Effect of hybridization on the creep and stress relaxation characteristics of pultruded composites

Pultrusion manufacturing process is a well established technique for the cost-effective production of high-modulus and lightweight composite materials having constant cross-sectional profiles. The pultruded composites are widely used as structural members viz., beams, trusses and stiffeners, owing to the presence of high proportion of axial fibers necessary to sustain large tractive forces. These structural members are subjected to a combination of static and dynamic loading conditions at wide temperature ranges and over longer periods. Since polymeric composites exhibit viscoelastic behavior, the effectiveness of these materials as structural members must be thoroughly evaluated to ensure long-term stability. In previous research, the dynamic performance characteristics of pultruded glass-graphite/epoxy hybrids were evaluated at room temperature. The effects of temperature, frequency, post-curing, along with the type and placement of fibers on the dynamic flexural properties of glass/epoxy and hybrid glass-graphite/epoxy were also investigated. This paper focuses on the evaluation of creep and stress relaxation performance characteristics of pultruded glass-graphite/epoxy hybrid composites. Dynamic mechanical analysis technique was adopted for the accelerated creep and stress relaxation testing. Time-temperature superposition principle, which greatly reduced the experimental time, was effectively utilized for predicting the creep and stress relaxation properties of the hybrid composites. Results indicate that the type and amount of fibers as well as their lay-up sequence plays a significant role in determining the flow and load bearing characteristics.