Tensile strength and impact toughness of carbon/glass fiber hybrid composites with surface crack

Tensile strength and impact toughness of inter-layer hybrid composites, made of twill woven E-glass fabric and unidirectional carbon fiber in epoxy resin matrix with/without surface crack, were experimentally investigated. Hybrid laminates with eight, ten and twelve layers were prepared by employing the vacuum-assisted resin transfer molding method, while carbon and E-glass layers were stacked in an alternating sequence. Specimens were cut for uniaxial tensile loading, low-velocity Charpy impact tests, and for resin burn-off process, while additional specimens with standard artificial surface crack were also prepared for the tensile tests. The results of the quasi-static tensile tests surprisingly showed that tensile properties are a function of number of layers. It is seen that, ultimate tensile stress increases with number of plies by 20 +/- 6% and 38 +/- 11%, on an average basis for the uncracked and cracked specimens, respectively. This increase is less pronounced for ultimate strain and initial tensile modulus. As for the impact toughness of the specimens, those with twelve layers showed the largest toughness. In all cases, the rate of increase in mechanical properties decreases with increasing number of layers. It is also shown that the existence of surface crack generally increases the ultimate strain at the expense of a drop in ultimate tensile stress, because the laminates are more glass dominated. Reinforcement efficiency factors for uncracked specimens were found to be the constant value of 0.37. A detailed failure analysis has been also presented by scanning electron microscopy of the fracture surface. Charpy impact tests revealed that impact toughness of hybrid laminates increases with the number of layers. Failure modes are reported qualitatively in macroscopic and microscopic scales.