Mechanical Instability in Soft Materials
- Author(s): Liang, Xudong
- Advisor(s): Cai, Shengqiang
- et al.
We are surrounded by soft materials in a variety of physical and chemical states, which can be easily deformed under external stimuli. When subjected to sufficiently large compression, electric voltage, gravity or impact, soft materials may undergo mechanical instabilities of various types. The instability modes can be either linear or nonlinear, depending on the form of perturbation when the instability set in. When the material is a pressurized dielectric elastomeric film under high voltage, snap-through instability is linear with finite change of volume, while the bulge-out mode is nonlinear with a localized deformation. In terms of surface instability, wrinkles are linear instability mode with undulations finite in space with infinitesimal strain deviating from the smooth state, while creases are localized nonlinear modes with large strain deviating from smooth state. If a soft material is subjected to high speed impact, both the viscoelastic behaviors of the material and inertial effect are involved, and the mechanical instability is coupled with the wave propagation, finally leading to highly nonlinear instability mode.
We start with the instability analysis of a pressurized dielectric elastomeric film subjected to high voltage. By adopting ideal dielectric elastomer (DE) constitutive model, we show that linear perturbation analysis can capture the shape bifurcation in a spherical DE balloon. However, nonlinear bulge-out shape with a highly localized deformation appears as constraints of the boundaries of the film is applied. A competition between the surface instability modes between the wrinkle and crease is studied in both experiment and theoretical analysis under a deformation mode called eversion, and crease is shown to form prior to wrinkle with lower critical strain to set in. A transition between the wrinkle and crease instability happens when gravity becomes important. We measure the dynamics of soft elastomeric blocks with stiff surface films subjected to high-speed impact, and observe wrinkles forming along with, and riding upon, waves propagating through the system.