ABSTRACT
It was necessary to learn how to deal with time-dependent properties in the materials, to discover which properties were controlling and controllable, to learn how to measure such properties, and to understand how to handle them in engineering design. What was needed was a rather ambitious undertaking, namely the construction of a fully three-dimensional theory that obeyed the laws of thermodynamics and that went beyond the assumption commonly known as linearity. The paper Stress Relaxation with Finite Strain, published in 1962 by Bernstein, Kearsley, and Zapas, was a major milestone in meeting this challenge. Essentially, a strain is a change in a length divided by a reference length. For linearly elastic models, the strain is simply proportional to the stress that produces it. For nonlinear models, such simple and unique proportionality does not occur. That was the fundamental problem for models of synthetic rubber and polymers.
