Micromechanics of Failure Waves in Glass: I, Experiments

Plate and bar impact experiments were performed on glass to investigate the mechanisms responsible for recently ob-served failure waves. In the present work we report obser-vations showing that the so-called failure wave is actually a propagating boundary of damaged material. It initiates at the specimen surface, without appreciable incubation time, and propagates to its interior. In-material gauge measure-ments show that the spall strength and shear resistance of the material are drastically reduced behind the failure wave front. The shear resistance interferometrically mea-sured in pressure-shear experiments is in agreement with the in-material gauge records. Normal stress measurements performed close to the impact surface show a progressive reduction in normal stress behind the failure wave. This feature suggests that the inelastic process responsible for the reduction in shear strength has well-defined kinetics. Recovery experiments performed on confined soda–lime glass rods reveal extensive material fragmentation. Micros-copy studies performed on fracture surfaces show features commonly seen in glass fractured under tensile loading. Post-test X-ray experiments reveal that the material retains its amorphous structure. Two interpretations of the failure process, consistent with the above features, are discussed. The first mechanism is the initiation of microcracks, at the glass surfaces being subjected to compressive tractions, which propagate along surfaces of maximum shear to its interior. Another mechanism, based on the inhomogeneous nature of plastic flow in amorphous materials, is the initia-tion of shear-induced flow surfaces at the impact plane which are punched into the bulk of the material. In the latest, microcracks nucleated at the intersection of these shear flow surfaces are believed to be responsible for the dramatic reduction in spall strength behind the failure wave. The experimental observations herein presented are especially suitable for the formulation and examination of constitutive models describing damage evolution behind the so-called failure waves