Interaction effect of adjacent small defects on the fatigue limit of a medium carbon steel

Abstract

Structural steels contain various material irregularities and natural defects which cause local stress concentrations from which fatigue cracks tend to initiate. Two defects in close proximity to each other may affect local stress distributions, and thus, begin to interact. In this paper, the effect of interacting small cracks on the fatigue limit is systematically investigated in a medium carbon steel. The growth of interacting cracks, as well as the characteristics of non‐propagating cracks and microstructural aspects, was closely examined via the plastic replica method. It was found that although the fatigue limit is essentially controlled by the mechanics of interacting cracks, based on their configuration, the local microstructure comprised ferrite and pearlite has a statistical scatter effect on the behaviour of interacting cracks and non‐propagating thresholds. With respect to the fatigue limit, when two defects were in close proximity, they behaved as a larger single defect. However, with greater spacing between defects, rather than mechanical factors, it is the local microstructure which determines the location and characteristics of non‐propagating cracks.

Effect of defects on the fatigue limit of Ni‐based superalloy 718 with different grain sizes

“…It has been reported that for various metallic materials, the fatigue limit as a crack‐growth threshold can accurately be predicted by the parameter model . This model states that Δ Κ th at a stress ratio of −1 depends on the size of the initial crack or defect and the hardness of the material, expressed by the following equation: The unit is MPa for Δ Κ th .…”

Micromechanical Modeling of Fatigue Crack Nucleation around Non-Metallic Inclusions in Martensitic High-Strength Steels

“…This characteristic debonding behavior of MnS inclusions was independently confirmed by Maciejewski [13]. Åman et al [14] investigated the effects of interacting microstructural defects on the fatigue limit for a medium carbon steel. The investigations showed the fundamental influence of the underlying microstructure on the fatigue crack nucleation and short crack behavior.…”

“…Furthermore, they concluded that the largest microstructural defect determines the fatigue limit. Based on the experimental works [9][10][11][12][13][14], it is possible to extract the following key points: (a) the investigated martensitic steels SAE 4150 and SAE 4140 show fatigue crack nucleation on free surfaces as well as at oxidic and sulfidic inclusions with varying sizes, shapes and orientations to loading axis, (b) the MnS inclusions reveal a complex delamination behavior in case of transversal loading and (c) the lower bound of fatigue properties is controlled by the maximum size of non-metallic inclusions within the high stressed volumes [15][16][17].…”

Effect of impact velocity and impact angle on residual stress fields caused by foreign object damage

“…This equation is widely used for the prediction of the fatigue limit of numerous metallic materials with small defects and cracks. [30][31][32][33][34][35][36][37] It is worth noting that the model has been proven to give an accurate forecast of the fatigue limit of the Ti-6Al-4V alloy with small defects and cracks of diverse sizes and dimensions. [38,39] In the prior study by the author et al, the local R value was calculated according to the residual stress distribution in Equation 2, thereby enabling the successful prediction of the fatigue limit of an FOD specimen.…”