Dynamic Mechanical Behaviors of a Short-glass-fiber Reinforced Polyamide in Hopkinson Bar Test

To investigate the dynamic mechanical property of a 50% short-glass-fiber reinforced polyamide, the quasi-static hydraulic testing machine and split Hopkinson pressure/tension bar are used to apply loading on specimens with diameters from 6 mm to 10 mm, and the average strain rate is between 0.0005 s^-1 and 1600 s^-1. The stress-strain curves and failure modes of specimens are obtained and investigated. Micro-mechanics of failure process at different strain rates are analyzed. It can be seen from the stress-strain curves that the strength of material under dynamic loading are obviously higher than that under quasi-static loading (the compression strengths are increased by 31%, 25% and 29% at the strain rates of 400 s^-1, 900 s^-1 and 1 600 s^-1, respectively; and the tensile strengths are increased by 46%, 47% and 28% at the strain rates of 400 s^-1, 800 s^-1 and 1200 s^-1, respectively), and the failure strain decreases significantly. The failure processes at different strain rates are also investigated. Under compression loading, the specimens experience the compaction of existing defect, elastic deformation and failure; under tensile loading, the specimens experience only elastic deformation and failure. The growth processes of micro-cracks are obviously different under static loading and quasi-static loading: under quasi-static loading, the micro-cracks assembly into macro-cracks. On the contrary, micro-cracks separately grow into macro-cracks. Optical observation and scanning electron microscope (SEM) are adopted to investigate the fracture surface. Results show that the fiber pull-out and fiber fracture are generated under dynamic compression, the fracture surface are smoother under quasi-static compression. And under tensile loading, dynamic surface shows fiber fracture, and quasi-static surface shows fiber pull-out.