Fatigue Mechanisms in the Sub-Creep Range

Abstract

A comprehensive description of the mechanisms of fatigue in metals is given for the temperature range in which creep processes are not important. The general response of materials to cyclic loading is discussed, including fatigue hardening/softening and the development of inhomogeneous plastic strains. The generation of fatigue cracks is treated by listing first the common sites for initiation and then discussing the various mechanisms for initiation which have been observed. Fatigue crack growth is described generally in terms of propagation modes and the appearance of fracture surfaces. Then the mechanisms of crack advance are considered in detail, and the effects of multiple load amplitude are included. Predictive theories of fatigue, from early “damage” theories to recent crack growth laws, are discussed in the light of fatigue mechanisms knowledge. Included is a discussion of the effects of crystal structure on fatigue strength. Finally, the practical implications of fatigue mechanisms knowledge are presented as they affect nondestructive inspection, repair of fatigue damage, the choice of fatigue-resistant materials, and development of new materials. There are 68 references to the fatigue literature.

On ‘multi‐stage’ fatigue life diagrams and the relevant life‐controlling mechanisms in ultrahigh‐cycle fatigue

“…Internal crack initiation can be excluded. The irreversibility of slip depends on the loading amplitude and decreases markedly with decreasing amplitude 16 , 17 . In HCF, and more so in UHCF, a major part of fatigue life is spent in the bulk microstructural processes leading to cyclic strain localisation, in subsequent (stage I) crack initiation at the surface and in the initially very slow crack growth, compare 16–20 …”

“…The irreversibility of slip depends on the loading amplitude and decreases markedly with decreasing amplitude. 16,17 In HCF, and more so in UHCF, a major part of fatigue life is spent in the bulk microstructural processes leading to cyclic strain localisation, in subsequent (stage I) crack initiation at the surface and in the initially very slow crack growth, compare. 16±20 The dominant type of cyclic strain localisation occurs in the form of persistent slip bands (PSBs) with welldefined threshold amplitudes of strain and stress 18,21 that are well known for a number of pure metals and alloys, compare.…”

On the life‐controlling microstructural fatigue mechanisms in ductile metals and alloys in the gigacycle regime

“…The existence of a fatigue threshold and its relation to a fatigue limit have traditionally been a central question in discussions of the fatigue problem 1 –6 . 10 –13 , 17 , 20 , 22 With regard to fatigue in the UHCF range, controversial results have been reported at the recent International Conference Euromech 382: ‘Fatigue Life in the Gigacycle Regime’ with respect to so‐called ‘two‐stage’ or ‘stepwise’ Wöhler curves.…”

Estudo da propagação da trinca por fadiga em um aço de alta resistência e baixa liga após o processo de soldagem por centelhamento

“…Comumente o material que passa pelo processo de soldagem é divido em regiões, Deve-se a C. Laird a proposição básica mais difundida do mecanismo de formação das estrias. A descrição do seu modelo, denominado "processo de arredondamento plástico", pode ser encontrado em diversos textos sobre fadiga (DIETER, 1976;GROSSKREUTZ, 1971;KLESNIL;LUKÁŠ, 1992;SURESH, 1998;TOMKINS, 1996) As aplicações práticas envolvendo os conceitos de falha segura e tolerância ao dano, requerem o conhecimento das propriedades de resistência à propagação da trinca dos materiais, que são determinadas ao se ajustar os dados obtidos em ensaios de laboratório ao modelo matemático empregado para sua descrição e, no caso da equação…”