Cyclic Slip Irreversibilities and the Evolution of Fatigue Damage

Abstract

In this article, the physical origin of fatigue crack initiation in ductile metals is discussed from a historical perspective. The main focus is to assess those cyclic slip irreversibilities in a microstructural sense that occur not only at the surface but also in the bulk at the dislocation scale and to show how they contribute to surface fatigue damage. The evolution of early fatigue damage, as evidenced experimentally in the last decades, is reviewed. The phenomenon of cyclic strain localization in persistent slip bands (PSBs) and models of the formation of extrusions, intrusions, and microcracks are discussed in detail. The predictions of these models are compared with experimental evidence obtained on mono-and polycrystalline face-centered-cubic (fcc) metals. In addition, examples of the evolution of fatigue damage in selected fcc solid solution alloys and precipitation-hardened alloys and in body-centered-cubic (bcc) metals are analyzed. Where possible, the cyclic slip irreversibilities p, defined as the fraction of plastic shear strain that is microstructurally irreversible, have been estimated quantitatively. Broadly speaking, p has been found to vary over orders of magnitude (0 < p < 1), being almost negligible at low loading amplitudes (high fatigue lives) and substantial at larger loading amplitudes (low fatigue lives).

Microstructural aspects of fatigue in Ni-base superalloys

“…It is also likely to be associated with a higher degree of slip reversibility, as shown in figure 22, where at lower temperatures the slip is more planar. The importance of slip reversibility in fatigue damage has also been pointed out by Mughrabi [31], who used shearing of γ particles demonstrated by Clavel & Pineau [27] to measure the degree of reversibility ( figure 24).…”

“…The dislocation structure of the slip bands formed during the cyclic deformation of Ni-base superalloys is seen to be very different from the persistent slip bands (PSBs) so extensively studied for Cu. Classical PSBs have been described [28][29][30] and reviewed [25,31,32] in detail elsewhere. [27].…”

“…Figure 24. Measurements [31] of slip reversibility for planar-slip PSBs in Waspaloy fatigued to failure at ε p = 4 × 10 −3 , N f = 550 cycles and T = 650 • C [27]. By algebraically measuring the net slip offset, it was determined that the slip was about 95% reversible.…”

Introduction

“…Very well-known and highly cited are Haël Mughrabi's seminal contributions to the understanding of the deformation mechanisms which govern metal fatigue, such as the discovery of the role of persistent slip bands (PSBs) and accompanying cell formation processes 1 and about the importance of deformation irreversibility. 2 In 1983 he developed his composite model, 3 consisting of hard dislocation walls of high dislocation density separated by soft regions of low dislocation density to describe the local and global deformation behavior during fatigue. The model shows, that substantial long-range internal stresses arise unavoidably as a natural consequence of compatibility requirements.…”

Creep-fatigue interactions in equiaxed and single crystal Ni-base superalloys

“…The superior strength properties of this class of alloys come from the combined effects of precipitation hardening by a high volume fraction of ordered coherently embedded Ni 3 Alγ phase particles and the strong solid solution hardening of the γ matrix. In fact, the great stability of the γ /γ composite microstructure at high temperature and under the application of mechanical and thermal loading accounts for the outstanding performance of these materials in service [1,2].…”