Static and High Strain Rate Response of a Glass Fiber Reinforced Thermoplastic

The mechanical response of an impact-modified, discontinuous fiber reinforced styrene-maleic anhydride (S/MA) polymer has been characterized at static and high strain rates and under both dry and wet test conditions. Five different material configurations were tested, including unreinforced S/MA as a reference material and composites incorporating fiber reinforcement with different average diameters, the presence or absence of an interfacial silane coupling agent, and fibers prepared with an acrylonitrile/butadiene latex coating.

The ultimate tensile strengths, strains to failure, fracture energies, and effective moduli for each of the materials were evaluated as a function of strain rate, which was varied between 1.67 × 10⁻³ and 6.0 mm/mm·s. The results of the tests performed on the unreinforced S/MA revealed a 60% increase in the ultimate strength and marked reductions of 80% and 50% in the strain to failure and fracture energy, respectively, with increasing strain rate. While all of the composites exhibited on the order of twice the strength, 2.5 times the stiffness, and less than a tenth of the strain to failure compared to the unreinforced S/MA, the dependence of these properties on the strain rate was much weaker. Significantly, the work of fracture more than doubled with the strain rate for all the composite configurations tested due to the comparatively small reduction in fracture ductility of about 25%. All of the materials showed some degradation in the mechanical properties when tested wet, a result that was particularly evident for the composites having no silane coupling agent which suffered about a 15% loss in strength and stiffness.

A simple rule of mixtures calculation revealed that as the rate of testing was increased, more efficient reinforcement by the fibers was realized. Fractographic observations using a scanning electron microscope, viewed in conjunction with the experimental results, indicated that fiber debonding during composite deformation was limited by the inhibition of viscoelastic flow in the matrix material at high strain rates.