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

Abstract

Background-Creep-fatigue damage is generally identified as the combined effect of fatigue and creep. This behaviour is macroscopically described by crack growth, wherein fatigue and creep follow different principles. Need-Although the literature contains many studies that explore the crack-growth path, there is a lack of clear models to link these disparate findings and to explain the possible mechanisms at a grain-based level for crack growth from crack initiation, through the steady stage (this is particularly challenging), ending in structural failure. Method-Finite element (FE) methods were used to provide a quantitative validation of the grain-size effect and the failure principles for fatigue and creep. Thereafter, a microstructural conceptual framework for the three stages of crack growth was developed by integrating existing crack-growth microstructural observations for fatigue and creep. Specifically, the crack propagation is based on existing mechanisms of plastic blunting and diffusion creep. Results-Fatigue and creep effects are treated separately due to their different damage principles. The possible grain-boundary behaviours, such as the mismatch behaviour at grain boundary due to creep deformation, are included. The framework illustrates the possible situations for crack propagation at a grain-based level, particularly the situation in which the crack encounters the grain boundary. Originality-The framework is consistent with the various creep and fatigue microstructure observations in the literature, but goes further by integrating these together into a logically consistent framework that describes the overall failure process at the microstructural level.

An Explicit Creep-Fatigue Model for Engineering Design Purposes

“…As mentioned in Section 1.2, the damage-accumulation-based models ignore the interaction between fatigue and creep. The microstructural interactions between creep and fatigue are beginning to be understood at a qualitative level, e.g., [24]. Various mathematical expressions for this interaction are also available, with some (albeit limited) basis in microstructure or loading partitioning, e.g., [25,26].…”

“…To achieve this, it would be necessary to better understand the microstructural processes of fatigue & creep-especially their interactions-and how those affect plastic strain and life. Some work is available in this area, e.g., [24], but there is still a long way to go before the values of coefficients in a creep-fatigue model can be predicted ab initio from microstructural inspection. In addition, as mentioned in Section 2, the explicit model ignores the dislocation creep and grain boundary sliding.…”

“…To achieve this, it would seem necessary to modify the formulation (especially, the creep component in this explicit model) to include new terms of as yet-unknown mathematical form. Conceptual works, e.g., [24], may be useful in identifying the basic form of these relationships.…”

A Microcrack Location Method Based on Nonlinear S0 Mode Lamb Wave and Probability Scan Positioning Imaging Matrix

“…However, the metal materials may generate defects due to fatigue loading. Studies have shown that the degradation of metal material performance has obvious stage characteristics [1,2]. Due to cyclic loading, a large number of dislocation groups are generated inside the material, and the damage accumulates, which in turn causes the resident slip zone and microcracks to be generated.…”

Failure Mechanisms in Alloys

“…"Progressive" or "delayed modes of failure" possess exceptional interest in various industrial sectors, such as the chemical, mechanical, and manufacturing industries and in plant machinery. Four basic categories can be distinguished(Figure 2a): a.Creep and Creep Fatigue failure modes[2,9,17,23]; b.…”