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Crack Development (crack + development)
Selected AbstractsSphere contact fatigue of a coarse-grained Al2O3 ceramicFATIGUE & FRACTURE OF ENGINEERING MATERIALS AND STRUCTURES, Issue 11 2006T. FETT ABSTRACT The opposite sphere test is an appropriate tool to determine crack-growth exponents for fatigue under repeated contact loading. Lifetime measurements for a coarse-grained Al2O3 are reported. To explain the fatigue exponents that strongly deviated from those obtained in cyclic bending tests, a fracture mechanics analysis was carried out. It was aimed at determining the correct stress intensity factor solution for the tests, including limited dimensions of test specimens deviating from the case of a cone crack in a half space. Cone crack development was observed microscopically and the related stress intensity factors were computed for the observed crack shape. For modelling the fatigue behaviour, it is assumed that the fatigue effect is influenced by a reduction of the shielding term of crack growth resistance due to periodical friction between the grain-interlock bridges in coarse-grained alumina. This results in a loss of traction at the junctions, crack tip shielding is reduced, and the effective load at the crack tip is increased. [source] Image analysis to reveal crack development using a computer simulation of wear and rolling contact fatigueFATIGUE & FRACTURE OF ENGINEERING MATERIALS AND STRUCTURES, Issue 10 2003D. I. FLETCHER ABSTRACT Plastic flow of near-surface rail material under contact loading is a feature of rail,wheel contact, and severe flow typically leads to both wear, and the initiation and development of small surface-breaking cracks. This paper presents results from a ratcheting based computer simulation, which has been developed to allow the simultaneous investigation of wear, crack initiation and early crack propagation. To identify repeatably small crack-like flaws, image analysis is applied to the visual representation of the wearing surface generated by the model. This representation shows a good similarity to traditional micrographs taken from sections of worn surfaces. The model clearly reveals the interaction of wear with crack development, processes which are linked because wear truncates surface-breaking cracks, and can completely remove small surface-breaking cracks. [source] A framework for fracture modelling based on the material forces concept with XFEM kinematicsINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING, Issue 13 2005Ragnar Larsson Abstract A theoretical and computational framework which covers both linear and non-linear fracture behaviour is presented. As a basis for the formulation, we use the material forces concept due to the close relation between on one hand the Eshelby energy,momentum tensor and on the other hand material defects like cracks and material inhomogeneities. By separating the discontinuous displacement from the continuous counterpart in line with the eXtended finite element method (XFEM), we are able to formulate the weak equilibrium in two coupled problems representing the total deformation. However, in contrast to standard XFEM, where the direct motion discontinuity is used to model the crack, we rather formulate an inverse motion discontinuity to model crack development. The resulting formulation thus couples the continuous direct motion to the inverse discontinuous motion, which may be used to simulate linear as well as non-linear fracture in one and the same formulation. In fact, the linear fracture formulation can be retrieved from the non-linear cohesive zone formulation simply by confining the cohesive zone to the crack tip. These features are clarified in the two numerical examples which conclude the paper. Copyright © 2005 John Wiley & Sons, Ltd. [source] Mechanical and fracture properties for predicting cracking in semi-sweet biscuitsINTERNATIONAL JOURNAL OF FOOD SCIENCE & TECHNOLOGY, Issue 4 2005Qasim Saleem Summary Mechanical and fracture properties required for predicting crack development in semi-sweet (,rich tea') biscuits have been experimentally determined. Pilot-scale biscuits of different fat concentrations were prepared and studied with commercial biscuits at different moisture contents. Bending modulus, fracture stress and strain were measured using three-point bending tests. All biscuit types showed considerable dependence on moisture content over a range of 4,12%. Young's modulus and failure stress showed a uniform decrease and failure strain showed an increase with increasing moisture content. For pilot-scale biscuits of different fat concentrations, an increase in fat level caused a decrease in modulus and failure stress values; however, the failure strains were very similar for all the fat types. The testing of the samples with top surface up and top surface down revealed that the sample orientation does not affect the measured parameters. The measured parameters also did not show any directional dependence within the plane, thus assuring that the assumption of an isotropic material would be valid for modelling. The mechanical and fracture properties measured in this study will serve as a very useful set of data to predict the stress state and cracking of the checked biscuits. [source] The influence of fibre placement and position on the efficiency of reinforcement of fibre reinforced composite bridgeworkJOURNAL OF ORAL REHABILITATION, Issue 8 2001A. E. Ellakwa The effect of placement of ultra-high molecular weight polyethylene (UHMWPE) fibres on the flexural properties and fracture resistance of a direct dental composite was investigated. The UHMWPE fibres are increasingly being used for the reinforcement of laboratory fabricated resin composite crown and bridgework. The aim of this study was to assess the effect of a commonly used laboratory fabrication variable on the in vitro strength of beam shaped specimen simulating a three-unit fixed bridge. Four groups (10 specimens per group) of Herculite XRV were prepared for flexural modulus and strength testing after reinforcement with UHMWPE fibres. Two groups of control specimens were prepared without any fibre reinforcement. Half the specimen groups were stored in distilled water and the other groups were stored dry, both at 37 °C for 2 weeks before testing. The results of this study showed that placement of fibre at or slightly away from the tensile side improved the flexural properties of the composite in comparison with the unreinforced control specimen groups whilst the mode of failure differed according to fibre position. Scanning electron microscope (SEM) investigation revealed that placement of the fibre slightly away from the tensile side favoured crack development and propagation within the resin bridging the interfibre spaces in addition to debonding parallel to the direction of fibre placement. Laboratory fabrication variables may effect the strength of fibre reinforced bridgework significantly. [source] Stratigraphic and environmental implications of a large ice-wedge cast at Tjæreborg, DenmarkPERMAFROST AND PERIGLACIAL PROCESSES, Issue 1 2004Else Kolstrup Abstract Exceptionally large ice-wedge casts and composite-wedge casts occur together with involutions (cryoturbations) in a gravel pit near Tjæreborg, western Jutland. The filling reveals distinctly different, vertically-orientated sedimentary units, suggesting discrete events. Variations in wedge structure and infill between the different exposures suggest differences with alternating pools and drier conditions over the former wedges. In an attempt to date and correlate crack development a perusal of local glacial history is given and optically-stimulated-luminescence (OSL) dates are presented. A review of Saalian and early Weichselian wedge casts and deep involutions in other areas in northwest Europe is provided for correlation. It is suggested that the cracking in Tjæreborg took place during the Saale or/and early Weichselian. The existence of such old wedges shows that the present land surface has probably existed since the Saalian, leaving the ground available for cracking and infilling during succeeding periods of permafrost. Copyright © 2004 John Wiley & Sons, Ltd. [source] Deformation Driven Homogenization of Fracturing SolidsPROCEEDINGS IN APPLIED MATHEMATICS & MECHANICS, Issue 1 2005Ercan Gürses The paper discusses numerical formulations of the homogenization for solids with discrete crack development. We focus on multi,phase microstructures of heterogeneous materials, where fracture occurs in the form of debonding mechanisms as well as matrix cracking. The definition of overall properties critically depends on the developing discontinuities. To this end, we extend continuous formulations [1] to microstructures with discontinuities [2]. The basic underlying structure is a canonical variational formulation in the fully nonlinear range based on incremental energy minimization. We develop algorithms for numerical homogenization of fracturing solids in a deformation,driven context with non,trivial formulations of boundary conditions for (i) linear deformation and (ii) uniform tractions. The overall response of composite materials with fracturing microstructures are investigated. As a key result, we show the significance of the proposed non,trivial formulation of a traction,type boundary condition in the deformation,driven context. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Influence of thickness and cooling rate on development of spontaneous cracks in porcelain/zirconia structuresAUSTRALIAN DENTAL JOURNAL, Issue 3 2010M Guazzato Abstract Background:, Clinical studies reporting the outcome of zirconia-based restorations indicate that the strength of the zirconia frameworks is sufficient to withstand masticatory forces. However, a significant incidence of cohesive fracture of the veneering porcelain has been reported. The aim of this study was to investigate spontaneous crack development (chipping, rupture) in a range of porcelains veneered to a zirconia core as a result of thermal stresses induced by changes in thickness and cooling rate. The hypothesis tested was that crack incidence would increase with increased veneer thickness and faster cooling rates. Methods:, Zirconia spheres (diameter 7.8 mm) were veneered with 1.5 gm (thickness ratio 1:2) and 2.5 gm (thickness ratio 1:1) of five nominally compatible commercially available porcelains. The manufacturers' firing cycles and a rapid cooling firing cycle were followed. Results:, Multiple regression analysis showed positive associations between the occurrence of cracks and the three covariates (materials, thickness and cooling rate). The incidence of cracks and rupture of the veneering porcelain increased with a faster cooling rate and increased thickness of the specimens in three porcelain,zirconia combinations. Conclusions:, Crack incidence increased with increased porcelain veneer thickness and faster cooling rates in nominally compatible porcelain/zirconia systems in the geometrically configured specimens tested. [source] | |||||||||||||