A New Theoretical and Practical Approach to Creep and Creep Fracture

Abstract

For several decades, theoretical and practical studies of creep and creep fracture have usually relied on a knowledge of only a limited number of parameters, such as the steady-state creep rate and the rupture life (tf). Unfortunately, numerous anomalies are introduced by the adoption of these traditional approaches, particularly when the properties of particle-strengthened alloys are considered. It is therefore proposed that these conventional approaches should be replaced by procedures offering constitutive equations which accurately describe the shape of the entire creep curve and which quantify the changes in creep curve shape resulting from changes in test conditions. Specifically, an analysis of the data recorded for 1/2Cr1/2Mo1/4V ferritic steel suggests that normal creep curves can be envisaged as the sum of the decaying primary and an accelerating tertiary component so that the increase in creep strain (ɛ) with time (t) can be described as ▪ where θ1 and θ3 define the extent with respect to strain and θ2 and θ4 quantify the curvatures of the primary and tertiary stages of creep respectively. Once the stress and temperature dependences of the θ functions are established, even the anomalous behaviour observed for particle-strengthened alloys can be rationalized directly in terms of the processes governing primary and tertiary creep behaviour. Furthermore, this new θ Projection Concept offers major practical advantages since high-precision data obtained in tests having a maximum duration of less than three months can be used to predict accurately the creep strain and creep life characteristics of the 1/2Cr1/2Mo1/4V steel for times up to 10 years and more.