DMA based characterization of stiffness reduction in long fiber reinforced polypropylene

This paper describes an experimental technique based on dynamic mechanical analysis (DMA) that is shown to be useful in separating nonlinear viscoelastic effects from the characterization of irreversible stress-induced damage responsible for stiffness reduction in fiber-reinforced polymer composites. In this work, we characterize the damage evolution of polypropylene (PP) reinforced with long, discontinuous glass fibers (GF) produced by the Direct Long-Fiber Thermoplastic/Compression Molding (D-LFT/CM) process. The experimental technique is comprised of three phases: 1) dynamic stabilization at low load to measure the time-dependent storage and loss moduli, followed by 2) a frequency sweep to provide rate-dependent viscoelastic properties and 3) a quasi-static application of a peak load. These three phases are repeated with the peak load of Phase 3 increased in each iteration. Experimental results for D-LFT/CM PP/GF30 presented here show a direct correlation between peak load and the irreversible stiffness reduction. Furthermore, the stabilization phase following peak load application is shown to lead to a stiffness recovery of up to 40% due to nonlinear viscoelastic recovery.