Stored Energy in Creep Deformed Graphite

Abstract

GRAPHITE which has been strained in tensile creep releases a very large amount of stored energy when the creep stress is reduced, as happens in the dip test¹. The very large amount of negative creep (actual shrinkage of the specimen while it is still under an appreciable applied tensile force) that takes place after a stress reduction is shown graphically in Fig. 1. Three individual stress reduction tests are shown. The work the specimen does against gravity in lifting the hanging weights is measured by the distance the specimen shrinks in negative creep multiplied by the remaining applied force. In a series of tests on ZTA graphite, the largest measured amount of energy released was 1.1 × 10⁶ ergs/cm³ (0.17 calories/g atom). This was released when one specimen was strained almost to fracture (20 per cent elongation) at 3,200 pounds inch⁻², at which point the applied stress was reduced to 1,600 pounds inch⁻². The negative creep strain after the load reduction was 1.2 per cent. In time the negative creep period ended, but additional negative creep, some four times greater than that already measured, then occurred after complete removal of the applied stress. If the total energy release were proportional to that measured after the initial stress reduction, it would have been about 5.5 × 10⁶ ergs/cm³. It was probably less than this. For further calculations, however, let us assume that the released energy was somewhere between 1.1 × 10⁶ and 5.5 × 10⁶ ergs/cm³.