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The prediction of crack initiation and propagation in ductile failure processes are challenging tasks for the design and fabrication of metallic materials and structures on a large scale. Numerical aspects of ductile failure dictate a sub-optimal calibration of plasticity- and fracture-related parameters for a large number of material properties. These parameters enter the system of partial differential equations as a forward model. In this work, we develop a step-wise Bayesian inversion framework for ductile fracture to provide accurate knowledge regarding the effective mechanical parameters. To this end, synthetic and experimental observations are used to estimate the posterior density of the unknowns. To model the ductile failure behavior of solid materials, we rely on the phase-field approach to fracture, for which we present a unified formulation that allows recovering different models on a variational basis. In the variational framework, incremental minimization principles for a class of gradient-type dissipative materials are used to derive the governing equations. The overall formulation is revisited and extended to the case of anisotropic ductile fracture. Three different models are subsequently recovered by certain choices of parameters and constitutive functions, which are later assessed through Bayesian inversion techniques. To estimate the posterior density function of ductile material parameters, three common Markov chain Monte Carlo (MCMC) techniques are employed: (i) the Metropolis-Hastings algorithm, (ii) delayed-rejection adaptive Metropolis, and (iii) ensemble Kalman filter combined with MCMC. To examine the computational efficiency of the MCMC methods, we employ the R-convergence tool., 61 pages, 21 figures, 10 tables

This paper presents a framework for modeling failure in quasi-brittle geomaterials under different loading conditions. A micromechanics-based model is proposed in which the field variables are linked to physical mechanisms at the microcrack level: damage is related to the growth of microcracks, while plasticity is related to the frictional sliding of closed microcracks. Consequently, the hardening/softening functions and parameters entering the free energy follow from the definition of a single degradation function and the elastic material properties. The evolution of opening microcracks in tension leads to brittle behavior and mode I fracture, while the evolution of closed microcracks under frictional sliding in compression/shear leads to ductile behavior and mode II fracture. Frictional sliding is endowed with a non-associative law, a crucial aspect of the model that considers the effect of dilation and allows for realistic material responses with non-vanishing frictional energy dissipation. Despite the non-associative law, a variationally consistent formulation is presented using notions of energy balance and stability, following the energetic formulation for rate-independent systems. The material response of the model is first described, followed by the numerical implementation procedure and several benchmark finite element simulations. The results highlight the ability of the model to describe tensile, shear, and mixed-mode fracture, as well as responses with brittle-to-ductile transition. A key result is that, by virtue of the micromechanical arguments, realistic failure modes can be captured, without resorting to the usual heuristic modifications considered in the phase-field literature. The numerical results are thoroughly discussed with reference to previous numerical studies, experimental evidence, and analytical fracture criteria., Comment: Postprint version. Minor corrections

A full understanding of the non-linear mechanical response of the polymer is essential for fibre-reinforced polymer composite design because an explicit definition of constitutive material models for the constituents (fibres, matrix, and interface) are prerequisite in micromechanical simulations. Unlike ductile metals, the material behaviour of polymer matrix is characterised by plasticity theories influenced by a combination of distortional and spherical energy dissipation. In this respect, an elastoplastic thermodynamic continuum model derivation is proposed using the paraboloidal yield criterion under isothermal conditions. A non-iterative scheme is developed for the numerical computation of the plastic strain increment multiplier. Both associated and non-associated flow rules are investigated following classical plasticity loading-unloading conditions. It thereby, evades conventional computationally demanding iterative process by replacing it with an exact determination of plastic strain increment. This novel approach highly improves the computational efficiency algorithmically. The real-sized numerical models are investigated and the comparison between simulated and experimental results shows the reliability and unprecedented accuracy of the proposed elastoplastic mathematical model.

Particulate composites are found in a wide range of applications. Their heterogeneous microstructure affects their bulk behavior and structural performance, however tools for predicting this important structure-property relationship are still lacking. In this study, a numerical method that can provide predictions of the mechanical response of a particulate polymeric matrix composite as a function of volume fraction and particle mean diameter is presented. The work is derived for an alumina trihydrate filled poly(methyl methacrylate) but the methodology is generic and can be used for any particulate composite. Representative Volume elements are determined through images obtained from scanning electron microscopy. The model takes into account the possibility of failure through interface debonding as well as cracks through the matrix. The model predictions for the modulus and fracture strength of the composites are validated through independent experiments on the composite. The numerical results are also used to qualitatively explain the trends measured regarding the fracture toughness of the composites. Compared to other literature on particulate composites, our study is the first to report accurate stress–strain distributions as well as fracture predictions whilst all the necessary model parameters defining the failure criteria are all derived through independent experiments. This paves the way for a relatively simple methodology for determining structure-property relationships in composites design, enabling smarter material utilization and optimal mechanical properties.

Friction stir welding (FSW) since its invention has been attracting relevant interest for joining aluminium alloys. Due to the nature of this process, the materials can be joint without melting. Thanks to this peculiar characteristic, the issues associated with the cooling from liquid phase are avoided or considerably reduced, such as cracking, porosity, and defects. However, as well as other well-established welding techniques, the FSW process gives rise to formation of residual stress in the welding region and surrounding volume: heat and thermo-mechanical affected zones. Presence of residual stress in a mechanical component is well-known to affect its performance, particularly regarding fatigue at high number of cycles. Another aspect that influences the fatigue life is the underlying microstructure. In this work, we firstly study the residual stress field and the underlying microstructural features arising in FSW butt joints and their effect on the fatigue performance of this type of weldments. The evaluation of residual stress field is carried out by means of modern experimental techniques. In the first instance, synchrotron X-ray powder diffraction was employed for two-dimensional full field maps of residual stress. Corroboration of these measurements was done by exploiting the capability of focused ion beam and digital image correlation (FIB-DIC), which is able to deliver pointwise absolute measurement of residual stress. A set of FSW samples were then tested under uniaxial fatigue loading at several loading ranges, in the high cycle fatigue regime, in order to understand whether the severity of loads affects the crack path and life endurance. Fractographic and electron backscattered diffraction (EBSD) analysis then revealed crack nucleation site and propagation mechanisms with the respect of the underlying microstructure. Outcome of these experimental studies is then thoroughly discussed.

This paper examines the structural response of reinforced concrete flat slabs, provided with fully-embedded shear-heads, through detailed three-dimensional nonlinear numerical simulations and parametric assessments using concrete damage plasticity models. Validations of the adopted nonlinear finite element procedures are carried out against experimental results from three test series. After gaining confidence in the ability of the numerical models to predict closely the full inelastic response and failure modes, numerical investigations are carried out in order to examine the influence of key material and geometric parameters. The results of these numerical assessments enable the identification of three modes of failure as a function of the interaction between the shear-head and surrounding concrete. Based on the findings, coupled with results from previous studies, analytical models are proposed for predicting the rotational response as well as the ultimate strength of such slab systems. Practical recommendations are also provided for the design of shear-heads in RC slabs, including the embedment length and section size. The analytical expressions proposed in this paper, based on a wide-ranging parametric assessment, are shown to offer a more reliable design approach in comparison with existing methods for all types of shear-heads, and are suitable for direct practical application.

Mechanoluminescence (ML) is the phenomenon describing the emission of light during mechanical action on a solid, leading to applications such as pressure sensing, damage detection and visualization of stress distributions. In most cases, this mechanical action releases energy that was previously stored in the crystal lattice of the phosphor by means of trapped charge carriers. A drawback is the need to record the ML emission during a pressure event. In this work, we provide a method for adding a memory function to these pressure-sensitive phosphors, allowing an optical readout of the location and intensity of a pressure event in excess of 72 h after the event. This is achieved in the BaSi2O2N2:Eu2+ phosphor, where a broad trap depth distribution essential for the process is present. By merging optically stimulated luminescence (OSL), thermoluminescence (TL) and ML measurements, the influence of light, heat and pressure on the trap depth distribution is carefully analysed. This analysis demonstrates that mechanical action can not only lead to direct light emission but also to a reshuffling of trap occupations. This memory effect not only is expected to lead to new pressure sensing applications but also offers an approach to study charge carrier transitions in energy storage phosphors., Light: Science & Applications: Sensors: Light-up materials remember when they’ve been touched Compounds that release light after experiencing mechanical stress can now provide optical readouts of pressure-induced events several days after they occur. Philippe Smet from Ghent University in Belgium and colleagues report that the europium-doped barium silicon oxynitride (BaSi2O2N2:Eu) phosphor exhibits a unique memory function after an initial exposure to ultraviolet light. The phosphor’s excited electrons fill ‘traps’, or defects in the crystal lattice, which can be emptied by applying weight to it, enabling the emission of light. Subsequent illumination of the BaSi2O2N2:Eu crystal with infrared radiation enabled the researchers to release trapped charges in states less accessible to mechanical stimulation. The resulting glow from the phosphor had increased signal strength at locations where pressure actions had previously occurred, even if the pressure action had occurred up to three days before.

In this work, we report a high-vanadium hierarchical coating prepared on the surface of nodular cast iron substrate by a low-cost plasma transferred arc (PTA) surface alloying process. The coating consists of a graded layer with an alloyed zone (AZ) rich in submicron sized granular (V-Ti-Nb-Cr-Mo) composite carbides on top of intermediate melted zone characterized by refined ledeburite and martensite. The dense spherical particles in the AZ are FCC structured MC-type (M = V, Ti and Nb) carbides which tend to aggregate while M7C3 and M2C carbides nucleate on MC. The super-lattice V8C7 maintains its cube-on-cube orientation relationship with TiC. The hardness of the AZ is 9.6 +/- 1.0 GPa, similar to 4 times that of the substrate. Nano- and micro-indentations point at a superior strength-toughness in the AZ, where cracks are deflected and bridged by the spherical MC carbides in a compressive residue stress state. The fracture mode appears to be rather ductile in the AZ whereas brittle failure appears in both the melted zone and substrate. TEM and EDS results confirm that such a micro architecture design, assisted by the rapid solidification rate of the PTA process, concurrently activates various strengthening micromechanisms including the precipitation hardening and grain refinement.

Dry-snow slab avalanches release due to the formation of a crack in a weak layer buried below cohesive snow slabs, followed by rapid crack propagation. The onset of rapid crack propagation occurs if stresses at the crack tip in the weak layer overcome its strength. In this study, we use the finite element method to evaluate the maximum shear stress τmax induced by a preexisting crack in a weak snow layer allowing for the bending of the overlaying slab. It is shown that τmax increases with increasing crack length, slab thickness, slab density, weak layer elastic modulus, and slope angle. In contrast, τmax decreases with increasing elastic modulus of the slab. Assuming a realistic failure envelope, we computed the critical crack length a꜀ for the onset of crack propagation. The model allows for remote triggering from flat (or low angle) terrain. Yet it shows that the critical crack length decreases with increasing slope angle., Geophysical Research Letters, 45 (16), ISSN:0094-8276, ISSN:1944-8007

The present paper describes improvements in the fracture resistance of epoxy polymers due to the addition of either (a) one-dimensional (1-D) carbon nanofibres (CNFs), or (b) two-dimensional (2-D) graphene nanoplatelets (GNPs), or (c) hybrid combinations of these carbon nanofillers (i.e. using both CNFs and GNPs). The effects of the dimensional shape and concentration (i.e. 0.0, 0.5, 1.0, 1.5 and 2.0 wt%) of the nanoscale carbon fillers are considered. The addition of CNFs, GNPs or hybrid combinations of CNFs and GNPs increases greatly the quasi-static fracture energy, GIc, of the epoxy due to these nanofillers inducing multiple intrinsic (e.g. interfacial debonding and void growth) and extrinsic (e.g. pull-out and bridging) toughening mechanisms. A mechanistic model is presented to quantify the contributions from the different toughening mechanisms induced by the CNF and the GNP fillers which result in the improvements observed in the fracture energy. The resistance of the epoxy, modified with either the GNPs or the CNFs, to cyclic-fatigue loading is also increased by the presence of the carbon nanofillers.

This contribution to the Festschrift for Professor Jing Zhu concentrates on in-situ electron microscopy, a topic which is key in her scientific work. In particular this paper delineates the possibilities of utilizing in-situ transmission electron microscopy to unravel size effects in the structure-property relationship. We have focused on in-situ compression, tension and cyclic fatigue experiments on non-crystalline metallic materials, also called metallic glasses (MG). In-situ quantitative compression tests revealed intrinsic and strong size effects of taper-free metallic glass nanopillars inside a transmission electron microscope (TEM) on different MG compositions. The deformation is defect-nucleation-controlled in larger pillars but becomes propagation-controlled in smaller pillars. A unique increase in strength and strain hardening was observed for smaller sized specimens in tension which is different compared to compression experiments with the same MG compositions. Under cyclic loading a very interesting new phenomenon was observed of increasing deformability depending on the number of cycles and size. The increase in deformability increases with increasing diameter whereas the number of cycles to reach half of the peak stress is decreasing with increasing diameter. The proposed experimental technique of loading and unloading cycles opens a new route to increase the ductility of metallic glasses.

We present a compelling finite element framework to model hydrogen assisted fatigue by means of a hydrogen- and cycle-dependent cohesive zone formulation. The model builds upon: (i) appropriate environmental boundary conditions, (ii) a coupled mechanical and hydrogen diffusion response, driven by chemical potential gradients, (iii) a mechanical behavior characterized by finite deformation J2 plasticity, (iv) a phenomenological trapping model, (v) an irreversible cohesive zone formulation for fatigue, grounded on continuum damage mechanics, and (vi) a traction-separation law dependent on hydrogen coverage calculated from first principles. The computations show that the present scheme appropriately captures the main experimental trends; namely, the sensitivity of fatigue crack growth rates to the loading frequency and the environment. The role of yield strength, work hardening, and constraint conditions in enhancing crack growth rates as a function of the frequency is thoroughly investigated. The results reveal the need to incorporate additional sources of stress elevation, such as gradient-enhanced dislocation hardening, to attain a quantitative agreement with the experiments.

We propose the new notion of Visco-Energetic solutions to rate-independent systems $(X,\mathcal E,\mathsf d)$ driven by a time dependent energy $\mathcal E$ and a dissipation quasi-distance $\mathsf d$ in a general metric-topological space $X$. As for the classic Energetic approach, solutions can be obtained by solving a modified time Incremental Minimization Scheme, where at each step the dissipation (quasi-)distance $\mathsf d$ is incremented by a viscous correction $\delta$ (e.g.~proportional to the square of the distance $\mathsf d$), which penalizes far distance jumps by inducing a localized version of the stability condition. We prove a general convergence result and a typical characterization by Stability and Energy Balance in a setting comparable to the standard energetic one, thus capable to cover a wide range of applications. The new refined Energy Balance condition compensates the localized stability and provides a careful description of the jump behavior: at every jump the solution follows an optimal transition, which resembles in a suitable variational sense the discrete scheme that has been implemented for the whole construction., Comment: 60 pages

Employing the technique of vanishing viscosity and time rescaling, we show the existence of quasistatic evolutions for elastoplastic materials with incomplete damage affecting both the elastic tensor and the plastic yield surface, in a softening framework and in small strain assumptions.

Cracks generate the largest strain gradients that any material can withstand. Flexoelectricity (coupling between strain gradient and polarization) must therefore play an important role in fracture physics. Here we use a self-consistent continuum model to evidence two consequences of flexoelectricity in fracture: the resistance to fracture increases as structural size decreases, and it becomes asymmetric with respect to the sign of polarization. The latter phenomenon manifests itself in a range of intermediate sizes where piezo- and flexoelectricity compete. In BaTiO3 at room temperature, this range spans from 0.1 to 50 nm, a typical thickness range for epitaxial ferroelectric thin films. Fil: Abdollahi, Amir. Universidad Politécnica de Catalunya; España Fil: Peco Regales, Christian. Universidad Politécnica de Catalunya; España Fil: Millán, Raúl Daniel. Universidad Politécnica de Catalunya; España. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Arroyo Balaguer, Marino. Universidad Politécnica de Catalunya; España Fil: Catalan, Gustau. Institut Catalá de Recerca I Estudis Avançats; España. Institut Catala de Nanociéncia i Nanotecnologia; España Fil: Arias, Irene. Universidad Politécnica de Catalunya; España. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina

The prediction of the response of reinforced concrete structures subjected to projectiles impact still presents open questions. These include the rate dependence of material properties, the interaction between concrete and steel reinforcement and the simulation of fracture and fragmentation. Because the appearance of discontinuities in the target structure is difficult to account using a continuum approach, the application of discrete models was developed as an appealing alternative. A version of the discrete model in which nodal masses are linked by an array of uniaxial elements, herein called discrete element method, is used in this study. This method was implemented in the system Abaqus to take advantage of its numerical and post-processing capabilities. A reinforced concrete rectangular plate subjected to impact of a projectile is examined in detail. Comparisons between experimental and numerical results are shown with the aim of validating the proposed method.

Acoustic emission (AE) supplies information on the fracturing behavior of different materials. In this study, AE activity was recorded during fatigue experiments in metal coupons. The plates were characterized by a symmetric V-shape notch and were loaded in tension-tension fatigue until final failure with concurrent AE activity monitoring. The relatively broad bandwidth of the sensors enabled the recording of a wide range of frequencies up to 1 MHz. AE parameters like energy and duration exhibited a certain increase with the accumulation of damage although the hit rate was not significantly influenced. Furthermore the behavior of RA value (ratio of rise time to amplitude of the waveforms) which quantifies the shape of the first part of the AE signals and has been used for characterization of the cracking mode, showed a certain shift indicating the transition from tensile mode to shear which can be confirmed by the visual observation of the crack geometry after the experiment. The time history of RA is similar to the crack propagation rate (da/dN) curve but exhibits the rapid hyperbolic growth consistently about 1000 cycles earlier than final failure. Therefore, the use of acoustic emission parameters is discussed both in terms of characterization of the damage mechanisms, as well as a tool for the prediction of ultimate life of the material under fatigue. (C) 2011 Elsevier Ltd. All rights reserved. Mechanics Research Communications

SUMMARYFish swimming is probably one of the most studied and best understood locomotor behaviors in vertebrates. However, many fish also actively exploit sediments. Because of their elongate body shape, anguilliform fishes are not only efficient swimmers but also very maneuverable. Consequently, many species live in complexly structured environments near the bottom and many are known to burrow into the sediment. To better understand burrowing and subsurface locomotion in anguilliform fish we provide descriptive kinematic data on subsurface locomotion in a burrowing eel (Pisodonophis boro) using videofluoroscopy. We also measured the maximal forces that can be exerted by this species during head-first and tail-first burrowing, and explored the implications of head-first burrowing on mechanical stress distribution in the skull. Our data show that P. boro uses lateral undulation to penetrate and move in sandy sediments under water. The kinematics of subsurface locomotion are different from those observed during swimming and are characterized by a very high slip factor. These observations differ considerably from recently published data in terrestrial sand-swimming lizards, and suggest that the sediment behaves like a solid rather than a frictional fluid. Finally, our finite element models show that the cranial shape and structure in the head-first burrowing P. boro is mechanically more suited for head-first burrowing than that of an obligate tail-first burrowing species, Heteroconger hassi.

We present a combined chemical (auger electron spectroscopy) and microscopic (optical microscopy, scanning electron microscopy and scanning probe microscopy) study of the work of adhesion and delamination mechanisms at interfaces between a glassy polymer (glycol-modified polyethylene terephthalate) and Al covered with different types of surface hydroxides. A clear correlation between the measured work of adhesion and the chemical nature of the Al surface, specifically the hydroxyl coverage and the iso-electric point is found. The magnitude of the work of adhesion points to important contributions from plastic deformation in the glassy polymer for some cases. Delamination is shown to be accompanied by the formation of microscopic shear bands at such interfaces. The non-monotonous stress-strain behaviour of the glassy polymer that gives rise to the shear bands is also shown to lead to peculiar pinning events at the crack front. Evidence indicates that the occurrence of protrusions in the crack front deriving for example from the presence of stress concentrators and crack initiation sites ahead of the front, combined with mode and rate dependence of the local energy release rate along the front may pin the front at positions adjacent to a protrusion. It is believed such microscopic mode-dependent pinning phenomena may be relevant for the adhesion on patterned interfaces. (C) 2010 Elsevier B.V. All rights reserved.

The roughness of fracture surfaces has been shown to exhibit self-affne scale invariance for a wide variety of materials and loading conditions. The range of scales over which this regime extends remains a matter of debate, together with the universality of the associated exponents. The topography of these surfaces is however often investigated with a contact probe that is larger than the micro-structure. In this case, we show that the correlation function of the roughness and the corresponding Hurst exponent $\zeta$ can only be measured down to a length scale $\Delta xc$ which depends on the probe size $R$, on $\zeta$ and on the surface topothesy $l$, and exhibit spurious behavior at smaller scales. First, we derive the dependence of $\Delta xc$ on these parameters from a simple scaling argument. Then we study this dependence numerically and verify our theoretical prediction. Finally, we establish the relevance of this analysis from AFM measurements on an experimental glass fracture surface and provide a metrological procedure for roughness measurements., Comment: 4 pages, 4 figures, submitted to PRL

The geometry of delamination fronts between a glassy polymer film and a metal substrate in an asymmetric double cantilever beam geometry is studied. Curved crack fronts are observed when tensile residual stresses are present in the polymer layer. Inward bending of the polymer due to the tensile residual stress locally leads to compressive residual stresses normal to the polymer-metal interface and counteracts the imposed crack opening. This modulates the local toughness at the crack front leading to crack front curvature. It is shown that for subcritical cracks under these conditions the critical energy release rate increases with "age" of the crack tip. It also appears that this ageing depends on the local stress state, and that locations with a higher initial toughness age faster. "Pinning points" with increased energy release rates are found to be associated with crack propagation through the polymer layer and also with local stick-slip movement. It is concluded that on these interfaces that are otherwise homogeneous, a distribution of stresses leads to differing local ageing characteristics and delamination mechanisms. The observations indicate that some macroscopic stick-slip phenomena observed in delamination may be due to inhomogeneous front ageing and movement. (C) 2008 Elsevier B.V. All rights reserved.

We study anomalous scaling and multiscaling of two-dimensional crack profiles in the random fuse model using both periodic and open boundary conditions. Our large scale and extensively sampled numerical results reveal the importance of crack branching and coalescence of microcracks, which induce jumps in the solid-on-solid crack profiles. Removal of overhangs (jumps) in the crack profiles eliminates the multiscaling observed in earlier studies and reduces anomalous scaling. We find that the probability density distribution $p(\Delta h(\ell))$ of the height differences $\Delta h(\ell) = [h(x+\ell) - h(x)]$ of the crack profile obtained after removing the jumps in the profiles has the scaling form $p(\Delta h(\ell)) = ^{-1/2} ~f(\frac{\Delta h(\ell)}{^{1/2}})$, and follows a Gaussian distribution even for small bin sizes $\ell$. The anomalous scaling can be summarized with the scaling relation $[\frac{^{1/2}}{^{1/2}}]^{1/\zeta_{loc}} + \frac{(\ell-L/2)^2}{(L/2)^2} = 1$, where $^{1/2} \sim L^{\zeta}$., Comment: 9 pages, 10 figures

We carried out classical atomistic studies of crack propagation in fully three-dimensional nanocrystalline alpha-Fe (body-centered cubic structure) to examine the influence of temperature and average grain size on the fracture mechanisms and properties. Digital samples with grain sizes ranging from 6 to 12 nm are reported at temperatures ranging from 100 K to 600 K using atomistic simulations. For all grain sizes, a combination of intragranular and intergranular fracture is observed. Mechanisms such as grain boundary accommodation, grain boundary triple junction activity, grain nucleation and grain rotation are observed to dictate the plastic deformation energy release. Intergranular fracture is shown to proceed by the coalescence of nanovoids formed at the grain boundaries ahead of the crack. The simulations also show that at an atomistic scale the fracture resistance and plastic deformation energy release mechanisms increase with increasing temperature. The observed fracture toughness increases with decreasing grain size. Office of Naval Research, Division of Materials Science Grant No. N00014J-1351

A method was developed and implemented in the finite element program ADINA for the numerical simulation of 2D- and 3D-crack propagation. The elastic-plastic 3D analyses results for different compact-tension specimens and a plate containing a semi-elliptical surface crack were compared with experimental findings. The results show the importance of consideration of the local stress state (constraint) in the crack-tip region in addition to the primary loading parameter J. Moreover, a creep crack growth experiment was simulated using the creep integral C∗ as a crack growth controlling parameter.

Fracture toughness tests are conducted at room temperature and 923 K (650 degrees C) on SENB (Single Edge Notched Bend) specimens of three different (alpha(2) + gamma ) TiAl-based alloys. One alloy is a conventional near fully lamellar Ti-45Al-2Mn-2Nb + 0.8%vol TiB2 XD alloy. The second and third have a composition of Ti -46Al-8Ta, with a novel "convoluted" cast microstructure developed for turbine applications and a coarse near fully lamellar microstructure, respectively. The novel alloy displays fracture toughness values of 13 +/- 2 MPa root m at room temperature and 19 MPa root m at 923 K (650 degrees C); these values approach data for lamellar microstructures, known to be optimal for this class of alloy. In particular, shear ligament toughening is observed with a role of retained lamellar colonies in toughening the convoluted microstructure. (C) 2011 Elsevier Ltd. All rights reserved.

Objectives. To understand the resistance to cyclic and static contact loading of feldspathic porcelain on dental zirconia (3Y-TZP), in order to understand the partial failure of porcelain (chipping or cracking).; Methods. Hertzian contact techniques were used to evaluate the appearance of damage as a ring crack in terms of applied load and number of cycles in air and simulated saliva.; Results. Static contact loading showed the presence of stress corrosion cracking in the porcelain; the environmental crack growth in air was determined from the results of time to damage under static load. There was also a contribution of fatigue effects due to the interactions of the crack with the microstructure. From the obtained results, a time to failure was estimated depending on the material counterpart. Cracking can occur on porcelain coatings if the contact counterpart is teeth or porcelain in a time interval of a few years, consistent with clinical studies.; Signcance. Contact loading, particularly against teeth or other ceramic materials can be a significant cause of failure and chipping of feldspathic porcelain on zirconia, especially if the patient suffers from bruxism. Protection, by e.g. a guard, against repetitive contact against the porcelain can increase the lifetime of the veneer. (C) 2014 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

We investigate the dynamic behavior of concrete in relation to its composition within a computational framework (FEM). Concrete is modeled using a meso-mechanical approach in which aggregates and mortar are represented explicitly. Both continuum phases are considered to behave elastically, while nucleation, coalescence and propagation of cracks are modeled using the cohesive-element approach. In order to understand the loading-rate sensitivity of concrete, we simulate direct tensile-tests for strain rates ranging 1–1000 s−1. We investigate the influence of aggregate properties (internal ordering, size distribution and toughness) on peak strength and dissipated fracture energy. We show that a rate independent constitutive law captures the general increase of peak strength with strain rate. However, a phenomenological rate-dependent cohesive law is needed to obtain a better agreement with experiments. Furthermore, at low rates, peak strength is sensitive to the inclusions' toughness, while the matrix dominates the mechanical behavior at high rates.

Silane-coated glass microspheres randomly embedded in an epoxy polymer matrix have been employed as a model system to investigate the degradation of disordered composite materials by water, and to test various models of deformation and fracture. Numerous composites containing sodalime (A) glass in the range 0 to 25% by volume were tested dry and immersed in saturated NaCl at 40 °C for periods up to 70 days before testing. Enhanced osmotic water uptake due to percolating interface damage was observed for composites containing more than 15% glass. The electrical resistance of similar composites filled with conducting spheres confirmed the existence of a percolation transition, though with high resistance values implying no direct contact of the spheres. Tensile measurements conducted on dry material at a nominal strain rate of about 10−3 s−1 showed an increase in elastic modulus and a decrease in the fracture strength with increasing glass content. New detail was apparent in these curves and confirmed by statistical analyses. For wet specimens, in addition to a general embrittlement effect of water absorption, there was a distinct plateau or small peak in fracture strength in the range 9 to 12% glass, and an abrupt drop between 12 and 15%. The plateau can be related to favorable crack interaction effects between disconnected clusters of interfaces just below the percolation threshold. The steep increase in elastic modulus with glass content seen in the dry material vanished entirely in wet material, which behaved like a porous polymer above 6% glass, owing to osmotic interface damage within particle clusters.

The two-dimensional (2D) debonding behavior of GFRP/balsa sandwich panels presenting embedded circular disbonds at the face sheet/core interface was experimentally investigated. The bottom of the panels was fixed and out-of-plane quasi-static tensile loads were applied to open the crack. Two different face sheet configurations were studied. The first consisted of a pure woven ply layup while in the second, layers of continuous filament mat (CFM) were placed above and below the woven plies. The inclusion of CFM layers at the face sheet/core interface resulted in an enhanced fracture behavior exhibiting improved load-bearing performance and significant crack arrest. In-plane stretching strains appeared in the deformed face sheets due to the 2D embedded nature of the crack. The magnitude of the stretching was quantified based on the in-plane strains monitored throughout the experiments. As a result of the stretching, shear mode contributions were activated, causing the initiation of secondary cracks in the pure woven configuration and crack migration in the woven/CFM configuration. The migration in the latter occurred from the face sheet/core interface to the woven/CFM interface, generating dense fiber-bridging. This intensified fiber-bridging was found to be the main cause of the already-mentioned improvement in the fracture performance of the woven/CFM configuration.

This paper presents an experimental and numerical study on quasi-static ductile tearing of thin plates of the aluminium alloy AA6016 in three tempers. Depending on the temper, the main fracture mechanism in the plate tearing tests changes from grain boundary failure to coalescence of voids nucleated at the constituent particles. The experiments are complemented by nonlinear finite element simulations using an enriched Gurson-TvergaardNeedleman (GTN) model to describe the material response. The onset of accelerated void growth is initiated either by incipient material softening (named the softening model) or by the occurrence of strain localization (named the localization model). It was found that strain localization takes place at a critical porosity f(c), which depends on the current hydrostatic and deviatoric stress states. While the failure strain depends on the stress path, the critical porosity appears to be path independent. A third method is proposed (named the f(c)( T, L) model), where a critical porosity surface f(c) = f(c) (T, L) is used to determine when accelerated void growth starts. The surface is generated beforehand by solving for strain localization under proportional stress states defined by the stress triaxiality T and the Lode parameter L. By comparing the simulations to the experiments, it was found that the localization model performed well for a wide range of stress states. The softening model does not portray dependence on the Lode parameter and is therefore less versatile. The localizationmodel and the f(c)(T, L) model gave similar predictions, but some minor differences were observed for two of the three tempers.