Application of Acoustic Emission Technique to Identifying Median Crack Initiation (Part 1): In Case of Soda-Lime Silicate Glass

Crack initiations in soda-lime silicate glass during Vickers indentation were observed with an optical microscope, and the accompanied acoustic emissions (AE) were measured to evaluate KIC by the indentation micro-fracture (IM) technique. A large amount of small amplitude AE, which corresponds to the surface cracks, was detected during indentation load increment. When the load exceeded about 10kgf, penny shaped median type cracks initiated accompanied by large amplitude AE, the wave of which was composed of longitudinal wave, the reflection at the side wall of the wave guide and transversal wave. Just before the load reaching zero, lateral type cracks initiated accompanied by the large amplitude AE, in the wave of which clear longitudinal wave could not be observed. When the load was decreased before median crack initiation, the median crack initiated at the load decrement. The amplitude of the longitudinal wave (ξ) increased with increasing maximum indentation load (P). The amplitude increased also proportionaly to the crack length, which means that the energy of AE increases proportionally to the crack area.