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

The indentation cracking behaviour of thermally tempered and of anneal ed soda-lime glass blocks has been studied experimentally. The indente rs were a Knoop pyramidal diamond and a tungsten carbide cone of apex angle 90-degrees. They were loaded normally on to the flats and partic ular 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 th e tempered and the annealed soda-lime blocks during loading to suffici ently 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 dur ing unloading. The shallow surface laterals always formed at close to complete unloading and caused chipping of the surface. The surface ext ent 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 f ormed in the tempered glass blocks during loading to sufficiently high loads. During unloading, the median cracks broke through to the surfa ce becoming like half-pennies. In the annealed glass blocks, the media n cracks broke through to the surface during loading to sufficiently h igh loads. Otherwise, the evolution of median and lateral cracks was s imilar 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 c racks formed even at loads of 100-200 N), but median cracks were obser ved to form during unloading. High-speed framing photography has been unable to resolve the location of initiation of the unloading median c racks. Observations of unloading median cracks under quasistatic inden tations have so far not been reported. From in situ measurements of th e depth of the median cracks generated by Knoop and conical (apex angl e, 90-degrees) indenters in the tempered and the annealed soda-lime gl ass blocks, the surface residual compressive stress in the tempered so da-lime glass blocks has been estimated. The Knoop and conical indente rs gave values of 56 +/- 22 and 82 +/- 28 MPa respectively for the str ess. By making measurements of the surface extent of the median and ra dial cracks produced by a Vickers indenter in lapped and in annealed N i-Zn ferrite blocks, the surface compressive residual stress in the la pped ferrite was also estimated. Furthermore, two other techniques, na mely 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 sh own that the pointed-indentation technique can give values which are c lose to those obtained using the deflection method and X-ray diffracti on. The observations of indentation cracking under Knoop and conical i ndenters 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 t he major experimental observations. A brief discussion of this discrep ancy is also given. Finally, the relative merits of using various poin ted indenters for determining the fracture toughness and residual stre sses in brittle materials are discussed.