Damage in elastomers: nucleation and growth of cavities, micro-cracks, and macro-cracks

The nucleation of internal cavities and their transition to cracks are examined at high spatial and temporal resolutions within polydimethylsiloxane (PDMS) elastomers of various cross-link densities under externally applied quasi-static mechanical loads. The focus here is on experiments where the initiation and propagation of internal damage are designed to occur in between two spherical glass beads that are firmly embedded within a matrix of the PDMS elastomer and are placed close to each other in order to generate a high triaxial stress state. An optical microscope is used to monitor the various processes of nucleation and growth of cavities and cracks at a spatial resolution of about 1μm and a temporal resolution of about 66.7 ms. In combination with corresponding full-field simulations, the experiments show that the nucleation of cavities—that is, the onset of cavitation—is an extremely fast process (involving stretch rates in excess of 100s-1) that is controlled primarily by the stiffening at large deformations of the underlying elastomer and, more critically, by its fracture properties. The experiments and simulations also show that cavitation is followed by two distinct events upon further macroscopic loading: the transition of the nucleated cavities to micro-cracks, and the further transition of some micro-cracks to macro-cracks. These two distinct events are also controlled primarily by the fracture properties of the underlying elastomer.