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Crack Extension (crack + extension)
Selected AbstractsFatigue as a process of cyclic brittle microfractureFATIGUE & FRACTURE OF ENGINEERING MATERIALS AND STRUCTURES, Issue 3 2005R. SUNDER ABSTRACT While fatigue crack growth in vacuum may occur by slip alone, environmental fatigue including crack growth in air is strongly influenced by crack-tip surface chemistry that adversely affects ductility. Cumulative diffusion, combined with adsorption and chemisorption in the loading half-cycle may promote instantaneous crack extension by brittle microfracture (BMF). Unlike slip, the BMF component will be sensitive to parameters that affect near-tip stresses, such as load history and constraint. BMF dominates near-threshold environmental fatigue. Being a surface phenomenon, it loses its significance with increasing growth rate, as slip-driven crack extension gains momentum and growth becomes less sensitive to environment. The BMF model provides for the first time, a scientific rationale for the residual stress effect as well as the related connection between stress,strain hysteresis and load-sequence sensitivity of metal fatigue including notch response. Experimental evidence obtained on a variety of materials under different loading conditions in air and vacuum appears to support the proposed model and its implications. [source] Effects of solidification structure on tear resistance of Al,7% Si,0.4% Mg cast alloysFATIGUE & FRACTURE OF ENGINEERING MATERIALS AND STRUCTURES, Issue 1 2004S.-W. HAN ABSTRACT The tear resistance behaviour of Al,7% Si,0.4% Mg cast alloys was examined using Kahn-type tear test specimens. Tests were performed for two permanent mould casts with an ordinary dendrite structure and a semi-liquid die cast with a globular cell and fine grain structure. The microstructure of the two permanent mould casts was controlled by the cooling rates and the addition of Ti elements. Tear resistance was evaluated by the ,pop-in' stress, the energies required for crack initiation, UEi and the crack propagation, UEp. Special attention was paid to an effective microstructural parameter for tear resistance improvement. Pop-in, indicating sudden crack extension and arrest, was observed in all specimens. Homogeneous deformation occurs near the notch tip of the semi-liquid die cast, characterized by a refined grain structure. Refinement of the grain size is more effective than that of the dendrite cell size or eutectic Si particle size to increase the energy for crack initiation. Unit propagation energy, UEp, can be converted into a critical stress intensity factor, Kc, which in the semi-liquid die cast was improved due to an increased amount of slant or shear fracture surface. [source] On the Fracture Toughness of Advanced MaterialsADVANCED MATERIALS, Issue 20 2009Maximilien E. Launey Abstract Few engineering materials are limited by their strength; rather they are limited by their resistance to fracture or fracture toughness. It is not by accident that most critical structures, such as bridges, ships, nuclear pressure vessels and so forth, are manufactured from materials that are comparatively low in strength but high in toughness. Indeed, in many classes of materials, strength and toughness are almost mutually exclusive. From a fracture-mechanics perspective, the ability of a microstructure to develop toughening mechanisms acting either ahead or behind the crack tip can result in resistance-curve (R-curve) behavior where the fracture resistance actually increases with crack extension; the implication here is that toughness is often developed primarily during crack growth and not for crack initiation. Biological materials are perfect examples of this; moreover, they offer microstructural design strategies for the development of new materials for structural applications demanding combinations of both strength and toughness. [source] Mode I crack propagation in concrete under fatigue: microscopic observations and modellingINTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS, Issue 13 2002A. Toumi Abstract In this study, three-point bending tests were carried out on notched beams to investigate mode I crack propagation in plain concrete under fatigue. The first part of the study focused on microscopic observations of the crack growth features. Microscopic observations were made using the replica method associated with scanning electron microscopy (SEM). Observations of fatigue crack growth both on the surface and inside the specimens are presented as a comparison between the observed crack lengths and those estimated by the compliance calibration method. In the second part, a finite element model of mode I crack propagation under fatigue is presented. According to the cohesive crack concept, a cohesive force distribution on the crack at various loading stages is assumed, according to both the stress-crack opening relation worked out by Hordijk (1991; Thesis, Technische Universiteit) and a new proposed relation with hysteresis loop. Finite element computation is used to evaluate the crack extension in the bending beams. Numerical predictions are discussed in comparison with experimental results. Copyright © 2002 John Wiley & Sons, Ltd. [source] Estimation of the High-Temperature R Curve for Ceramics from Strength Measurements Including Specimens with Focused Ion Beam NotchesJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 9 2010Stefan Fünfschilling For failure assessment of natural cracks caused by surface treatment, the crack resistance for very small crack extensions is necessary. In ceramics with strongly rising R curves, the initial part of a few micrometer crack propagation is of special interest. For this purpose, the use of very small sharp starter notches introduced by a focused ion beam is recommended. As an example, the initial R curve for gas-pressure-sintered silicon nitride at high temperature will be estimated. Whereas for V-notched bending bars a fracture toughness of about 6,6.3 MPa·m1/2 was determined, the toughness for small strength-relevant cracks was found to be KIc,5.1 MPa·m1/2. [source] | ||||||||||