CRACK INITIATION BY INDENTATION FATIGUE IN LEAD ALKALI AND SODA-LIME GLASS

An indentation technique using a conventional Vickers microhardness te ster was used to evaluate fatigue properties of lead-alkali and soda-l ime silica glasses. The specimens were indented repeatedly at the same point with subcritical loads until radial cracks were initiated. The number of cycles to initiate the cracks at different subcritical loads demonstrated typical fatigue curves for both glasses. The uniqueness of the experiment was that the diagonal lengths of the deformed cavity were observed to increase with the number of cycles. This increase of the deformed cavity for a certain number of cycles prior to the visib ility of crack initiation was analyzed, correlating the elastic-plasti c phenomenon and the accumulation of the residual stress in each cycle .