Rupture Time Under Creep Conditions

Abstract

The problem of long time strength (evaluation of time to rupture) is of obvious relevance for various machine parts working under high temperature conditions. The existing data on specimens tested under uniaxial tension are insufficient for general loading conditions and inhomogeneous stress states: intensive experimental investigations are being conducted in this direction. Theoretical modelling of long time strength in the framework of continuum mechanics appears to be important. In recently published work of Hoff (1953), the moment of failure of a rod under tension is defined as the one at which the cross-sectional area becomes zero as a result of quasiviscous flow. His result is in satisfactory agreement with experimental data. A similar concept was used by Katz (1957) and by Rosenblum (1957) in their studies of times to rupture of thick-walled pipes and of a rotating disk with a hole. We note, however, that the experimental data points typically lie below the theoretical predictions and that rupture occurs at elongations not exceeding several tens per cent. The concept of Hoff has certain limitations. It predicts, for example, that creep under torsion does not result in rupture, contrary to observations. Also, his scheme does not explain fractures at small strains (‘brittle’ ruptures) and the change of character of rupture (from ‘viscous’ to ‘brittle’) if the material is not sufficiently stable. Here, we suggest a theoretical model for the time to rupture with the account of embrittlement. We emphasize that the physics of such phenomena is very complex and has not been studied sufficiently. Although we discuss microcracking, the results can be interpreted in a more general way, in terms of development of damage.