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EN
An analysis of the internal fish-eye crack growth in a plasma-nitrided high-strength lowalloy steel specimen has been performed. The study combines information obtained from the fractographical observation of the striations field located near the centre of the fish-eye crack on the specimen subjected to combined bending-torsion loading with the linear elastic fracture mechanics modelling of the fatigue crack propagation. The results suggest that the propagation stage constitutes only a minor part of the total fatigue life, the major part of which is spent on the fatigue crack initiation. This conclusion is consistent with the high cycle and very high cycle fatigue behaviour of specimens without any surface hardening layer.
PL
W pracy przedstawiono analizę wzrostu wewnętrznego pęknięcia typu „rybie oko” w plazmowo azotowanej, niskostopowej próbce wykonanej ze stali o wysokiej wytrzymałości. W badaniach powiązano dane otrzymane z obserwacji fraktograficznej pola prążkowanego w pobliżu środka pęknięcia w próbce poddanej złożonemu stanowi obciążenia giętno-skręcającemu z modelowaniem mechanicznym procesu pękania i propagacji szczeliny zmęczeniowej. Otrzymane wyniki wskazują, że faza propagacji stanowi jedynie drobną część całego procesu pękania zmęczeniowego, którego dominującym etapem jest inicjacja pęknięcia. Wniosek ten pozostaje w zgodzie z badaniami nad wysoko- i bardzo wysoko-cyklowym zmęczeniem próbek bez powierzchni utwardzanych.
EN
An assessment of the residual fatigue life of plasma-nitrided high-strength low-alloy steel specimens subjected to symmetrical in-phase bending-torsion loading has been performed. All specimens failed due to the internal fish-eye type of fracture initiated at non-metallic inclusions within the specimen interior. The deflection of the crack plane from the plane perpendicular to the specimen axis was almost negligible in the radial direction and varied as a function of the loading regime in the tangential direction. In all cases, the crack propagated predominantly under a local mode I loading.
3
Content available remote Dislocation-based and continuum models of crack tip shielding in fatigue
EN
Along with external forces and the macro-geometry of cracked bodies, the local stress intensity factors k at fronts of brittle and fatigue cracks are also determined by internal residual stress fields and the crack front mi- crogeometry (extrinsic shielding). Consequently, the values of k that represent a real crack driving force can be significantly different from those of the remote. This means that, even in the frame of two-parameter linear-elastic fracture mechanics, a characterization of the crack-tip stress field only by K and T is not sufficient at all.
4
Content available remote Tortuous cracks under remote mode I: the statistical LEFM approach
EN
Principles of the statistical approach to the stability of microtortuous cracks under the remote mode I loading are presented within the framework of linear elastic fracture mechanics. The basic idea of this approach lies in the suggestion that branching occurs only at those sites along the crack front, where the characteristic structural parameter (grain size, interparticle distance) is comparable to or larger than the plastic zone size. By using procedures introduced in the paper, extrinsic contribution to the fracture toughness or to the fatigue threshold can be separated from experimentally obtained KIc or [delta]Kth values. Consequently, it enables to determine the intrinsic resistance against crack initiation and growth as a more relevant measure of materials quality. Moreover, this theory elucidates quantitatively well some interesting phenomena observed in fracture and fatigue of metallic materials.
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