A novel indentation based method to determine the threshold stress intensity factor for sub-critical crack growth in glass

A new indentation based approach is proposed to measure the threshold stress intensity factor, K-th (static fatigue limit) in glass. The method involves indenting glass samples and then ageing them in air until no further crack growth is observed at room temperature. 50% of the samples are then annealed and tested in bending while the other 50% are tested in bending without annealing. In the former case failure is spontaneous, whilst in the latter it is activated. In all cases the crack lengths are measured prior to testing and, although the actual crack length at failure in the latter case is not readily accessible, by combining the analyses of the two situations it is shown that K-th can be obtained from the measured failure stresses and the crack lengths prior to failure. The K-th values of soda-lime-silica glass and E-glass were found to be 0.14 +/- 0.02 MN m(-3/2) and 0.17 +/- 0.01 MN m(-3/2), respectively.