ANALYSIS OF CRACK PROPAGATION IN A THICK-WALLED CYLINDER UNDER FATIGUE LOADING

Abstract

Reliability of materials and structures in the form of thick-walled cylinders is of critical importance to many industries including power, nuclear, chemical, armament, and food processing industries. Catastrophic failure of these cylinders can put the human life and the surroundings at very high risk. For this reason, the integrity of the cylinder should be guaranteed. The integrity of nearly all engineering structures is threatened by the presence of cracks. Structural failure occurs if a crack larger than a critical size exists. Although most well designed structures initially contain no critical cracks, subcritical cracks can grow to failure under fatigue loading, called fatigue crack propagation. Fatigue failure that is failure under repeated or cyclic loading is a serious concern of engineering design. Under fatigue loading the component may fail at a stress level that is far below its yield strength. In present research the fatigue crack propagation, in a thickwalled cylinder, is analyzed through detailed experimental work and finite element analysis and the fatigue crack growth life of the cylinder, with crack at the bore surface, has been predicted. iii Extrusion process induces microstructural anisotropy in the thick-walled cylinder. The intensive experimental work, with the help of laboratory tests on the material under investigation, explores the details of the material and the microstructure-properties relationship in the longitudinal and transverse orientations. The yield and tensile strength in two orientations are not significantly different. However, percent elongation, reduction in area, impact strength and fracture toughness of the material are superior in the axial direction. A marked impact of anisotropy is found on the fatigue properties and shorter fatigue life in the transverse direction was obtained, which is 41 to 62 % lower in the tested stress range of 129 to 47 MPa. The theoretical part of the study includes modeling and simulation based on finite element method and the numerical technique is employed for the simulation of fatigue crack propagation. The finite element analysis, based on linear elastic fracture mechanics (LEFM) combined with the Paris law, suitably predicts the fatigue life and provides the results that are in good agreement with the experimental results. Both the experimental and numerical results of the crack growth data at different stress levels were found in good agreement all along the Paris regime. In the near threshold region the predicted values are conservative. With implementation of the present scheme of work the fatigue crack growth life of the thick-walled cylinder, with internal axial crack, has been predicted.