The Unified Creep-Fatigue Equation for Stainless Steel 316

Abstract

Background-The creep-fatigue properties of stainless steel 316 are of interest because of the wide use of this material in demanding service environments, such as the nuclear industry. Need-A number of models exist to describe creep-fatigue behaviours, but they are limited by the need to obtain specialized coefficients from a large number of experiments, which are time-consuming and expensive. Also, they do not generalise to other situations of temperature and frequency. There is a need for improved formulations for creep-fatigue, with coefficients that determinable directly from the existing and simple creep-fatigue tests and creep rupture tests. Outcomes-A unified creep-fatigue equation is proposed, based on an extension of the Coffin-Manson equation, to introduce dependencies on temperature and frequency. The equation may be formulated for strain as ε p = C 0 c T, t, ε p N −β 0 , or as a power-law ε p = C 0 c (T, t) N −β 0 b(T,t) . These were then validated against existing experimental data. The equations provide an excellent fit to data (r 2 = 0.97 or better). Originality-This work develops a novel formulation for creep-fatigue that accommodates temperature and frequency. The coefficients can be obtained with minimum experimental effort, being based on standard rather than specialized tests.

Crack Propagation Mechanisms for Creep Fatigue: A Consolidated Explanation of Fundamental Behaviours from Initiation to Failure

“…The concept of 'fatigue capacity' indicates that the total fatigue capacity is gradually consumed by creep effect. This process is numerically presented by taking the form of "1-X" (❹ in Figure 8) [43,44].…”

“…in Figure 8) [43,44]. Alternatively, the tip-crush effect during the process of crack closure may lead to crack growth along the grain boundary.…”

Testing of Microalloyed Steel Quality under Creep Conditions for a New Intended Use

“…The failure development stages of metal materials during high-temperature loading do not differ from a common fracture process, which is characterized by gradual local and time accumulation of individual forms of failure [19,20]. The characteristics of the failure of integrity and of fracture are functions of stress, temperature, and creep rate [21][22][23]. The creep rate, and, hence, the failure characteristic under constant stress and temperature are functions of the structure of steel.…”

A Unified Creep-Fatigue Equation with Application to Engineering Design

“…These limitations have been recently improved by the strain-based unified creep-fatigue equation [10,11]. The next sections describe the advantages of this new model, and present a case study illustrating its applicability to engineering design.…”

“…(8)) [10,11] is based on the underlying physical mechanisms of fatigue and creep. It provides a linear relationship between temperature and applied loading, based on the observation of creep-diffusion phenomenon [12], and it includes a power-law relation between number of cycles and applied loading which is consistent with crack-growth behavior [13].…”