Creep Range Research Articles

Effect of Surface Condition on Creep of Some Commercial Metals

Abstract The relative creep rate of specimens of 2S aluminum and commercial drawn copper were investigated with the oxide surface layers removed and then, under the same conditions, after the oxide was permitted to form. The oxide was removed mechanically by stripping the test specimens in an inert atmosphere furnace so equipped that an axial stress could be applied and the creep rate determined without disturbing the atmosphere. After the oxide-free specimen was observed to be creeping essentially linearly with time (i.e., the quasiviscous, secondary creep range was established), air was admitted to the furnace. The extension versus time plot was continued after admission of air and the temperature held constant. Since all of the known parameters affecting creep rate (temperature, grain size, degree of work hardening, stress, and so on) except the gaseous environment were the same before and after admitting air, the effect of this change could be isolated. A decrease in creep rate was observed after air had been admitted to the furnace. These experimental results indicate that the presence of an oxide surface layer on the commercial grades of pure copper and aluminum increases the creep strength of these metals. The manner in which this relatively thin surface layer contributes to the strength of the parent metal is not clear, but the phenomenon is consistent with the theory of dislocations as presented by G. I. Taylor if it is assumed that the oxide inhibits the generation and/or migration of dislocations.

Formula for Pipe Thickness

Abstract The so-called Lamé formula, either in its true form, or in an equivalent but less complex modified form, is advanced as the most suitable formula for the design of pipe that is stressed in the elastic range by internal pressure. In the creep range, the use of the Bailey-Nadai formula is proposed, and it is shown that the so-called Lamé formula in either form is a good approximation of the Bailey-Nadai formula. In the selection of these two formulas, the so-called Lamé formula and the Bailey-Nadai formula, an examination has been made of the common, average (membrane), Barlow, Clavarino, Guest, Beltrami, and von Mises formulas. The origin and meaning of all of these expressions are discussed. Their significance for design purposes is explained, and their relationship to the so-called Lamé formula is demonstrated. The need for additional pure shear experimental data on creep and on stress rupture is emphasized.