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

This article presents the results of an investigationof crack nucleation and propagation in atransparent polydimenthylsiloxane (PDMS) elastomer.The main objective of the investigation is to characterizequantitatively the evolution of crack nucleationand propagation behavior not just through the usualmacroscopic load and displacement data, but with synchronizedoptical images at high spatial and adequatetemporal resolution that will resolve the evolution ofthe failure processes. This is augmented with x-ray computedtomography (CT) scans to characterize the threedimensionalgeometry of the cracks nucleated in the interiorof the elastomer.Towards this goal, we reproduce the classical pokerchipexperiment of Gent and Lindley (1959) in whichthe specimen’s diameter-to-thickness ratio is varied overa broad range to cover crack nucleation, propagation,and their coalescence. These experiments are performedon transparent PDMS with different compositions, firstin a specially built loading machine that is fitted witha high magnification microscopic camera that permitsthe measurement of the load while simultaneously providingimages of the specimen configuration and subsequentlyin an apparatus built for in situ observationsusing an x-ray CT scanning system.These experiments reveal that nucleation of multiplemicrocracks dominates when the diameter-to-thicknessaspect ratio α is sufficiently large, because the incompressibilityof the material induces substantial, nearlyuniform hydrostatic tension in the specimen. In contrast, specimens with smaller aspect ratio tend to nucleatefewer cracks, and are dominated by the growth ofthese cracks. At even smaller α, the hydrostatic stress issignificantly lowered and failure is dominated by surfaceflaws. The three-dimensional geometry, and the spatialdistribution of the nucleated cracks were evaluated usingoptical microscopy and x-ray CT scans. This revealedcracks of three different shapes, one of which wasconfined in a layer near to the upper or bottom boundaryof the poker-chip, another was across the thickness,but with a tilt relative to the axis of the specimen, andthe last was propagating along the radial direction.