Cumulative Damage in Creep-Rupture Tests of a Carbon Steel

Abstract

A life-fraction rule is often used to predict the rupture life of a piece of elevated-temperature equipment exposed to several temperature and stress conditions. It is based on the assumption that the fraction of rupture life used up in a specific time at one stress and temperature is independent of previous conditions and therefore can be estimated from tests in which exposure conditions were held constant. Test data are needed to substantiate the validity of this rule. The results of 23 constant-stress creep-rupture tests on an ASTM A201A carbon steel have shown this rule to be valid when all of the test conditions caused appreciable creep strain. Even so, it may not be valid for prolonged service above 1100 F followed by service at lower temperatures. Creep rates are reasonably predictable for constant-stress conditions when only temperature is changed. The transient creep rate accompanying a change in stress continues long enough to make creep-rate predictions unreliable. Periodic shutdowns during tests at constant stress and constant temperature have little effect on either rupture life or creep rate.