Nanoindentation technique for measuring residual stress field around a laser-induced crack in fused silica

A nanometre scale indentation technique using microprobe indentations to measure residual stresses at selected positions near u.v.-laser-induced cracks in fused silica is presented. The approach is based on the observation that the nanoindentations' penetration depths are affected by the residual stress field emanating from the laser-induced crack. A simple theoretical model based on the change of the nanoindentation penetration depth as well as the change in Young's modulus and hardness of the material is derived. The results show good agreement with the inclusion model [15] suggesting that the residual stress field around a laser-induced crack in fused silica is of shear nature. An exploratory test made on an unstressed sample (free of a laser-induced crack), yielding values for Young's modulus and hardness in accordance with handbook values, shows the high accuracy of this nanoindentation diagnostic.