Low-strain fatigue in 316L steel surface grains: a three dimension discrete dislocation dynamics modelling of the early cycles. Part 2: Persistent slip markings and micro-crack nucleation

Abstract

The early stages of formation of persistent slip markings in fatigue are analysed, using threedimensional discrete dislocation dynamics modelling. The surface displacements due to slip are computed using a specific post-processing method. Fatigue simulations under various strain ranges and grain sizes have been performed. The resulting surface slip markings and their evolutions are analyzed quantitatively, in terms of the marking height and thickness. A detailed scheme for persistent slip marking formation and morphology is proposed, in relation to the persistent slip-band dislocation arrangements present within the grain. The simulations show the crucial role of these arrangements for the extrusion-intrusion growth and localisation of slip at the band edges. Local stress concentrations inside the crystal and their relations to damage initiation are also analysed. The results provide insights for an original micro-crack initiation scheme, combining different initiation mechanisms as described in the literature.

Numerical investigations of the free surface effect in three-dimensional polycrystalline aggregates

“…This work showed that extrusion steps height increases significantly during the early fatigue cycles before developing with a three times slower growth rate, proportional to the number of cycles N . Discrete Dislocation Dynamics simulations showed that the extrusion height remains proportional to √ N in the case of symmetric cyclic loading, while it is linearly related to N in the presence of a positive mean strain [7]. This statement is consistent with the fact that fatigue life decreases for a given stress amplitude, when a mean stress is applied.…”

Micro-crack initiation in thermal fatigue — an analysis based on experimental and DD simulation results

“…The effect of the thermo-mechanical induced stress state on micro-crack initiation is now investigated, thanks to 5REHUWVRQ HW DO 7UDQV ,,0 9RO ,VVXHV $SULO -XQH microstructures are available elsewhere [1,2]. In the selected conditions, slip systems "03" i.e.…”

“…Surface grains are represented by appropriate properties of the boundaries [1]. The escaping dislocations print surface steps that are computed using a specific post-treatment method [2]. The calculations to be described in this section are performed under isothermal conditions at 300 K, using an equi-biaxial applied stress tensor (with σ ZZ = σ θθ and σ RR .…”

Stage I surface crack formation in thermal fatigue: A predictive multi-scale approach

“…One possible alternative to direct measurement of extrusion growth is dislocation dynamics modelling. With this method, the kinetics of individual extrusion growth can be computed using different simulations setups, where the grain size, depth and plastic strain range are systematically varied [23]. In symmetrical tension-compression conditions, the results obtained by DD simulations lead to the following microcrack initiation criterion, in combination with expressions (1) and (2):…”