The Properties of Metals: A Study of the Influence of Time Effects on the Tensile Strength at High Temperatures

This is an investigation on tensile testing dealing with the serious weakening effect which occurs at elevated temperatures and which is only realized when the metal is stressed for a prolonged period. The extension of the metal in these prolonged tests has been designated "Creep" or "Flow". The rate at which the metal extends may be scarcely detectable but if it persists, it will ultimately cause failure. The stress, if any, at which this creep ceases is known as the "Creep Limit Stress." The most direct methods for estimating this stress are very tedious and an effort has been made to materially shorten the test. A consideration of evidence given by various investigators, leads to certain definite conclusions, and from these is devised a development of the "Progressive Stress Test" which reduces the time to determine the limit stress to about a fortnight. Certain prolonged tests on wires at normal temperature, supply further evidence of this time effect. They also show that there are only two phases of creep. An "Initial Creep" which represents the gradual stabilising of conditions within the metal, if the final condition is perfect stability the creep ceases, giving the creep limit; but if the final condition is instability, the second phase "Final Creep" commences, this producing continued reduction in cross sectional area till failure takes place. The creep limit stress is not yet an established fact, but in practice the acceptance of a restricted life provides a working limit stress