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Fracture Mechanics (fracture + mechanic)
Kinds of Fracture Mechanics Terms modified by Fracture Mechanics Selected AbstractsEnhancement of fracture toughness due to energy screening effectin the early stages of non-elastic failureFATIGUE & FRACTURE OF ENGINEERING MATERIALS AND STRUCTURES, Issue 8 2003M. P. WNUK ABSTRACT A triaxiality-dependent cohesive zone model for a stationary and a quasistatic crack is proposed. The model is rooted in the mesomechanical approach to Fracture Mechanics and is inspired by the quantum law concerning emission of light, which was postulated by Max Planck at the end of the 19th century. The model provides an extension of the early concepts of Barenblatt, Dugdale and the Bilby,Cotrell,Swinden team. It also incorporates the experimental observations of the pre-fracture states due to Panin and his school in Tomsk. Relations between micro- and macroparameters that characterize the deformation and fracture processes in dissipative media are described in detail. The analysis suggests that the ratio of the ,true' work of fracture to the total energy dissipated during the course of the irreversible deformation contained within the end zone can be used as a measure of material resistance to a quasistatically propagating fracture. [source] Weight Function Analysis on the R-Curve Behavior of Multilayered Alumina,Zirconia CompositesJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 6 2002Robert J. Moon The single-edge V-notched-beam (SEVNB) testing geometry was used to measure the crack growth resistance (R -curve) behavior of multilayered alumina,zirconia composites. Fracture mechanics weight function analysis was applied to predict the R -curve behavior of multilayered composites having a stepwise change in composition. These results were then used to differentiate the influence of residual stresses from crack-bridging stresses on the measured R -curve behavior. [source] Reliability analysis of universal joint of a compliant platformFATIGUE & FRACTURE OF ENGINEERING MATERIALS AND STRUCTURES, Issue 7 2010M. M. ZAHEER ABSTRACT The paper describes a methodology for computation of fatigue reliability of universal joint in an articulated offshore tower. Failure criteria were formulated using the conventional Palmgren-Miner rule (S-N curve approach) and the fracture mechanics (F-M) principle. The dynamic analysis of double hinged articulated tower under wind and waves is carried out in time domain. The response histories of hinge shear stresses are employed for the reliability analysis. Advanced first-order reliability method and Monte Carlo simulation method were used to estimate the reliability. Various parametric studies were carried out, which yield important information for the reliability based design. The S-N curve approach yields a significantly conservative estimate of probability of failure when compared to the F-M approach. [source] Fatigue life prediction of cracked padded platesFATIGUE & FRACTURE OF ENGINEERING MATERIALS AND STRUCTURES, Issue 3-4 2008K. GUO ABSTRACT The fatigue crack propagation analyses of padded plates are conducted to predict the crack growth behaviour under various loading conditions. The fatigue life of a padded plate with a single edge crack originating from the weld toe is calculated using the weight function approach. The fatigue strength of padded plates with different pad thickness under remote loading conditions was investigated and compared to the T-plate joint. The improvement of the fatigue strength of the pad design is verified. The thickness effect of the padded plate was investigated using the fracture mechanics approach. The geometrically similar model pairs with different initial crack sizes were investigated under remote loading conditions. It was shown that the thickness effect depends on both stress concentration and initial crack size. [source] Incremental model for fatigue crack growth based on a displacement partitioning hypothesis of mode I elastic,plastic displacement fieldsFATIGUE & FRACTURE OF ENGINEERING MATERIALS AND STRUCTURES, Issue 7 2007S. POMMIER ABSTRACT The mode I displacement field in the near crack tip region is assumed to be depicted by its partition into an elastic field and a plastic field. Then, each part of the displacement field is also assumed to be the product of a reference field, a function of space coordinates only, and of an intensity factor, function of the loading conditions. This assumption, classical in fracture mechanics, enables one to work at the global scale since fracture criteria can be formulated as a function of the stress intensity factors only. In the present case, the intensity factor of the plastic part of the displacement field measures crack tip plastic flow rate at the global scale. On the basis of these hypotheses, the energy balance equation and the second law of thermodynamics are written at the global scale, i.e. the scale of the K-dominance area. This enables one to establish a yield criterion and a plastic flow rule for the crack tip region. Then, assuming a relation between plastic flow in the crack tip region and fatigue crack growth allows an incremental model for fatigue crack growth to be built. A few examples are given to show the versatility of the model and its ability to reproduce memory effects associated with crack tip plasticity. [source] The perturbation method and the extended finite element method.FATIGUE & FRACTURE OF ENGINEERING MATERIALS AND STRUCTURES, Issue 8 2006An application to fracture mechanics problems ABSTRACT The extended finite element method has been successful in the numerical simulation of fracture mechanics problems. With this methodology, different to the conventional finite element method, discretization of the domain with a mesh adapted to the geometry of the discontinuity is not required. On the other hand, in traditional fracture mechanics all variables have been considered to be deterministic (uniquely defined by a given numerical value). However, the uncertainty associated with these variables (external loads, geometry and material properties, among others) it is well known. This paper presents a novel application of the perturbation method along with the extended finite element method to treat these uncertainties. The methodology has been implemented in a commercial software and results are compared with those obtained by means of a Monte Carlo simulation. [source] Fatigue of cantilevered pipe fittings subjected to vibrationFATIGUE & FRACTURE OF ENGINEERING MATERIALS AND STRUCTURES, Issue 8 2003M. HAMBLIN ABSTRACT Vibration fatigue failures in piping systems often occur at cantilevered small-bore fittings (SBF) such as pressure tappings and drain valves. Piping vibration has often been assessed by measuring vibration displacement or velocity. However, in the case of straight cantilevered fittings with a concentrated mass, a better method exists. This paper presents a simple robust method of calculating vibration induced stress for cantilevered fittings at both the fitting branch neck and the main pipe. The method is based on measurement of acceleration at the concentrated mass (e.g., valve) and simple mechanics calculations. Extensive laboratory and field verification of the technique is presented. The choices of a fatigue strength criterion, such as ASME BPVC VIII Div. 2, BS7608, or a fracture mechanics approach such as BSI PD6493, are discussed. Several case studies are presented as well as simple ,good engineering practice' rules-of-thumb for SBF design. These ,rules-of-thumb' are now incorporated in the piping specifications of the author's company. [source] Strength estimation of ceramic,metal joints with various interlayer thicknessFATIGUE & FRACTURE OF ENGINEERING MATERIALS AND STRUCTURES, Issue 5 2003M. TAKAHASHI ABSTRACT Residual stresses generated by the mismatch of thermal expansion coefficients of ceramics and metals affect the strength of ceramic,metal joints. An interlayer metal can be inserted between the ceramic and metal in order to relax this stress. An analysis was carried out of the residual stresses produced during joint-cooling and in 4-point bending tests. The effects of interlayer thickness on ceramic,metal joint strength were then studied by considering a superimposed stress distribution of the residual stress and the bending stress. Finally, joint strength was estimated from fracture mechanics and strength probability analysis by considering the residual stress distribution, defect size and position of pre-existing defects in the ceramic parts. As a result of this study, we suggest an optimum material selection and interlayer thickness for ceramic,metal joint structures. This approach is generally suitable for the design of electrical and mechanical structures. [source] Application of interface finite elements to three-dimensional progressive failure analysis of adhesive jointsFATIGUE & FRACTURE OF ENGINEERING MATERIALS AND STRUCTURES, Issue 5 2003J. P. M. GONÇALVES ABSTRACT The paper presents a new model for three-dimensional progressive failure analysis of adhesive joints. The method uses interface elements and includes a damage model to simulate progressive debonding. The interface finite elements are placed between the adherents and the adhesive. The damage model is based on the indirect use of fracture mechanics and allows the simulation of the initiation and growth of damage at the interfaces without considering the presence of initial flaws. The application of the model to single lap joints is presented. Experimental tests were performed in aluminium/epoxy adhesive joints. Linear elastic and elastoplastic analyses were performed and the predicted failure load for the elastoplastic case agrees with experimental results. [source] The effect of contact load reduction on the fatigue life of pearlitic rail steel in lubricated rolling,sliding contactFATIGUE & FRACTURE OF ENGINEERING MATERIALS AND STRUCTURES, Issue 8 2000D. I. Fletcher Twin-disc contact simulation tests were carried out to investigate the influence of contact pressure variation on rail steel fatigue life. Both a colloidal suspension of molybdenum disulphide in an oil carrier fluid (similar to a commercial flange lubrication product) and water were used as lubricants. It was found that the reduction from 1500 to 900 MPa of the maximum Hertzian contact pressure (at which a molybdenum,disulphide-lubricated and previously worn rail sample was tested) extended the fatigue life of the rail steel by over five times. For water lubrication a similar reduction in contact pressure produced only a marginal increase in fatigue life. The results were found to be in qualitative agreement with the predictions of the newly developed Three Mechanism (TM) model of rolling contact fatigue, which is introduced here. This model combines the mechanisms of ratcheting and the fracture mechanics-based mechanisms of both shear stress- and tensile stress-driven, fluid-assisted, crack growth. [source] The effect of pitting corrosion on fatigue lifeFATIGUE & FRACTURE OF ENGINEERING MATERIALS AND STRUCTURES, Issue 7 2000Dolley Fatigue testing of pre-pitted 2024-T3 aluminium alloy specimens is performed in laboratory air at 22 °C and 40% RH to characterize the effect of pitting corrosion on fatigue life. Specimens, pre-corroded in a 0.5 M NaCl solution from 48 to 384 h, have fatigue lives that are reduced by more than one order of magnitude after 384 h pre-corrosion as compared to those of uncorroded specimens. The reduction in fatigue life is interpreted in terms of the influence of the time of exposure to the corrosive environment or pit size. The crack-nucleating pit sizes, ranging from 20 to 70 ,m, are determined from post-fracture examinations by scanning electron microscopy. Fatigue lives are estimated using a fracture mechanics approach and are shown to be in good agreement with the actual data. A probabilistic analysis shows that the distribution of fatigue life is strongly correlated to the distribution in nucleating pit size. [source] A numerical method for the study of shear band propagation in soft rocksINTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS, Issue 13 2009Marta Castelli Abstract This paper investigates the possibility of interpreting progressive shear failure in hard soils and soft rocks as the result of shear propagation of a pre-existing natural defect. This is done through the application of the principles of fracture mechanics, a slip-weakening model (SWM) being used to simulate the non-linear zone at the tips of the discontinuity. A numerical implementation of the SWM in a computation method based on the boundary element technique of the displacement discontinuity method (DDM) is presented. The crack and the non-linear zone at the advancing tip are represented through a set of elements, where the displacement discontinuity (DD) in the tangential direction is determined on the basis of a friction law. A residual friction angle is assumed on the crack elements. Shear resistance decreases on elements in the non-linear zone from a peak value at the tip, which is characteristic of intact material, to the residual value. The simulation of a uniaxial compressive test in plane strain conditions is carried out to exemplify the numerical methodology. The results emphasize the role played by the critical DD on the mechanical behaviour of the specimen. A validation of the model is shown through the back analysis of some experimental observations. The results of this back analysis show that a non-linear fracture mechanics approach seems very promising to simulate experimental results, in particular with regards to the shear band evolution pattern. Copyright © 2009 John Wiley & Sons, Ltd. [source] Transient solution for a plane-strain fracture driven by a shear-thinning, power-law fluidINTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS, Issue 14 2006D. I. Garagash Abstract This paper analyses the problem of a fluid-driven fracture propagating in an impermeable, linear elastic rock with finite toughness. The fracture is driven by injection of an incompressible viscous fluid with power-law rheology. The relation between the fracture opening and the internal fluid pressure and the fracture propagation in mobile equilibrium are described by equations of linear elastic fracture mechanics (LEFM), and the flow of fluid inside the fracture is governed by the lubrication theory. It is shown that for shear-thinning fracturing fluids, the fracture propagation regime evolves in time from the toughness- to the viscosity-dominated regime. In the former, dissipation in the viscous fluid flow is negligible compared to the dissipation in extending the fracture in the rock, and in the later, the opposite holds. Corresponding self-similar asymptotic solutions are given by the zero-viscosity and zero-toughness (J. Numer. Anal. Meth. Geomech. 2002; 26:579,604) solutions, respectively. A transient solution in terms of the crack length, the fracture opening, and the net fluid pressure, which describes the fracture evolution from the early-time (toughness-dominated) to the large-time (viscosity-dominated) asymptote is presented and some of the implications for the practical range of parameters are discussed. Copyright © 2006 John Wiley & Sons, Ltd. [source] Comparison between cohesive zone modelsINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN BIOMEDICAL ENGINEERING, Issue 11 2004K. Y. Volokh Cohesive zone models (CZMs) are widely used for numerical simulation of the fracture process. Cohesive zones are surfaces of discontinuities where displacements jump. A specific constitutive law relating the displacement jumps and proper tractions defines the cohesive zone model. Within the cohesive zone approach crack nucleation, propagation, and arrest are a natural outcome of the theory. The latter is in contrast to the traditional approach of fracture mechanics where stress analysis is separated from a description of the actual process of material failure. The common wisdom says that only cohesive strength,the maximum stress on the traction,separation curve,and the separation work,the area under the traction,separation curve,are important in setting a CZM while the shape of the traction,separation curve is subsidiary. It is shown in our note that this rule may not be correct and a specific shape of the cohesive zone model can significantly affect results of the fracture analysis. For this purpose four different cohesive zone models,bilinear, parabolic, sinusoidal, and exponential,are compared by using a block-peel test, which allows for simple analytical solutions. Numerical performance of the cohesive zone models is considered. It appears that the convergence properties of nonlinear finite element analyses are similar for all four CZMs in the case of the block-peel test. Copyright © 2004 John Wiley & Sons, Ltd. [source] Conserving Galerkin weak formulations for computational fracture mechanicsINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN BIOMEDICAL ENGINEERING, Issue 12 2002Shaofan Li Abstract In this paper, a notion of invariant Galerkin-variational weak forms is proposed. Two specific invariant variational weak forms, the J-invariant and the L-invariant, are constructed based on the corresponding conservation laws in elasticity, one of which is the conservation of Eshelby's energy-momentum (Eshelby. Philos. Trans. Roy. Soc. 1951; 87: 12; In Solid State Physics, Setitz F, Turnbull D (eds). Academic Press: New York, 1956; 331; Rice, J. Appl. Mech. 1968; 35: 379). It is shown that the finite element solution obtained from the invariant Galerkin weak formulations proposed here can conserve the value of J-integral, or L-integral exactly. In other words, the J and L integrals of the Galerkin finite element solutions are path independent in the discrete sense. It is argued that by using the J-invariant Galerkin weak form to compute near crack-tip field in an elastic solid, one may accurately calculate the crack extension energy release rate and subsequently the stress intensity factors in numerical computations, because the flux of the energy-momentum is conserved in discrete computations. This may provide an alternative means to accurately simulate crack growth and propagation. Copyright © 2002 John Wiley & Sons, Ltd. [source] An optimally convergent discontinuous Galerkin-based extended finite element method for fracture mechanicsINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING, Issue 6 2010Yongxing Shen Abstract The extended finite element method (XFEM) enables the representation of cracks in arbitrary locations of a mesh. We introduce here a variant of the XFEM rendering an optimally convergent scheme. Its distinguishing features are as follows: (a) the introduction of singular asymptotic crack tip fields with support on only a small region around the crack tip (the enrichment region), (b) only one and two enrichment functions are added for anti-plane shear and planar problems, respectively and (c) the relaxation of the continuity between the enrichment region and the rest of the domain, and the adoption of a discontinuous Galerkin (DG) method therein. The method is provably stable for any positive value of a stabilization parameter, and by weakly enforcing the continuity between the two regions it eliminates ,blending elements' partly responsible for the suboptimal convergence of some early XFEMs. Moreover, the particular choice of enrichment functions results in a surprisingly sparse stiffness matrix that remains reasonably conditioned as the mesh is refined. More importantly, the stress intensity factors can be extracted with a satisfactory accuracy as primary unknowns. Quadrature strategies required for the optimal convergence are also discussed. Finally, the DG method was modified to retain stability based on an inf-sup condition. Copyright © 2009 John Wiley & Sons, Ltd. [source] An enriched element-failure method (REFM) for delamination analysis of composite structuresINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING, Issue 6 2009X. S. Sun Abstract This paper develops an enriched element-failure method for delamination analysis of composite structures. This method combines discontinuous enrichments in the extended finite element method and element-failure concepts in the element-failure method within the finite element framework. An improved discontinuous enrichment function is presented to effectively model the kinked discontinuities; and, based on fracture mechanics, a general near-tip enrichment function is also derived from the asymptotic displacement fields to represent the discontinuity and local stress intensification around the crack-tip. The delamination is treated as a crack problem that is represented by the discontinuous enrichment functions and then the enrichments are transformed to external nodal forces applied to nodes around the crack. The crack and its propagation are modeled by the ,failed elements' that are applied to the external nodal forces. Delamination and crack kinking problems can be solved simultaneously without remeshing the model or re-assembling the stiffness matrix with this method. Examples are used to demonstrate the application of the proposed method to delamination analysis. The validity of the proposed method is verified and the simulation results show that both interlaminar delamination and crack kinking (intralaminar crack) occur in the cross-ply laminated plate, which is observed in the experiment. Copyright © 2009 John Wiley & Sons, Ltd. [source] Robust and direct evaluation of J2 in linear elastic fracture mechanics with the X-FEMINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING, Issue 10 2008G. Legrain Abstract The aim of the present paper is to study the accuracy and the robustness of the evaluation of Jk -integrals in linear elastic fracture mechanics using the extended finite element method (X-FEM) approach. X-FEM is a numerical method based on the partition of unity framework that allows the representation of discontinuity surfaces such as cracks, material inclusions or holes without meshing them explicitly. The main focus in this contribution is to compare various approaches for the numerical evaluation of the J2 -integral. These approaches have been proposed in the context of both classical and enriched finite elements. However, their convergence and the robustness have not yet been studied, which are the goals of this contribution. It is shown that the approaches that were used previously within the enriched finite element context do not converge numerically and that this convergence can be recovered with an improved strategy that is proposed in this paper. Copyright © 2008 John Wiley & Sons, Ltd. [source] Extrinsic cohesive modelling of dynamic fracture and microbranching instability in brittle materialsINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING, Issue 8 2007Zhengyu (Jenny) Zhang Abstract Dynamic crack microbranching processes in brittle materials are investigated by means of a computational fracture mechanics approach using the finite element method with special interface elements and a topological data structure representation. Experiments indicate presence of a limiting crack speed for dynamic crack in brittle materials as well as increasing fracture resistance with crack speed. These phenomena are numerically investigated by means of a cohesive zone model (CZM) to characterize the fracture process. A critical evaluation of intrinsic versus extrinsic CZMs is briefly presented, which highlights the necessity of adopting an extrinsic approach in the current analysis. A novel topology-based data structure is employed to enable fast and robust manipulation of evolving mesh information when extrinsic cohesive elements are inserted adaptively. Compared to intrinsic CZMs, which include an initial hardening segment in the traction,separation curve, extrinsic CZMs involve additional issues both in implementing the procedure and in interpreting simulation results. These include time discontinuity in stress history, fracture pattern dependence on time step control, and numerical energy balance. These issues are investigated in detail through a ,quasi-steady-state' crack propagation problem in polymethylmethacrylate. The simulation results compare reasonably well with experimental observations both globally and locally, and demonstrate certain advantageous features of the extrinsic CZM with respect to the intrinsic CZM. Copyright © 2007 John Wiley & Sons, Ltd. [source] Dual boundary element method for anisotropic dynamic fracture mechanicsINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING, Issue 9 2004E. L. Albuquerque Abstract In this work, the dual boundary element method formulation is developed for effective modelling of dynamic crack problems. The static fundamental solutions are used and the domain integral, which comes from the inertial term, is transformed into boundary integrals using the dual reciprocity technique. Dynamic stress intensity factors are computed from crack opening displacements. Comparisons are made with quasi-isotropic as well as anisotropic results, using the sub-region technique. Several examples are presented to assess the accuracy and efficiency of the proposed method. Copyright © 2004 John Wiley & Sons, Ltd. [source] An enriched meshless method for non-linear fracture mechanicsINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING, Issue 2 2004B. N. Rao Abstract This paper presents an enriched meshless method for fracture analysis of cracks in homogeneous, isotropic, non-linear-elastic, two-dimensional solids, subject to mode-I loading conditions. The method involves an element-free Galerkin formulation and two new enriched basis functions (Types I and II) to capture the Hutchinson,Rice,Rosengren singularity field in non-linear fracture mechanics. The Type I enriched basis function can be viewed as a generalized enriched basis function, which degenerates to the linear-elastic basis function when the material hardening exponent is unity. The Type II enriched basis function entails further improvements of the Type I basis function by adding trigonometric functions. Four numerical examples are presented to illustrate the proposed method. The boundary layer analysis indicates that the crack-tip field predicted by using the proposed basis functions matches with the theoretical solution very well in the whole region considered, whether for the near-tip asymptotic field or for the far-tip elastic field. Numerical analyses of standard fracture specimens by the proposed meshless method also yield accurate estimates of the J -integral for the applied load intensities and material properties considered. Also, the crack-mouth opening displacement evaluated by the proposed meshless method is in good agreement with finite element results. Furthermore, the meshless results show excellent agreement with the experimental measurements, indicating that the new basis functions are also capable of capturing elastic,plastic deformations at a stress concentration effectively. Copyright © 2003 John Wiley & Sons, Ltd. [source] Enrichment of enhanced assumed strain approximations for representing strong discontinuities: addressing volumetric incompressibility and the discontinuous patch testINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING, Issue 1 2004J. E. Dolbow Abstract We present a geometrically non-linear assumed strain method that allows for the presence of arbitrary, intra-finite element discontinuities in the deformation map. Special attention is placed on the coarse-mesh accuracy of these methods and their ability to avoid mesh locking in the incompressible limit. Given an underlying mesh and an arbitrary failure surface, we first construct an enriched approximation for the deformation map with the non-linear analogue of the extended finite element method (X-FEM). With regard to the richer space of functions spanned by the gradient of the enriched approximation, we then adopt a broader interpretation of variational consistency for the construction of the enhanced strain. In particular, in those elements intersected by the failure surface, we construct enhanced strain approximations which are orthogonal to piecewise-constant stress fields. Contrast is drawn with existing strong discontinuity approaches where the enhanced strain variations in localized elements were constructed to be orthogonal to constant nominal stress fields. Importantly, the present formulation gives rise to a symmetric tangent stiffness matrix, even in localized elements. The present modification also allows for the satisfaction of a discontinuous patch test, wherein two different constant stress fields (on each side of the failure surface) lie in the solution space. We demonstrate how the proposed modifications eliminate spurious stress oscillations along the failure surface, particularly for nearly incompressible material response. Additional numerical examples are provided to illustrate the efficacy of the modified method for problems in hyperelastic fracture mechanics. Copyright © 2003 John Wiley & Sons, Ltd. [source] Influence of processing conditions and physicochemical interactions on morphology and fracture behavior of a clay/thermoplastic/thermosetting ternary blendJOURNAL OF APPLIED POLYMER SCIENCE, Issue 6 2010M. Hernandez Abstract This study provides information on the mechanical behavior of epoxy-poly(methyl methacrylate) (PMMA)-clay ternary composites, which have been prepared using the phase separation phenomenon of PMMA and the introduction of organophilic-modified montmorillonites (MMTs), the continuous matrix being the epoxy network. Two dispersion processing methods are used: a melt processing without any solvent and an ultrasonic technique with solvent and a high-speed stirrer. TEM analysis shows that phase separation between PMMA and the epoxy network was obtained in the shape of spherical nodules in the presence of the clay in both process methods used. Nanoclay particles were finely dispersed inside thermosetting matrix predominantly delaminated when ultrasonic blending was used; whereas micrometer-sized aggregates were formed when melt blending was used. The mechanical behavior of the ternary nanocomposites was characterized using three-point bending test, dynamic mechanical analysis (DMA), and linear elastic fracture mechanics. The corresponding fracture surfaces were examined by scanning electron microscopy to identify the relevant fracture mechanisms involved. It was evidenced that the better dispersion does not give the highest toughness because ternary nanocomposites obtained by melt blending present the highest fracture parameters (KIc). Some remaining disordered clay tactoids seem necessary to promote some specific toughening mechanisms. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010 [source] Micromechanical analysis of silicon nitride: a comparative study by fracture mechanics and Raman microprobe spectroscopyJOURNAL OF RAMAN SPECTROSCOPY, Issue 9 2002Shigemi Tochino Raman microprobe spectroscopy was used to characterize in situ microstress fields which develop during fracture of a toughened silicon nitride (Si3N4) polycrystal. Maps of microscopic stress were collected in the neighborhood of a propagating crack both at zero and at critical loading conditions. Micromechanics results by Raman spectroscopy were analyzed and compared with conventional fracture mechanics assessments, such as the evaluation of rising R -curve behavior and crack opening displacement. Outcomes of these assessments illustrate that, despite the approximations involved in the piezo-spectroscopic equations used for calculating the microstress field from a local Raman shift, Raman microprobe spectroscopy is a viable method for semi-quantitative investigations of microfracture mechanisms in advanced ceramic materials. Copyright © 2002 John Wiley & Sons, Ltd. [source] Mechanism for Salt ScalingJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 4 2006John J. Valenza II Over the past 60 years, concrete infrastructure in cold climates has deteriorated by "salt scaling," which is superficial damage that occurs during freezing in the presence of saline water. It reduces mechanical integrity and necessitates expensive repair or replacement. The phenomenon can be demonstrated by pooling a solution on a block of concrete and subjecting it to freeze/thaw cycles. The most remarkable feature of salt scaling is that the damage is absent if the pool contains pure water, it becomes serious at concentrations of a few weight percent, and then stops at concentrations above about 6 wt%. In spite of a wealth of research, the mechanism responsible for this damage has only recently been identified. In this article, we show that salt scaling is a consequence of the fracture behavior of ice. The stress arises from thermal expansion mismatch between ice and concrete, which puts the ice in tension as the temperature drops. Considering the mechanical and viscoelastic properties of ice, it is shown that this mismatch will not cause pure ice to crack, but moderately concentrated solutions are expected to crack. Cracks in the brine ice penetrate into the substrate, resulting in superficial damage. At high concentrations, the ice does not form a rigid enough structure to result in significant stress, so no damage occurs. The morphology of cracking is predicted by fracture mechanics. [source] Crack Toughness Behaviour of Multiwalled Carbon Nanotube (MWNT)/Polycarbonate NanocompositesMACROMOLECULAR RAPID COMMUNICATIONS, Issue 15 2005Bhabani K. Satapathy Abstract Summary: The morphology and fracture behaviour of polycarbonate (PC)/multiwalled carbon nanotube (MWNT) composites have been studied by AFM and post-yield fracture mechanics. The essential work of fracture (EWF) method has been used to distinguish between two terms representing the resistance to crack initiation and crack propagation. A maximum in the non-essential work of fracture was observed at 2 wt.-% MWNT, demonstrating enhanced resistance to crack propagation compared to pure PC. At 4 wt.-% MWNT, a tough-to-brittle transition has been observed. The time-resolved in-situ strain field analysis revealed that the onset of crack initiation was shifted to a shorter time for nanocomposites with 4 wt.-% MWNT compared to that with 2 wt.-%, and thus explained the existence of a tough-to-brittle transition in these nanocomposites. [source] Spatially resolved defect studies on fatigued carbon steelPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 10 2007P. Eich Abstract Several samples of the common ferritic tool steel AISI 1045 were fatigued in cyclic load tests. The local distribution of the Von-Mieses stress ,VM was simulated using the finite elements method (FEM). In the regions of interest, where ,VM reaches maximum values, the defect distribution was measured spatially resolved by Doppler-spectroscopy (DBAR) employing the Bonn Positron Microprobe (BPM). The lateral distribution of the S-parameter, which could be described by a simple model derived from linear fracture mechanics, corresponds well with the simulated Von-Mieses stress. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Fracture and failure behavior of fabric-reinforced all-poly(propylene) composite (Curv®),POLYMERS FOR ADVANCED TECHNOLOGIES, Issue 2 2007Gábor Romhány Abstract The in-plane static fracture of a fabric reinforced all-poly(propylene) (all-PP, Curv®) composite was studied at ambient temperature using the concept of the linear elastic fracture mechanics. The apparent fracture toughness was determined on single-edge notched tensile specimens (SEN-T) considering the maximum load. The related values did not differ much from those determined by the resistance curve (KR) method. The crack growth, requested to construct the KR curves, was traced by the movement in the center of gravity of the cumulative amplitude of the located acoustic emission (AE) events. The quality of consolidation of the all-PP composite was reflected by the force-displacement curve (appearance of pop-in), course of the cumulative AE events during loading, extension and change of the estimated damage zone during fracture. The failure behavior was studied also by fractography and is discussed. Copyright © 2006 John Wiley & Sons, Ltd. [source] Re-entrant corner problems for isotropic materials and layered compositesPROCEEDINGS IN APPLIED MATHEMATICS & MECHANICS, Issue 1 2005E. Schnack Modern materials like composites in different types require new methods in computational fracture mechanics. Besides of classical fracture mechanics we have to solve the asymptotic solution around crack tips. This can be done on the basis of the Kondratievs theorem with the Pietrov-Galerkin method to solve the unknown eigenvalues for those problems. Additionally, we have to de.ne instead classical modes I, II and III, mode I* up to mode IV* to interpret the computational results for the unknown eigenvalues. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Beanspruchungsanalyse von Holzbauteilen durch 2D-PhotogrammetrieBAUTECHNIK, Issue 2 2005Bettina Franke Dipl.-Ing. Für die Bewertung der Tragsicherheit von Bauteilen aus Voll- und Brettschichtholz in Lasteinleitungs- und Störbereichen mittels der Bruchmechanik ist die Kenntnis von kritischen Bruchkennwerten Voraussetzung. Realitätsnahe Kennwerte können aus der Kombination experimenteller Untersuchungen zur Bestimmung der Rißaufweitung und der Rißlänge mit daran anschließender FE-Simulation gewonnen werden. Aufgrund der, bisher bei konventionellen Meßverfahren, nicht ausreichenden Zuordnung der Meßwerte taktil applizierter Meßaufnehmer lag es nahe das Rißwachstum mit Hilfe der Photogrammetrie zu untersuchen. Mit dem entwickelten Meßsystem ist nunmehr die Möglichkeit der exakten Vermessung der Rißgeometrie zur Bestimmung von bruchmechanischen Kennwerten gegeben. Zusätzlich ermöglicht der Einsatz der Photogrammetrie die Verdichtung der Anzahl variierender diskreter Meßpunkte, so daß ein deutlich vollständigeres Bild der örtlichen Verformungen gewonnen, und damit die Aussagekraft eines Versuchs wesentlich verbessert werden kann. Strain analysis of solid wood and glued laminated timber members by close range photogrammetry. The assumption of critical fracture mechanics parameters for the evaluation of the load-bearing safety of structural units of full and board laminated timber in loaded areas and disturbance ranges the use of fracture mechanics is required. Realistic parameters can be gained only from experimental investigations that are transferred to the determination of the crack opening and the crack length with subsequent FE-simulation. Due to the insufficient appropriation of measured data gained by tactile position encoders in conventional measuring procedures it is advisable to investigate crack growth with the help of the photogrammetry. Using the developed measuring system it is now possible to measure the crack geometry exactly for the determination of fracture mechanics parameters. Additionally a complete picture of the local deformations and the validity of a test can be improved considerably by the possibility of the varying number of discrete measuring points. [source] | ||||||||||||||||