Experimental Mechanics for Prognosis of Material State Changes in Heterogeneous Materials for Energy Systems

Abstract

The concept of prognosis is typically discussed in terms of mechanical characteristics such as structural integrity, durability, damage tolerance, fracture toughness, etc. These familiar concepts are usually addressed by considering balance equations, crack growth relationships, and constitutive equations with constant material properties, and constant or cyclically applied load conditions. Loading histories are represented by changing stress (or strain) states, only. But for many situations, especially associated with high performance engineering structures, the local state of the material may also change during service, so that the properties used in those equations are functions of time and history of applied conditions. But for many energy systems, a broader definition is required. For example, in fuel cells, properties such as conductivity and electrochemical character are altered by material degradation, so that “property fields” replace the global constants in multiphysics balance equations, and time and history enter into the governing equations. The present paper will examine a small set of such problems which involve novel experimental methods of following the accumulation of distributed damage and changes in material state. Specifically, the paper discusses this problem in the context of material state changes measured by impedance variations as a method of following and interpreting those changes in terms of functional performance. The application of these concepts is extended from mechanical structures to energy systems, wherein the material state changes result in variations in electrochemical properties that directly control functional performance of devices such as fuel cells.