Experimental observations of intersonic crack growth in asymmetrically loaded unidirectional composite plates

Some recent experimental observations of highly dynamic crack growth events in thick unidirectional composites are presented. The specimens used in this study were 48-ply thick unidirectional graphite-epoxy composite plates which were either symmetrically (mode I) or asymmetrically (mode II) loaded by impact in a one-point bend configuration with an edge pre-notch machined in the fibre direction. Moderate impact speeds of up to 57 m s(-1) were used. The lateral shearing interferometric technique of coherent gradient sensing in conjunction with high-speed photography was used to visualize the failure process in real time. Mode-I cracks propagated subsonically with crack speeds increasing to the neighbourhood of the Rayleigh wave speed. For asymmetric mode-II types of loading the results revealed highly unstable and intersonic shear-dominated crack growth along the fibres. These cracks propagated with unprecedented speeds reaching 7400 m s(-1): a speed which is more than three times the shear wave speed of the composite and almost equal to the dilatational wave speed of the composite along the fibres. For intersonic crack growth, the interferograms featured a shock wave structure typical of disturbances travelling with speeds higher than one of the characteristic wave speeds in the solid. Evidence of large-scale frictional contact is also presented.