Abstract
An experimental study of the indentation cracking behaviour of thermally tempered soda-lime glass flats is described. The indenters were steel and tungsten carbide spheres of radii in the range 0·5 to 2·5 mm and a Vickers diamond pyramid. They were loaded normally on to the flats and particular attention was paid to the evolution (i.e. initiation and growth) of the Hertzian cone cracks formed under the spherical indenters and to the median cracks which were generated under the Vickers indenter.
With the spherical indenters, the Hertzian ring‐cone cracks have been shown to initiate most frequently at the circle of contact between the indenter and the specimen surface. Moreover, with increasing indenter load, the skirt of the cone crack has been found to extend almost parallel to the indented surface; this is shown to be consistent with the principal stress trajectories within the tempered glass. The implication of such cone cracks for the strength degradation of the tempered glass is discussed.
Within the range of spherical indenter sizes studied, it has been found that in the tempered glass the critical load required to cause a ring-cone fracture is proportional to the indenter radius. A comparison of these observations with those from the same glass specimens, but this time in an annealed state, has also been made. By employing a simple analysis, the surface compressive stress in the tempered glass has been determined to be 280 ± 180 MPa.
Under the Vickers indenter, a full penny-shaped median crack has been found to form in the tempered glass at a sufficiently high load. During loading, the shape of this crack remains approximately unaltered with increasing indenter load. During unloading, the penny-shaped median crack becomes like a half-penny. On the other hand, in the annealed glass, the penny-shaped median crack clearly changes into the half-penny shape with increasing indenter load during the loading cycle. Also, in both types of glass shallow radial cracks have been observed to initiate during loading as well as during unloading.
From measurements of the surface extent of the median-radial cracks in the thermally tempered glass and in the annealed glass, a second method of analysis has also been employed to estimate the magnitude of the surface compressive stress in the former. This method has given a value of 60 ± 5 MPa for the compressive stress, which is considerably smaller than that determined using the spherical indenters. A brief discussion of the possible reasons for the discrepancy is given.
