Low Velocity Impact Experiments of S-2 Glass-Epoxy Composites Under Different Environmental Conditions

Abstract

S-2 glass-epoxy composites are used in damage tolerant structural applications such as military ground vehicles due to their high specific strength, stiffness, and energy-absorbing capabilities. Composites can be susceptible to delamination failure caused due to high interlaminar shear stresses when subjected to transverse impact loading. While fiber fracture, fiber pullout, matrix cracking, and delaminations are the major damage mechanisms, delamination is a significant energy absorbing failure mode in low-velocity impacts (LVI). LVI leads to a reduction of stiffness and residual strength, which is critical for structural integrity. To improve the delamination resistance, researchers have used various types of interlayers for composites subjected to impacts. In this research, LVI experiments are conducted on composites made using plain weave S-2 glass and SC-15 epoxy resin. Two types of specimens are tested, 'baseline' (without interlayer) and 'interlayer' (with thermoplastic interlayer UAF 472). A Dynatup 9200 drop tower with a hemi-cylindrical 12.7 mm impactor is used. The specimens are impacted (40J) at three temperatures, -55C, RT, and 76C. The stiffness before and after the LVI test is evaluated, and the influence of temperature and impacts on stiffness change is discussed, along with the mechanisms that cause the change in stiffness. The interlayer specimens have significantly smaller delamination areas, and the stiffness loss due to impact is also reduced (compared to baseline) at each temperature. However, since the properties of the resin and the TPU deteriorate at elevated temperatures, there is a drastic stiffness loss in interlayer specimens at elevated temperatures in the pristine, non-impacted samples.