Modelling the role of non-metallic inclusions on the anisotropic fatigue behaviour of forged steel

Abstract

is an open access repository that collects the work of Arts et Métiers ParisTech researchers and makes it freely available over the web where possible. AbstractForged components are known to show high cyclic and monotonic mechanical properties. This is mainly due to a better compactness and a finer microstructure introduced by the forming process. However, this good mechanical behaviour is sometimes a source of anisotropy especially when the microstructural heterogeneities are not randomly distributed and/or oriented. This study aims at describing the high cycle fatigue response of a forged bainitic steel. This material contains a lot of elongated manganese sulphide (MnS) inclusions, oriented as a function of the rolling or forging direction. Specimens with different orientations relative to the rolling direction are tested in fatigue under push-pull uniaxial and torsion loads.The influence of "inclusion clusters" is clearly demonstrated via the observation of the failure surfaces. Experiments show that the anisotropic fatigue behaviour is due to a change in the crack initiation mechanism. At 0°, when the inclusions are parallel to the applied stress, micro-crack initiation is controlled by the material matrix. At 45° and 90°, elongated manganese-sulfide inclusion clusters are the origin of crack initiation and the fatigue strength drops significantly.A statistical approach based on the competition between two different crack initiation mechanisms is proposed. One mechanism is modelled by local elastic shakedown concepts and the other by linear elastic fracture mechanics. This approach leads to a Kitagawa type diagram and explains the anisotropy in the material. The approach developed in this study demonstrates a framework using both the elastic shakedown concept and the weakest link theory to account for the loading mode, loading path and data scatter in High Cycle Fatigue.

A mechanistic approach to the Kitagawa–Takahashi diagram using a multiaxial probabilistic framework

“…However, it has been observed that for other metallic materials the parameters m 1 and m 2 can vary significantly. For example, for the hot rolled steel, MetascoMC (25MnCrSiVB6), there is more scatter associated with the fatigue strength of the material without defects than when defects are present (m 1 = 30 and, m 2 = 48) [14,15,16].…”

“…This work is only briefly described below as it has been well documented elsewhere. [12,13,14,15,16].As such, the objective of this section is to highlight the usefulness and flexibility of the proposed modelling framework.…”

“…The modelling framework was originally proposed by Pessard et al [14,15,16], in order to model the anisotropic fatigue behaviour of the hot rolled bainitic steel (Metasco MC) used in forging applications. For this material it was shown experimentally that two distinct fatigue damage mechanism can be activated.…”

“…However, real metallic materials often contain microstructural heterogeneities, many of which can be classified as material defects. Two striking examples are the presence of micro-shrinkage pores in cast aluminium alloys [12,13] and clusters of elongated MnS inclusions in forged steels [14,15,16]. In these cases the distinction between the two fatigue damage mechanisms is less clear.…”

Brief review on high-cycle fatigue with focus on non-metallic inclusions and forming

“…The inclusions found from the fracture surfaces were vastly different especially in the aspect ratio -in parallel the inclusions were mostly regular oxides and also failures from the matrix occurred whereas, in the other direction, aspect ratios were shown to be in the range of 5-30, being mostly oxides, oxide clusters and sulfides. The manganese sulfides are known for their ability to deform and thus emphasize the anisotropy created in such deformation [26]. Makino highlighted that, for such elongated inclusions, the √ area overestimates the maximum stress intensity factor.…”