Crack propagation in a PVA dual-crosslink hydrogel: Crack tip fields measured using digital image correlation

As a non-contact, full field deformation measurement technique, 2D digital image correlation (DIC) is a good candidate for the measurement of the crack tip fields of gels. We apply 2D DIC to study the crack propagation of a Poly(vinyl) alcohol (PVA) hydrogel specimen immersed in mineral oil. Practical issues such as the quality of the speckle pattern and the possible optical distortion are discussed. The accuracy of the setup in measuring the high-gradient crack tip strain field is carefully validated. A hydrogel specimen with an existing crack is loaded to fracture and the strain field near the tip of the propagating crack is measured using DIC. We find that the crack propagates under steady state conditions. The moving crack induces high strain rates at material points ahead of the crack.