress (0. 3 to 0.7 sigma(r)), temperature (250 to 300 degrees C), and time (5 an d 1300 h). The creep compliance has been modeled in the field where th e behavior is linear, using the Burgers model, together with a retarda tion time distribution for the viscoelastic region. The viscoelastic b ehavior is described satisfactorily by the Kohlrausch function, and co mbined with the Maxwell relation, enables the creep compliance of PMR- 15 polyimide to be modeled in the temperature range considered for sho rt loading times. For long times at 250 degrees C and 0.3 a,, the poly mer undergoes slow plastic deformation, but nevertheless, conserves it s overall viscoelastic properties. There is no equivalence between tim e and temperature. The master curve, constructed. graphically on the b asis of linear viscoelastic strains, short times, and a reference temp erature of 250 degrees C, deviates quite significantly from the experi mental curve at longer times. In the latter case, the creep compliance curve can be correctly fitted to a. simple power law. (C) 1998 John W iley & Sons, Inc.