Impact energy absorption of unidirectional glass fiber‐epoxy composites

The influence of resin and fiber properties on the impact behavior of composites can be assessed in a three‐point drop‐weight impact test by varying the length‐to‐thickness ratio of the specimen. The fracture initiation energy per unit deformed volume, w i , can be described by the expression:\documentclass{article}\pagestyle{empty}\begin{document}$$w_i = \frac{{\tau _{11}^2 }}{{18E_{11} }} + \frac{{4\tau _{12}^2 }}{{15G_{12} }}$$\end{document}where τ 11 is the tensile stress, τ 12 is shear stress; E 11 is tensile modulus; and G 12 is shear modulus. A unidirectional glass fiberepoxy composite was tested at impact velocities of 2.2 m/s (5 mph) and 4.5 m/s (10 mph). The energy to initiate fracture, w i , was in the range of 2 to 3.5 MJ/m 3 , apparently independent of impact velocity. The total energy absorbed by the impacted composite was also found to be independent of impact rate but very sensitive to the length to thickness ratio: about 13 and 3.5 MJ/m3 at the corresponding ratios of 4.6 and 23. It was generally observed that high fracture energy is associated with extensive specimen delamination, i.e. failure in shear.