The Influence of Loading Rate on the Mode II Interlaminar Fracture Toughness of Composite Materials

Abstract

The effect of varying loading rate on the mode II interlaminar fracture toughness of a range of composite materials has been investigated. A screw-driven universal testing machine and a fully instrumented drop-weight carriage have been used to obtain crosshead speeds in the range 0.01 mm/min to 3 m/s. The results have shown that the value of GIIc for many composites is rate-sensitive, tending to increase with increasing loading rate. Damage development in the crack tip region has been investigated by examining the polished edges of a number of test specimens in a scanning electron microscope. It has also been shown that offsetting the center plies by a few degrees in order to reduce fiber bridging results in a greater degree of rate-sensitivity in GIIc. It is believed that the presence of fiber-bridging in standard unidirectional fracture-mechanics samples conceals much of the intrinsic viscoelastic response of the polymeric matrix. Failure mechanisms at the crack tip have been investigated using the Double Edge Notch Flexure specimen. A scanning electron microscope study of this region has shown that the size and nature of this zone is dependent upon the crosshead displacement rate.