Parameter Identification and Validation of Shape-Memory Polymers within the Framework of Finite Strain Viscoelasticity

Abstract

Shape-Memory Polymers (SMPs) can be stretched to large deformations and recover induced strains when exposed to an appropriate stimulus, such as heat. This emerging class of functional polymers has attracted much interest and found applications in medicine and engineering. Nevertheless, prior to any application, their physical and mechanical properties must be thoroughly studied and understood in order to make predictions or to design structures thereof. In this contribution, the viscoelastic behavior of a polyether-based polyurethane (Estane) and its rate- and temperature-dependent behavior have been studied experimentally and by the mean of simulations. The model-inherent material parameters are identified with the assumption of the thermo-rheological complexity. Here, the numerical results of uni-axial stress relaxations were compared with the associated experiments in conjucation with the Levenberg-Marquard optimization method to determine the parameters of the Prony equation. The ability of the model to simulate the thermo-mechanical properties of Estane was evaluated by data-rich experimental observations on tension and torsion in various temperature ranges. Heterogeneous tests are included into the experimental program to cover a broader spectrum of loading scenarios.

Cardiovascular Stents: A Review of Past, Current, and Emerging Devices

“…Specifically, in the case of shape-memory alloys (e.g., nitinol), there is a reversible transformation from the austenite phase to the martensite phase, which takes place over a specific temperature range depending on the alloy composition [157][158][159]. Similarly, polymeric smart materials that recover from a deformed state to their original shape under external stimuli have also been considered [160][161][162]. Stents from shape-memory polymers (e.g., poly(tert-butyl acrylate) and poly(ethylene glycol) dimethylacrylate) could be manufactured to preserve shape storage at ambient temperature and become fully activated at body temperature [156].…”

Creep relaxation in nonlinear viscoelastic twisted rods

“…The large strain kinematics is specified based on the Sidoroff multiplicative decomposition [1] of the total deformation gradient 𝐅 into elastic 𝐅 𝑒 and inelastic 𝐅 𝑐 parts [12] 𝐅 = 𝐅 𝑒 𝐅 𝑐 = (𝐕 𝑒 𝐑 𝑒 ) (𝐑 𝑐 𝐔 𝑐 )…”

“…The large strain kinematics is specified based on the Sidoroff multiplicative decomposition [1] of the total deformation gradient F into elastic $${{{\bf F}}}^e$$ and inelastic $${{{\bf F}}}^c$$ parts [12] $$\begin{equation}{{\bf F}} = {{{\bf F}}}^e{{{\bf F}}}^c = \left( {{{{\bf V}}}^e{{{\bf R}}}^e} \right)\left( {{{{\bf R}}}^c{{{\bf U}}}^c} \right)\end{equation}$$…”

“…Nevertheless, the obtained results make it possible to calculate the relaxation of the torque, as well as the axial force (second-order effects) for an arbitrary creep law and elastic potential with low computational costs. This is important in the context of utilizing the optimization algorithms to identify material parameters in the creep laws, since when using these algorithms it is necessary to repeatedly calculate the discrepancy between the real (experimentally observed) and calculated material response [1,11].…”

“…Torsion is a widespread material testing method. In particular, it serves to determine the constitutive equations for creep of different materials, including those that are capable to sustain large deformation [1]. Torsion of a thin hollow cylinder results in a stress state close to homogeneous.…”