An experiment in stress redistribution caused by creep

Abstract

A simple redundant structure subjected to a non-uniform temperature distribution is simulated experimentally by means of two tensile-creep test machines holding specimens of Nimonic Alloy 90 at different temperatures in the range 800-900°C. The machines are linked together so that although the total load applied to the specimens is constant the share of that load taken by each varies owing to dissimilar creep rates, as it would in the structure. This redistribution of load is observed for several initial stress conditions and several temperature differences. In each case the results are compared with calculations based on creep-strain data obtained from constant-stress tests. An important practical feature of the results is the establishment of stable (fully redistributed) stresses. These stresses are insensitive to the initial conditions, can be estimated quite simply from constant-stress creep data, and are always more uniform than the thermal stresses appropriate to the same temperature difference. There is some indirect evidence that the creep process itself (primary creep in particular) is a manifestation of stress redistribution on a microscopic scale.