Prefracture instabilities govern generation of self-affine surfaces in tearing of soft viscoelastic elastomeric sheets

Abstract

We present an experimental study on fracture behavior of soft viscoelastic network solid films of polydimethylsiloxane (PDMS) and morphology of the fracture-generated surfaces. The most interesting behavior is the generation of fractal patterns when the storage and loss moduli are comparable. We find the formation of isolated defects and cracks ahead of the fracture-front. The observed morphology of the torn surfaces is correlated to the prefracture behavior by studying the crack propagation and coalescence. Tearing of PDMS containing 2% cross-linking agent with a terminal elastic modulus, G′, of 10 kPa, shows distinct crack patterns at different tearing speeds. Slower tearing leads to the formation of dendritic patterns of propagating fingers, whereas at the higher deformation-rates, more frequent nucleation of bubble like defects occurs. The propagating fingers develop undulations along its sides. The effects of tearing speed on the length scale of the undulations and the bubbles are studied. The formation of undulations can be explained as a result of Saffman–Taylor instability with a modified effective surface tension which accounts for the dissipative energy loss. We also study the rate of formation of the defects, which is shown to be an activated process in that they form beyond a threshold value of the local stress.