CONSTITUTIVE EQUATION FOR CREEP FRACTURE UNDER CONSTANT, VARIABLE OR CYCLIC POSITIVE STRESS.

This paper deals with the development and application of a constitutive equation for creep fracture of RR58 aluminum alloy at 180 degree C under variable stress. Stress changes were arranged to take place approximately at two stages during primary creep, one stage corresponding to minimum creep rate and one stage in established tertiary creep. The creep fracture curve for virgin material was also available, thus giving five levels of damage. A datum stress level was selected and specimens were damaged to each of the above stages of creep, whereupon the stress was changed and the residual life, at a series of new stress levels, determined. Correlation and regression analyses of the results from 32 tests gave an exponential equation, the constants being dependent upon the degree of damage. This equation predicts the residual life and the total time to fracture is obtained by summation of the stage times and the residual life. The concept of effective damage is introduced to apply the equation to multi-change variable and cyclic tests. Acceleration of initial damage is shown to be highest during early primary creep which is when the percentage anelasticity is highest.