Fracture Mechanic

This paper presents elastic stress distributions near a cracktip in a continuous fiber composite. The material heterogeneity is explicitly accounted for by using the finite element method and a new Mesh Superposition Technique. This new technique superposes a fine mesh with heterogeneous material properties over a coarse mesh with homogeneous ones. The results indicate that the load transferred by fibers near a cracktip may be well described by the homogeneous orthotropic elastic K~ field. A technique to postprocess the K~ field to accurately obtain the detailed stress distributions within the fiber and matrix is also presented.

The study of defected axisymmetric structures is among important industrial applications. Detection of such defects, and or the evaluation of intrinsic parameter leads to a better design of those mechanical parts. The first part of the conducting research concerns the evaluation of the stress intensity factors (SIF) in axisymmetric elastic structures with internal or circumferential edge crack using the dual boundary element method (DBEM). Its application to axisymmetric problems requires a stress (hypersingular) boundary integral equation together with the displacement (standard) boundary integral equation, one applied to each side of the crack. This process requires a great algebraic handling due to the complexity of the axisymmetric kernels. Crack surfaces are discretized with discontinuous quadratic boundary elements to satisfy the existence of the finite-part integrals and the continuity of the unit outward normal at corners. SIF evaluation is done using displacements extrapolation at the crack tip. Examples of axisymmetric geometries are analyzed and obtained results are compared to others researchers.

Hat-shaped specimen geometries were developed to generate high strain, high-strain-rates deformation under prescribed conditions. These geometries o↵er also the possibility to investigate the occurrence of ductile rupture under low or negative stress triaxiality, where most failure models fail. In this work, three tophat geometries were designed, by means of extensive numerical simulation, to obtain desired stress triaxiality values within the shear region that develops across the ligament. Material failure was simulated using the Continuum Damage Model (CDM) formulation with a unilateral condition for damage accumulation and validated by comparing with quasi-static and high strain rate compression tests results on OFHC copper. Preliminary results seem to indicate that ductile tearing initiates at the specimen corner location where positive stress triaxiality occurs because of local rotation and eventually propagates along the ligament.

In the framework of CDM, the evolution of the plastic strain, at which damage processes initiates, as a function of stress triaxiality, making the assumption of constant damage dissipated work is derived. Based on this, the CDM model proposed by Bonora (Eng. Frac. Mech., 58, 1-2, 1997) has been used to predict the occurrence of ductile failure at different stress triaxiality conditions, under both quasi-static and dynamic loading conditions. This solution allows the possibility to correctly predict the conditions for which ductile damage can initiate under uniaxial strain (such as that for the flyer plate impact test) and mixed conditions such as those in the Taylor impact test. The solution offers the possibility to correlate continuum damage model parameters to microstructural features such as grain size and purity grade.

Ductile damage evolution assessment in high purity copper and stainless steel subjected to different shock-loading profiles using cohesive modeling ANDREW RUGGIERO, NICOLA BONORA, LUCA ESPOSITO, University of Cassino, GEORGE T. (RUSTY) GRAY III, Los Alamos National Laboratory -At continuum level, the triaxiality of the stress state has the major effect to reduce the ductility that the material can exhibit. As a result of this, ductile metals under uniaxial strain loading condition, such as that experienced in flyer plate impact experiment, should show a very limited ductility that is often in contrast with the energy dissipation estimated from the pull-back signal amplitude. In this paper, it is proposed that the developing of damage in form of voids causes a dramatic drop of the stress triaxiality giving the material the ability to sustain much larger deformation and consequent plastic work dissipation. Thus, a numerical model using cohesive elements has been developed to account for this dissipation in the energy balance. The model has been used to investigate the damage evolution and plastic strain accumulation in plate impact experiments with different loading profiles, for both high purity copper and stainless steel, in order to achieve a better comprehension of the processes related to spall fracture. Comparisons of numerical results with experimental data, available in term of both velocity profiles and optical metallography of soft-recovered samples, seem to confirm the capability of the proposed model to correctly capture distinctive features of spall event.

Recently, the continuum damage mechanics model proposed by Bonora (Eng. Fract. Mech. 58, 1997) has been updated to account for stress triaxiality effect on model parameters, (Bonora et al., AIP Conf. Proc. 1195. This model enhancement allows to predict ductile damage initiation under varying stress states (uniaxial stress, uniaxial strain, and complex load paths) and dynamic loading conditions. In this work, the model has been used to investigate ductile damage developments in Taylor anvil and symmetric Taylor impact (rod-on-rod) configuration. Although the two configurations are equivalent for right scaled impact velocities, experimental evidences show that when ductile damage occurs in rod-on-rod not necessarily also develops in Taylor anvil impact. It has been found that, in the two impact configurations, the stress triaxiality builds up differently with plastic strain leading to different conditions for ductile damage initiation. Taylor impact tests have been designed and performed with the gas-gun facility at the University of Cassino. Damage investigation results obtained on recovered samples have been compared with rod-on-rod data reported in the literature and used to validate the proposed model predictions.

One of the most important features of micromechanical models, with respect to other global approaches to fracture, is that progressive damage and failure can be described only by parameters characteristic of the material and not of the geometry. In spite of the large number of papers on micromechanical modeling presented in literature, a detailed assessment the geometry transferability of model parameters has been addressed in a limited number of works. In most of the cases, the model parameters transferability is analyzed only for one geometrical configuration other than that -usually a uniaxial tensile bar -on which the parameters set has been identified. In this paper, the continuum damage mechanics approach, as proposed by . A non-linear CDM model for ductile failure. Engineering Fracture Mechanics 58, 11-28] is used to model ductile damage processes in ferritic steels. The geometry transferability of the damage parameters is demonstrated both in the range of low and high stress triaxiality. The possibility to accurately predict constraint effects on material crack resistance curve is demonstrated.

In various fields of engineering, the assembly and repair of hydraulic installations are accomplished by joining the pipes. In such applications, the ring swaging method is used to connect the fittings to the pipes by means of a hydraulic hand tool. The basis to develop a swaging tool relies on the knowledge of the design parameter that influence plastic deformation of the swaging ring. In addition to build control over the design parameters, it is necessary to join pipes under severe conditions such as cryogenic vacuum and constrained space which require an intact sealing. In this study, the effect of swaging ring designs on sealing and strength has been examined and different swaging methods have been investigated by finite element modeling methods. Based on the obtained results, the analysis methodology of ring swaging and the characteristic impact of swaging ring design on the sealing of pipe connection are shown. The prime novelty of the study is to report the impact of swaging ring design and geometry on sealing efficiency of the pipe connection. The results of the study open new avenues for the development of efficient tools for designing swaging rings.

Since the development of the wedge splitting test (WST), techniques have been used to extract material properties that can describe the fracture behavior of the tested materials. Inverse analysis approaches are commonly used to estimate the stress-crack width relationship; which is described by the elastic modulus, tensile strength, fracture energy, and the assumed softening behavior. The stress-crack width relation can be implemented in finite element models for computing the cracking behavior of cementitious systems. While inverse analysis provides information about the material properties of various concrete mixtures there are limitations to the current analysis techniques. To date these techniques analyze the result of one WST specimen, thereby providing an estimate of material properties from single result. This paper utilizes a recent improvement to the inverse analysis technique, which enables the stress-crack width response to be determined simultaneously from multiple experimental tests. The effect of water-to-cement ratio and aggregate size are discussed. A comparison of epoxy-impregnated cracked WST specimen and material properties indicate a relationship between fracture properties and characteristics of load induced cracks.

The shear creep of the Al–4Al4C3 composite material was investigated for the first time at different temperatures ranging from 523 K to 773 K, and different shear stress levels ranging from 25 to 40 MPa in comparison to pure aluminum. The specimens were loaded in pure shear using specially designed patented specimen geometry and a prototype shear-testing machine. The experimental results indicate that the Al4C3 composite aluminum exhibits shear creep resistance more than four orders of magnitude compared to that of the aluminum, and can be loaded at shear stress levels exceeding two times those of pure aluminum material.

The paper investigates and models the mechanism of pre-split cutting, a technique used in excavations in hard rock and based on creating large splits by simultaneous blasting or slow pressurising of closely placed holes. Only the case of initially intact rock (no discontinuities comparable with the dimension of pre-split fracture) is considered. The critical spacing between the holes is found separating the case of splitting from the case of bulk fracturing of the rock. The determination of the critical spacing is based on the comparison between the hole pressure required to start the pre-existing cracks with the pressure required to bring the cracks to the length at which the interaction between them will make their propagation unstable. If the initial cracks are relatively large such that the pressure of the crack start is less than the pressure of unstable growth, other cracks will appear fracturing the bulk of material. If the initial cracks are small enough, the starting pressure will be high, hence the cracks will propagate dynamically until the coalescence with the neighbours, which produces splitting. For the case of pre-splitting prepared parallel to a free boundary, the influence of the latter is minor, so the dominant mechanism of pre-splitting is the crack interaction.

The Weibull stress is widely used as a measure of the probability of cleavage failure. In this work analytical and semi-analytical expressions for the Weibull stress are developed in terms of the remote loading parameters, J or K, and material properties. Results are presented for sharp cracks and notches in elastic and elastic-plastic materials under plane stress and plane strain

The welding process induces Residual tensile stress that is detrimental to Fatigue life. Tensile stress act to stretch or pull apart the surface of the material. With enough loads cycle at a high enough tensile stress, a metal surface initiate a crack. Significant improvement in Fatigue life can be obtained by modifying the Residual stress level in the material. The intent of this paper is for Butt welded similar and dissimilar joint, evaluate Residual stress level by using most accurate and best developed X-Ray diffraction method, before and after surface modification. In this paper, welding compressive residual stress can be modified by Shot peening, which required simple equipment and treatment is extensively employed as a method to improve Fatigue strength. This study uses MATLAB software to analyze two experimental data sets using a neural network fitting tool. According to the results, artificial neural networks (ANNs) are accurate and results show that ANN models can be used successfully for predicting surface roughness changes during shot Peening Process.

We study the Γ-convergence of damage to fracture energy functionals in the presence of low-order nonlinear potentials that allows us to model physical phenomena such as fluid-driven fracturing, plastic slip, and the satisfaction of kinematical constraints such as crack non-interpenetration. Existence results are also addressed.

HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L'archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

The last decade has been witness to phenomenal growth in endodontic technology. The introduction of these new technologies has resulted in endodontics becoming easier, faster and most importantly, better. NiTi alloys overall are softer than stainless steel, are not heat treatable, have a low modulus of elasticity (about one fourth to one fifth that ofstainless steel) but a greater strength, are tougher and more resilient, andshow shape memory and superelasticity. The latter two properties arethe main reasons why NiTi alloys have succeeded in endodontics. Thepurpose of this article is to review the design features of different rotary instruments used for pulp space preparation. Important mechanical features include thevariability of taper, rake angle, cross-sectional geometry, tip configuration, design of blades, helical angle and pitch. These design features influence flexibility, cutting efficiency and safety. In this review, design features of commonlyused NiTi rotary systems are ...

The influence of primary and secondary orientations on the elastic response of a hollow core, (00 1 Joriented nickel base single-crystal superalloy turbine blade, was investigated under combined thermal and mechanical loading conditions. Finite element technique is employed through MARC finite element code to conduct the analyses on a hollow core SSME turbine blade made out of PW A 480 single crystal material. Primary orientation of the single crystal superalloy was varied in increments of 2°, from 0 to 10°, from the (001] direction. Two secondary orientations (0 and 45 °) were considered, with respect to the global coordinate system, as the primary orientation angle was varied. The stresses developed within the single crystal blade were determined for different orientations of the blade. The influence of angular offsets such as the single crystal's primary and secondary orientations and the loading conditions on the elastic stress response of the PW A 1480 hollow blade are summarized. The influence of the primary orientation angle, when constrained between the bounds considered, was not found to be as significant as the influence of the secondary orientation angle.

This paper present a study of the electro-mechanical impedance spectroscopy method (EMIS) used in structural health monitoring with piezoelectric wafer active sensors, with and without wire lead connections, bonded on thin plates, at intermediate and high frequencies. For simple geometries like circular piezoelectric wafer active sensor bonded to circular plates, a simplified 2D axisymmetric analytic model exist, and is briefly presented. However when cracks exist the 2D model could not be applied. Usually the low frequencies (10kHz−150kHz) are used in the EMIS method. A 3D finite element method is used to analyze ability of the EMIS method to evaluate the distance to the crack at intermediate (150kHz−300kHz) and high (300kHz−450kHz) frequencies. The influence of the asymmetry position on the wire lead piezoelectric wafer active sensors related to the crack at intermediate and high frequencies is also studied. Quantifications by classical damage metrics are done.

This paper shows and discusses a generic implementation of the global-local analysis toward generalized finite element method (GFEM gl ). This implementation, performed into an academic computational platform, follows the object-oriented approach presented by the authors in a previous work for the standard version of GFEM in which the shape functions of finite elements are hierarchically enriched by analytical functions, according to the problem behavior. In global-local GFEM, however, the enrichment functions are constructed numerically from the solution of a local problem. This strategy allows the use of a coarse mesh even when the problem produces complex stress distributions. On the other hand, a local problem is defined where the stress field presents high gradients and it is discretized using a large number of elements. The results of the local problem are used to enrich the global problem which improves the approximate solution. The great advantage is allowing a well-refined description of the local problem, when necessary, avoiding an overburden for the computation of the global solution. Details of the implementation are presented and important aspects of using this strategy are highlighted in the numerical examples.

This study therefore, explored the effects of the section sizes ranging from 10mm -16 mm in diameters. Chemical compositions of the steel samples were carried out using XRF. Tests samples were machined using lathe machine and their tensile, hardness and impact properties were determined. Results showed that their carbon contents ranged from 0.01% to 0.25% and the ultimate tensile strength and the yield strength properties decreased with increase in sample diameter. Hence, the diameters of the rods have great influence on the mechanical properties of the materials. The results of this study are relevant to steel industries that produce low carbon steel and structural engineers that make use of this steel in construction.

We studied the rearran gement of ion-implanted hydrogen in <100 > oriented n-type silicon wafers upon annealing and its effect on the crystal damage . The silicon samples were implanted with 42 keV protons to a dose of 2 x 1016 H /cm2 and subsequently vacuum annealed at temperatures r anging from 200 °C to 500 °C . The evolution of the H-concentration and the crystal damage depth profiles during the heat treatments were investigated through the combined use of elastic recoil detection (ERD) analysis, secondary ion mass spectroscopy (SIMS), and Rutherford backscattering spectroscopy (RBS) in channeling mode . The obtained results reveal information about the damage accumulation caused by the thermally induced rearrangement o f

Crack evolution in wood from Norway Spruce during mode I wedge splitting tests was measured using a Digital Image Correlation (DIC) system. The resulting series of deformation fields were post-processed in order to obtain kinematically based crack tip location histories throughout the loading procedure, by a purposely made algorithm. The developed algorithm is based on generic mode patterns which are fitted to the observed deformation fields and optimized. The developed procedure and its application are explained and the resulting crack paths are presented. Subsequently, fracture energies, critical stress intensity factors and experimental resistance curves were derived based on the crack path data. Comparisons of the obtained material parameters for fracture mechanics with literature values show that the proposed method is a powerful alternative to the more traditional methods. Moreover, experimentally derived crack tip location histories give more detailed insight into the behaviour of wood (Norway Spruce) during mode I fracture and may replace indirect methods and crack length assumptions.

Finite-element analysis based on elasticeperfectly plastic material was conducted to examine the influence of structural deformations on collapse loads of circumferential through-wall critically cracked 90 pipe bends undergoing in-plane closing bending and internal pressure. The critical crack is defined for a through-wall circumferential crack at the extrados with a subtended angle below which there is no weakening effect on collapse moment of elbows subjected to in-plane closing bending. Elliptical and semioval cross sections were postulated at the bend regions and compared. Twice-elastic-slope method was utilized to obtain the collapse loads. Structural deformations, namely, ovality and thinning, were each varied from 0% to 20% in steps of 5% and the normalized internal pressure was varied from 0.2 to 0.6. Results indicate that elliptic cross sections were suitable for pipe ratios 5 and 10, whereas for pipe ratio 20, semioval cross sections gave satisfactory solutions. The effect of ovality on collapse loads is significant, although it cancelled out at a certain value of applied internal pressure. Thinning had a negligible effect on collapse loads of bends with crack geometries considered.

Better understating of predicting failure in tough ductile polymers is desirable because of their increasing use in load bearing applications. In this treatise, the use of several methodologies, all of which assume the material to behave elastically. were used to predict failure loads for various injection moulded articles that contained cracks. The motivation for this is to produce a simple means of predicting failure in normally thin walled injection moulded articles. It was found that a new simple model could predict safe failure loads for thin walled polymer articles with short crack lengths. The model could be applied to articles which are also complex in geometry. This model supposes that sharp flaws in thin walled injection moulded articles blunt themselves to a radius of curvature p and this parameter p is a characteristic of each material. Brittle polymeric materials such as polystyrene were found to have low values of p. whilst tough polymers such as poly-vinyl chloride had high p values (up to fifty times the value of polystyrene). Once the parameter p is found from simple fracture tests, it is able to be used in predicting failure loads for cracked articles (as in this thesis) or it may also be utilised as a design tool. In design, the simple failure model informs the engineer of the size of the flaw which a component is able to withstand (under service loads) before failing. This allows for better control over the amount of material used and correct allocation of material to regions of probable failure in the manufacture of load bearing components. Nine types of polymers were studied and four loading situations were investigated. Even under severe loading conditions, such as a crack running along the base of the rib in a ribbed specimen, where a plane strain situation exists, the simple model reliably predicted safe working loads. The cracks are in the order of 15% or less of the article's width. The work covered in this thesis is therefore concerned primarily with cases where the crack is small in comparison to the size of the object. The author feels that consumers will tend to discard components which have large cracks in them thereby making analysis for those cases inconsequential.

Better understating of predicting failure in tough ductile polymers is desirable because of their increasing use in load bearing applications. In this treatise, the use of several methodologies, all of which assume the material to behave elastically. were used to predict failure loads for various injection moulded articles that contained cracks. The motivation for this is to produce a simple means of predicting failure in normally thin walled injection moulded articles. It was found that a new simple model could predict safe failure loads for thin walled polymer articles with short crack lengths. The model could be applied to articles which are also complex in geometry. This model supposes that sharp flaws in thin walled injection moulded articles blunt themselves to a radius of curvature p and this parameter p is a characteristic of each material. Brittle polymeric materials such as polystyrene were found to have low values of p. whilst tough polymers such as poly-vinyl chloride had high p values (up to fifty times the value of polystyrene). Once the parameter p is found from simple fracture tests, it is able to be used in predicting failure loads for cracked articles (as in this thesis) or it may also be utilised as a design tool. In design, the simple failure model informs the engineer of the size of the flaw which a component is able to withstand (under service loads) before failing. This allows for better control over the amount of material used and correct allocation of material to regions of probable failure in the manufacture of load bearing components. Nine types of polymers were studied and four loading situations were investigated. Even under severe loading conditions, such as a crack running along the base of the rib in a ribbed specimen, where a plane strain situation exists, the simple model reliably predicted safe working loads. The cracks are in the order of 15% or less of the article's width. The work covered in this thesis is therefore concerned primarily with cases where the crack is small in comparison to the size of the object. The author feels that consumers will tend to discard components which have large cracks in them thereby making analysis for those cases inconsequential.

The very large glued laminated timber columns that are needed for tall timber buildings are too large to be physically tested in most facilities. To safely design these columns, it is necessary to identify and extrapolate behaviour from the physical testing of smaller specimens. Compression testing of 27 glue-laminated timber columns showed a reduction in strength with increased member size. This phenomenon is known as a size effect. The laminated timber exhibited a compressive strength much higher than the characteristic strength that it was graded it. Comparisons between columns of different lengths and widths, suggests that the homogenisation of laminated timber may mitigate the size effect. An extrapolation of the size effect from the column sizes tested, to an ultra-large column for a timber skyscraper, indicated that the magnitude of the effect could be large enough to reduce the compressive strength of the glulam to below its characteristic strength.

Surface roughness is known to have great influence on fatigue life. The traditional approach is to relate the average roughness to the endurance life through a material parameter such as ultimate tensile strength. Such approaches have however shown a large degree of scatter, indicating that this approach is insufficient for relating surface topography to fatigue strength. This is the motivation for the current research, where surface roughness is modelled by finite elements obtained from measured topography. The results can be used directly in fatigue life prediction or as a foundation for evaluating and developing prediction methods. An aluminium alloy used in a wrought automotive front wheel suspension arm is used for testing and evaluation.

HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L'archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

An extensive literature review was conducted primarily to develop a comprehensive understanding of the quantitative behavior of residual stress (RS) distribution in weld joints. Based on prior data, various levels of the peak RS and distribution profiles were applied to a finite element analyses (FEA) model as an initial loading to evaluate the effect of RS on the Mode-I stress intensity factor (SIF), K I . The RS was not found to have a significant effect on the SIF values when the peak RS was less than 300 MPa. The enhanced effect of RS on the K I values was found to be more pronounced at a lower crack ratio, while the analytical form of the RS distribution had a minor effect. The effective boundary corrections function, F Residual , was derived under the effect of RS for 1T CT specimen for various crack geometries of the crack ratio, (a/W) of 0.2, 0.40, 0.50, 0.60, and 0.75, and with peak RS varying from 100 MPa to 600 MPa. The obtained effective function of K I can be employed to quantitatively evaluate the effect of RS on the fatigue performance of welded joints.

The 6061-T651 aluminium alloy is one of the most common aluminium alloys for marine components and general structures. The stress intensity factor (SIF) is an important parameter for estimating the life of the cracked structure. In this paper, the stress intensity factors of a slant-cracked plate, which is made of 6061-T651 aluminum, have been calculated using extended finite element method (XFEM) and finite element method (FEM) in ABAQUS software and the results were compared with theoretical values. Numerical values obtained from these two methods were close to the theoretical values. In simulations of crack growth at different crack angles, the crack propagation angle values were closer to the theoretical values in XFEM method. Also, the accuracy and validity of fatigue crack growth curve were much closer to the theoretical graph in XFEM than the FEM. Therefore, in this paper the capabilities of XFEM were realized in analyzing issues such as cracks.

An experimental investigation was conducted for a better understanding of fatigue and fracture behaviour of carburized steel used in the manufacture of vehicle transmission elements such as crown wheel and pinion. An attempt was made to study the fatigue strength of En 353 based on failure at 10 7 cycles after subjecting to three different treatments namely conventional heat treatment, shallow cryogenic treatment and deep cryogenic treatment. Rotating bending fatigue test was performed in air at room temperature for all specimens. Fractured specimen surfaces were examined by scanning electron microscope to identify the mechanism involved during fatigue. The study concludes that shallow and deep cryogenically treated samples show an improvement with an overall fatigue life of 71% and a reduction of 26% over conventionally heat treated samples respectively. The combined presence of retained austenite and fine carbides has resulted in enhanced fatigue strength of shallow cryogenically treated specimens over the conventionally heat treated and deep cryogenically treated specimens.

This study aims at evaluating the influence of fatigue strain range on hydrogen embrittlement of resulfurized high strength steel on low-cycle fatigue tests. Previously, samples were charged with hydrogen (H) under different conditions. The presence of H causes softening effects on the cyclic behavior and a reduction in fatigue life of samples in comparison with uncharged samples. The explanation of this behavior is based on quantitative analyses of fracture surfaces as well as the microstructure by optical microscopy and scanning electron microscopy (SEM). Moreover, enthalpy changes, associated with the main H trapping sites, were determined by differential scanning calorimetry (DSC).This study has enhanced the understanding of the variables that most influence the fatigue behavior of this type of steel charged with hydrogen.

This paper analyzes the reliability of the fracture mechanics approach to environmentally assisted cracking in engineering design. A wide collection of experimental evidences of uncertainty in the fracture mechanics characteristics of the phenomenon -the crack growth kinetics vWcurve and the threshold stress intensity factor K,, -is presented. Although these basic fracture mechanics items are supposed to depend so/e/y on the material and the environment, they are notably sensitive to the influence of a wide family of test/service variables, producing loss of confidence in materials evaluation and structural integrity assessment.

Experimental and numerical databases are presented for thick pipes (10<1)0/t<40) under pure bending and combined tension and bending loads. Equations to predict collapse moment, critical curvature, moment-tension and curvature-tension interactions are proposed based on the databases. The model uncertainty analyses have confirmed that the proposed equations are accurate.

AM&act-The Welding Institute Level-3 crack tip opening displacement (CTOD) method was ezpanded to be applicable to plane strain (Planbstrain Level-3 CLOD method). The delinition of the reference strain is a major source of uncertainty in the application of the Level-3 CTOD method. Two different measures of the reference strain were proposed. Applied CTOD estimates of the surface cracked wide plate in bending were performed using the Level-3 CLOD methods. It was found that the proposed Plane-strain Level-3 CTOD method can provide highly accurate predictions of applied CIOD with a safety factor approximately equal to 1.0 for the surface crack in bending.

As research of the high cycle fatigue of carburized gear steel could not meet the status quo of longer and longer service lives, research of very high cycle fatigue (VHCF) performance has become the focus of current research. The VHCF properties of case-hardening steel 18CrNiMo7-6 after being carburized with gradient hardness distribution were investigated by means of ultrasonic fatigue tests. The results showed that the carburized specimens with a case hardness of 705 HV and core hardness of 530 HV showed VHCF phenomenon, and the fatigue lives continuously increased to even 109 cycles as the stress amplitude decreased to about 500 MPa. Observations of the fracture surfaces of the fatigue specimens showed that the fatigue crack initiation sites were located in the transition area with the hardness at about 580 HV. It was found that the transition area had low VHCF properties, since the core did not show VHCF phenomenon, and the case had a higher hardness. A fine microstructure was o...

ausgelost uferden, werden leicht zu handhabende Formeln hergeleitet. Ferner werden ihre Anu,endung bei der Losung von Ra7~duieriprobleinen und der EinjluJ der elastischen Konstanten diskutiert. Spezielle Auf?nerksnmkeit ooird der Behandl?ipbg cines elastischen Slretfens yewidmet, der ein periodisches Frltl von Lo'chern enthalt. Convenient formulae are provided for the stress fields produced by isolated or periodically distributed forces and dislocations i n plane orthotropic elasticity. Their application to the solution of boundary oaluc proble?rl.s nnd the influence of thr elastic constants are discussed. Special attention is given to the problem of an elastic strip containing a periodic nrray o j holes.

Round robin tests carried out under the direction of the Technical Committee 4 of the European Structural Integrity Society (ESIS TC4) have shown that, for determining the fracture resistance of ductile polymers at low loading rates, the multi-specimen methodology based on the construction of the material crack growth resistance curve often does not provide reliable data due to the uncertainties associated with the measurement of crack advancement (Da). With the aim of strengthening this multi-specimen methodology, the ESIS TC4 attention has been recently focused on the analysis of a testing scheme based on the load separation criterion, which does not require the measurement of Da. The present work gives the results of a multi-laboratory round-robin testing exercise carried out by ESIS TC4 in order to assess the degree of reproducibility of the fracture parameters obtainable with the application of this load separation criterion based testing scheme. Encouraging results have been obtained.

In steel fiber reinforced concrete (SFRC), the fibers are generally considered to be oriented isotropically. However, it is known that the procedures used in the specimen fabrication during quality control and material characterization may influence the distribution of the fibers significantly. This has an important effect on the properties determined from the specimens, which has to be considered when the results are used to analyze the behavior of other elements. In the present work, the orientation and segregation of fibers in cylindrical specimens and prisms, which are normally used in the mechanical characterization of SFRC, are evaluated. The type of compaction (i.e., table vibration, hand tamping and internal vibration) is seen to have a considerable influence on the distribution of the fibers. In the prisms, table vibration increases the tendency for the fibers to be oriented horizontally. In the cylindrical specimens, hand tamping appears to cause the least nonuniformity of the fiber distribution. The study was conducted on conventional concretes with 40 kg/m 3 of fibers. 1359-5997 9 2004 RILEM. All rights reserved. Dans le bgton arm~ avec des fibres' d'acier (BAFA), les fibres sont gdnOralement considOrOes comme dtant orientOes de mani~re isotropique. Toutefois, les procOdures utilisges clans la fabrication d~chantillons peuvent influencer, de maniOre significative, la distribution des' fibres. Etles jouent sur les propri~t~s mesurdes sur les OchantilIons, un rdle important, qui doit &re pris en compte lorsque les rdsultats sont utilisOs pour analyser le comportement des' autres glOments. Dam' le prgsent travail, sont Otudi~es l'orientation et la sOgrOgation des fibres darts des dchantillons cylindriques et prismatiques, lesquels sont gdnOralement utilis~s lots de la caractOrisation des BAFA. Le type de compactage (c 'est-h-dire, table vibrante, compactage manuel et vibration interne) est conside'rg comme co/ant une influence consMgrable sur la distribution des" fibres. Darts les prismes, la table vibrante cr~e la meilleure disposition pour l'alignement horizontal des fibres. Dans un ~chantillon cylindrique, te compactage manuel apparait comme la cause de la non uniformit~ de la distribution des fibres. L 'dtude a OtO rOalisOe sur des bdtons conventionnels ayant une densitO de fibres de 40 kg/m s,

Failure of a ply due to transverse loading is one of the mechanisms accounted for in physically-based failure criteria, used in composites design. Experimental data, however, is scarce and the measurement techniques used in the past are time consuming and involve lot of specimen handling during testing. Some physical information is currently well consolidated (such as the dependence of the strength on ply thickness, or in-situ strength) but relevant open questions remain. This work presents a methodology, which does not interfere with the tensile test, to detect transverse cracks by optical means. Four different configurations of CFRP are considered. The results show that there is a dependence of the in-situ strength on the ply thickness and the orientation of the adjacent layers. In the case of thick transverse plies, the strength is controlled by transverse cracks whereas, in thin plies, longitudinal ply splitting occurs before transverse matrix cracking.

In this paper, we present a general method for the calculation of the various stress intensity factors in a material whose constitutive law is elastic, linear and varies continuously in space. The approach used to predict the stress intensity factors is an extension of the interaction integral method. For this type of material, we also develop a systematic method to derive the asymptotic displacement fields and use it to achieve better-quality results. A new analytical asymptotic field is given for two special cases of graded materials. Numerical examples focus on materials with spacedependent Young modulus.

Institute of Mathematics of the Academy of Sciences of the Czech Republic provides access to digitized documents strictly for personal use. Each copy of any part of this document must contain these Terms of use. This paper has been digitized, optimized for electronic delivery and stamped with digital signature within the project DML-CZ: The Czech Digital Mathematics Library 54 (2009) APPLICATIONS OF MATHEMATICS No. 3, 177-180 IVAN HLAVÁČEK IS SEVENTY-FIVE* Without doubt Ivan Hlaváček is one of the top numerical analysts in the Czech Republic. He made several fundamental contributions to the theory of elastic and elasto-plastic bodies, the dual finite element method, to numerical solution of variational inequalities, optimal shape design, superconvergence of the finite element method, Galerkin approximation of nonlinear partial differential equations, the worst scenario method, and many other topics. Moreover, as he is still very active, new items may appear on his conto in the years ahead of us.

The performance and the reliability of many devices are controlled by interfaces between thin films. In this study we investigated the use of patterned, nanoscale interfacial roughness as a way to increase the apparent interfacial toughness of brittle, thin-film material systems. The experimental portion of the study measured the interfacial toughness of a number of interfaces with nanoscale roughness. This included a silicon interface with a rectangular-toothed pattern of 60-nm wide by 90-nm deep channels fabricated using nanoimprint lithography techniques. Detailed finite element simulations were used to investigate the nature of interfacial crack growth when the interface is patterned. These simulations examined how geometric and material parameter choices affect the apparent toughness. Atomistic simulations were also performed with the aim of identifying possible modifications to the interfacial separation models currently used in nanoscale, finite element fracture analyses. The fundamental nature of atomistic tractionseparation for mixed mode loadings was investigated.

In this work we apply a Lagrangian kernel-based estimator of continuum fields to atomic data in order to estimate the J-integral for the analysis of cracks and dislocations. We show that this method has the properties of: consistency between the energy, stress and deformation fields; path independence of the contour integrals of the Eshelby stress; and excellent correlation with linear elastic fracture mechanics theory for appropriately constructed simulations. We discuss the appropriate reference configuration and reference energy for this type of analysis. Lastly, we use canonical examples to demonstrate that the proposed method is a direct and rational approach for estimating the configurational forces on atomic defects.