INDENTATION CRACKING IN SODA LIME GLASS AND NI-ZN FERRITE UNDER KNOOP AND CONICAL INDENTERS AND RESIDUAL-STRESS MEASUREMENTS

The indentation cracking behaviour of thermally tempered and of annealed soda-lime glass blocks has been studied experimentally. The indenters were a Knoop pyramidal diamond and a tungsten carbide cone of apex angle 90-degrees. They were loaded normally on to the flats and particular attention was paid to the evolution (i.e. initiation and growth) of the median and lateral cracks generated under the indenters. Under the Knoop indenter, a half-penny-shaped median crack formed in both the tempered and the annealed soda-lime blocks during loading to sufficiently high loads. During unloading, the median crack grew in length at the indented surface while its depth appeared to decrease (not heal). Lateral cracks, both the subsurface and the shallow types, formed during unloading. The shallow surface laterals always formed at close to complete unloading and caused chipping of the surface. The surface extent of the lateral cracking extended beyond the surface traces of the median cracks in the thermally tempered glass blocks. With the conical indenter, one or more approximately full-penny-shaped median cracks formed in the tempered glass blocks during loading to sufficiently high loads. During unloading, the median cracks broke through to the surface becoming like half-pennies. In the annealed glass blocks, the median cracks broke through to the surface during loading to sufficiently high loads. Otherwise, the evolution of median and lateral cracks was similar to that in the case of the Knoop indenter. With all the pointed indenters, namely Knoop, cone and Vickers, when the maximum load was not sufficiently high, no cracks formed during loading (sometimes no cracks formed even at loads of 100-200 N), but median cracks were observed to form during unloading. High-speed framing photography has been unable to resolve the location of initiation of the unloading median cracks. Observations of unloading median cracks under quasistatic indentations have so far not been reported. From in situ measurements of the depth of the median cracks generated by Knoop and conical (apex angle, 90-degrees) indenters in the tempered and the annealed soda-lime glass blocks, the surface residual compressive stress in the tempered soda-lime glass blocks has been estimated. The Knoop and conical indenters gave values of 56 +/- 22 and 82 +/- 28 MPa respectively for the stress. By making measurements of the surface extent of the median and radial cracks produced by a Vickers indenter in lapped and in annealed Ni-Zn ferrite blocks, the surface compressive residual stress in the lapped ferrite was also estimated. Furthermore, two other techniques, namely a deflection method and X-ray diffraction (in ferrite only), have been used to determine the residual stress in the tempered glass and in the lapped ferrite blocks. The residual stress measurements have shown that the pointed-indentation technique can give values which are close to those obtained using the deflection method and X-ray diffraction. The observations of indentation cracking under Knoop and conical indenters in the tempered and the annealed soda-lime glass blocks have been analysed in the context of the predictions made by various models of indentation. None of the indentation models seems to explain all the major experimental observations. A brief discussion of this discrepancy is also given. Finally, the relative merits of using various pointed indenters for determining the fracture toughness and residual stresses in brittle materials are discussed.