162 Technical modeling and performance metrics

To construct a technical model, performance metrics Pi, are identified and evaluated for the foam and for competing materials or systems. A performance metric is a measure of the performance offered by the material in a particular application. In minimum weight design the performance metric is the mass: the lightest material which meets the specifications on stiffness, strength, etc. is the one with the greatest performance. In design for energy-mitigation, in which it is desired that a protective packaging should crush, absorbing a specified energy, and occupy as little volume as possible, the performance metric is the volume. In design to minimize heat loss, the metric might be the heat flux per unit area of structure. In design for the environment, the metric is a measure of the environmental load associated with the manufacture, use and disposal of the material. Each performance metric defines an objective: an attribute of the structure that it is desired to maximize or minimize. Models are developed for the performance metrics following the method of Chapter 5; the resulting performance equations contain groups of material properties or 'material indices'. The material that maximizes the chosen performance metric has the highest technical merit. But for this to be a viable choice, the performance must be balanced against the cost.

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