The stress field near the tip of a plane stress crack in a gel consisting of chemical and physical cross-links

We study the time-dependent asymptotic stress fields near the tip of a mode I plane stress crack in a hydrogel. The analysis is based on a three-dimensional continuum model which describes the viscoelastic behaviour of a hydrogel gel with permanent and transient cross-links. The viscoelasticity results from the breaking and healing of the transient cross-links in the gel network. We show that the crack tip fields satisfy a local correspondence principle—that is, the spatial singularities of these fields are identical to a hyperelastic cracked body with the same but undamaged networks. Asymptotic results compare very well with finite-element simulations on a single-edge crack specimen loaded under constant stretch rate. We also compare the theoretical results (crack opening profile and crack tip strain field) with experiments and find excellent agreement.

Optimization of the Charging Process for Dielectric Elastomer Generators

Energy harvesting using dielectric elastomers is an upcoming possibility to convert ambient energy into electric energy. So far energy harvesting cycles are known and described, in which the charging and discharging of the polymer is realized during a constant stretch phase. In real applications a continuously changing stretch must be assumed, so that the time frames of the charging- and discharging-intervals have a considerable influence on the amount of harvested energy. The contribution of this paper is related to the calculation of the optimal charging- and discharging-intervals to maximize the energy gain. Therefore the physical model of a lossy generator is investigated to derive the converted energy as a function of the timing of the charging- and discharging-interval. Afterwards the optimization is carried out for the most significant harvesting cycles with constant electric field and constant charge. The achievable energy gain of these optimized harvesting cycles are compared to each other and control laws for the realization of harvesting cycles are quoted.