Showing papers on "Fracture toughness published in 2001"
TL;DR: In this paper, the authors revisited the maximum tensile stress (MTS) criterion to predict brittle fracture of polymethylmethacrylate (PMMA) using angled cracked plates.
Abstract: The purpose of this paper is to revisit the maximum tensile stress (MTS) criterion to predict brittle fracture for mixed mode conditions. Earlier experimental results for brittle fracture of polymethylmethacrylate (PMMA) using angled cracked plates are also re-examined. The role of the T-stress in brittle fracture for linear elastic materials is emphasized. The generalized MTS criterion is described in terms of mode I and II stress intensity factors, K I and K II and the T-stress (the stress parallel to the crack), and a fracture process zone, r c . The generalized MTS criterion is then compared with the earlier experimental results for PMMA subjected to mixed mode conditions. It is shown that brittle fracture can be controlled by a combination of singular stresses (characterized by K) or non-singular stress (T-stress). The T-stress is also shown to have an influence on brittle fracture when the singular stress field is a result of mode II loading.
501 citations
TL;DR: In this paper, intercalated nanocomposites of modified montmorillonite clays in a glassy epoxy were prepared by crosslinking with commercially available aliphatic diamine curing agents.
Abstract: Intercalated nanocomposites of modified montmorillonite clays in a glassy epoxy were prepared by crosslinking with commercially available aliphatic diamine curing agents. These materials are shown to have improved Young's modulus but corresponding reductions in ultimate strength and strain to failure. The results were consistent with most particulate-filled systems. The macroscopic compressive behavior was unchanged, although the failure mechanisms in compression varied from the unmodified samples. The fracture toughness of these materials was investigated and improvements in toughness values of 100% over unmodified resin were demonstrated. The fracture-surface topology was examined using scanning electron and tapping-mode atomic force microscopies and shown to be related to the clay morphology of the system. © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 1137–1146, 2001
344 citations
TL;DR: In this paper, the fracture properties of thin brittle films on compliant substrates for flexible optoelectronic devices were evaluated using experimental methods and their relevant analysis to evaluate fracture properties.
322 citations
TL;DR: In this paper, a notch was inserted into the culm and node specimens using a razor blade with a thickness of 0.4mm, and tensile tests were carried out to evaluate fracture toughness.
Abstract: Bamboo is a typical natural composite material, which is longitudinally reinforced by strong fibers. The fibers are distributed densely in the outer surface region, and sparsely in the inner surface region, and their volume fraction changes with respect to radius. The structure of bamboo has been characterized by tensile tests and its mechanical properties have been related to its structure. This paper presents the fracture toughness of bamboo culms and nodes. A notch is inserted into the culm and node specimens using a razor blade with a thickness of 0.4 mm. Tensile tests are carried out to evaluate fracture toughness. The average value obtained was 56.8 MPa m1/2, which is higher than that of Al-alloy. It was concluded that the fracture toughness of the bamboo culm depends on the volume fraction of fibers.
262 citations
TL;DR: In this article, the influence of stress triaxiality on austenite transformation kinetics and the coupling between martensitic transformation and damage was investigated using double edge notched (or cracked) plate specimens tested in tension.
166 citations
TL;DR: In this paper, the fracture strength of porous Al2O3-based porous ceramics fabricated from pure Al 2O3 powder and the mixtures with Al(OH)3 was investigated.
Abstract: The mechanical properties of Al2O3-based porous ceramics fabricated from pure Al2O3 powder and the mixtures with Al(OH)3 were investigated. The fracture strength of the porous Al2O3 specimens sintered from the mixture was substantially higher than that of the pure Al2O3 sintered specimens because of strong grain bonding that resulted from the fine Al2O3 grains produced by the decomposition of Al(OH)3. However, the elastic modulus of the porous Al2O3 specimens did not increase with the incorporation of Al(OH)3, so that the strain to failure of the porous Al2O3 ceramics increased considerably, especially in the specimens with high porosity, because of the unique pore structures related to the large original Al(OH)3 particles. Fracture toughness also increased with the addition of Al(OH)3 in the specimens with higher porosity. However, fracture toughness did not improve in the specimens with lower porosity because of the fracture-mode transition from intergranular, at higher porosity, to transgranular, at lower porosity.
163 citations
TL;DR: In this article, polypropylene (PP) was injection molded several times to mimic the effect of recycling procedures and the influence of the recycling was studied by following changes in chemical structure, melt viscosity, crystallisation behavior, and tensile and fracture properties.
Abstract: Polypropylene (PP) was injection moulded several times to mimic the effect of recycling procedures. The influence of the recycling was studied by following changes in chemical structure, melt viscosity, crystallisation behaviour, and tensile and fracture properties. The main effect of recycling is the lowering of the melt viscosity, which is attributed to molecular weight decrease. Recycled PP exhibits greater crystallisation rate, higher crystallinity and equilibrium melting temperature than those measured for virgin PP. Elastic modulus and yield stress increase with the number of recycling steps. However, elongation at break and fracture toughness decrease.
160 citations
TL;DR: In this paper, the molecular criteria for efficient stress transfer, by connecting chains and by broad interfaces, are reviewed with a special emphasis on the role of the molecular architecture (diblock, triblock or random copolymers) and molecular weight of the chains present at the interface.
Abstract: In order to obtain a measurable fracture toughness, a joint between two immiscible polymer glasses must be able to transfer mechanical stress across the interface. This stress transfer capability is very weak for narrow interfaces and a significant reinforcement can be achieved, either by the use of connecting chains (block copolymers), or by a broadening of the interface (random copolymers). In both cases, the stress is transferred by entanglements between polymer chains. The molecular criteria for efficient stress transfer, by connecting chains and by broad interfaces, are reviewed here with a special emphasis on the role of the molecular architecture (diblock, triblock or random copolymers) and molecular weight of the chains present at the interface. Recent theoretical developments in the relationship between macroscopic fracture toughness and interfacial stress transfer are also discussed, and the essential role of bulk plastic deformation properties of the polymers on either side of the interface are specifically addressed.
159 citations
Book•
01 Jan 2001TL;DR: In this article, the authors present an analysis of crack problems in homogeneous piezoelectrics or on the interfaces between two dissimilar bimaterial materials based on the continuity of normal electric displacement and electric potential across the crack faces.
Abstract: This paper presents an analysis of crack problems in homogeneous piezoelectrics or on the interfaces between two dissimilar piezoelectric materials based on the continuity of normal electric displacement and electric potential across the crack faces. The explicit analytic solutions are obtained for a single crack in an infinite piezoelectric or on the interface of piezoelectric bimaterials. For homogeneous materials it is found that the normal electric displacement D2, induced by the crack, is constant along the crack faces which depends only on the remote applied stress fields. Within the crack slit, the perturbed electric fields induced by the crack are also constant and not affected by the applied electric displacement fields. For bimaterials, generally speaking, an interface crack exhibits oscillatory behavior and the normal electric displacement D2 is a complex function along the crack faces. However, for bimaterials, having certain symmetry, in which an interface crack displays no oscillatory behavior, it is observed that the normal electric displacement D2 is also constant along the crack faces and the electric field E2 has the singularity ahead of the crack tip and has a jump across the interface. Energy release rates are established for homogeneous materials and bimaterials having certain symmetry. Both the crack front parallel to the poling axis and perpendicular to the poling axis are discussed. It is revealed that the energy release rates are always positive for stable materials and the applied electric displacements have no contribution to the energy release rates.
147 citations
TL;DR: In this article, the authors studied the impact and fracture properties of hollow-glass micro-sphere/epoxy resin composites in terms of fracture toughness, fractography, flexural properties and impact force.
Abstract: Fracture and impact behaviours of hollow-glass micro-sphere/epoxy resin composites are studied in terms of fracture toughness, fractography, flexural properties and impact force. Volume fraction of micro-spheres for the composites was varied up to 0.65. The addition of micro-spheres did not enhance the specific fracture toughness of the composites despite the presence of a pinning mechanism at relatively low volume fractions. Performance in reducing the impact force was enhanced as the content of micro-spheres increased, but at the expense of other properties such as specific fracture toughness and specific flexural strength, while specific flexural modulus marginally increased at some high volume fractions of micro-spheres.
146 citations
TL;DR: In this article, a damage variable that is consistent with the fatigue damage mechanism is defined and a corresponding fatigue damage evolution equation is derived by connecting the damage variable with the static toughness exhaustion during fatigue, which is further examined in both theory and experiment.
TL;DR: In this paper, a theoretical fracture model for brittle piezoelectric and dielectric materials is developed consistent with standard features of elasticity and Dielectricity, and the influence of electric field and mechanical loading is considered in this approach.
Abstract: A theoretical fracture mechanics for brittle piezoelectric and dielectric materials is developed consistent with standard features of elasticity and dielectricity. The influence of electric field and mechanical loading is considered in this approach and a Griffith style energy balance is used to establish the relevant energy release rates. Results are given for a finite crack in an infinite isotropic dielectric and for steady state cracking in a piezoelectric strip. In the latter problem, the effect of charge separation in the material and discharge in the crack are considered. Observations of crack behavior in piezoelectrics under combined mechanical and electrical load are discussed to assess which features of the theory are useful.
TL;DR: In this paper, two series of zirconia-hydroxyapatite composites, Z4H6 and Z6H4 with 40 and 60 vol.% of the ZIRconia content respectively, were prepared by powder uniaxial pressing at 700 MPa and sintering in air at 1200-1500°C for 3 h.
TL;DR: In this article, the mechanical behavior of Nylon-6 blends modified by two types of CaCO3 particles of 0.7 and 3.5μm diameter with particle volume fractions ranging from 0.05 to 0.28 was studied between −30 and 60°C in slow tension, and at 20°c in bending impact.
TL;DR: In this article, the mode I constant displacement rate tests were conducted on epoxy-bonded CFRP joints at −50, 22, and 90°C and the critical strain energy release rate was seen to increase with temperature and the failure locus transferred from predominantly in the composite substrate to predominantly in adhesive.
TL;DR: In this paper, a sol ± gel coating of ZrO2 powders by thin-film coating of high-fracture-toughness oxides effectively suppresses the lattice-constant changes during electrochemical cycling and thereby suppresses phase transitions.
Abstract: nonuniform strain during delithiation produces a new-class of cathode materials, which are extremely tolerant to electrochemical cycling. Note that the lattice constant c of the ZrO2coated LiCoO2 exhibits negligible shift in the range 0< x< 0.7, which indicates that the original hexagonal symmetry is unaffected by delithiation. A sol ± gel coating of ZrO2, which has the highest fracture toughness, leads to the formation of a fracture-toughened thin-film solid solution (tens of nm thick) near the particle surface. This film significantly improves the structural stability of the cathode material by suppressing c-axis expansion (or phase transition), thereby preventing capacity fading during electrochemical cycling. In conclusion, the encapsulation of LiCoO2 powders by thin-film coating of high-fracture-toughness oxides effectively suppresses the lattice-constant changes during electrochemical cycling and thereby suppresses phase transitions. The order of capacity retention correlates well with the fracture toughness of the coated thin-film oxides. This innovative technology can be applied to any cathode material that has an accompanying lattice strain (or phase transition) during cycling. The commercial potential of a rechargeable Li-ion battery is enormous.
TL;DR: In this paper, the microstructure and room temperature fracture toughness of binary Nbss/Nb5Si3 and ternary nbss-Nb 5Si3 in situ composites alloyed with Mo are investigated at hypo-and hypereutectic compositions.
TL;DR: In this article, the fracture toughness of chevron-notched alloys with the nominal composition Mo-12Si-8.5B (at.%) was determined from the three-point fracture of Chevron notched specimens, and the results indicated that the relatively high room temperature toughness is consistent with the deformation of the particles observed on fracture surfaces.
TL;DR: In this article, the authors review the mechanical properties of agglomerates in the context of continuum solid and fluid mechanics and fracture mechanics, focusing on practical measurements of tensile strength, yield strength, hardness and fracture toughness.
TL;DR: In this article, the authors investigated the major energy dissipation mechanisms in the fracture of glass bead filled epoxies and established micro-shear banding as the major and most effective toughening mechanism.
TL;DR: In this article, the authors considered the limits of strength and toughness from two perspectives: theoretical and practical: the first perspective is theoretical, and the second perspective is practical: how to maximize the toughness of high-strength steel.
Abstract: The ideal structural steel combines high strength with excellent fracture toughness. In this paper we consider the limits of strength and toughness from two perspectives. The first perspective is theoretical. It has recently become possible to compute the ideal shear and tensile strengths of defect-free crystals. While the ferromagnetism of bcc Fe makes it a particularly difficult problem, we can estimate its limiting properties from those of similar materials. The expected behavior at the limit of strength contains many familiar features, including cleavage on {100}, slip on multiple planes, "conditionally" brittle behavior at low temperature and a trend away from brittle behavior on alloying with Ni. The behavior of fcc materials at the limit of strength suggests that true cleavage will not happen in austenitic steels. The results predict an ideal cleavage stress near 10.5 GPa, and a shear strength near 6.5 GPa. The second perspective is practical: how to maximize the toughness of high-strength steel. Our discussion here is limited to the subtopic that has been the focus of research in our own group: the use of thermal treatments to inhibit transgranular brittle fracture in lath martensitic steels. The central purpose of the heat treatments described here is grain refinement, and the objective of grain refinement is to limit the crystallographic coherence length for transgranular crack propagation. There are two important sources of transgranular embrittlement: thermal (or, more properly, mechanical) embrittlement at the ductile–brittle transition, and hydrogen embrittlement from improper heat treatment or environmental attack. As we shall discuss, these embrittling mechanisms use different crack paths in lath martensitic steels and, therefore, call for somewhat different remedies.
TL;DR: In this paper, the dynamic fracture toughness of Fangshan gabbro and Fangshans marble subjected to high temperature was measured by means of the split Hopkinson pressure bar (SHPB) system.
TL;DR: In this article, the authors examined the fracture toughness and fracture growth rate of Boron-containing molybdenum intermetallics and found that they exhibited a relatively high intrinsic (crack-initiation) toughness at 800 up to 1200 � C( � 10 MPa p m), but only limited extrinsic R-curve toughness.
TL;DR: In this article, the effect of irradiation on the tensile and fracture toughness properties of candidate tritium materials, 316L and 304L stainless steel (annealed), modified (Mod) 9Cr-1Mo steel, and Alloy 718 (precipitation hardened), was measured on various materials irradiated at the Los Alamos Neutron Science Center accelerator.
TL;DR: To really understand the functional and ecological significance of leaf texture in relation to herbivory, a more reductionist approach is needed and only then can the authors move on to the larger scales of pattern that many ecologists are seeking.
Abstract: Leaf biomechanical properties have the potential to act as antiherbivore defences. However, compared with studies on chemical defences, there are few studies that have demonstrated that the physical or biomechanical structure of plants can prevent or influence herbivory. This difference in focus by ecologists may relate to the dominant paradigm of plant chemical defences in ecological research and the perceived difficulties that ecologists have with the engineering principles embedded in biomechanics. The advantage of using materials engineering concepts is that each property is precisely defined and quantifiable, although the latter may be difficult in leaves because of their composite and anisotropic nature. Most herbivory studies have used simple penetrometers to measure leaf properties, often termed 'toughness'. As defined in materials engineering, the measured properties are 'force to fracture' and 'strength', not toughness. Measurement of strength, the resistance to crack initiation, is relevant to understanding herbivory. Measurement of toughness' as defined by materials engineering is also relevant. Toughness is the resistance to crack propagation and is a measure of the energy required to fracture the leaf. This requires more sophisticated equipment than simple penetrometers because it requires a simultaneous measure of the punch displacement. In addition, purists would argue that a punch cannot be used to measure true toughness because the crack is not controlled and plastic deformation is also involved. However, it may be the only method that allows detection of fine-scale pattern in mechanical properties across a leaf surface at a scale that is relevant to herbivory. There is very little work on the scale at which these properties vary, particularly with regard to different sized herbivores. In addition, few studies have investigated a broad range of relevant biomechanical properties in relation to herbivory. Therefore, it is not possible yet to be definitive about the relative merits of the various types of tests. A single test might show a pattern in relation to herbivore damage at a gross level. However, to really understand the functional and ecological significance of leaf texture in relation to herbivory, a more reductionist approach is needed. Only then can we move on to the larger scales of pattern that many ecologists are seeking.
TL;DR: In this paper, it has been shown that pressureless sintering of SiC to theoretical density is possible with Sintering additives from the system AlN-Y2O3, resulting in very small mass loss during densification which only depends on the oxygen content of the starting powder.
Abstract: It has been shown that pressureless sintering of SiC to theoretical density is possible with sintering additives from the system AlN–Y2O3. While commonly a combination of oxides is used such as Al2O3–Y2O3 (–SiO2), the oxynitride additives offer the advantage that only a nitrogen atmosphere is required instead of a powder bed for thermochemical stabilisation at the sintering temperature. The thermal decomposition of AlN is suppressed quite effectively when a moderate nitrogen overpressure is applied, resulting in very small mass loss during densification which only depends on the oxygen content of the SiC starting powder. By varying the mass ratio of β-SiC to α-SiC and applying dedicated post-densification heat treatments, a platelet-strengthened microstructure is obtained which shows enhanced fracture toughness. The platelet formation is attributed to a solution / precipitation process with simultaneous phase transformation from β-SiC to α-SiC, followed by anisotropic grain growth of α-SiC. In the present work, recent progress in the mechanical properties of these materials is reported. By means of a simple surface treatment-annealing in air — it is possible to obtain four-point bending strengths in excess of 1 GPa in liquid phase sintered SiC. The strength retention at temperatures around 1200°C is significantly improved.
TL;DR: In this paper, the role of higher order strain gradients in the mechanical behavior of elastic-perfectly brittle materials, such as rocks, is given that is based on a special grade-2 elasticity theory with surface energy.
TL;DR: In this article, the fracture behavior of compositionally graded glass-filled epoxy is studied relative to homogeneous counterparts made of identical constituents and the apparent stress intensity factors prior to crack initiation are determined using dynamic equivalent of the stationary fields for FGMs while crack tip fields for steadily growing cracks in FGMs are used for post-initiation situations.
TL;DR: In this article, the effects of annealing and changes in stress state on the toughness of Zr-Ti-Ni-Cu-Be alloy have been determined in amorphous state, both notched and fatigue precracked specimens have been tested.
Abstract: The effects of annealing and changes in stress state on the toughness of both 4 mm thick and 7 mm thick plates of a Zr-Ti-Ni-Cu-Be alloy have been determined In the amorphous state, both notched and fatigue precracked specimens have been tested The effects of changing the notch root radius from a fatigue precrack to that of a blunt notch on the fracture toughness are dramatic The toughness increases from approximately 179 ± 18 MPa√m in the fatigue precracked specimens to in excess of 130 MPa√m in the notched specimens These results are compared to similar tests on a range of structural materials, including aluminum alloys, steels, Ti alloys, and metal matrix composites The increased toughness obtained by increasing the notch root radius in this bulk metallic glass far exceeds that typically observed in other structural materials Possible reasons for this are presented In addition, the effects of changes in loading rate and various annealing treatments on the toughness are presented and rationalized via both crack path and fracture surface observations Annealing of this bulk metallic glass at temperatures below Tg produces increases in strength/hardness, rapid decreases in toughness, and a corresponding change in the fracture morphology Changes in loading rate did not have a significant effect on the toughness for either notched or fatigue precracked specimens
TL;DR: In this article, a polycrystalline Ti 3 SiC 2 sample was fabricated by reactively hot-isostatic pressing (HIPing) a mixture of elemental Ti, Si and C powders.