Gregory B. McKenna and Martin Y.M. Chiang, Materials Science and Eng. Lab.
Robert S. Chambers, Sandia National Laboratories, Albuquerque
Timothy J. Burns, ACMD
When glass-forming materials (polymers, inorganic glasses) are cooled from a temperature which is above their glass transition temperature to one which is below it, they remain in a nonequilibrium thermodynamic state, from which their properties (thermodynamic, mechanical, dielectric, etc.) evolve slowly towards equilibrium. This process, which is called physical aging, must be taken into account in order to predict accurately the long-term performance of glass-forming materials, especially in composite systems such as glass-to-metal seals and electronic packaging. There are a number of nonlinear viscoelastic constitutive laws which have been proposed as models for the physical aging process in solid polymers and glasses in composite systems over a range of the stress, strain, and temperature fields which occurs during the processing of these materials near the glass-transition temperature. In particular, there are several models which incorporate the concept of a material clock, i.e., a time-like variable which depends on stress, volume, strain, or temperature. The goal of this project is to find a model of this class which does an acceptable job of predicting creep and relaxation in specific materials of interest to U.S. industry. Once this is done, the model will be incorporated into a Sandia finite element solid dynamics code, and simulations of the mechanical behavior of composite systems which include glass-forming materials will be performed. Attention over the past year has been focused on convincing managers in Sandia's Technology Transfer Program in Engineering Sciences to commit scarce R&D funds to enable Chambers to participate in this collaboration. Recently, this effort has been successful, and Chambers now has three years of internal Sandia funding to work part-time on this project. In addition to gaining from Chambers' experience in the modeling of polymer mechanics, this relationship will enable NIST staff members to have access to Sandia's extensive finite-element software libraries.