Congratulations to Dr. Jingkai Guo for his successful dissertation defense, “Modeling the Mechanical Behavior of Soft Active Materials“.  Abstract below.

Soft active materials are capable of converting chemical or physical energy into mechanical work in response to various environmental stimuli such as pH, solvent, salt, heat, humidity, electric or magnetic field, and light. These materials are generally inexpensive, lightweight and their properties can be tailored based on requirements of applications. These attractive features give them great potentials in a wide range of applications, such as biomimetic systems inspired from living organisms. The objective of this work was to develop theoretical models that are capable of accurately describing the physical- mechanical behavior of soft active materials. A common set of methods that can be applied to the study of wide range of soft materials was developed. The methods covered different stages of the study, from material characterization to constitutive modeling, from simulation to experiments.

In this work, I focused on three kinds of soft active materials: dielectric elastomers, thermoresponsive gels and shape memory polymers. In the first part of this work, we investigated the temperature dependent viscoelastic behavior of dielectric elastomers and the effects of viscoelasticity on the electro- actuation behavior. We measured the viscoelastic relaxation spectrum of VHB 4905 and applied the results to a discrete multi-process viscoelastic model. The model generally showed good quantitative agreements with experimental measurements in terms of both pure-mechanical and electro-mechanical behaviors of the material. The model was able to qualitatively capture the dependence of the electric breakdown time on voltage and pre-stretch. In the second part of this work, we studied the thermoresponsive bilayer plates with soft and stiff segments that exhibit bidirectional and biaxial curving. We investigated the mechanism underlying the deformation modes through finite element simulation and explored the effects of geometry factors such as the aspect ratio and the segment spacing. In the final part of this work, we developed a constitutive model based on the effective temperature theory that is capable of describing strain hardening behavior in a thermodynamically consistent manner. The model incorporated two mechanisms: one represents the stretching and orientation of the polymer network, which leads to the development of a backstress; the other one represents the molecular reptation, which accounts for the temperature and rate dependence of strain hardening. The model was implemented into finite element programs and was applied to simulate the thermomechanical behavior of polycarbonate. All of the model parameters were determined through standard thermomechanical tests. The simulation results showed good agreements with experiments and the temperature dependence, strain rate dependence and strain state dependence of hardening were quantitatively captured.