Time-Dependence in Materials

Abstract

The last ten or fifteen years have seen some highly significant changes in what needs to, and what can be, achieved in the area of the characterization of materials with time-dependent mechanical properties. On the one hand we have seen a tremendous increase in the use and variety of materials with time-dependent properties such as composites and semi-crystalline engineering materials, as well as the development and application of multiphase materials (e. g., block and graft copolymers, and polyblends), requiring new or improved methods of characterization. On the other hand, there have been notable advances in the availability and sophistication of state-of-the-art sensors, and of methods of data acquisition and manipulation. It appears that the need for better characterization has not been universally recognized, and that, consequently, the possibilities for implementing it have not been fully exploited. The lecture examines these issues and suggests some new-old approaches. The mechanical properties (and other properties: dielectric, optical, etc.) of ALL materials are time-dependent. This is simply a necessary consequence of the Second Law of Thermodynamics according to which a part of the energy of deformation is always dissipated by viscous forces even while the rest is stored elastically. Time-dependent behavior may range from virtually purely elastic to virtually purely viscous behavior. Polymeric materials are typically viscoelastic and thus fall in-between. As a consequence their mechanical properties must be described by time- (or, equivalently, frequency-) dependent material response functions, not elastic or viscous constants.