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Sand production in Wells is the main contradiction facing the explo让ation of unconsolidated sandstone oil and gas reservoirs・ Resin consolidated artificial wellbore is the most widely used chemical sand control method in oil fields at present because of its excellent effect on fine sand control, which can achieve screen-free sand control under complex well conditions ・ The extended finite element method (XFEM) was used to simulate the crack passing through a single particle, a pair of particles and a plurality of evenly distributed particles, and the influence of nanoparticles on the crack growth path and damage dissipation energy was studied・ It was found that the influence range of nano-SiO2 on the crack was 5 nanoparticle diameters, and the best mass fraction of nano-SiO2 was 3% of the resin matrix. The results of measuring the impact strength of resin spline and the compressive strength of resin sand control body show that when the mass fraction of nano-SiO2 is 3%, the impact strength of resin spline increases about 39% and the compressive strength of resin sand control body increases about 36%, which significantly improves the mechanical properties of phenolic resin. [ABSTRACT FROM AUTHOR]

Introduction: The finite element method has become popular to determine fracture mechanics parameters, but this type of numerical analysis may or not lead to accurate results, depending on the model quality and the implementation of the established methods. Objective: Implement and validate the calculation of the stress intensity factor and the J integral through the displacement extrapolation, domain integral and interaction integral methods, through finite element modeling of cracked part problems in 2D and 3D. Materials and methods: The stress intensity factor and the integral J were calculated in a compact cracked specimen and a plate with a semi-circumferential surface crack, by means of 2D and 3D finite element models, and these results were verified with those obtained by means of published analytical expressions. Results and Discussion: The error of the results provided by the finite element models does not exceed 3% for the two-dimensional models and 7% for the three-dimensional ones, which demonstrates their quality and precision. Conclusions: The implementation of the calculation of the fracture mechanics parameters is considered validated, which makes it possible to extend its use in the future for constructions and equipment that present cracking in service. [ABSTRACT FROM AUTHOR]

The case history of a deep-seated slope movement in a complex rock formation (Marly-Arenaceous Formation) is analyzed. The movement, monitored for more than 20 years, was recognized after the discovery of intense cracking in the concrete lining of a hydraulic tunnel running across the slope. The time history of displacements shows that the ongoing deformation process is essentially a stationary creep phenomenon, also influenced by transient variations in pore pressure distribution. The shearing zone is mainly formed by tectonized clay gouge and is characterized by a mobilized strength close to residual. The slope has been modelled (by DEM approach) as a complex blocky structure defined by several joint sets: bedding planes, inclined and sub-vertical joints. Different hypotheses about the geometry of the slip surface, compatible with field evidences, are discussed as well as their influence on the critical friction angle of the slope. The type of model adopted for rock mass, with continuous or staggered joints, influences the pre-failure deformation mode, but the failure conditions are not as much affected. Finally, the response of DEM models to the increase in water level at the toe of the slope and to the rise of the groundwater table inside the slope has been analyzed. The results of simplified models, without flow calculation, and coupled hydro-mechanical analyses are compared, finding some relevant differences. [ABSTRACT FROM AUTHOR]

A study of cracking failure of three-way T junctions between inlet manifold and branch pipes in large-scale hydrogen reformer furnaces in terms of pipe material, operation medium, properties of cracking zone material, and vertical displacements of inlet manifold and branch pipes is presented. The results revealed that cracking failures of three-way T junctions can be attributed to the stress concentration, which was caused by incongruous vertical deformations of inlet manifold and branch pipes, as well as variations in horizontal deformation heights of branch pipes. The incongruous displacements of inlet manifold and branch pipes were significantly mitigated by optimization design of the suspension device of the inlet pipe using the finite element method. The vertical displacement differences between inlet manifold and branch pipes and the east-west vertical displacement differences between the five branch pipes were reduced by 48 and 91.5%, respectively. Bending of three-way T junctions was relieved and torsions were eliminated, resulting in mitigation of stress concentration. As a result, the maximum equivalence stress was reduced by 40%, satisfying strength requirements. In this way, cracking failures of inlet manifold in hydrogen reformer furnace were prevented and stable hydrogen production was guaranteed. [ABSTRACT FROM AUTHOR]

A computational constitutive model is presented to predict matrix cracking evolution in laminates under in-plane loading. Transverse cracks are treated as separate discontinuities in the micro-model that provides damage parameters for the macro-model. Both micro- and macro-models are implemented using finite element analysis, specifically, ANSYS, to avoid limitation of analytical micro-modeling. The computational cost of the micro-model is limited to constructing a database of micro-model predictions a priori. The macro-model is simply a finite element analysis discretization of the structure using plane stress or shell elements in ANSYS. The macro-model queries the database, which effectively becomes a constitutive model. The damage surfaces in the database are obtained from the results of large number of finite element micro-scale (unit-cell) analyses. The proposed procedure is implemented in ANSYS as a usermaterial subroutine for transverse crack initiation and propagation in symmetric cross-ply and [0r/(θ / −θ)s/0n]s laminates under in-plane loads. This method is also examined to study matrix crack evolution in tensile specimen with open hole, and the results found to be in good agreement with available experimental data. [ABSTRACT FROM AUTHOR]

Peridynamic (PD) theory is used to investigate the dynamic responses of electronic packages subjected to impact loading arising from drop-shock. The capability of the PD theory to predict failure is demonstrated by simulating a drop test experiment of a laboratory-type package. The failure predictions and observations are exceptionally similar. For the drop test simulation of a production-type package, the finite element method (FEM) and PD theory are coupled via a submodeling approach. The global modeling is performed using the FEM while the PD theory is employed for the submodeling and failure prediction. The analysis yielded the outermost solder joint as the critical joint, with failure at the interface between the solder and copper pad on the printed circuit board side. [ABSTRACT FROM AUTHOR]

The paper proposes a model relating the level of reinforcement corrosion (in terms of loss of rebar radius) with the width of corrosion-induced cracks. The model is developed using a simulation of the process of corrosion-induced crack opening by means of finite-element (FE) modelling. Concrete is treated as a linear elastic material, a vertical crack is introduced above a reinforcing bar and expansive behaviour of corrosion products is modelled using a thermal analogy. Corrosion products are divided into those that penetrate (dissipate) into cracks (no crack propagation) and those that cause crack opening. The amount of corrosion products penetrating into cracks at a specified crack width is evaluated using FE modelling and analyses of experimental results. It was found that a corrosion loss of 0.1-0.4 mm dissipated into the cracks when the crack width was 1.0 mm. The influence of geometric parameters such as rebar diameter, spacing and location, and thickness of top and edge concrete covers were investigated. The limits of applicability of the proposed relationships were established using non-linear FE modelling. [ABSTRACT FROM AUTHOR]

Abstract: A three-dimensional extension of a previously published two-dimensional cracked finite element [Potirniche, G.P., Hearndon, J., Daniewicz, S.R., Parker, D., Cuevas, P., Wang, P.T., Horstemeyer, M.F., 2008. A two-dimensional damaged finite element for fracture applications. Engineering Fracture Mechanics 17(13), 3895–3908] is presented in this paper. The new element has an embedded edge crack, and was developed to model damage in three-dimensional structures using the finite element method. The element simulates the presence of a crack without physically inserting it in the three-dimensional finite element mesh. The method involves the derivation of a modified stiffness matrix that accounts for the change in the element flexibility due to the crack presence. The cracked element was analytically formulated and implemented in the finite element code ABAQUS Standard as a User-defined Element (UEL) subroutine. Tests of various cracked beam configurations were used to estimate the accuracy of the element by comparing two models: one with a UEL and another with an embedded edge crack. Beam deflections and natural frequencies were analyzed and compared for the two models. The results indicate that the new element has a good potential in modeling cracks in three-dimensional parts. Moreover, the method using this UEL computes the global response of damaged structures, in which cracks can be placed at various locations and in an unlimited number. [Copyright &y& Elsevier]

Abstract: The objective of this paper is to propose a novel methodology for determining dynamic fracture toughness (DFT) of materials under mixed mode I/II impact loading. Previous experimental investigations on mixed mode fracture have been largely limited to qusi-static conditions, due to difficulties in the generation of mixed mode dynamic loading and the precise control of mode mixity at crack tip, in absence of sophisticated experimental techniques. In this study, a hybrid experimental–numerical approach is employed to measure mixed mode DFT of 40Cr high strength steel, with the aid of the split Hopkinson tension bar (SHTB) apparatus and finite element analysis (FEA). A fixture device and a series of tensile specimens with an inclined center crack are designed for the tests to generate the components of mode I and mode II dynamic stress intensity factors (DSIF). Through the change of the crack inclination angle β (=90°, 60°, 45°, and 30°), the K II/K I ratio is successfully controlled in the range from 0 to 1.14. A mixed mode I/II dynamic fracture plane, which can also exhibit the information of crack inclination angle and loading rate at the same time, is obtained based on the experimental results. A safety zone is determined in this plane according to the characteristic line. Through observation of the fracture surfaces, different fracture mechanisms are found for pure mode I and mixed mode fractures. [Copyright &y& Elsevier]

Abstract: Coatings of transition metal compounds find widespread technological applications where adhesion is known to influence or control functionality. Here, we, by first-principles calculations, propose a new way to assess adhesion in coatings and apply it to analyze the TiN coating. We find that the calculated adhesion energies of both the (111) and (001) orientations are small under no residual stress, yet increase linearly once the stress is imposed, suggesting that the residual stress is key to affecting adhesion. The strengthened adhesion is found to be attributed to the stress-induced shrinkage of neighbouring bonds, which results in stronger interactions between bonds in TiN coatings. Further finite elements simulation (FEM) based on calculated adhesion energy reproduces well the initial cracking process observed in nano-indentation experiments, thereby validating the application of this approach in quantifying adhesion energy of surface coating systems. [Copyright &y& Elsevier]

Abstract: Radial fretting tests on human femur cortical bone with a ball-on-flat configuration were carried out in vitro under different normal contact loads. The kinetics behaviors and damage characteristics of the cortical bone were analyzed using the load versus displacement (F–D) curves. The fretting scars were examined using laser confocal scanning microscopy (LCSM) and scanning electron microscopy (SEM). A nano-indenter was used to characterize the mechanical property of the osteon of the cortical bone. Morphologies showed that the primary damage form was due to micro-cracking. Around the osteon, three kinds of micro-crack configurations were observed; i.e. annular cracks—located at the concentric lamellae; radial cracks—initiated from the Harversion canal; and interstitial cracks—initiated and propagated between the interstitial lamellae. Plenty of interstitial cracks occurred in the wear scar, which indicated that the interstitial tissue was the weak zone in the structure of cortical bone. In addition, the contact stresses under the radial fretting condition were calculated by the finite element method analysis (FEMs) and the behaviors of cracking were explained. The results demonstrated that the stress concentration that occurred around the Harversian canal and osteon system resulted in a better resistance than that from the interstitial tissue. The concentric lamellae presented a better radial-fretting resistance and contact fatigue resistance than that from the interstitial lamellae. [Copyright &y& Elsevier]

Abstract: The scratch behavior of a thermoset solid polymer exhibiting brittle behavior in tension is investigated. The surfaces are scratched and an imaging system is used to record real time photographs. The 3D crack pattern is analyzed using fluorescence confocal laser scanning microscopy. A finite element simulation is performed to determine the indenter/specimen contact conditions. They are used as input data for the 3D crack network analysis based on combined 3D localized multigrid and X-FEM/level set techniques. The computed stress distributions within the 3D cracked specimen are of great interest to understand the crack network formation observed during the experiments. [Copyright &y& Elsevier]

ABSTRACT Research on fatigue crack formation from a corroded 7075-T651 surface provides insight into the governing mechanical driving forces at microstructure-scale lengths that are intermediate between safe life and damage tolerant feature sizes. Crack surface marker-bands accurately quantify cycles ( Ni) to form a 10-20 μm fatigue crack emanating from both an isolated pit perimeter and EXCO corroded surface. The Ni decreases with increasing-applied stress. Fatigue crack formation involves a complex interaction of elastic stress concentration due to three-dimensional pit macro-topography coupled with local micro-topographic plastic strain concentration, further enhanced by microstructure (particularly sub-surface constituents). These driving force interactions lead to high variability in cycles to form a fatigue crack, but from an engineering perspective, a broadly corroded surface should contain an extreme group of features that are likely to drive the portion of life to form a crack to near 0. At low-applied stresses, crack formation can constitute a significant portion of life, which is predicted by coupling macro-pit and micro-feature elastic-plastic stress/strain concentrations from finite element analysis with empirical low-cycle fatigue life models. The presented experimental results provide a foundation to validate next-generation crack formation models and prognosis methods. [ABSTRACT FROM AUTHOR]

ABSTRACT Sliding contact of a rigid rough surface with a semi-infinite medium including a horizontal subsurface crack was investigated by using linear elastic fracture mechanics and finite element method (FEM). The fractal geometry was used to characterize the rigid rough surface. The propagation of crack was studied with the shear and tensile stress intensity factors. The effect of surface roughness, crack length to depth ratio and friction at the contact and crack interfaces was investigated by using the FEM. It was shown that increasing friction coefficient at the contact interface increases both KII and KI. [ABSTRACT FROM AUTHOR]

Aiming at the most dangerous crack damage in structural health, a crack identification method based on wavelet finite element model and determinant transformation method is conducted. Natural frequencies of the structure with various crack locations and depths are first accurately obtained by means of wavelet finite element methods. Then, the actual structures are measured to gain vibration characteristics. Measured natural frequencies are used in a crack detection process, while crack location and size can be identified using determinant transformation method. The experimental example of a static beam with rectangle cross-section indicates that the current method is effective and accurate. The whole approach provides a powerful technology for practical application. [ABSTRACT FROM AUTHOR]

Creep studies were carried out on 2.25Cr-1Mo steel base metal and its fusion-welded weldments at 823 K over the stress range 100-240 MPa. The weldment possessed lower creep rupture strength than the base metal due to type IV failure at the outer edge of the heat-affected zone (HAZ). Premature failure of the weldment was associated with pronounced creep cavitation accompanied with localized creep deformation in the soft intercritical region of the HAZ that was sandwiched between relatively higher creep deformation-resistant microstructural regions. The cavitation was associated with coarse intergranular precipitates in the intercritical region of the HAZ. The type IV cracking in the intercritical region of the HAZ was found to initiate deep inside the weldment and propagate towards the specimen surface. Finite element analysis of stress and strain distributions across the weldment was carried out considering the micro-mechanical strength inhomogeneity across it to explain the observed features of type IV cracking. The estimated higher von-Mises and principal stresses deep inside the intercritical region of the HAZ of the weldment led to the localized creep deformation and preferential cavity nucleation and growth, resulting in type IV failure of the weldment. The role of intergranular precipitate particles in the intercritical region of the HAZ in facilitating creep cavity nucleation by the exhaustion of creep ductility of the material close to the precipitate was corroborated from finite element analysis of stress and strain distribution around the precipitates. [ABSTRACT FROM AUTHOR]

Rapid casting (RC) of freeform-surface parts can be realized via replacing wax patterns by stereolithography (SL) patterns to overcome the disadvantage of time-and-cost consuming in traditional investment casting. It has a promising application prospect in single and small batch production. But ceramic shell cracking during the pyrolyzing of SL patterns limits the application and popularization of RC. In this paper, a simplified thermo-mechanical model was first built and the distribution rule of circumferential stress on the contact boundary between SL patterns and ceramic shell was theoretically derived using displacement method to predict dangerous area of freeform-surface parts. Subsequently, the variation rule of circumferential stress at the predicted dangerous area with changing temperature was revealed by applying transient thermo-mechanical finite element analysis (FEA) on a turbine blade cross section. The comparison of theoretical and FEA results indicated that the circumferential stress was enlarged by the freeform shape of turbine blade instead of being relieved due to the softening of resin. Dangerous temperature range was then determined, so polyimide was added to increase strength under corresponding temperature to prevent shell cracking. An experimental validation was offered to verify our point. [ABSTRACT FROM AUTHOR]

Abstract: Nowadays, numerical simulation of 3D fatigue crack growth is easily handled using the eXtended Finite Element Method coupled with level set techniques. The finite element mesh does not need to conform to the crack geometry. Most difficulties associated to complex mesh generation around the crack and the re-meshing steps during the possible propagation are hence avoided. A 3D two-scale frictional contact fatigue crack model developed within the X-FEM framework is presented in this article. It allows the use of a refined discretization of the crack interface independent from the underlying finite element mesh and adapted to the frictional contact crack scale. A stabilized three-field weak formulation is also proposed to avoid possible oscillations in the local solution linked to the LBB condition when tangential slip is occurring. Two basic three-dimensional numerical examples are presented. They aim at illustrating the capacities and the high level of accuracy of the proposed X-FEM model. Stress intensity factors are computed along the crack front. Finally an experimental 3D ball/plate fretting fatigue test with running conditions inducing crack nucleation and propagation is modeled. 3D crack shapes defined from actual experimental ones and fretting loading cycle are considered. This latter numerical simulation demonstrates the model ability to deal with challenging actual complex problems and the possibility to achieve tribological fatigue prediction at a design stage based on the fatigue crack modeling. [Copyright &y& Elsevier]

Abstract: A three-dimensional methodology for simulation of fatigue crack propagation is presented. The method is leveraged by the use of enriched crack tip elements to compute the mixed-mode stress intensity factors. The crack growth model used and the crack propagation life calculation are also described. As examples, fatigue crack propagation of a mode-I surface crack and crack advancements of mixed-mode surface cracks are simulated. The predicted results showed excellent agreement with experimental data from the literature. Thus, it is concluded that the crack propagation method developed allows efficient and accurate simulation of three-dimensional fatigue crack propagation problems. [Copyright &y& Elsevier]

Abstract: It is well known that the life of material composite depends on the hardness of the interface between the fiber and the matrix. Herein, the aim of this work is the calculation by finite element method interaction effect between a crack and an interface by energy release rate approach. The first part of this work is devoted to the crack normal to the interface and the second one deals with the crack terminated at the interface. In this study, two cases are considered using a copper matrix with different fibers. An axisymmetric model has been used to evaluate the conditions for the crack deflection/penetration by the interface as well as the effects of the volume percent of fibers and the effects of the distance between the crack tip and the interface were highlighted as well as the effects of the elastic properties of two bonded materials. [Copyright &y& Elsevier]

Composite plates are finding increasing application in aerospace, naval, and automobile industries and other high-tech designs because of their high specific stiffness and strength. However, during its vibration the composite plate can undergo cracks. Thus the natural frequencies of the plate will be influenced by these cracks. In this paper, a new element of hp FEM using the Mindlin plate theory from tree-node plate elements has been proposed for solving the free vibration of cracked orthotropic plates. This element performs well for elastic analysis and has the advantage that the crack can be easily incorporated in any location of the plate. The objective of the present work is to use this model on the triangular orthotropic plate, to investigate the variation of natural frequencies according to the dimension and location of the crack. [ABSTRACT FROM AUTHOR]

Abstract: The aim of this study was to investigate the Coefficient of Thermal Expansion (CTE) of anodic films on 7175 T7351 aluminium alloy and to evaluate the influence of the film characteristics on this value. In particular, effects of porosity and post-treatments, such as coloring and sealing, were studied. Beam bending analysis was used as the experimental method and a numerical finite element model was developed to verify theoretical relationships hypotheses. In particular, the errors induced by the use of theoretical relationships between the curvature of the sample and stresses in the film were not negligible. A relation based on a finite element model was then developed and used to calculate stresses. The experimental value of CTE obtained by beam bending test was then validated by comparing the experimental cracking temperature of anodic films with a theoretical value directly depending on the previously determined CTE. The CTE of anodic films was found to be 13.0±1.010−6 K−1 independent from the porosity of the films and from the post-treatment (inorganic coloring and sealing). [ABSTRACT FROM AUTHOR]

The objective of this study was to follow the crack propagation in the tooth foot of a spur gear by using Linear Elastic Fracture Mechanics (LEFM) and the Finite Element Method (FEM). The tooth foot crack propagation is a function of Stress Intensity Factors (SIF) that play a very crucial role in the life span of the gear. A two-dimensional quasi-static analysis is carried out using a program that determines the gear geometry, coupled with the Finite Element Code (ANSYS). The study estimates the stress intensity factors and monitors their variations on the tooth foot according to crack depth, crack propagation angle, and the crack position. An appropriate methodology for predicting the crack propagation path is applied by considering gear tooth behavior in bending fatigue. The results are used to predict/prevent catastrophic rim fracture failure modes from occurring in critical components. [ABSTRACT FROM AUTHOR]

Abstract: This paper is dedicated to simulation of dynamic analysis of fixed cracks in orthotropic media using an extended finite element method. This work is in fact an extension to dynamic problems of the recently developed orthotropic extended finite element method for fracture analysis of composites. In this method, the Heaviside and near-tip enrichment functions are used in the framework of the partition of unity for modeling crack discontinuity and crack-tip singularities within the classical finite element method. In this procedure, elements that include a crack are not required to conform to crack edges. Therefore, mesh generation can be performed without any need to comply to crack edges and the method is capable of modeling the crack propagation without any remeshing. To determine the fracture properties, mixed-mode dynamic stress intensity factors (DSIFs) are evaluated by means of domain separation integral (J ′-integral) method. Results of the proposed method are compared with other available analytical and computational results. [ABSTRACT FROM AUTHOR]

Central crack development in a metal stip under tension is considered using a regular-finiteelement model. Crack development is modeled by removing the finite element in which the fracture conditions are satisfied. It has been shown that this approach allows one to qualitatively trace crack development and to represent some experimentally observed effects, such as stepwise crack development in metal. [ABSTRACT FROM AUTHOR]

The article focuses on the assessment of adaptive finite element method and photoelastic technique in identifying stress intensity factors, crack propagation and trajectory which are essential to predict the life of a product. It states the use of single, double and slant-edge crack plates as standard problems to assess the performance of both techniques. Moreover, results show that both methods provide accurate prediction of the stress intensity factors and initial crack trajectory.

Abstract: Elimination of edge cracking is one of the major challenges in flanging of advanced high-strength steels (AHSS). Several studies show that edge cracking occurs at lower strains than those predicted by the forming limit curves (FLC) and it is influenced significantly by the sheared or blanked edge quality. This study focuses on FEM modeling and experiments on blanking and hole expansion of AHSS DP590. The FEM model of blanking was developed to characterize the edge quality for different punch/die clearances. Hole expansion was simulated to demonstrate the effect of sheared edge upon stretchability. Thus, it was possible to demonstrate how metal flow, strains and stresses in blanking affect the part quality and potential edge cracking in stretch flanging. [Copyright &y& Elsevier]

Abstract: The crack repair technique by “crack grinding”, due to its efficiency, has been widely used to extend the service life of the cracked structural components. The “crack grinding” technique is carried out by removing the materials which contains the crack. So it is of great importance to study the effects of this repair technique. In this paper, the finite element method is used to analyse the behavior of repaired cracks in mode I by computing the stress-intensity factors at the crack tip. 3D numerical simulation was carried out to model the steam governor valve in a coal-fired thermal power plant with a semi-elliptic crack. Since no extra auxiliary solutions are required for the computation, the proposed method appears to be visual and can be used for evaluation of the associated stress-intensity factors. The plot of the stress-intensity factors according to the crack length in mode I, shows that the stress-intensity factor exhibits an asymptotic behavior as the crack length increases. It’s to be noted that a transition zone allows connecting the refined specific crack tip mesh to the global structural discretization. So this technique can be used for complex industrial structures. [Copyright &y& Elsevier]

Abstract: The CFD–DEM coupled approach was used to simulate the complex gas–solid reacting flows in fluid catalytic cracking (FCC) processes accommodated in riser or downer reactors. Considering the solid catalyzed gas-phase reactions, the model particularly incorporated the descriptions for heat transfer behaviors between particles and between gas and particles, the instantaneous catalyst deactivation, and the lumped kinetics in the gas phase for FCC process, together with the governing equations for the hydrodynamics. The distinct advantage of the present approach is that the catalyst activity can be calculated in time by tracking the history of the particle movement with the occurrence of heat transfer and chemical reactions. The simulation results captured the major features of FCC process very well either in riser or in downer, which had reasonable agreement with the experimental data in the literature. The reduced selectivity to the desired intermediate products in risers, especially under high catalyst-to-oil ratios, can be clearly understood from the simulated backmixing behavior of solid catalysts and the deactivation of catalysts at different locations in the reactor, which caused the non-ideal reaction progress inside the reactor space. It can be concluded that this type of modeling approach forms a solid basis for the cross-scale modeling of general multi-phase catalytic reacting flows. [Copyright &y& Elsevier]

Abstract: In this paper, the influence of aggregate size and volume fraction on shrinkage induced micro-cracking and permeability of concrete and mortar was investigated. Nonlinear finite element analyses of model concrete and mortar specimens with regular and random aggregate arrangements were performed. The aggregate diameter was varied between 2 and 16mm. Furthermore, a range of volume fractions between 0.1 and 0.5 was studied. The nonlinear analyses were based on a 2D lattice approach in which aggregates were simplified as monosized cylindrical inclusions. The analysis results were interpreted by means of crack length, crack width and change of permeability. The results show that increasing aggregate diameter (at equal volume fraction) and decreasing volume fraction (at equal aggregate diameter) increase crack width and consequently greatly increases permeability. [Copyright &y& Elsevier]

The objective of this paper is to investigate geometric and material parameters that affect the interaction of multiple pre-existing (initiated) cracks due to rolling contact fatigue (RCF) loading conditions. In particular, the behaviour of short cracks (head checks) in railway rail is studied. Parameters of interest are initial crack angle and spacing, distribution of initial crack lengths, width of the load/contact zone, and the material properties (in particular, the friction coefficient). Furthermore, the sensitivity of the finite element (FE)-mesh density is investigated. An open question of particular interest is the effect of crack interaction, e.g. shielding, on the prevailing crack spacing. In view of all uncertainties of the material model as well as the three-dimensional geometric complexity of the RCF problem, it is important to obtain a good understanding of the sensitivity of parameter changes. This is achieved in the presented investigation, although it is carried out using simplifying assumptions such as plane strain, linear elasticity, and Hertzian pressure distribution. [ABSTRACT FROM AUTHOR]

The article presents a study which aims to study thermal-cracking phenomenology to determine the suitability and reliability of direct-bond-copper (DBC) substrates for high-temperature packaging under thermal cycling. Study used a finite element method with the copper's Chaboche model to analyze the distribution of thermal plastic strain at the edge of DBC substrates. Thermal analyses which involve different lengths of edge tail indicate the impact of the DBC substrate geometry in DBC's susceptibility to cracking. Study reveals the interface cracking at the bonded copper's short edge which is caused by thermal plastic strain's accumulation near short edge.

Abstract: Cracks appear on slab corner which is an important quality problem in some steel works. The closure and growth of transversal crack on slab corner during multi-pass vertical–horizontal (V–H) rolling was simulated with the explicit dynamic finite element method (FEM) and the updating geometric method. The influences of the friction coefficient, the crack size, the edger roll shape, and the fillet radius of grooved edger roll on the closure and growth of cracks were analyzed with the crack width and the contact pressure on crack surfaces. For vertical rolling, the contact pressure on crack surfaces when the grooved edger roll is used is larger than that when the flat edger roll is used. However, after horizontal rolling, the cracks open again when using the grooved edger roll, which may close well when using the flat edger roll. With increase of the fillet radii of the grooved edger roll, the contact pressure on crack surfaces increases during V1 pass and decreases during V2, V3 pass. Reduction of the friction coefficient between the slab and the rolls is helpful for the crack closure for both the flat edger roll and the grooved edger roll during vertical rolling. The experiments were performed for the behavior of transversal crack on the pure lead slab corner, and the calculated results have the same tendency of the crack behavior as the experimental ones. [Copyright &y& Elsevier]

Fatigue crack closure is strongly influenced by the plastic wake developed because of the action of precedent cycles. It is related to the load sequence and is an additional difficulty for its determination. Finite-element method has been an alternative for the study of crack closure, commonly in constant-amplitude loading. However, such analyses are complex and computationally expensive, the length of the wake simulated being a critical parameter. In this paper, a comprehensive study of the effect of the plastic wake in fatigue crack closure is made. It is structured in two parts: one regarding the correct numerical modelling process and the other focussed on the influence of the wake under moderate variable-amplitude loading. After a brief description of the numerical methodology, results for different wake lengths are reported. The plastic wake is modelled with a new approach in which the final crack tip is fixed. Information regarding potential error is provided. Second, the load sequence effect is discussed. Crack closure is obtained for several load ratios, with wakes developed with different stress ratios. This permits the analysis of the influence of the load sequence over the crack-driving force compared with constant-amplitude loading. [ABSTRACT FROM AUTHOR]

Abstract: This work discusses an application of a meshfree, semidiscrete finite element method to solving the ‘thermal shock’ problem for a thin, cylindrical plate made of functionally graded ceramics. The unsteady heat conduction equation is discretized in space with the partition of unity FEM. The temporal discretization is realized by using an explicit finite difference method. The computations are performed on two model samples: the homogenous one, made of the pure material, and the composite plate, made of the alumina/zirconium layers with variable weight content of . The heat transfer coefficient on the surface subjected to the ‘thermal shock’, as well as thermal conductivity and thermal diffusivity of the material, are modelled with the theoretical distribution function based on the experimental findings. The discrete solution is first checked for accuracy against the analytical solution to an exemplary test problem, yielding very advantageous results. Finally, it is verified in comparison with the jet-impingement-cooling experimental data, showing good accordance. [Copyright &y& Elsevier]

The fracture strength of five HVOF coatings, which are made of hard metals, Tribaloy alloy, and superalloys, respectively, coated on 1018 low carbon steel substrate, is studied under Vickers indentation, associated with FEA stress computation. The cross sections of the coating specimens are examined on a Hitachi Model S-570 scanning electron microscope (SEM), which investigates the quality and measures the geometry of the coatings. The mechanical properties of the coatings and the substrate are determined in the cross sections using the nano-indentation technique. The cracking behavior of the coatings under different indentation loads is investigated using a Vickers hardness tester. Three-dimensional finite element analysis (FEA) simulation of the Vickers indentation test is conducted to determine the stress fields in the coating/substrate systems in order to understand the fracture mechanisms of the coatings under the indentation loads using the ABAQUS software package. The FEA stress results are in good agreement with the experimental observation of Vickers indentation. [ABSTRACT FROM AUTHOR]

Abstract: In the present work a novel micro-mechanical approach to analyze the influence of micro-crack evolution and contact on the effective properties of elastic composite materials is proposed, based on homogenization techniques, interface models and fracture mechanics concepts. By means of the finite element method, enhanced non-linear macroscopic constitutive laws are developed by taking into account changes in micro-structural configuration associated with the growth of micro-cracks and with contact between crack faces. Numerical simulations are carried out for the cases of a porous composite with edge cracks and of a debonded fibre reinforced composite, loaded along extension/compression uniaxial macro-strain paths. Micro-crack propagation is modelled by using an original methodology based on the J-integral technique in conjunction with an interface model taking into account the unilateral contact of crack faces. In the context of a micro-to-macro transition obtained by controlling the macro-deformation of the micro-structure, the effects of adopting three types of boundary conditions on the macroscopic constitutive law, namely linear deformation, uniform tractions and periodic deformations and anti-periodic tractions, are studied. As a consequence, the proposed method can be applied to a large class of problems including periodic, locally periodic and irregular composite materials. Micro-crack and contact evolution result in a progressive loss of stiffness and can lead to failure for homogeneous macro-deformations associated with unstable crack propagation. [Copyright &y& Elsevier]

Web holes are often made in wood I-joists to allow passage of services. Design properties of wood I-joists with a web hole are commonly developed through testing which leads to empirical design information. This paper describes a mechanics-based method to predict the strengths of wood I-joists with a circular web hole that is suitable for implementation as a design tool. Curved beam theory is used to compute stresses around a hole. Peak tensile stresses obtained from curved beam analysis agree well with those predicted using the finite-element method. Failure load is attained by the application of fracture mechanics based finite area method. The method developed is used to predict failure loads of wood I-joists of different depths with different web hole sizes. Simplification of the method leads to the development of a simple expression that can be used for engineering design. It is found that the predicted strengths are in a good agreement with the corresponding test results. The calculation method developed can potentially replace the testing approach generally adopted by the wood I-joist industry. [ABSTRACT FROM AUTHOR]

On the physical nature, most crack tips are not ideally sharp but have a small curvature radius. Both surface energy and crack-root curvature affect the stress and displacement fields in the vicinity of the crack tip. In the present paper, a numerical method, which incorporates the effect of surface elasticity into the finite element method, is employed to study the surface effects on the mode-II crack tip fields. It is found that when the curvature radius of the crack root decreases to micro-/nanometers, surface elasticity has a significant influence on the stresses near the crack tip. For a mode-II crack, surface effects alter both the magnitude and position of the maximum stresses, as is different from a mode-I crack, in which case only the stress magnitude is influence by surface stresses. [ABSTRACT FROM AUTHOR]

Following previous work carried out in Building Research Institute in Japan, finite element analyses of conceptual column designs are performed in this paper. Tile effectiveness of the numerical model is evaluated by experimental tests and parametric studies are conducted to determine influential factors in conceptual column designs. First, three different column designs are analysed: bonded, unbonded, and un-bonded with additional reinforcing bars. The load-displacement curves and cracking patterns in concrete are obtained and compared with experimental ones. The comparisons indicate that the finite element model is able to reflect the experimental results closely. Both numerical and experimental results show that, the introduction of on-bonded zones in a column end can reduce cracking strains, accordingly reduce the stiffness and strength as well; the addition of extra reinforcement in the un- bonded zones can offset the losses of the stiffness and strength. To decide the proper length of the un- bonded zones and the sufficient amount of the additional reinforcing bars, parametric studies are carried out on their influences. It has been found that the stiffness of un-bonded designs slightly decreases with increasing the length of the un-bonded zones and increases with the size of the additional reinforcing bars. [ABSTRACT FROM AUTHOR]

This paper deals with two aspects of the characterization of the fracture process zone (FPZ) in quasi-brittle materials such as concrete. An overview is given of the possibility of using a destructive technique, such as the scanning electron microscope, and a numerical model, such as the nonlocal isotropic damage model (NLIDM), to detect FPZ characteristics, e.g., length and width of the FPZ. The fracture of concrete requires the consideration of progressive damage, which is usually modelled by a constitutive law and can be studied by a numerical method. The object-oriented finite element method (OOFEM) has recently been used in damage studies, and thus the FPZ is calculated on the basis of one of the damage models (the NLIDM). The results obtained from the experimental investigation are similar to those obtained using the NLIDM, which has proven to be a useful tool for analysis of the cracking process. [ABSTRACT FROM AUTHOR]

The article presents a study which examines the deformation characteristics of billets with liquid core during soft reduction. This study is also aimed at clarifying the correlation between internal cracks and deformation of the billet in the mushy zone which is a fully coupled thermo-mechanical finite element model. Researchers used the casting and reduction tests in a laboratory strand casting machine. Results of the study are discussed.

Abstract: Thin brittle films on polymer substrates are finding increasing use as gas barriers for example in the medical and food packaging industries and also for the next generation of ultra-light displays based on flexible polymer substrates. In order to determine the durability of the barrier under thermal and mechanical loads, test procedures and corresponding data reduction methods are needed to feed the analysis models. One of the tests frequently employed for this kind of multi-layer material systems is the fragmentation test, whose designation comes from the progressively denser pattern of parallel cracks developing when the specimen is loaded under uniaxial tension. From the crack-density versus strain data obtained, a critical strain for crack growth and an assessment of the adhesion of the coating to substrate can be obtained. However, no accepted data reduction methods exist to extract material properties from the test or inversely, successfully predict the crack density as a function of a set of material properties without fitting parameters. In an earlier paper, the authors presented a finite element based analysis methodology to determine the fracture toughness of both the coating and the interface from the fragmentation data. In the simulations, the plastic constitutive behavior of the substrate and the debonding of the coating from the substrate were explicitly included, the latter by use of a cohesive zone model. In this paper an extension of this methodology is presented that enables crack-density evolution with strain to be predicted. The results presented comprise comparisons with experiments to validate the methodology and the influence of (i) coating toughness, (ii) interface toughness and (iii) coating thickness on crack density versus strain. [Copyright &y& Elsevier]

Matrix cracks in composite laminates are almost invariably the first damage mechanisms in composite laminates and are the precursor to more serious forms of damage. The results presented in this paper include experimental investigations into the occurrence of these cracks in a wide range of laminates containing various angle-plies. Ultrasonic detection of these cracks using polar backscattering is found to be successful and shows promise as a method which could be applied outside the laboratory. Further work includes finite element simulations which lead to an understanding of aspects of the accumulation of damage in composite laminates. [ABSTRACT FROM AUTHOR]

On-line identification of cracks occurring in large structures has attracted the attention of many researchers. Identification becomes difficult when there is no easy access available for investigators to employ conventional nondestructive evaluation techniques. In this paper, a scheme is presented for identifying the crack location and its extent using vibration response. Numerical studies were carried out on a 1/20th-scale model of the side shell of the structure of a ship. The first natural frequency of the numerical model was 584.3 Hz. The numerical and physical models were excited by a random force having a dominant spectral frequency of 2 Hz. Cracks under investigation occurred at the connection between a horizontal and bulkheads and at the intersection between horizontals and a web frame. A scheme using the root mean square of vibration response is presented for identifying cracks occurring in the flange and web of the horizontal. It is demonstrated that the scheme can identify the crack location and size. [ABSTRACT FROM AUTHOR]

Abstract: The study was done using notched two-colony thick tensile specimens of a directionally solidified cast fully lamellar TiAl alloy. In-situ observations of fracture processes in scanning electron microscope (SEM) were combined with section-to-section related observations of fracture surfaces to investigate the crack growth process. Finite element method (FEM) calculations are carried out to evaluate the stresses for propagating cracks. The results reveal that: (1) the reason why enhancement of applied load is required to propagate the main crack, was attributed to that the main crack observed at the surface did not extend all the way through the specimen''s thickness thus the stress field was still controlled by the notch, in which a definite stress required for extending a crack tip should be kept by increasing the applied load. (2) Crack propagation resistance is enhanced at colony boundaries, only when a change occurs from an inter-lamellar propagation to a trans-lamellar propagation (3) Ligament bridging toughening phenomena can be integrated into aforementioned mechanism. As a whole the processes of new crack nucleation with bridging ligament formation decreases the crack propagation resistance rather than increasing it. (4) In case the majority of microcracks are surface cracks, the effect of microcrack shielding is not obvious. [Copyright &y& Elsevier]

The multiple-cracking phenomenon of the Fe-Zn intermetallic coating layer on the hot-dip galvannealed (GA) steels under thermal and tensile stresses was studied experimentally by tensile tests and analytically by means of the finite-element analysis. The multiple cracking of the coating layer had occurred in the as-supplied samples, and it progressed with increasing applied strain. Based on the calculated dependence of the stress of the coating layer on the crack spacing and applied strain, the multiple cracking in the as-supplied samples was accounted for by the thermally induced residual stress, and the further multiple cracking with increasing applied strain was accounted for by the increased stress of the coating layer. The experimentally observed decrease of the average crack spacing with increasing applied strain was described well, and the tensile strength of the coating layer was estimated to be 260 MPa, by application of the calculated relation between the increased stress of the coating layer and applied strain. The influences of the thickness of the coating layer and the substrate material on the multiple cracking were discussed based the stress analysis. It was revealed that the thinner the coating layer and the higher the flow stress of the substrate, the higher the stress of the coating layer becomes and, therefore, the smaller the crack spacing becomes. [ABSTRACT FROM AUTHOR]

The present work investigates problems: (1) How are the plastic strain and the stress (triaxiality) re-distributed after a short crack initiated, extended and blunted at the pre-crack tip? (2) How do the above changes put a crucial effect on the triggering of the cleavage fracture? Based on the previous observations of configuration changes and fracture surfaces of pre-crack tips, Finite element method (FEM) simulations of a short crack initiated, extended and blunted at a pre-crack tip and calculations of distributions of stress, strain and triaxiality are carried out for 3PB pre-cracked HSLA steel specimens tested at -130°C. The results reveal that: as long as the fatigue pre-crack is only blunted, in its vicinity a region where the accumulated strain is sufficient to nucleate a crack, and a region where the stress (triaxiality) is sufficient to propagate a crack nucleus are separated by a distance. The nucleated crack cannot be propagated and the cleavage fracture cannot be triggered. While a short crack produced at the fully blunted fatigue pre-crack, the strain retains, the stress (triaxiality) is rebuilt. An initiated and significantly extended and then blunted short crack makes a tip configuration, which on one hand is much sharper than that of the fully blunted original pre-crack tip, on other hand is wide enough to spread its effects into the high stress covered region. This sharpened crack tip configuration re-builds a ‘sharper’ distribution of stress (triaxiality) and makes two regions metioned above closer. Finally the two regions overlap each other and a cleavage crack can be initiated and propagated at a distance ahead of the blunted fatigue pre-crack. [ABSTRACT FROM AUTHOR]

The maturity method based on the Arrhenius law makes it possible to predict the concrete''s compressive strength evolution in a structure on the basis of a 20 °C characterization. This technique has been successfully used and improved now for more than 20 years. Examples are mentioned in this paper. The recent French National Project CALIBE (for “quality of concrete”) produced a handbook that synthesizes the improvement of the 20 years on site experience of the technique and shows the way to use the maturity method in a conservative way to predict the concrete''s compressive strength in situ.This technique can also be applied successfully to predict the characteristics (Young''s modulus, tensile strength, thermal coefficient) needed to control the risk of cracking in cement based materials through the use of numerical tools such as the finite element program CESAR-LCPC. [Copyright &y& Elsevier]