Indentation plasticity and the ensuing fracture of glass

A systematic study of spherical indentations on various glasses has shown that for small indenters (<or approximately=1 mm) plastic flow as well as Hertzian cone cracking occurs. What is of more interest however, is that on unloading, a series of radial cracks forms about the area of contact. With continued unloading, these cracks grow and just before complete unloading, an entirely independent system of lateral cracks, which are responsible for material removal processes, occurs. The indentation process was followed using an inverted microscope and indentation stress-strain curves were produced. These curves were compared with recent indentation correlation expressions and found to fit reasonably well. The radial cracking about the contact site was analysed in terms of a stress field which was a composite of the normal Hertzian loading situation coupled with the expansion of a cavity adjacent to a free surface. This model agrees reasonably well with experiment and provides a qualitative treatment of lateral cracking phenomena.