Topic1:　 Fatigue Mechanisms in the Ultrahigh Cycle Regime

Abstract

The increasing interest in fatigue failures occurring after very high numbers of cycles to failure of 109 or more in the so-called Ultra-High Cycle Fatigue (UHCF) or Very High Cycle Fatigue (VHCF) regime has several reasons. It is now clear that UHCF failures can occur unexpectedly at stress levels below the conventional high-cycle fatigue (HCF) limit. The major goal of this work is to summarize our current understanding of the microstructural UHCF failure mechanisms. In addition to the most widely discussed case of subsurface 揻ish-eye?failures in UHCF of high-strength steels containing inclusions, possible damage mechanisms occurring in ductile single-phase materials at amplitudes below the threshold for formation of persistent slip bands (PSBs) will be discussed. The importance of crack initiation rather than propagation will be emphasized. An attempt will be made to review and classify the failure modes investigated so far in the form of suitable 搈ultistage?fatigue life diagrams. The research direction of future studies which will be necessary in order to make further progress in understanding UHCF will be indicated. 

Topic2:  Role of Geometrically Necessary Dislocations in Inducing Long-Range Internal Stresses and Lattice Plane Misorientations in Deformed Crystals

Abstract

Long-range internal stresses and lattice plane misorientations are commonly observed in the heterogeneous dislocation patterns which develop in deformed crystals. In the present work, studies which clarify the role played by geometrically necessary dislocations (GNDs) in the evolution of the deformation-induced dislocation microstructure in unidirectionally and cyclically deformed copper crystals will be reviewed. The results of pertinent X-ray diffraction experiments (line broadening, rocking curves) will be presented and analyzed. On the modelling side, the composite models of single/multiple slip serve to interpret the experimentally measured long-range internal stresses quantitatively in terms of densities of GNDs. Using simple dislocation models, the density of those GNDs that are responsible for the observed lattice plane misorientations can be quantified and is found to be a small fraction of the total dislocation density. Quite generally, it is concluded that one and the same GND array can give rise to both internal stresses and misorientations. From the evolution of the misorientations, it is possible to classify specific dislocation boundaries as either incidental dislocation boundaries (IDBs) or geometrically necessary boundaries (GNBs). 

Topic3: 　Cyclic Deformation and Fatigue of Ultrafine-Grained Metals

Abstract

The results of recent investigations of the cyclic deformation and fatigue properties of different ultrafine-grained (UFG) metals and alloys with submicron grain size, produced by ECAP (ECAP: equal channel angular pressing) are discussed with respect to the microstructural mechanisms controlling fatigue life. Because of their high strength, UFG materials always exhibit a superior fatigue performance, when displayed in the W鰄ler S-N plot, than conventional grain size (CG) materials. However, because UFG materials are normally less ductile, they usually exhibit shorter fatigue lives in a Coffin-Manson plot than CG materials. The fatigue lives can generally be enhanced by suitable annealing treatments which sometimes lead to a bimodal grain size distribution which can have positive effects on the fatigue life. A comparison of the fatigue behaviour of different (commercial) UFG metals and alloys (Cu, Al, AA6061, a-brass) shows that the types of fatigue damage and fatigue crack initiation differ substantially from case to case and that specific strategies must be developed in each case in order to optimize the fatigue resistance.

 

Curriculum Vitae -- Hael Mughrabi