Tuesday  |  Salon 11  |  10:20 AM–10:40 AM

#16090, Experimental Calibration of a Thermomechanics Constitutive Model

A thermodynamics based, nonlinear viscoelastic constitutive model is presented. This thermomechanics model monitors the change in free energy from mechanical deformation from strain history. Stress is then the derivative of energy with respect to strain. One particular challenge of the approach is experimentally extracting the volume specific free energy function, because the function is based on internal state variables that can not be measured directly.

To capture strain induced orthotropy from initially isotropic materials, the free energy function is split into distortional and dilatational components, all of which are defined in terms of principal strains. The 3 distortional contributions require a novel shear strain definition. The 3 dilatational contributions involve a novel concept of orthotropic dilatation, where isotriaxial volume change is related to three principal stresses instead of simply to hydrostatic pressure.

The basic forms of the free energy functions can be gleaned from Materials Science. For example, the Lennard-Jones potential motivates the shape of the dilatation contribution. Gent hyperelasticity can motivate the distortional contributions. Theoretically predicting the basic shapes of these functions reduces the number of fitting parameters.

Three steps are required to experimentally extract the free energy function. First, a novel uniaxial dynamic mechanical analysis provides a thermodynamically self-consistent set of bulk and shear modulus master curves. Second is a series of ramp tests to failure. Materials fail in either dilatation through some sort of cavitation, or they fail in distortion through a form a slip. If the uniaxial failure reveals dilatational failure, a distortion dominated pure shear Arcan is used to measure the large strain dilatation energy function. It uniaxial tests fail in distortion, a dilatation dominated test provides the information needed to extract the dilatational contribution. This test could be tension on an adhesively bonded thin layer.

Alex Arzoumanidis   Psylotech, Inc.