Abstract
The present study consists of two parts: (a) finding the high strain rate response of novel auxetic Kevlar®/epoxy laminated composites, (b) developing a mathematical model for impact problem of viscoelastic material. A comprehensive experimental investigation was performed to study the dynamic compressive constitutive response of novel auxetic KevlarÒ/epoxy laminated composites. Strain rate response was investigated using split Hopkinson pressure bar (SHPB) test setup. Laminated composites were fabricated using vacuum infusion process. Short Nylon fibers were flocked between the laminates with different flock density and flock length. For obtaining dynamic force equilibrium in SHPB experiments, copper pulse shaper was used to increase the rising time of incident pulse. To have comparison, woven KevlarÒ/epoxy composites were also characterized at same strain rates. In addition, quasi-static tests were also performed on both woven and auxetic laminated composites for completeness of the study. For quasi-static loading conditions, auxetic composites showed higher peak strain and lower peak stress compared to woven composites. For no flock condition, although both auxetic and woven composites showed rate dependency, woven composites provided 353% increase in peak stress and auxteic composites showed only 155% increase in peak stress when the strain rate increased from low (1200s-1) to high (3300s-1). For flock conditions, woven composites showed rate dependency for all strain rates, but auxetic composites demonstrated rate dependency only from low to medium strain rates. Both auxetic and woven composites had shear failure under quasi-static compression, where auxetic composites failed at higher shear angle of 37o, but woven composites had a failure angle of 30o. For impact loads, under no flocking condition, woven composites did undergo severe edge failure at all strain rates, but auxetic composites showed a sign of edge failure only at high strain rates. With flocking condition, auxetic composites had through thickness shear failure and woven composites experienced splitting and fibrillation of Kevlar fibers. A linear physics based model is developed to investigate the one dimensional impact on a viscoelastic material. Generalized model with three Maxwell elements were considered to describe the viscoelastic material. An analytical method based on Laplace transformation was used to solve the impact problem. To have a comprehensive understanding, drop weight impact was also considered in this study. Reduction of the impact force as well as greater energy absorption can be achieved with the increase of the loss tangent of viscoelastic material. Moreover, high stiff material absorbs more impact energy and experiences high impact force as compared to low stiff material.