Creep

Abstract

In the preceding chapters it has been assumed that there exists for any material a stress—strain relationship which is independent of time. In a tensile test, on applying a load in the plastic region, there is an immediate extension, but this is followed in time by further extension. In mild steel, for example, at room temperature after applying an increment of load, the strain varies rapidly at first and then at a decreasing rate. In lead, the strain can continue for a long time at a steady rate even under very low stresses. This is seen in the slow movement of lead on sloping roofs and the gradual sagging of unsupported lead pipes. The strain—time variation at constant load is known as creep. At high temperatures, steel and other materials show this steady creep, which becomes of importance in components which are exposed to high temperature during service.