EXPERIMENTAL STUDIES OF THE MECHANICAL BEHAVIOR OF A PVA DUAL-CROSSLINK HYDROGEL
This study aims to understand the mechanical behavior of a PVA dual-crosslink hydrogel using both experimental and theoretical tools, with more focus on the experimental aspects. This PVA hydrogel is crosslinked by both chemical (permanent) and physical (transient) crosslinks. The chemical crosslinks remain attached during loading while the physical crosslinks can break and reform, leading to the viscoelasticity. This material serves as a model system for understanding the behavior of such materials. Firstly, we studied the effects of temperature and loading rates on the mechanical response of this material. The breaking and reforming rates of the physical crosslinks are sensitive to the temperature change. Both large strain uniaxial tension tests and small strain torsional rheometry tests were performed at different rates in a temperature range of 13 to 50 Celsius. The rheometry data show that time-temperature superposition can be used to condense the data to a master curve. It is found that by allowing the model parameters to be temperature dependent, a previously developed constitutive model fits the tension and rheometry data well. Horizontal (time) and vertical (amplitude) shift factors calculated directly from the rheometry test data and from tension test data agree well with each other, establishing the connection between the effects of temperature on different types of experiments. Secondly, we developed a 2D digital image correlation (DIC) system to measure the deformation near the crack tip of a cracked specimen. Experimental details such as how to prepare a durable speckle pattern on a material that is 90% water are discussed. DIC is used to measure the strain field in tension loaded samples containing a central hole, a circular edge notch and a sharp crack. These experiments are modeled using the finite element method (FEM). Excellent agreement between FEM and DIC results for all three geometries suggests that the DIC measurements are accurate up to strains of over 10, even in the presence of very high strain gradients near a crack tip. The method is then applied to verify a theoretical prediction that the deformation field in a cracked sample under relaxation loading, i.e. constant applied boundary displacement, is stationary in time even as the stress relaxes by a factor of three. We further utilized DIC to study the crack propagation of this PVA gel. We show that the crack propagates in steady state. The moving crack induces a very high strain rate ahead of the moving crack tip, which leads to high stress that causes the fracture of the material. Thirdly, we developed a predictive fracture criterion for this material. We loaded the specimen with an edge crack to fracture under two loading conditions: constant applied stress (creep) and constant stretch rate. The stress fields near the crack tip were simulated using finite element method (FEM) and a stress-intensity-factor-like crack tip parameter was obtained. Using this parameter in a kinetic fracture model in which the rate of bond breaking depends exponentially on the stress level, results from creep fracture tests are used to develop a fracture criterion that is then applied to predict failure under constant stretch rate loading.
Fracture; Mechanical engineering; Mechanics; digital image correlation; large deformation; viscoelastic; hydrogel
Zehnder, Alan Taylor
Hui, Chung-Yuen; Warner, Derek H.
Ph.D., Mechanical Engineering
Doctor of Philosophy
dissertation or thesis