Structures designed for extreme environments must be designed not only for the magnitude of the load that they will experience, but also time for which that load acts upon them. At the core of the problem lies the loading impulse experienced by materials and the operating deformation mechanisms that are excited. Our experience of materials' physics, gathered by investigating response to mechanical loads, has suggested a series of descriptive constructs within which we build our picture of behaviour. This paper suggests a framework by which to interpret data collected on the response of metals. It suggests that strength is a quantity that decays over time and that fundamentally approaches zero in the limit of infinite time. Controlling this decay is the business of engineering, to design structures that will survive in the environments for the time windows of importance. © 2011 Elsevier Ltd. All rights reserved.