The thermal acceleration method involves laboratory testing at temperatures much higher than the actual service temperature expected. The data are plotted as stress versus time for a family of constant temperatures where the creep strain produced is constant for the whole plot. It is important to recognize that such extrapolations are not able to predict the potential of failure by creep rupture prior to reaching the creep life design. In any testing method it should be noted that creep-testing guidelines usually dictate that test periods of less than I of the expected life are not deemed to give significant results. Tests extending to at least 10% of the expected life are preferred where feasible. Several different theories have been proposed to correlate the results of short – time elevated-temperature tests with long-term service performance at more moderate temperatunes. One of the more accurate and useful of these proposals is the Lusen-Miller theory.
The Larson-Miller theory3? postulates that for each combination of material and stress level there exists a unique value of a parameter P that is related to temperature and time by the equation P = (B+460)(C+logo t) (19) where P = Larson-Miller parameter, constant for a given material and stress level B = temperature, 0F C = constant, usually assumed to be 20 J = time in hours to rupture or to reach a specified value of creep strain This was investigated for both creep equation and rupture for some 28 different materials by Larson and Miller with good success. By using (19) it is a simple matter to find a short term combination of temperature and time that is equivalent to any desired long term service requirement. For example, for my given material at a specified stress level the test conditions listed in Table 3 should be equivalent to the operating conditions. We provide Professional technology, excellent product quality and intimate after-sales service when you purchase Vibratory feeder, Rotary kiln, from our company.