Development of Constitutive Relation for Plastic Deformation From a Dislocation Model

Further analysis of a model in which creep is attributed to the movement generation, immobilization, and recovery of dislocations has permitted the anelastic (recoverable) component of strain to be calculated. The earlier conclusions are unaffected by this refinement and so the model equations continue correctly to predict the effects of raising, reducing, removing and reversing the stress during the creep of a range of materials including stainless steel, zirconium alloys, copper, tungsten, nickel, and aluminum. In addition, however, it is now (correctly) predicted that removal of a tensile stress will permit compressive creep to occur, producing an anelastic strain whose magnitude increases towards an upper asymptote as the magnitude of the prior tensile stress increases. Anelasticity is found to become an increasingly large proportion of the total strain at low stresses: the model predicts this. It also predicts the pattern of behavior observed in “stress-dip” tests: in particular that there is a critical magnitude of stress reduction which causes momentary cessation of creep.