Creep of Metals at Elevated Temperatures—the Hyperbolic-Sine Relation Between Stress and Creep Rate

Abstract This paper recommends further use of the hyperbolic sine to express the relation between stress and creep rate at constant temperature. Evidence is submitted to show that the commonly used straight-line relation in the log-log plot becomes unreliable when extrapolated over a considerable range into the field of low stresses. Reference is made to theoretical considerations in favor of the hyperbolic sine and to tests on a wide variety of materials which appear to support its use. For specific examples, comparative curves are shown to illustrate application to creep data representing a wrought 18 Cr-8 Ni alloy tested at 1500 F. The two methods of extrapolation are compared with special reference to their reliability for selection of safe working stresses. To stimulate application of the hyperbolic-sine relation to practical problems, a graphical solution is proposed, and a chart is given to facilitate determination of the required constants from creep tests at two or more stresses at the same temperature.