جستجوی مقالات مرتبط با کلیدواژه « ترک زیرسطحی » در نشریات گروه « مکانیک »

The Griffith crack, a central crack in an infinite plane under uniform loading, is converted to a subsurface one by moving close to a loaded edge of the plane. Subsurface cracks initiate under rolling contact fatigue conditions. In this paper, first, finite element model of the Griffith crack has been developed and validated by calculating stress intensity factors (SIFs) under uniform tension and shear loadings. Then, by moving the crack close to a parallel edge of the plane, mixed mode SIFs of the subsurface crack have been determined for a wide range of the cracks depths. Non-symmetrical geometry with respect to the crack edge causes coupling between fracture modes and so, considerable shear and tension fracture modes under tension and shear loadings, respectively. The ratio of SIF for the coupling mode to the direct mode is creased up to 69% for the length to depth ratio of 20. Also, by fitting third-degree polynomials to the mixed mode SIFs, four geometry correction factors have been obtained for SIFs of subsurface cracks under uniform loadings. These approximate equations can be used easily and efficiently by engineers. Also, the relations can be utilized as a primary estimation for non-uniform loadings, especially when the crack length as well as the load variation along it is small.

Elliptical subsurface cracks, under rolling contact fatigue, are special due to the coupling of the fracture modes and the crack closure. In this paper, the ability of the matrix of two dimensional weight function, to analyze the completely closed subsurface crack is investigated. Weight function method has a super position base and so, the ability is evaluated for the frictionless contact between crack surfaces. To do this, mixed mode stress intensity factors of a subsurface crack front under semi-ellipsoid of contact pressure are calculated using the matrix of two dimensional weight function and subsurface stress field. The results are compared to finite element stress intensity factors with high accuracy. It is concluded that the matrix of weight function is able to quick analyze the completely closed subsurface crack with high accuracy if the compression normal stress is vanished. Also, the effects of the aspect ratio and depth of the crack on the stress intensity factors are investigated.

In the last decade, decrease of the wear rate, increase in the axel load, and the velocity of the trains have changed the main damage mechanism of railway wheels from wear to rolling contact fatigue (RCF). RCF may lead to surface and subsurface cracks. The interaction between wear and RCF, removes the surface cracks, but the subsurface crack propagate gradually, and may lead to deeper and more dangerous fractures. In this study, by considering an elliptical subsurface crack, the lifetime of an Iranian railway system wheel has been predicted using FEM, and under RCF condition. To this end, a 3D model of a cracked R7T wheel with 920mm diameter in contact with UIC60 rail has been prepared and analyzed in Abaqus FEM software. The histories of the stress intensity factors of the subsurface crack have been extracted in one cycle wheel rolling. Then, equivalent stress intensity factors have been calculated using Tanaka and Liu-Mahadevan models. Finally, wheel lifetime has been predicted by Walker relation. This paper provides a local criterion of critical crack length and crack growth lifetime that is essential for frequent inspections of railroad wheel.