PSI - Issue 18

Matthias Hell et al. / Procedia Structural Integrity 18 (2019) 823–836 Author name / Structural Integrity Procedia 00 (2019) 000–000

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The cyclic stress-strain behaviour as well as the fatigue behaviour is determined by the generation and the annihilation of dislocations, the formation of dislocation structures and strain localizations within the microstructure of the material [10]. The slip character in this respect describes the tendency towards the formation of three dimensional dislocation structures. In the case of a planar slip character, the dislocations are bound to single slip planes. Up to highest load magnitudes, no cross slip occurs and the dislocation substructures stay in a planar configuration, which is considered to be unstable towards cyclic deformations. In case of the planar slip character, the cyclic stress-strain relation depends mainly on the load magnitude and only marginally on the load sequence [11]. The wavy slip character is marked by the formation of three dimensional, cyclically stable dislocation structures. At low load magnitudes, dislocation pile ups and dipoles as well as persistent slip bands are the dominating dislocation substructures. With rising load magnitude, the dislocations emerge from the planar configurations and form cell like structures. In case of the wavy slip character, the load sequence has a pronounced influence on the stabilization of the stress-strain behavior [9,11]. Although the slip behaviour is determined by localized phenomena, the effects of the slip character on the stress-strain behaviour may be qualitatively assessed by a comparison between the stress-strain behaviour under constant amplitude loading and during incremental step tests. Fig. 6 shows the schematic evaluation of the slip character and the results for the steel 42CrMo4+QT, which exhibits a strong tendency towards a wavy slip behaviour.

Fig. 6: Schematic evaluation of the slip character (left), wavy slip character of the heat treatable steel 42CrMo4+QT

In case of a tendency towards the wavy slip behaviour, the stress-strain relation derived by incremental step tests corresponds more accurately to the actual stress-strain behaviour under variable amplitude loading and should therefore be used for the assessment of the local stress-strain state. Neglecting the effects of the load sequence and the load magnitude on the stress-strain behaviour, will result in a misinterpretation of the actual stress-strain state and a misevaluation of the fatigue strength of the component and may lead to either over-dimensioning or severe reductions in safety margins.