Influence of temperature dependent matrix properties on the high-rate impact performance of thin glass fiber reinforced composites

The influence of measurement temperature on the high velocity (>100 m/s) impact performance was investigated for three thermosetting epoxy resin S-2 glass composite systems. These three model resins were chemically similar but have different glass transition temperatures (T-g) and molecular weights between crosslinks (M-c) through the use of different diamine curing agents (Jeffamine (R) D230, D400, and D2000). Impact performance was quantified by the projectile kinetic energy absorbed (KE50) as calculated from the characteristic ballistic velocity (V-50) of the composites during high velocity impact. Among the resin systems, the KE50 remained essentially constant over a broad range of temperatures for each composite set and modestly increased with decreasing M-c. The superficial damage area associated with delamination showed remarkable sigmoidal behavior as a function of the testing temperature relative to the T-g (T-T-g). Damage was high for low T-T-g values (glassy resin) and decreased as the resin traversed its Tg into the rubbery region. These damage area trends were found to depend on the resin M-c, with higher M-c values resulting in lower overall damage area and a lower inflection point temperature. High speed videography of the back surface of the samples showed that lower damage areas correlated with an increased back face deflection, which enabled energy absorption with relatively less delamination. Composite mechanical tests were performed to validate the impact performance and explain the deformation mechanisms observed during impact energy dissipation. Our results illustrate the critical importance of the resin architecture and temperature-dependent viscoelastic behavior on the impact properties of composites for impact-resistance applications.