Slow change in the dimensions of a material from prolonged stress. Most common metals exhibit creep behaviour. In the creep test, loads below those that ordinarily cause plastic flow or fracture are applied to the material, and the deformation over a period of time (creep strain) under constant load is measured, usually with an extensometer or strain gauge. Time to failure is also measured against stress. Once creep strain versus time is plotted, various mathematical techniques are available for extrapolating creep behaviour beyond the test times; thus, designers can use thousand-hour test data, for example, to predict ten-thousand-hour behaviour. See alsotesting machine.