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Toughness
Kinds of Toughness Terms modified by Toughness Selected AbstractsCOMPETITION FOR MARKET SHARE OR FOR MARKET SIZE: OLIGOPOLISTIC EQUILIBRIA WITH VARYING COMPETITIVE TOUGHNESS,INTERNATIONAL ECONOMIC REVIEW, Issue 3 2007Claude D'Aspremont For an industry producing a composite commodity, we propose a comprehensive concept of oligopolistic equilibrium, allowing for a parameterized continuum of regimes varying in competitive toughness. Each firm sets simultaneously its price and its quantity under two constraints, relative to its market share and to market size. The price and the quantity equilibrium outcomes always belong to the set of oligopolistic equilibria. When firms are identical and we let their number increase, any sequence of symmetric oligopolistic equilibria converges to the monopolistic competition outcome. Further results are derived in the symmetric CES case, concerning in particular the collusive solution enforceability. [source] Biodegradable Polymer Crosslinker: Independent Control of Stiffness, Toughness, and Hydrogel Degradation RateADVANCED FUNCTIONAL MATERIALS, Issue 19 2009Chaenyung Cha Abstract Hydrogels are being increasingly studied for use in various biomedical applications including drug delivery and tissue engineering. The successful use of a hydrogel in these applications greatly relies on a refined control of the mechanical properties including stiffness, toughness, and the degradation rate. However, it is still challenging to control the hydrogel properties in an independent manner due to the interdependency between hydrogel properties. Here it is hypothesized that a biodegradable polymeric crosslinker would allow for decoupling of the dependency between the properties of various hydrogel materials. This hypothesis is examined using oxidized methacrylic alginate (OMA). The OMA is synthesized by partially oxidizing alginate to generate hydrolytically labile units and conjugating methacrylic groups. It is used to crosslink poly(ethylene glycol) methacrylate and poly(N -hydroxymethyl acrylamide) to form three-dimensional hydrogel systems. OMA significantly improves rigidity and toughness of both hydrogels as compared with a small molecule crosslinker, and also controls the degradation rate of hydrogels depending on the oxidation degree, without altering their initial mechanical properties. The protein-release rate from a hydrogel and subsequent angiogenesis in vivo are thus regulated with the chemical structure of OMA. Overall, the results of this study suggests that the use of OMA as a crosslinker will allow the implantation of a hydrogel in tissue subject to an external mechanical loading with a desired protein-release profile. The OMA synthesized in this study will be, therefore, highly useful to independently control the mechanical properties and degradation rate of a wide array of hydrogels. [source] On the Fracture Toughness of Advanced MaterialsADVANCED MATERIALS, Issue 20 2009Maximilien E. Launey Abstract Few engineering materials are limited by their strength; rather they are limited by their resistance to fracture or fracture toughness. It is not by accident that most critical structures, such as bridges, ships, nuclear pressure vessels and so forth, are manufactured from materials that are comparatively low in strength but high in toughness. Indeed, in many classes of materials, strength and toughness are almost mutually exclusive. From a fracture-mechanics perspective, the ability of a microstructure to develop toughening mechanisms acting either ahead or behind the crack tip can result in resistance-curve (R-curve) behavior where the fracture resistance actually increases with crack extension; the implication here is that toughness is often developed primarily during crack growth and not for crack initiation. Biological materials are perfect examples of this; moreover, they offer microstructural design strategies for the development of new materials for structural applications demanding combinations of both strength and toughness. [source] Artifical Muscles: Nanocomposite Hydrogel with High Toughness for Bioactuators (Adv. Mater.ADVANCED MATERIALS, Issue 17 200917/2009) The inside cover shows a nanofibrous hydrogel based on ferritin for a bioinspired nanocomposite actuator, reported in work by Seon Jeong Kim and co-workers on p. 1712. The ferritin-based nanofibrous hydrogels demonstrate synergy between the ferritin protein and a synthetic polymer matrix, as the protein shell of ferritin behaves like an elastic nanospring in the polymer. The actuator is reversibly actuated by chemical energy under external tensile stress, showing improved response speed in comparison to bulk and microfiber hydrogels, coming closer to the goal of mimicking the performance of natural muscle. [source] Nanocomposite Hydrogel with High Toughness for BioactuatorsADVANCED MATERIALS, Issue 17 2009Min Kyoon Shin Ferritin-based nanofibrous hydrogels that demonstrate synergy between the ferritin protein and the synthetic polymer matrix are fabricated. The hybrid hydrogels showed enhanced mechanical properties and repeated expansion and contraction without showing severe creep during pH switching. The ferritin nanoparticles incorporated into the hydrogel nanofibers improved the actuation stability of a hydrogel actuator by acting as elastic nanosprings in a nanoscale polymer. [source] Toughness of Spider Silk at High and Low Temperatures,ADVANCED MATERIALS, Issue 1 2005Y. Yang The toughness of the major ampullate silk of spiders is shown to increase at low temperatures, unlike synthetic fibers. This temperature dependence of the mechanical properties of spider silk, together with other remarkable properties, demonstrates the potential usefulness of such a super-fiber in harsh environments. The Figure shows a single fiber of Nephila edulis spider silk fractured in liquid nitrogen. [source] Toughness, minimum degree, and spanning cubic subgraphsJOURNAL OF GRAPH THEORY, Issue 2 2004D. Bauer Abstract Degree conditions on the vertices of a t -tough graph G (1,,,t,<,3) are presented which ensure the existence of a spanning cubic subgraph in G. These conditions are best possible to within a small additive constant for every fixed rational t ,[1,4/3),[2,8/3). © 2003 Wiley Periodicals, Inc. J Graph Theory 45: 119,141, 2004 [source] Fracture Force, Deflection, and Toughness of Acrylic Denture Repairs Involving Glass Fiber ReinforcementJOURNAL OF PROSTHODONTICS, Issue 4 2008Ioannis Kostoulas DDS Abstract Purpose: Fractures in acrylic resin dentures occur quite often in the practice of prosthodontics. A durable repairing system for denture base fracture is desired to avoid recurrent fracture. The purpose of this study was to evaluate the fracture force, deflection, and toughness of a heat-polymerized denture base resin repaired with autopolymerized resin alone (C), visible light-polymerizing resin (VLC), or autopolymerizing resin reinforced with unidirectional (Stick) (MA-FS) and woven glass fibers (StickNet) (MA-SN). Another group was repaired with autopolymerized resin after wetting the repair site with methyl methacrylate (MA-MMA) for 180 seconds. A group of intact specimens was used as control. Materials and Methods: Heat-polymerizing acrylic resin was used to fabricate the specimens. The specimens (10 per group) were sectioned in half, reassembled with a 3-mm butt-joint gap, and repaired. A cavity was included when glass fibers were used. Three-point bending was used to test the repaired site, and data were analyzed with one-way ANOVA and the Tukey's post hoc test (,, 0.05). Results: Fracture force, deflection, and toughness for the repaired groups without reinforcement (MA: 46.7 ± 8.6 N, 2.6 ± 0.3 mm, 0.08 ± 0.001 J; MA-MMA: 41.0 ± 7.2 N, 2.7 ± 0.4 mm, 0.07 ± 0.002 J) were significantly lower (p < 0.05) than the control group (C: 78.6 ± 9.6 N, 5.9 ± 0.4 mm, 0.27 ± 0.003 J). Repair with visible light-polymerizing resin (VLC, 15.0 ± 4.0 N, 1.2 ± 0.4 mm, 0.02 ± 0.0001 J) resulted in significant reduction of mechanical properties (p < 0.05). Reinforcement with glass fibers restored (MA-SN: 75.8 ± 9.2 N) or increased (MA-FS: 124.4 ± 12.5 N) the original strength. Conclusion: The most effective repair method was the use of autopolymerized resin reinforced with unidirectional glass fibers. [source] Crack,Tip Toughness of a Soft Lead Zirconate TitanateJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 11 2003Alain B. Kounga Njiwa Crack,opening displacement (COD) measurements were performed on a commercial lead zirconate titanate (PZT). The intrinsic fracture toughness (or crack,tip toughness) of this material was determined using a new evaluation procedure, which takes into account the near,tip CODs and complete crack profile CODs. The crack,tip toughness KI0 was determined from an extrapolation of COD data obtained at various loading stages, thus avoiding the complications caused by subcritical crack growth in PZT. Results for plane strain and plane stress condition are presented. [source] Effect of Heating Schedule on the Microstructure and Fracture Toughness of ,-SiAlON,Cause and SolutionJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 7 2002Misha Zenotchkine The effect of heating schedule on microstructure and fracture resistance has been investigated in single-phase Nd-, Y-, and Yb-,-SiAlON. Such effect is strongly system dependent, reflecting the strong influence of phase stability on ,-SiAON nucleation and the amount of transient/residual liquid during processing. The addition of 1% of ,-SiAlON seeds to the starting powders nearly completely obliterates such effect, while it simultaneously improves microstructure homogeneity and fracture resistance. SENB toughness of 7 MPa·m1/2 and peak R -curve toughness of ,11 MPa·m1/2 have been obtained for seeded Y-,-SiAlON ceramics using heating rates from 1°C/min to 25°C/min. [source] Relationship between Microstructure and Fracture Toughness of Toughened Silicon Carbide CeramicsJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 6 2001Sung-Gu Lee Different microstructures in SiC ceramics containing Al2O3, Y2O3, and CaO as sintering additives were prepared by hot-pressing and subsequent annealing. The microstructures obtained were analyzed by image analysis. Crack deflection was frequently observed as the toughening mechanism in samples having elongated ,-SiC grains with aspect ratio >4, length >2 ,m, and grain thickness (t) <3 ,m (defined as key grains 1). Crack bridging was the dominant toughening mechanism observed in samples having grains with thickness of 1 ,m < t < 3 ,m and length >2 ,m (key grains 2). The values of fracture toughness varied from 5.4 to 8.7 MPa·m1/2 with respect to microstructural characteristics, characterized by mean grain thickness, mean aspect ratio, and total volume fraction of key grains. The difference in fracture toughness was mainly attributed to the amount of key grains participating in the toughening processes. [source] Comparison of Methods to Determine the Fracture Toughness of Three Glass-Ceramics at Elevated TemperaturesJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 12 2000Janet B. Quinn Two standardized methods for measuring kIc in ceramics are compared for use at high temperatures (precracked beam and surface crack in flexure). Results from a third technique involving the measurement of cracks around Vickers indentations are also presented. Three dental glass-ceramics, differing primarily in microstructure, were used as model materials in this study. They emphasized the importance of microstructure in determining high-temperature kIc values and the challenges in measuring them. The measured fracture toughnesses decreased with temperature and increased with imbedded grain size for all three methods. [source] Effects of Grain-Boundary Structure on the Strength, Toughness, and Cyclic-Fatigue Properties of a Monolithic Silicon CarbideJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 8 2000Da Chen An in situ -toughened silicon carbide (ABC-SiC) has been examined in the as-processed condition, where the grain-boundary films are predominantly amorphous, and following thermal exposure at a temperature of 1300°C, where the films become fully crystalline. Previous work has shown that, at elevated temperatures (up to 1300°C), after the grain-boundary films crystallize in situ, only a marginal reduction in strength, fracture toughness, and cyclic-fatigue crack-growth properties is observed, in comparison with those of the as-processed microstructure at 25°C. In the present study, the effect of such crystallization on the subsequent strength, toughness, and fatigue properties at 25°C is examined. Little or no degradation is observed in the room-temperature properties with the crystallized grain-boundary films/phase; in fact, although the strength shows a small reduction (,3%), the fracture toughness and fatigue-crack-growth threshold both increase by ,20%, compared with that of the as-processed structure with amorphous grain-boundary films. [source] On Toughness and Stiffness of Poly(butylene terephthalate) with Epoxide-Containing Elastomer by Reactive ExtrusionMACROMOLECULAR MATERIALS & ENGINEERING, Issue 8 2004Zhong-Zhen Yu Abstract Summary: To obtain a balance between toughness (as measured by notched impact strength) and elastic stiffness of poly(butylene terephthalate) (PBT), a small amount of tetra-functional epoxy monomer was incorporated into PBT/[ethylene/methyl acrylate/glycidyl methacrylate terpolymer (E-MA-GMA)] blends during the reactive extrusion process. The effectiveness of toughening by E-MA-GMA and the effect of the epoxy monomer were investigated. It was found that E-MA-GMA was finely dispersed in PBT matrix, whose toughness was significantly enhanced, but the stiffness decreased linearly, with increasing E-MA-GMA content. Addition of 0.2 phr epoxy monomer was noted to further improve the dispersion of E-MA-GMA particles by increasing the viscosity of the PBT matrix. While use of epoxy monomer had little influence on the notched impact strength of the blends, there was a distinct increase in the elastic stiffness. SEM micrographs of impact-fracture surfaces indicated that extensive matrix shear yielding was the main impact energy dissipation mechanism in both types of blends, with or without epoxy monomer, and containing 20 wt.-% or more elastomer. SEM micrographs of freeze-fractured surfaces of PBT/E-MA-GMA blend illustrating the finer dispersion of E-MA-GMA in the presence of epoxy monomer. [source] Influence of Domain Size on Toughness of Poly(styrene- block -butadiene) Star Block Copolymer/Polystyrene BlendsMACROMOLECULAR RAPID COMMUNICATIONS, Issue 24 2004Ralf Lach Abstract Summary: The toughness of poly(styrene- block -butadiene) star block copolymer/polystyrene (PS) blends have been investigated using the essential-work-of-fracture approach. The blends show a co-continuous or layer-like structure of polystyrene-rich and polybutadiene-rich domains arising from the used extrusion process. A tough-to-brittle transition at a critical domain size of polystyrene-rich domains of about 50 nm and a maximum in the non-essential work of fracture at 20,30% PS (co-continuous morphology) have been found. Non-essential work of fracture as a function of the mean thickness of polystyrene-rich domains, demonstrating a tough-to-brittle transition at a critical domain thickness about 50 nm. AFM micrograph of a star block copolymer/PS-blend containing 40% PS. [source] Properties of epoxy systems with clay nanoparticlesMACROMOLECULAR SYMPOSIA, Issue 1 2003Markéta Zelenková Mysková Abstract Low molecular epoxy resin was mixed with different concentrations of montmorillonite clay with alkylamine-modified surface. The presence of the clay nanoparticles influences the gel time of the amine cured resin as well as the mechanical properties. Toughness and modulus, both in the glass and the rubberlike region, increase with clay concentration. An increasing amount of lower mobility phase with increased clay concentration was observed. [source] Improvement of Hardness and Toughness of TiAlN Coating by Nanoscale Multilayered Structurization with Si3N4PLASMA PROCESSES AND POLYMERS, Issue S1 2007Jong-Keuk Park Abstract The Ti(Al)-Si based nitride coating material has been studied due to its excellent mechanical properties such as hardness and oxidation resistance as in the Ti(Al)N/a-Si3N4 nanocomposites with high hardness over 50 GPa. In this study, the effect of microstructure, especially layer thickness of TiAlN and Si3N4 phase, on the mechanical properties of the TiAlN/Si3N4 nanoscale multilayered coating has been investigated. By nanoscale multilayered structurization with a thin Si3N4 layer (,0.3 nm), the hardness and toughness of TiAlN coating were greatly improved. It is known that for the TiAlN/Si3N4 nanoscale multilayered coatings, thickness of the Si3N4 layer is the most important factor and should be carefully controlled to obtain coatings with high hardness and toughness. [source] Properties of polypropylene/aluminum trihydroxide composites containing nanosized organoclayPOLYMER ENGINEERING & SCIENCE, Issue 12 2005Noora Ristolainen Montmorillonite is a promising substitute for aluminum trihydroxide in flame-retardant polypropylene/aluminum trihydroxide (PP/ATH) composites. Study was made of the partial substitution of organoclay for ATH in PP/ATH composites. The total concentration of filler was kept at 30 wt%. The composites were compatibilized with two types of compatibilizer: commercial maleic anhydride functionalized polypropylene (PP- g -MA) and hydroxyl-functionalized polypropylene (PP- co -OH) prepared with metallocene catalyst. The effect of compatibilization on the morphology was studied by the transmission electron microscopy and the scanning electron microscopy. Mechanical properties were characterized by tensile and impact measurements, and flammability properties with a cone calorimeter. Addition of compatibilizer and stearic acid (SA) treatment of the ATH particles contributed to the dispersion of the fillers. Both compatibilizers produced organoclay with exfoliated structure and improved adhesion between the fillers and the matrix. Toughness improved and decomposition and flammability were reduced. POLYM. ENG. SCI. 45:1568,1575, 2005. © 2005 Society of Plastics Engineers [source] Dynamic matching of forager size to resources in the continuously polymorphic leaf-cutter ant, Atta colombica (Hymenoptera, Formicidae)ECOLOGICAL ENTOMOLOGY, Issue 6 2006E. CLARK Abstract 1.,Ergonomic optimisation theory proposes that by increasing variation in worker morphology, social insect colonies may increase their dietary breadth; however, little is known about how this relationship operates at the colony level. This study examines the colony-level pattern of forager size allocation to resource sites in a natural setting. 2.,Using a biologically relevant measure of toughness, it is shown that leaf-cutter ant colonies exploit a variety of plant resources that vary significantly in toughness at any given time. 3.,Forager size is shown to be matched to the toughness of plant material, with larger ants harvesting tougher material. 4.,Furthermore, outbound foragers travelling to a harvest site are matched in size to the toughness of plant material contained within the site and are not a random selection of available foragers. The match between forager size and plant toughness may reduce the number of wasted trips and ill-matched foragers. 5.,The observed colony-level pattern of forager allocation could be the result of learning by individual foragers, or the result of information shared at the colony level. [source] Tritrophic interactions and trade-offs in herbivore fecundity on hybridising host plantsECOLOGICAL ENTOMOLOGY, Issue 3 2004Maria V. Cattell Abstract., 1. Interspecific plant hybridisation can have important evolutionary consequences for hybridising plants and for the organisms that they interact with on multiple trophic levels. In this study the effects of plant hybridisation on the abundance of herbivores and on the levels of herbivore parasitism were investigated. 2. Borrichia frutescens, B. arborescens, and their hybrid (B. × cubana) were censused for Asphondylia borrichiae galls and Pissonotus quadripustulatus plant hoppers in the Florida Keys. Levels of egg parasitism were determined by dissecting parental and hybrid stems and galls for herbivore and parasite eggs and larvae. Stem toughness and gall size are plant-mediated modes of protection from parasitism and these were also measured. For gall midges, fly size was measured as an estimate of fecundity. 3. Field censuses indicated that herbivore abundances varied on hybrid hosts relative to parent plant species and that the different herbivore species exhibited different patterns of abundance. Asphondylia borrichiae gall numbers followed the additive pattern of abundance while P. quadripustulatus numbers most closely resembled the dominance pattern. 4. Parasitism of P. quadripustulatus eggs was high on B. frutescens and the hybrids, and low on B. arborescens, which also had significantly tougher stems. Asphondylia borrichiae suffered the highest levels of parasitism on B. frutescens, the host plant which produced the smallest galls. On B. arborescens, which produced the largest galls, levels of A. borrichiae parasitism were lowest. Both parasitism and gall size were intermediate on the hybrid plants. Galls on B. arborescens and hybrid plants produced significantly smaller flies then those from B. frutescens suggesting that, when selecting hosts from among parent species and hybrids, gall flies may face a trade-off between escape from natural enemies and maximising fecundity. [source] Effect of Decrease of Hydride-Induced Embrittlement in Nanocrystalline Titanium,ADVANCED ENGINEERING MATERIALS, Issue 8 2010M.A. Murzinova Abstract The room-temperature impact toughness, strength and ductility of nanocrystalline (NC) and microcrystalline (MC) titanium with hydrogen content ranging from 0.1 to 16,at.-% (0.002 to 0.450,wt.-%) are studied. NC titanium has higher strength and lower sensitivity to hydride-induced brittle fracture than the MC material. In contrast to MC titanium, the elongation and impact toughness in the NC material does not decrease dramatically with increasing hydrogen content. Moreover, the fracture toughness in hydrogenated NC condition is found to be higher than that in MC titanium. This unusual result may be associated with the precipitation of equiaxial nanoscale hydrides in the interior of ,-grains in the NC material, while platelet hydrides are formed in MC titanium. One can expect that the risk of hydride-induced embrittlement is lower in NC than in MC titanium, making the NC material attractive for potential application under conditions that may cause hydrogen saturation above the permissible level for MC titanium. [source] Processing of Carbon Nanofiber Reinforced ZrB2 Matrix Composites for Aerospace Applications,ADVANCED ENGINEERING MATERIALS, Issue 7 2010Jorge Barcena Ceramic matrix composites (CMCs) based on zirconium diboride (ZrB2) reinforced by vapor grown carbon nanofibers are a potential constituent of reusable thermal protection systems. A manufacturing procedure was devised that involved the fabrication of thin films by tape casting to obtain a layer that could be integrated into a more complex system. Higher thermal conductivities and improved toughness can be expected for nanofiber additions, as compared to the matrix alone. Consolidation by hot-pressing was more effective than pressureless sintering, in terms of the final relative density and flatness of specimens. Examination of microstructures showed that few carbon nanofibers were present in the matrix after consolidation by sintering, which was attributed to a reaction between the nanofibers and zirconium oxide present on the surface of the ZrB2 powder. As a solution, oxygen impurities from the boride powders were removed by reduction with carbon coatings derived from phenolic resin. The deleterious reaction was avoided, but residual carbon remained at the grain boundaries, likely from decomposition of the binder. The use of an alternative binder (PMMA vs. PVB) will be used in future studies to reduce the residual carbon content. Further, consolidation by Spark Plasma Sintering (SPS) will be explored to further reduce the reaction of surface oxides with the nanofibers. Finally, characterization of the microstructure at the nanometric level and further determination of the mechanical and thermal properties will be conducted as part of future studies. [source] Microstructure Evolution and Mechanical Properties of Linear Friction Welded Ti-5Al-2Sn-2Zr-4Mo-4Cr (Ti17) Titanium Alloy Joints,ADVANCED ENGINEERING MATERIALS, Issue 1-2 2010Wen-Ya Li The microstructural evolution, microhardness, tensile properties and impact toughness of Ti-5Al-2Sn-2Zr-4Mo-4Cr (Ti17) alloy joints welded by linear friction welding (LFW) are investigated. A narrow, sound weld is formed, consisting of a superfine ,,+,, structure in the weld center. The structure gradually changes from the weld center to the parent Ti17 in the TMAZ, with the highly deformed , and , phases oriented along the deformation direction, owing to the uneven deformation and temperature distribution. The microhardness of the TMAZ is the lowest of the distinct zones and presents a valley-like shape. The tensile strengths of the joints are comparable to that of the parent Ti17 but with a much lower plasticity and impact toughness. The microstructure variation contributes to the resultant properties. [source] Three-Dimensional Printing of Complex-Shaped Alumina/Glass Composites,ADVANCED ENGINEERING MATERIALS, Issue 12 2009Wei Zhang Abstract Alumina/glass composites were fabricated by three-dimensional printing (3DPÔ) and pressureless infiltration of lanthanum-alumino-silicate glass into sintered porous alumina preforms. The preforms were printed using an alumina/dextrin powder blend as a precursor material. They were sintered at 1600,°C for 2,h prior to glass infiltration at 1100,°C for 2,h. The influence of layer thickness and sample orientation within the building chamber of the 3D-printer on microstructure, porosity, and mechanical properties of the preforms and final composites was investigated. The increase of the layer thickness from 90 to 150,µm resulted in an increase of the total porosity from ,19 to ,39,vol% and thus, in a decrease of the mechanical properties of the sintered preforms. Bending strength and elastic modulus of sintered preforms were found to attain significantly higher values for samples orientated along the Y -axis of the 3D-printer compared to those orientated along the X - or the Z -axis, respectively. Fabricated Al2O3/glass composites exhibit improved fracture toughness, bending strength, Young's modulus, and Vickers hardness up to 3.6,MPa m1/2, 175,MPa, 228,GPa, and 12,GPa, respectively. Prototypes were fabricated on the basis of computer tomography data and computer aided design data to show geometric capability of the process. [source] Increase in the fracture toughness and bond energy of clay by a root exudateEUROPEAN JOURNAL OF SOIL SCIENCE, Issue 5 2008B. Zhang Summary Root exudates help drive the formation of the rhizosphere by binding soil particles, but the underlying physical mechanisms have not been quantified. This was addressed by measuring the impact of a major component of root exudates, polygalacturonic acid (PGA), on the interparticle bond energy and fracture toughness of clay. Pure kaolinite was mixed with 0, 1.2, 2.4, 4.9 or 12.2 g PGA kg,1 to form test specimens. Half of the specimens were washed repeatedly to remove unbound PGA and evaluate the persistence of the effects, similar to weathering in natural soils. Fracture toughness, KIC, increased exponentially with added PGA, with washing increasing this trend. In unwashed specimens KIC ranged from 54.3 ± 2.5 kPa m,1/2 for 0 g PGA kg,1 to 86.9 ± 4.7 kPa m,1/2 for 12.2 g PGA kg,1. Washing increased KIC to 61.3 ± 1.2 kPa m,1/2 for 0 g PGA kg,1 and 132.1 ± 4.9 kPa m,1/2 for 12.2 g PGA kg,1. The apparent bond energy, ,, of the fracture surface increased from 5.9 ± 0.6 J m,2 for 0 g kg,1 to 12.0 ± 1.1 J m,2 for 12.2 g kg,1 PGA in the unwashed specimens. The washed specimens had , of 13.0 ± 1.9 J m,2 for 0 g kg,1 and 21.3 ± 2.6 J m,2 for 12.2 g PGA kg,1. Thus PGA, a major component of root exudates, has a large impact on the fracture toughness and bond energy of clay, and is likely to be a major determinant in the formation of the rhizosphere. This quantification of the thermodynamics of fracture will be useful for modelling rhizosphere formation and stability. [source] Microstructures and Mechanical Properties of Hot-Pressed ZrB2 -Based Ceramics from Synthesized ZrB2 and ZrB2 -ZrC Powders,ADVANCED ENGINEERING MATERIALS, Issue 3 2009Wei-Ming Guo The influence of ZrC on the microstructure and mechanical properties of ZrB2 -SiC ceramics was investigated. SEM observation showed that the presence of ZrC greatly inhibited the grain growth of ZrB2 and SiC phases. With the introduction of ZrC, the Vickers' hardness, fracture toughness, and bending strength of the sintered ceramics increased significantly. [source] Measurement of the Debonding Resistance of Strongly Adherent Thick Coatings on Metals via In-plane Tensile Straining,ADVANCED ENGINEERING MATERIALS, Issue 5 2007S. Ryelandt When the ratio hc/hs of coating and substrate thicknesses is large enough, interfacial debonding can be induced to propagate from the root of a transverse crack under in-plane loading. An energy balance analysis accounting for the flow rule of the substrate allows translating the load for steady state debonding into an upper bound for the debonding toughness. The method is validated by FEM simulations using a cohesive zone model. [source] Properties of ZrB2 -Reinforced Ternary Composites,ADVANCED ENGINEERING MATERIALS, Issue 9 2004D. Sciti Different amounts of ZrB2 particles are added to a matrix constituted by AlN+SiC for the production of novel electro-conductive composites by hot pressing. The presence of ZrB2 particles makes the composites electro-conductive and improves many relevant mechanical properties, such as toughness, hardness and strength. The new materials can reach a fracture toughness of 4 MPa.m1/2 and strength of 504 MPa at 1400°C. [source] Microstructure and Properties of an HfB2 -SiC Composite for Ultra High Temperature Applications,ADVANCED ENGINEERING MATERIALS, Issue 5 2004F. Monteverde An ultra-high-temperature ceramic (UHTC) based on HfB2 was produced. The microstructure consisted of fine and regular diboride grains (2 ,m average size), with SiC particulate distributed intergranularly, not rarely in clustered formation, and low levels of secondary phases were identified. The resulting thermo-mechanical properties proved interesting results for microhardness and fracture toughness. The microstructural alteration experienced within the explored temperature range renders the material unsuitable for service in extreme conditions of temperature and pressure. [source] Determination of the Gurson,Tvergaard damage model parameters for simulating small punch testsFATIGUE & FRACTURE OF ENGINEERING MATERIALS AND STRUCTURES, Issue 11 2010I. I. CUESTA ABSTRACT The objective of the final small punch test (SPT) is to determine the fracture properties of materials, such as fracture toughness, when not enough material is available for the conduct of conventional fracture tests. The damage model developed by Gurson, and subsequently modified by Tvergaard and Needleman (GTN), allows for the numerical simulation of the elastic-plastic behaviour until fracture. This model is based on several constitutive material parameters that must be calibrated if the model is to be properly applied. In this paper, we develop a consistent methodology for the identification of the GTN damage parameters based on the adjustment of the load-displacement curve obtained in the SPTs. The methodology presented is applicable to simulating other different SPTs with different thicknesses and test temperatures. Also, the three-dimensional modelling developed will be useful in the future for analysing the possible anisotropy exhibited by some materials. The next step in the simulation will be to determine its validity in other stress fields with different triaxiality ratios, like the one present in CT specimens, the ultimate goal being to allow for the estimation of the material fracture toughness. [source] | |||||||||||||||||||||||||||||||||||||