Creep rupture tests data extrapolation

The need often arises for engineering creep data that are impractical to collect from normal laboratory tests. This is especially true for prolonged exposures (on the order of years). One solution to this problem involves performing creep and/or creep rupture tests at temperatures in excess of those required, for shorter time periods, and at a comparable stress level, and then making a suitable extrapolation to the in-service condition. A commonly used extrapolation procedure employs the Larson-Miller parameter, m, defined as... 

Consult or plot, from data available in the creep chart, the creep modulus curve of the material of interest for the temperature at which the part will be used, extrapolating where necessary. Like creep rupture, creep modulus varies greatly with temperature. In addition it is subject to another variable—stress level. For each material grade, creep modulus data are tabulated in the creep chart, first by test temperature and second by applied (test) stress. For very rigid plastics, such as thermosets, pilled thermoplastics, and amorphous thermoplastics at room temperature and below, the creep modulus curves show minor variation with applied stress. However, for the more flexible and ductile plastics, the creep modulus curves will vary significantly and systematically with stress level. The higher the stress, the lower the creep modulus. This is a consequence of viscoelasticity. 

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