Fatigue crack growth under non-proportional mixed-mode I + II. Role of compression while shearing

Abstract

Mode I + II fatigue crack growth tests are run, following sequential or pseudo-sequential loading paths, representative of those computed for "squat type" cracks in rails. Stereo DIC provides the near-tip displacements, from which ∆K ef f ective I and ∆K ef f ective II are derived. Compression while shearing extends coplanar growth, by slowing down the wear-induced rise of the effective mode mixity ratio, up to the critical value for which bifurcation occurs. The consideration of plasticity and contact and friction stresses improves crack paths predictions, compared to the maximum tangential stress criterion. The coplanar crack growth rate correlates well with a combination of ∆K ef f ective I and ∆K ef f ective II .

Fatigue crack growth in bearing steel under cyclic mode II + static biaxial compression

“…They stopped growing once the singular stress field around the main crack front moved away. The fracture surface of specimen #2 submitted to a constant shear stress range and a constant biaxial compression of 200 MPa over the whole test looks specially worn and oxidized near the notch root, where the crack faces were subjected to a larger number of shear cycles with a higher CSD range, than near the final crack tip, where both the number of cycles and CSD were smaller, as also reported by Bonniot et al [20]. In addition, oxygen molecules might more easily access the crack faces, kept closed by normal compression, through the open notch.…”

“…They focus on crack kinking from the tip, while -as shown below-branches also develop from the crack flanks, and compete with the main crack, especially when compression is present. Besides, crack face friction in these works is taken into account using Coulomb's law, with a constant friction coefficient, which, as observed in this work and in others [19,20], does not capture the effects of normal compression on the tribological phenomena. In RCF of bearing parts, two stages can be distinguished: i.…”

“…However, compression may also favor the trapping of wear debris, restrict the access of oxygen between contacting surfaces, thus changing -and sometimes actually reducing-the friction coefficient, and favor adhesive rather than abrasive wear [19]. Besides, as discussed by Bonniot et al [20] based on Archard's wear law [47], for a fixed CSD, normal compression should enhance wear, but on the other hand, it should also reduce ΔK , and thus the CSD, so that the resulting effect is not a systematic enhancement of crack face wear, but depends on the relative magnitude of the applied shear stress range, and the product of the friction coefficient by the compressive stress. During test #2, run under constant shear stress range and static biaxial compression, a quasi-linear and significant rise of the apparent friction coefficient was observed during crack propagation (see Fig.…”

Fretting tests for cyclic plastic law identification: Application to a 1XXX aluminium crossed wire contact

“…This trend is present at all points of the contact but the difference during the maximum and the minimum of the tangential force is more or less present depending on the analyzed location. It is worth noting that the initiation and propagation of cracks is strongly influenced by this type of multiaxial and non-proportional mechanical state, and this point is still an issue for modeling [23][24][25]. Moreover, the identification of the properties was carried out for uniaxial stress as well as for deformations not exceeding 1.1%.…”

Load Direction-Dependent Influence of Forming-Induced Initial Damage on the Fatigue Performance of 16MnCrS5 Steel

“…Further investigations were conducted with respect to compression loads applied while shearing under torsional loading conditions. Doing so, the extension of coplanar crack growth was indicated as the underlying mechanism, whereby the growth rate was found to continuously decrease up to the bifurcation of the cracks [19]. Said et al highlighted the crack propagation up to failure for negative stress intensity factors by mode shear mechanisms under compressive load during fretting fatigue [20].…”