Indentation fracture of silicone gels

Indentation tests were performed, using a flat punch probe, on silicone gels to induce failure under compression. The silicone gels were formed from networks of vinyl-terminated polydimethylsiloxane (PDMS) with molecular weights of 800 and 28,000 g/mol and a sol fraction of trimethylsiloxy-terminated PDMS with molecular weights ranging from 1250 to 139,000 g/mol. Cone cracks were observed in samples that fractured from defects at the sample surface, but failure more commonly originated from the corners of the indenter. Ring cracks were observed for the most highly compliant samples that fractured at indentation depths approaching the overall thickness of the sample. In these cases we generally observed a delayed fracture response, with a time delay that increased with increasing sol fraction and decreased with increasing indentation load.