5. An Experimental Method to Induce and Measure Crack Propagation in Brittle Polymers with Heterogeneities

Facture mechanics of heterogeneous brittle solids is a field of active research due to the recent developments in additive manufacturing to fabricate components with complex engineered microstructures. The majority of experimental work in crack propagation uses data from a single plane, usually on free surfaces, to measure the displacement field around the crack and the crack tip location. These measurements are used to determine the crack tip fields and fracture toughness which provides insights about the failure of a material. However, it is well known from three dimensional theory and experiments that the crack front shape and stress distribution is not constant through the thickness of a specimen. When toughening heterogeneities are added to a material, the theories and mechanics become significantly more complex. To better understand these stresses and shapes for both homogeneous and heterogeneous materials, an experimental method has been developed to induce steady-state crack propagation in thin, brittle hydrogel polymers. A microfilament needle inserted into the specimens allows for fluid to enter the crack and exert pressure on the crack surface, which effectively wedges the crack open. Distributed fluorescent microspheres serve as a speckle pattern for Digital Volume Correlation (DVC) of volumetric images captured using confocal microscopy. The DVC displacement field allows for determination of the 3D crack tip fields. This study seeks to provide an enhanced understanding of the three dimensional nature of crack interactions with heterogeneities and renucleation events, which can significantly improve our ability to design material toughness.