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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]

Unfilled elastomers often suffer from poor fracture resistance at high temperature where viscoelastic dissipation is low. A molecular design based on multiple interpenetrating networks composed of a brittle filler network isotropically prestretched to a value λ0 by swelling it in an extensible matrix leads to a dramatic increase of fracture energy Γc, typically attributed to sacrificial bond scission creating a dissipative damage zone ahead of the propagating crack. However, the molecular mechanisms controlling the size of the damage zone when the crack propagates are currently unknown. Here, we combine fluorogenic mechanochemistry with quantitative confocal mapping and mechanical testing to characterize both and the extent of bond scission in the sacrificial network detected on the fracture surfaces for different stretch rates and temperatures. We find that increasing the prestretch λ0 of the filler network leads to a large increase in Γc mainly at temperatures well above the glass transition temperature of the elastomers, where viscoelasticity is inactive, but also at lower temperatures where both mechanisms are coupled. Yet, we show that there is no direct linear relation between the extent of filler network scission and Γc. We mainly attribute the large increase in Γc to the dilution of highly stretched strands in the entangled and unstretched matrix, which delocalizes stress upon bond scission and effectively protects the matrix network from scission and the material from crack growth. Delaying the localization of bond scission by network design is a promising strategy that will guide molecular designs able to toughen elastomers even in the absence of viscoelastic dissipation. [ABSTRACT FROM AUTHOR]

The article discusses the main methods of modeling processes of skew rollers. Factors influencing the formation of internal cracks are discussed. The tendency of a material to crack in skew rolling processes decreases as the temperature of material increases, the number of rolling cycles decreases, and the number of rolls increases. In two-roll skew rolling, internal cracks appear around a central axial zone of a rolled specimen. The size and density of inclusions in a metal sample directly affect the formation of internal cracks. [ABSTRACT FROM AUTHOR]

Highlights • Hydrocracking and hydrotreating reactions kinetics are based on lumping technique. • The industrial model is validated with the performance of an experimental reactor. • Axial and radial dispersions are used to represent the slurry-phase reactor dynamic's. • Dynamic and steady-state simulations for hydrocracking of heavy oils are presented. • A sensitivity analysis of model parameters and operating conditions is presented. Abstract Modeling and simulation of a commercial size unit for hydrocracking of an atmospheric residue (312 °C+) in a slurry-phase reactor are reported. The model of the industrial reactor is formulated taking into account axial and radial gradients of the state variables: composition and temperature. The mathematical model was partially discretized with finite central differences in the positional derivatives generating a matrix system of ordinary differential equations which was solved by a Runge-Kutta method. The hydrocracking reaction kinetic model is based on lumping technique and hydrotreating reactions kinetics are described by Langmuir-Hinshelwood and power-law approaches. All the intrinsic kinetic parameters and correlations used in the simulations were taken from the literature. Dynamic and steady-state simulations were performed with the objective to find a distribution of composition and temperature in the reactor as a function of time. Also a parametric sensitivity study was elaborated in order to analyze the effects of uncertainties of model parameters in the dynamic and steady-state model responses. [ABSTRACT FROM AUTHOR]

The present manuscript reviews state-of-the art models of hydrogen-assisted cracking (HAC) with potential for application to remaining life prediction of oil and gas components susceptible to various forms of hydrogen embrittlement (HE), namely, hydrogen-induced cracking (HIC), sulfide stress cracking (SSC), and HE-controlled stress corrosion cracking (SCC). Existing continuum models are compared in terms of their ability to predict the threshold stress intensity factor and crack growth rate accounting for the complex couplings between hydrogen transport and accumulation at the fracture process zone, local embrittlement, and subsequent fracture. Emerging multiscale approaches are also discussed, and studies relative to HE in metals and especially steels are presented. Finally, the challenges that hinder the application of existing models to component integrity assessment and remaining life prediction are discussed with respect to identification of model parameters and limitations of the fracture similitude, which paves the way to new directions for further research. [ABSTRACT FROM AUTHOR]

Internal pores and fractures inside oil shale undergo complex changes during pyrolysis in superheated steam. After a simulated in-situ pyrolysis in superheated steam, the pore characteristics of 30 oil shale samples from different zones between the injection and production wells were investigated using mercury intrusion porosimetry (MIP). The results showed that the porosity of oil shale exceeded 27.65% in each position of horizontal fractures. In different types of pores, the proportions of mesopore (0.1 µm < d ≤ 1 µm) and micropore (d ≤ 0.015 µm) were the largest and the smallest, respectively. Moreover, mesopores outnumbered micropores. The pore structure of pyrolyzed oil shale after convection heating was complex. After the pyrolysis in superheated steam, the proportion of the oil shale ore body with a porosity ranging from 23 to 31% was 74.95% of total. This finding proved that oil shale was transformed from dense rock to a porous medium under the effect of superheated steam. [ABSTRACT FROM AUTHOR]

Figuring out the mining-crack-evolution characteristics of overburden strata is very important to roof control, gas drainage, disaster prevention and high efficient mining. In order to obtain the mining-crack-evolution characteristics of overburden strata in deep coal mining with large dip, in this study, No.1221 coal mining face of Zhao coal mine is chosen as the research object to establish an in-house experiment system of similarity simulation. The experiments could measure the stress and displacement of overburden strata, which can provide useful information to investigate the mining-crack-evolution characteristics, displacement variations and movement characteristics of overburden strata associated the coal mining face. Experimental results show that with the advancing of coal mining face, the scope of gob increases gradually, and formation of the false roof of overburden strata basically reflects the evolution process of collapsing. The overburden strata weight is constantly transferred to the front and rear of coal mining face, which forms the supporting pressures on both sides of coal pillars, and causes the gangue's collapsing in gob. The large dip of coal seam results in pressure disequilibrium in stress-increasing zone. Thus, the stress on the underside of gob becomes larger and larger, while the stress on upper side of gob decreases. The strata separation appears in the overburden strata of roof. Pressure-relief zone is mainly concentrated in the side of outlet roadway. Along the inclination direction of coal mining face, the crack development and strata separation are obvious, which create the passages for gas flow and migration. The similarity simulation results provide fundamental information for better understanding those mining-crack-evolution characteristics of overburden strata, which has become more and more important to control the roadway stability in coal mining and optimize the layout of gas drainage boreholes for improving mining safety. [ABSTRACT FROM AUTHOR]

Proppant embedment in hydraulic fracturing can reduce remarkably the fracture width and thus damage the fracture conductivity. This paper describes a study of the proppant embedment mechanism. Two groups of experiments were carried out. A proppant embedment mechanism was proposed. An analysis showed that proppant embedment is not an elastic process but could be a plastic or even failure process depending on the surface microstructure of the formation core. [ABSTRACT FROM AUTHOR]

In considering a site for gas storage, it will be important to evaluate the effects of gas storage on the formation, so as to minimize the risk of a breach occurring in the system. Gas injection will result in an increase in formation fluid pressure, especially around the injection source, which in turn results in redistribution of the stress field. The induced deformations within the reservoir can potentially result in a damage zone within the caprock formation. This mechanical failure may involve shear along many of the existing fractures or creation of new fractures that reduce the sealing properties of the caprock system. The main objective of this paper is to develop a model to estimate the growth and extension of cracks in the caprock. In order to achieve this, the smeared crack approach is used to model the process of cracking in the caprock. Smeared cracking is a continuum approach for damage mechanics which is based on the idea that a crack is modeled by modifying the strength and stiffness of the material. The main model presented in this paper has three sub-models, which are the reservoir model, the caprock model and the smeared crack model. The reservoir model is a simplified coupled hydro-mechanical model that numerically simulates the radi al fluid flow and analytically estimates the associated stress and strain within the reservoir. The results of the reservoir model are used as boundary conditions for the caprock model that estimates the stress and strain within the sealing caprock due to the deformation of the reservoir. Using the calculated stress and strain, the smeared crack model predicts the growth and extension of cracks within the caprock. The caprock is assumed to be initially crack free and impermeable. The developed model is then used to study the Yort-e-shah aquifer caprock in Iran to predict the growth and extension of cracks. [ABSTRACT FROM AUTHOR]

The physical interpretation of the J integral (i.e., J integral) is clear. J integral is the energy release rate associated with crack extension (i.e., translation of the crack in the x direction). However, the physical interpretation of the J integral remains unclear. In this study, different crack translations in the x direction were selected and tested by calculating the energy release rates associated with the crack translations and comparing them with the theoretical value of the J integral. [ABSTRACT FROM AUTHOR]

Phosphorus-modified ZSM-5 (P-ZSM-5) catalyst was active for steam cracking of n -hexane to produce propylene and ethylene; however, the P-ZSM-5 catalyst deactivated because of both dealumination and coke deposition, which were strongly influenced by the steam to n -hexane ratio in the reactant flow. The amount of coke deposition increased with decreasing the steam to n -hexane ratio from the TG-DTA measurement. The steam could reduce the reactions of the light olefins to BTX and coke and enhance the yield of the light olefins; however, the excess steam deactivated the ZSM-5 catalysts irreversibly via dealumination. [ABSTRACT FROM AUTHOR]

The development of alternative sources for energy and chemicals, particularly the use of plant biomass as a renewable resource for fuel or chemical feed stocks, has received much recent attention. The resin (biocrude) obtained fromPinus roxburghii, a resinous tree, was investigated as a potential source for liquid fuels in a fluidized bed reactor. The feed was studied for yield conversion data under different catalysts to feed ratio at different temperatures. High conversions of biocrude to liquid fuels were observed with maximum middle distillates selectivity at a catalyst/oil ratio of 1.8 and reaction temperature of 460°C. Gas chromatography analysis results indicated that the middle distillates obtained by catalytic cracking ofP. roxburghiibiocrude has a normal distribution of carbon number with high values from C10–C18carbon atoms. [ABSTRACT FROM PUBLISHER]

Developments in modeling of the fluid catalytic cracking (FCC) process have progressed along two lines. One emphasizes composition-based kinetic models based on molecular characterization of feedstocks and reaction products. The other relies on computational fluid dynamics. The aim is to develop an FCC model that strikes a balance between the two approaches. Specifically, we present an FCC riser model consisting of an entrance-zone and a fully developed zone. The former has four overlapping, fan-shaped oil sprays. The model predicts the plant data of Derouin et al. and reveals an inherent two-zone character of the FCC riser. Inside the entrance zone, cracking intensity is highest and changes rapidly, resulting in a steep rise in oil conversion. Outside the entrance zone, cracking intensity is low and varies slowly, leading to a sluggish increase in conversion. The two-zone model provides a computationally efficient modeling approach for FCC online control, optimization, and molecular management. © 2014 American Institute of Chemical Engineers AIChE J, 61: 610-619, 2015 [ABSTRACT FROM AUTHOR]

Compression-shear fractured rock under high pore fluid pressure was studied to determine an appropriate model for describing the Initial Cracking Law and the Evolution Law of the stress intensity factor at the tip of a wing crack. Considering the interaction of wing cracks and the additional stress caused by rock bridge damage, this paper proposes an intensity factor Evolution Equation of multiple rock cracks which combines the action of compression-shear stress and pore fluid pressure. The interaction of multiple wing cracks resulting in rock bridge damage makes the stress intensity factor at the crack tip larger than that of a single wing crack, and wing crack propagation under high pore fluid pressure takes a turn from stable expansion to unstable expansion. Based on the rock fracture mechanics criterion, the damage fracture mechanical models of a compression-shear rock mass are established when the rock bridge happens axial transfixion failure, tension-shear combined failure, or wing crack shear connection failure under the combined action of compression-shear stress and high pore fluid pressure. This theory provides the basis for the quantitative investigation of the hydraulic fracturing process. [ABSTRACT FROM AUTHOR]

Main failure modes and application limit of low alloy steel oil country tubular goods (OCTG) for sour service by relevant international standard were analyzed in this paper. The influences of metallurgical and environmental factors upon sulfide stress cracking (SSC) resistance property were discussed, and selection of SSC evaluation methods were described from the point f stress analysis and fitting for service. [ABSTRACT FROM AUTHOR]

The development of alternate sources of energy, particularly the use of plants as renewable sources for fuel, has received much attention recently. The potential of two indigenous arid land petrocrops,E. royleanaandE. neriifolia, as a source of hydrocarbon fuels was explored. Biocrudes extracted from these plant species were catalytically upgraded to liquid fuels using fluidized catalytic cracking. Naphtha fractions obtained by cracking of biocrudes were analyzed to give the detailed and group-wise distribution of paraffinic, isoparaffinic, oleofinic, naphthenic, and aromatic components. These naphtha fractions are similar in composition to the naphtha obtained from petroleum crudes. [ABSTRACT FROM PUBLISHER]

Lump kinetics models and reaction mechanism of fluid catalytic cracking (FCC) gasoline are introduced. Nine lump kinetics model, eight lump kinetics model, ten lump kinetics model, thirteen lump kinetics model, seventeen lump kinetics model, and twenty lump kinetics model have been put forward on the basis of lump theory and the reaction mechanism. In the network, the aromatization reaction species were first lumped into 9, 8, 10, 13, 17, and 20 parts, respectively. A mathematical method is first introduced to calculate a lump model of FCC gasoline. The results from experimental data are in accordance with the quantitative analytical conclusions drawn from the calculated data. [ABSTRACT FROM PUBLISHER]

The article focuses on a research which aims to develop crack growth predictions for reheat cracking using simulations of Finite Element (FE). The research highlights that Advanced Gas-Cooled Reactor (AGR) plants contain steel components which operates at a temperatures range of 500-650°C. The research further mentions that fracture mechanics specimens are required to conduct controlled laboratory tests for understanding the influence of residual stresses on crack growth behaviour.

The hot-tearing susceptibility of Mg-6Al-0.5Mn alloy decreased with the increasing Sr content (≤0.2 wt%). The related mechanism is described as follows: Sr refines the grain and reduces the tendency of divorced eutectic; Sr decreases the eutectic temperature and results in higher intergranular strength. Both of the above reasons could improve the filling capacity of the melt, resulting in the best hot-tearing-resistant property of the alloy. [ABSTRACT FROM AUTHOR]

Abstract: Experimental and numerical investigations have been conducted to evaluate the effect which the clamping force, resulting from torque tightening a nut and bolt, has on the mixed mode (I and II) fracture strength and effective stress intensity geometry/loading factor of a plate having an edge crack. The experimental tests were carried out on three batches of simple edge crack and bolt torque tightened edge crack specimens made from poly methyl-methacrylate (PMMA) rectangular plate. The specimens’ fracture strengths were obtained using a tensile testing machine in different loading angles using a modified Arcan fixture. In numerical part, finite element simulations were used to model the three test specimens variants used and thereby establish their stress intensity geometry/loading factors. The results show that the bolt tightening torque, and hence the plate clamping force, has a significant effect on reducing the equivalent mixed mode stress intensity factor of all loading angles and the equivalent factor, and thus the joint fracture strength compared to bolt-less simple edge crack specimens. [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: 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: Discrete crack models with cohesive binding forces in the fracture process zone have been widely used to address failure in quasi-brittle materials and interfaces. However, the numerical concerns and limitations stemming from the application of interface cohesive zone models in a quasi-static finite element framework increase considerably as the relative size of the process zone decreases. An excessively fine mesh is required in the process zone to accurately resolve the distribution of tractions in a relatively small moving zone. With a moderate mesh size, inefficient path-following techniques have to be employed to trace the local discretization-induced snap-backs. In order to increase the applicability of cohesive zone models by reducing their numerical deficiencies, a self-adaptive finite element framework is proposed, based on a hierarchical enrichment of the standard elements. With this approach, the planar mixed-mode crack growth in a general three-dimensional continuum, discretized by a coarse mesh, can be modeled while the set of equations of the non-linear system is solved by a standard Newton–Raphson iterative procedure. This hierarchical scheme was found to be most effective in reducing the oscillatory behavior of the global response. [Copyright &y& Elsevier]

The Neuber-Novozhilov approach is used to obtain necessary and sufficient fracture criteria. Using a modified Leonov-Panasyuk-Dugdale model, simple relations for the critical fracture parameters are derived for opening mode edge cracks for the case where the diameter of the prefracture zone coincides with the diameter of the plasticity zone. These relations are suitable for studying fracture where the crack length is negligibly small. A fracture diagram using critical stresses under both criteria is proposed for a wide range of crack length. At a certain level of loading, three regions are identified, in the first of which the crack is stable, in the second, the crack extends but remains stable, and in the third, the crack is unstable. Experimental data on the fracture of specimens with edge cracks are obtained. It is established that the theoretical critical fracture curves are in good agreement with the obtained critical parameters for flat tensile specimens with two collinear edge cracks. [ABSTRACT FROM AUTHOR]

Observations of fracture surfaces and deformation beneath fracture surfaces produced by hydrogen-assisted cracking in pipelines steels, which were published in a recent paper (M.L. Martin, I.M. Robertson, and P. Sofronis, Acta Materialia, 59 (2011) 3680–3687), are discussed, and it is argued that the conclusions made regarding mechanisms of fracture are not justified. [ABSTRACT FROM AUTHOR]

ABSTRACT In this paper, a model using the energy approach was established to investigate the formation of fish-eye shape in the gigacycle fatigue failure. The model was used to predict the size of the fish-eye formed by a given subsurface inclusion. The predicted results were comparable with the fractography of SNCM439 steel and other experimental results of gigacycle fatigue available in the literature. The current model can capture the fish-eye shape and can be used to predict the life of crack initiation during fatigue failure. [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]

Abstract: Traditional hydraulic fracturing techniques generally form main hydraulic cracks and airfoil branch fissures, but main hydraulic cracks are relatively few in number. Hydraulic fracturing after water pressure control blasting can transform the structure of coal and rock mass. Experiments prove that it is an effective method for increasing the number and range of hydraulic cracks, as well as for improving the permeability of coal seams. The technical principle is as follows. First, a hole is drilled in the coal seam and is injected with a gel explosive (a mining water-proof explosive). Then, water is injected into the hole to seal it, at low enough pressure to prevent cracks from forming. Third, water pressure blasting is done by detonating the explosive. The water shock waves and bubble pulsations produced by the explosion cause a high strain rate in the rock wall surrounding the hole. When the stress imposed on the rock wall surrounding the hole exceeds its dynamic critical fracture strength, the surrounding rock breaks and numerous circumferential and radial fractures propagate outward. Lastly, water injection processes, such as general injection, pulse injection, and/or cyclic injection, are carried out to promote hydraulic fracturing. Depending on the fissure water pressure, detonation fissures continue to expand and additional hydraulic fractures with a wider range are formed. Under the effect of detonation pressure, joints and fissures in the coal mass open and propagate, leading to reduced adhesive forces on structural surfaces and thereby enhancing coal cutting. Therefore, this method improves the permeability of the coal seam, effectively weakens the strength of the coal and rock mass, and reduces the surrounding rock stress of the weakened area, effectively solving the problem of having a small number of big cracks. It is a useful technical approach for improving top coal caving, preventing rock burst, preventing coal and gas outbursts, and raising the gas extraction efficiency in colliery. [Copyright &y& Elsevier]

Abstract: This paper investigates the effect of confining stress on rock fragmentation under TBM cutters by numerical simulation. Firstly, based on the characteristics of the force–penetration curve, the confining stress conditions for TBM tunneling are divided into three general classes through an index of confining ratio defined as the ratio of confining stress to uniaxial compressive strength. Then, the rock fragmentation processes with different confining stress induced by TBM cutters are studied by using a micromechanics-based numerical code. It is found that the confining stress has significant impact on the key factors for rock fragmentation, including the chipping force, crack angle, effective crack length and energy dissipation. Specifically, the chipping force and the crack angle increase with the rising of confining ratio. However, for the effective crack length, there exists a critical confining ratio at which it reaches a minimum value. Hence, when confining ratio is larger than the critical value, it will promote rock fragmentation, and vice versa. For energy consumption, similarly, the same critical confining ratio is also found, at which the specific energy reaches its peak value. This study provided an explanation for the field observations, where special condition of confining stress will better facilitate fragmentation process by TBM cutters. [Copyright &y& Elsevier]

Using the relation between plastic zone size in the crack tip vicinity, loading cycle parameters, and material properties, which has been derived in Part 1, we have elaborated fatigue crack propagation rate prediction for low-cycle mechanical loading combined with cyclic heating with account of the phase shift effect. The data for governing equation were obtained from simple tests. Application of the proposed method yields a good fit of calculated and experimental results. [ABSTRACT FROM AUTHOR]

pproaches of linearized solid mechanics are used to solve the axisymmetric problem of interaction of a periodic set of coaxial penny-shaped mode II cracks in an unbounded prestressed material. Two nonclassical fracture mechanisms are considered: fracture of a body with prestresses parallel to the crack plane and fracture of a body compressed along cracks. The fracture parameters for hyperelastic materials with elastic potentials of different types are calculated and their dependence on the loading conditions, the mechanical characteristics of the materials, and the geometrical parameters of the problem is studied [ABSTRACT FROM AUTHOR]

This paper investigates the low velocity impact response of cotton, flax/cotton and glass/cotton hybrid composites. The composites were prepared by compression moulding and cut to dimensions specified by the Boeing impact test specification. All the specimens were impacted at 17, 26, 44, 62 and 77J energy levels on an Instron dynatup 9250HV drop weight impact tower by a hemispherical impactor head. Impulse software was used in data acquisition and analysis of results. The cotton and flax/cotton composites mainly failed through cracking, tensile fracture and delamination, while glass/cotton hybrid composites failed by deflecting and delamination. The glass/cotton composites were not penetrated and did not fracture even at high impact energy levels. [ABSTRACT FROM AUTHOR]

The materials response under high energy impact loads was studied using a gas gun. The projectiles were pins 1.2-1.5 mm in diameter and weighing 0.347-0.435 g. The projectile velocity was in the range 100-1300 m/s. The remnant load carrying capability of composite samples after high velocity impact tests was measured to quantify high energy impact induced microstructural damage. The composites retained some of their load bearing capacity even after penetration of the projectile, since structural damage caused by projectiles remained localised, preventing catastrophic failure, particularly for continuous fibre reinforced Alpure matrix composites. Penetration by the projectile occurred at impact energy of about 62-65 J for the conditions investigated. The experimental findings show that the energy absorbing capacity of such composites and their ability to withstand a given blow are largely functions of fibre type and greatly influenced by the matrix ductility, fibre-matrix interfacial bonding and volume fraction of reinforcing fibre. Understanding crack propagation and damage development under high energy impact loads may open new opportunities for the use of these composites in lightweight armour applications. [ABSTRACT FROM AUTHOR]

Abstract: The fracture behavior of titanium open foam is characterized and the R-curves of crack propagation from pre-cracks are measured. The crack growth has been optically observed, the measured initiation toughness, J IC, has been analyzed and the effect of material morphology on the J IC is discussed. The fracture toughness was found to be dependent on the expanding crack bridging zone at the back of the crack tip. The compact tension specimens also have some plastic collapse along the ligaments and it has shown that the titanium foam with a higher relative density is tougher. The non-uniform stressing within the plastic zone at the crack tip and the plastic collapse of cell topology behind the tip was found to be the primary cause of the R-curve behavior in low relative density titanium foams. [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]

Abstract: The aim of this paper was to address the effect of the residual stresses within the brittle film on the substrate on the film strength, fracture energy, and interfacial shear strength (IFSS). Special analyses were performed on the SiO x film/polyethulene terephthalate substrate systems. The residual stresses were evaluated by using the curvature method. The film strength, fracture energy, and IFSS were estimated on the basis of the multiple cracking analyses. In the multiple cracking analyses, the system was subjected to the combination of the residual stresses and the unidirectionally applied stress. Results showed that the relationship between the crack density in the film and the applied strain can be predicted by adopting the energy criterion on the basis of the knowledge on the residual stress distributions in the film segment. The film strength and fracture energy for the initiation of film cracking were almost proportional to the compressive residual stresses in the film. With increasing the compressive residual stresses within the film, the IFSS also increased. [ABSTRACT FROM AUTHOR]

Abstract: The thermodynamic setting for the formulation of the “crack-driving force” for a singular crack in conjunction with rate-independent material response is discussed. One key ingredient is the introduction of a fixed (absolute) configuration, relative to which both physical and (virtual) configurational and spatial changes can be described. Only quasistatic and isothermal conditions are considered in this paper. A variational framework is established for the rate of global energy dissipation (integrated over the whole material domain) due to the combined action of a (discrete) crack extension and continuum inelasticity, whereby the material time derivative of internal variables and the rate of crack extension are coupled. The classical assumption (previously adopted in the literature) is that there is no coupling, i.e. the internal variables are considered as fixed (material) fields just like an inhomogeneous material property. The other (extreme) assumption is that the internal variables fields are convected with the configurational motion due to the virtual crack extension. Both cases are investigated in this paper for a simple 2D example of an edge crack in a plate in a setting of small strains and hardening plasticity. In particular, we consider convergence issues from mesh refinement. [Copyright &y& Elsevier]

Practical residual strain field around the microcrack tip of fully lamellar γ-TiAl alloy was estimate the by digital image correlation (DIC) technology with in-situ SEM observation. And the macro plastic deformation before fracture of such low ductility alloys was observed. The results showed that the size of plastic zone around microcrack tip was 2 ~ 10µm which was much more smaller than the calculated value by fracture mechanics. During the magnified observation on the plastic deformation region, a series of microcracks were observed in the specimen without macro cracking. Furthermore, the size of distribution region of the microcracks were in the same order of magnitude with the calculated plastic zone near primary crack tip which just reflected the comprehensive effect of microcracks and the plastic deformation on the tips of them. [ABSTRACT FROM AUTHOR]

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]

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Abstract: The role of prism decussation on the crack growth resistance of human enamel is evaluated. Miniature inset compact tension (CT) specimens embodying a section of cuspal enamel were subjected to Mode I cyclic or monotonic loads. Cracks were grown in either the forward (from outer enamel inwards) or reverse (from inner enamel outwards) direction and the responses were compared quantitatively. Results showed that the outer enamel exhibits lower resistance to the inception and growth of cracks. Regardless of the growth direction, the near-threshold region of cyclic extension was typical of “short crack” behavior (i.e. deceleration of growth with an increase in crack length). Cyclic crack growth was more stable in the forward direction and occurred over twice the spatial distance achieved in the reverse direction. In response to the monotonic loads, a rising R-curve response was exhibited by growth in the forward direction only. The total energy absorbed in fracture for the forward direction was more than three times that in the reverse. The rise in crack growth resistance was largely attributed to a combination of mechanisms that included crack bridging, crack bifurcation and crack curving, which were induced by decussation in the inner enamel. An analysis of the responses distinguished that the microstructure of enamel appears optimized for resisting crack growth initiating from damage at the tooth’s surface. [Copyright &y& Elsevier]

Abstract: In this work, we suggest a new and simple method named single gradient notched beam (SGNB) method for determining the fracture toughness of Ti3Si(Al)C2 and Al2O3 with four-point bending specimens. For the specimen with a gradient notch, a sharp natural crack will initiate and extends from the tip of the triangle under increasing load. Based on the straight through crack assumption or on the slice model, the stress intensity factor coefficient for this notched beam was derived. The fracture toughness can be calculated from the maximum load and the minimum of the stress intensity factor coefficient without knowing the crack length. To verify the feasibility and reliability of this suggested method, the SGNB method and two other conventional methods, e.g. the chevron notched beam (CNB) method and single edge notched beam (SENB) method, were performed to determine the fracture toughness of Ti3Si(Al)C2 and Al2O3. The measured fracture toughness values obtained from the SGNB method agreed well with those from conventional fracture toughness tests. [Copyright &y& Elsevier]

To extend the predictive capability of existing crack growth models for fibre metal laminates under constant amplitude fatigue loading to variable-amplitude loading, further research on variable-amplitude fatigue mechanisms in fibre metal laminates is necessary. In response to this need, an experimental study into the effects of multiple overloads, underloads and various block-loading sequences on crack growth in the fibre metal laminate Glare was investigated. Crack growth retardation effects were observed in the tests; however, the magnitude of these effects was lower than seen in monolithic aluminium because of fibre bridging. As a result, predictions of the observed behaviour were attempted using an existing constant-amplitude fatigue crack growth model for Glare in combination with a linear damage accumulation law. [ABSTRACT FROM AUTHOR]

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: 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]

In this paper we present a model of a propagating fatigue crack based on energy dissipated within the process zone. This model heavily relies on the mesomechanical concepts applicable in the region immediately adjacent to the leading edge of the propagating crack. The model applied here, based on the cohesive crack concepts, seems to be a good approximation of the mesomechanical phenomena that govern nonlinear discrete deformation and fracture processes. The model suggests that the displacement just ahead of the crack tip is not zero, but it gradually approaches zero over a finite length, which measures the extent of the decohesion zone. Consequently, there is no singular stress at the crack tip. A certain distribution of the cohesive stress is used to model the finite stresses within highly nonlinear end zone preceding the crack. Such approach appears to be in agreement with Panin''s concepts of the sequence of deformation and pre-fracture processes occurring at the mesomechanical level. Analytical methods such as a power law and an exponential law were used to describe high cycle and low cycle fatigue processes, respectively. Numerical values for the ratio of the upper plateau to the threshold level of the cyclic R-curve derived from Wnuk''s “final stretch” model of subcritical crack propagation are provided. The relations derived here and based on the principles underlying mechanics of fracture at nanolevels, can be used as a bridge between continuum description of material response to fracture and the more fundamental, microstructural representation of material behavior. [Copyright &y& Elsevier]

Understanding and predicting the process of material fracture and failure in heterogeneous materials such as rocks, concrete, ceramics and other composites is an extremely challenging scientific problem, central to a large number of applications and crucial in predicting the long-term structural integrity of structures. The complex phenomena occurring at the micro- and mesoscale can be monitored by using acoustic emission techniques that reveal some universal features in the damage evolution and fracture. In particular, extensive research works and studies on concrete fracture and failure have shown that fracture growth and damage evolution can be characterized through a single synthetic parameter, namely the b-value of the Gutenberg-Richter law, which changes systematically with the different stages of fracture growth. In this paper, we propose two different interpretations for the variation of the b-value during the evolution of damage, focused on the spatial development of cracks in the material. The first one, called the self-similarity approach, is based on fractal geometry and the statistical characterization of the cracks inside a material by means of two power-law distributions: one for the spatial arrangement of crack barycentres, and the second for the crack length distribution. The second interpretation is based on the Yule process, originally proposed to explain the power-law size distribution of biological taxa. Both modelling idealizations capture the transition from the condition of criticality, in which b =1.5, to that of imminent failure, characterized by b =1.0, in terms of damage localisation. As a case study we present the method used by the authors to determine the conditions of the materials and the crack patterns in the structures of the Syracuse Cathedral, built in the 17th century on the structures of the ancient Greek “Temple of Athena” (5th century B.C.). In particular, the acoustic emission technique was used to evaluate the onset of critical conditions in a monitored pillar, which is part of the vertical load-bearing structures. The b-value trends are shown by several acoustic emission tests carried out on specimens of different dimensions extracted from the pillar. In addition, these results are compared to the acoustic emission data obtained from the in situ monitored pillar; it is shown that the b-value can be used both in the laboratory specimens and in the in situ measurements as a reliable indicator of the structural integrity. [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]