The sub-critical indentation fracture process in soda-lime-silica glass

Abstract

The sub-critical propagation of indentation cracks in soda-lime silicate glass was investigated. Vickers indentations were produced using a force of 9.8 N, and both as-indented and annealed samples were studied. Indented bars were subjected to various flexural loading and unloading cycles in a water environment. Fractographical analysis revealed that the radial crack propagation process could be divided into two successive stages. Initially, the radial crack growth is strongly influenced by interaction with the lateral crack, which obstructs crack growth. This “pinning” effect of the lateral crack leads to bowing of the radial crack front into a complex shape. In the second stage, the radial cracks escape from the lateral cracks interaction and assume a semi-elliptical shape. Measurement of radial crack speed from the markings left on the fracture surface allowed the crack shape factor, ψ and the indentation-residual stress factor, χ, to be calibrated as a function of the crack size, c. The shape factor was found to be a decreasing function of c during the first stage of propagation, then it abruptly increases when radial cracks escape from lateral crack interaction. For further crack extension, ψ decreases again, assuming absolute values larger than those measured during the first stage. Conversely, the residual stress factor is constant during the first phase of radial crack growth while it suddenly decreases and becomes zero for larger c. The implications of the trend in ψ( c) and ψ( c) on the stable growth and on the time-to-failure predictions in static fatigue tests are discussed.