On crack nucleation and propagation in elastomers: I. In situ optical and X-ray experimental observations

Abstract

This article presents the results of an investigation of crack nucleation and propagation in a transparent polydimenthylsiloxane (PDMS) elastomer. The main objective of the investigation is to characterize quantitatively the evolution of crack nucleation and propagation behavior not just through the usual macroscopic load and displacement data, but with synchronized optical images at high spatial and adequate temporal resolution that will resolve the evolution of the failure processes. This is augmented with X-ray computed tomography (CT) scans to characterize the three-dimensional geometry of the cracks nucleated in the interior of the elastomer. Towards this goal, we reproduce the classical poker-chip experiment of Gent and Lindley (Proc R Soc Lond A 249(1257):195–205, 1959) in which the specimen’s diameter-to-thickness ratio is varied over a broad range to cover crack nucleation, propagation, and their coalescence. These experiments are performed on transparent PDMS with different compositions, first in a specially built loading machine that is fitted with a high magnification microscopic camera that permits the measurement of the load while simultaneously providing images of the specimen configuration and subsequently in an apparatus built for in situ observations using an X-ray CT scanning system. These experiments reveal that nucleation of multiple microcracks dominates when the diameter-to-thickness aspect ratio $\alpha$ is sufficiently large, because the incompressibility of the material induces substantial, nearly uniform hydrostatic tension in the specimen. In contrast, specimens with smaller aspect ratio tend to nucleate fewer cracks, and are dominated by the growth of these cracks. At even smaller $\alpha$, the hydrostatic stress is significantly lowered and failure is dominated by surface flaws. The three-dimensional geometry, and the spatial distribution of the nucleated cracks were evaluated using optical microscopy and X-ray CT scans. This revealed cracks of three different shapes, one of which was confined in a layer near to the upper or bottom boundary of the poker-chip, another was across the thickness, but with a tilt relative to the axis of the specimen, and the last was propagating along the radial direction.