Dynamic initiation and propagation of cracks in unidirectional composite plates

Dynamic crack growth along weak planes is a significant mode of failure in composites and other layered/sandwiched structures and is also the principal mechanism of shallow crustal earthquakes. In order to shed light on this phenomenon dynamic crack initiation and propagation characteristics of a model fiber-reinforced unidirectional graphite/epoxy composite plate was investigated experimentally. Dynamic fracture experiments were conducted by subjecting the composite plates to in-plane, symmetric and asymmetric, impact loading. The lateral shearing interferometric technique of coherent gradient sensing (CGS) in conjunction with high-speed photography was used to visualize the failure process in real time. It was found that mode-I cracks propagated subsonically with crack speeds increasing to the neighborhood of the Rayleigh wave speed of the composite. Also in mode-I, the dependence of the dynamic initiation fracture toughness on the loading rate was determined and was found to be constant for low loading rates and to increase rapidly above . The dynamic crack propagation toughness, K_ID, was observed to decrease with crack tip speed up to the Rayleigh wave speed of the composite.; For asymmetric, mode-II, types of loading the results revealed highly unstable and intersonic shear-dominated crack growth along the fibers. These cracks propagated with unprecedented speeds reaching 7400 m/s which is the dilatational wave speed of the composite along the fibers. For intersonic crack growth, the interferograms, featured a shock wave structure typical of disturbances traveling with speeds higher than one of the characteristic wave speeds in the solid. In addition high speed thermographic measurements are conducted that show concentrated hot spots behind the crack tip indicating non-uniform crack face frictional contact.; In addition, shear dominated dynamic crack growth is investigated along composite/Homalite interfaces subjected to impact loading. The crack growth phenomenon was observed using dynamic photoelasticity in conjunction with high-speed photography. Three quantized intersonic and supersonic crack tip speed regimes were identified. First conclusive evidence of crack growth at supersonic speeds with respect to lower speed material and sonic speeds with respect to the unidirectional composite was obtained. Furthermore, this investigation documents the first experimental observation of a mother/daughter crack mechanism allowing a subsonic crack to evolve into an intersonic crack.