Showing papers on "Brittleness published in 2001"
TL;DR: A structural polymeric material with the ability to autonomically heal cracks is reported, which incorporates a microencapsulated healing agent that is released upon crack intrusion and polymerization of the healing agent is triggered by contact with an embedded catalyst, bonding the crack faces.
Abstract: Structural polymers are susceptible to damage in the form of cracks, which form deep within the structure where detection is difficult and repair is almost impossible. Cracking leads to mechanical degradation of fibre-reinforced polymer composites; in microelectronic polymeric components it can also lead to electrical failure. Microcracking induced by thermal and mechanical fatigue is also a long-standing problem in polymer adhesives. Regardless of the application, once cracks have formed within polymeric materials, the integrity of the structure is significantly compromised. Experiments exploring the concept of self-repair have been previously reported, but the only successful crack-healing methods that have been reported so far require some form of manual intervention. Here we report a structural polymeric material with the ability to autonomically heal cracks. The material incorporates a microencapsulated healing agent that is released upon crack intrusion. Polymerization of the healing agent is then triggered by contact with an embedded catalyst, bonding the crack faces. Our fracture experiments yield as much as 75% recovery in toughness, and we expect that our approach will be applicable to other brittle materials systems (including ceramics and glasses).
3,786 citations
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TL;DR: In this article, the authors show how the algorithm can be improved by basing the artificial stress on the signs of the principal stresses in the dispersion relation for elastic waves in a uniform material, and apply the algorithm to oscillating beams, colliding rings and brittle solids.
558 citations
TL;DR: In this article, the brittle failure of polycrystalline ice at temperatures > 0.8Tmp and strain rates (∼10−7−10−1 s−1) is discussed.
298 citations
TL;DR: In the absence of sufficient segregated impurity, hydrogen produces enhanced cracking at relatively high stresses by concentrated plastic flow, a displacement-controlled phenomenon as mentioned in this paper, which can be demonstrated in steels that have been embrittled by the elements responsible for temper embrittlement and tempered-martensite embrittlements.
259 citations
TL;DR: In this paper, the role of cell wall microstructure in influencing the mechanical behavior of metallic foams was examined, and it was concluded that these differences can substantially affect the micro-mechanisms of deformation and failure under different types of loading and can also have an influence on the macroscopic mechanical response.
231 citations
TL;DR: In this article, the effect of coarse aggregate type on the compressive strength, splitting tensile strength, fracture energy, characteristic length, and elastic modulus of concrete produced at different strength levels with 28-day target compressive strengths of 30, 60, and 90 MPa, respectively.
229 citations
TL;DR: In this paper, 12 thin-film single crystal silicon specimens were tested to failure in a controlled air environment (30/spl +mn/0.1/spl deg/C, 50/spl plusmn/2% relative humidity). Damage accumulation and failure of the notched cantilever beams were monitored electrically during the fatigue life test.
Abstract: When subjected to alternating stresses, most materials degrade, e.g., suffer premature failure, due to a phenomenon known as fatigue. It is generally accepted that in brittle materials, such as ceramics, fatigue can only take place in toughened solids, i.e., premature fatigue failure would not be expected in materials such as single crystal silicon. The results of this study, however, appear to be at odds with the current understanding of brittle material fatigue. Twelve thin-film (/spl sim/20 /spl mu/m thick) single crystal silicon specimens were tested to failure in a controlled air environment (30/spl plusmn/0.1/spl deg/C, 50/spl plusmn/2% relative humidity). Damage accumulation and failure of the notched cantilever beams were monitored electrically during the "fatigue life" test. Specimen lives ranged from about 10 s to 48 days, or 1/spl times/106 to 1/spl times/1011 cycles before failure over stress amplitudes ranging from approximately 4 to 10 GPa. A variety of mechanisms are discussed in light of the fatigue life data and fracture surface evaluation.
227 citations
TL;DR: In this article, the authors assess how pressurized fluids may be impounded by layers of low-permeability phyllosilicate-rich fault gouge and conclude that small earthquakes and fault creep are the most likely modes of failure, facilitated by the buildup of high-pressure fluids in cells.
Abstract: The nucleation of earthquakes on weak faults is still poorly understood. Favored models for weakening have invoked the presence of high-pressure fluids contained within the fault. Here, from detailed field mapping, permeability measurements, and modeling, we assess how pressurized fluids may be impounded by layers of low-permeability phyllosilicate- rich fault gouge. Constraints are made on the permeability anisotropy, fluid-flow pathways, and fluid-loss rates from a large transcurrent fault zone. It is concluded that phyllosilicate-rich fault gouges having permeabilities ranging from 10−17 to 10−21 m2 and cumulative layer thicknesses of ∼50–200 m (from field observations) need fluid fluxes at the base of the brittle part of a large transcurrent fault (assumed to extend from 0 to 15 km depth) of ∼0.1 m3·m−2·yr−1. Furthermore, permeability anisotropy must exceed three orders of magnitude for overpressures to develop. This finding implies that vertical permeability must be enhanced by relatively permeable inclusions of fractured protolith within the fault zone, enclosed by walls of low-permeability gouge. For cross-zone flow, the lateral continuity of the phyllosilicate-rich fault-gouge layers provides effective barriers to fluid flow both into and out of the fault zone. This geometry restricts the origin of fluids in large fault zones as it favors fluids derived from deep sources. However, the fluid flux over time of mantle-derived CO2 and water produced from dehydration reactions has been estimated and appears inadequate. Given the frictional characteristics of phyllosilicate-rich fault gouge, small earthquakes and fault creep are the most likely modes of failure, facilitated by the buildup of high-pressure fluids in cells, producing spatial weakening of the fault.
223 citations
TL;DR: In this article, the thermomechanical behavior of two common polymers, polymethyl methacrylate (PMMA) and polycarbonate (PC), subjected to compressive dynamic loading was investigated.
197 citations
TL;DR: In this article, the performance of a series of 1.5 m-long reinforced concrete beams with CFRP plates bonded to the soffit is analyzed and an analytical formulation to predict the failure load corresponding to the concrete ripping failure mode is proposed based on composite theory in combination with a strain limiting criterion.
Abstract: The performance of a series of 1.5 m-long reinforced concrete beams with CFRP plates bonded to the soffit is presented. Effects of the plate length, the reinforcing steel ratio, and the thickness of the concrete cover on behavior of the beams are discussed with particular emphasis on the brittle failure mode of the concrete ripping. It was found that before the brittle failure, the composite action of the beam could be divided into three distinct zones based on the distribution of the strains along the CFRP plate: (1) The “destressed” zone, (2) the “bond development” zone; and (3) the “fully composite” zone. Concrete ripping may be prevented by limiting the strain at the transition point of the composite zone of the plate. An analytical formulation to predict the failure load corresponding to the concrete ripping failure mode is proposed based on composite theory in combination with a strain limiting criterion. The strain limit can be determined based on simple bond tests with various bond lengths for a g...
183 citations
TL;DR: In this paper, microstructural characterization of the spatial distribution of damage in failed samples is presented, which confirms that compaction bands and high-angle conjugate shears can develop in sandstones with porosities ranging from 13% to 28%.
Abstract: Since dilatancy is generally observed as a precursor to brittle faulting and the development of shear localization, attention has focused on how localized failure develops in a dilatant rock. However, recent geologic observations and reassessment of bifurcation theory have indicated that strain localization may be pervasive in a compactant porous rock. The localized bands can be in shear or in compaction, and oriented at relatively high angles (up to 90°) to the maximum compression direction. Here we report microstructural characterization of the spatial distribution of damage in failed samples which confirms that compaction bands and high-angle conjugate shears can develop in sandstones with porosities ranging from 13% to 28%. These failure modes are generally associated with stress states in the transitional regime from brittle faulting to cataclastic ductile flow. The laboratory results suggest that these complex localized features can be pervasive in sandstone formations, not just limited to aeolian sandstone in which they were first documented. They may significantly impact the stress field, strain partitioning and fluid transport in sedimentary formations and accretionary prisms. While bifuraction theory provides an useful framework for analyzing the inception of localization, our data rule out a constitutive model that does not account for the activation of multiple damage mechanisms in the transitional regime.
TL;DR: In this article, a series of experiments were performed to test the applicability of the stress-induced hydride mechanism, the hydrogen-enhanced localized plasticity mechanism, and the decohesion mechanism of hydrogen embrittlement.
TL;DR: The model predictions for both the brittle compressive strength and the brittle–ductile transition are consistent with data from a variety of crystalline materials, offering quantitative evidence for universal processes in brittle failure and for the broad applicability of the model.
Abstract: Brittle failure limits the compressive strength of rock and ice when rapidly loaded under low to moderate confinement. Higher confinement or slower loading results in ductile failure once the brittle-ductile transition is crossed. Brittle failure begins when primary cracks initiate and slide, creating wing cracks at their tips. Under little to no confinement, wing cracks extend and link together, splitting the material into slender columns which then fail. Under low to moderate confinement, wing crack growth is restricted and terminal failure is controlled by the localization of damage along a narrow band. Early investigations proposed that localization results from either the linkage of wing cracks or the buckling of microcolumns created between adjacent wing cracks. Observations of compressive failure in ice suggest a mechanism whereby localization initiates owing to the bending-induced failure of slender microcolumns created between sets of secondary cracks emanating from one side of a primary crack. Here we analyse this mechanism, and show that it leads to a closed-form, quantitative model that depends only on independently measurable mechanical parameters. Our model predictions for both the brittle compressive strength and the brittle-ductile transition are consistent with data from a variety of crystalline materials, offering quantitative evidence for universal processes in brittle failure and for the broad applicability of the model.
TL;DR: In this paper, the authors present a model of fracture statistics for brittle-matrix composites and the resulting model of mechanical behavior requires a limited number of parameters which consist in intrinsic constituent properties including elastic properties and flaw strength parameters.
TL;DR: Although many dental-restorative materials are routinely evaluated, only limited use has been made of fatigue data collected in vitro: Wear of materials and the survival of porcelain restorations has been modeled by both fracture mechanics and probabilistic approaches.
Abstract: Failure due to fatigue manifests itself in dental prostheses and restorations as wear, fractured margins, delaminated coatings, and bulk fracture. Mechanisms responsible for fatigue-induced failure depend on material ductility: Brittle materials are susceptible to catastrophic failure, while ductile materials utilize their plasticity to reduce stress concentrations at the crack tip. Because of the expense associated with the replacement of failed restorations, there is a strong desire on the part of basic scientists and clinicians to evaluate the resistance of materials to fatigue in laboratory tests. Test variables include fatigue-loading mode and test environment, such as soaking in water. The outcome variable is typically fracture strength, and these data typically fit the Weibull distribution. Analysis of fatigue data permits predictive inferences to be made concerning the survival of structures fabricated from restorative materials under specified loading conditions. Although many dental-restorative materials are routinely evaluated, only limited use has been made of fatigue data collected in vitro: Wear of materials and the survival of porcelain restorations has been modeled by both fracture mechanics and probabilistic approaches. A need still exists for a clinical failure database and for the development of valid test methods for the evaluation of composite materials.
TL;DR: In this paper, it is demonstrated that a soft storey mechanism is formed in such structural systems if the intensity of ground motion is above a certain level, and it is likely that collapse will occur if the global ductilities of the bare frames, as well as the ductility of the structural elements, are low, and if the infill walls are relatively weak and brittle.
Abstract: A large number of multi-storey reinforced concrete frame buildings with masonry infill walls, which were uniformly distributed over the height of the building, collapsed in the 1999 Kocaeli (Turkey) earthquake, due to complete failure of the first storey or the bottom two stories. In the paper it is demonstrated that a soft storey mechanism is formed in such structural systems if the intensity of ground motion is above a certain level. It is likely that collapse will occur if the global ductilities of the bare frames, as well as the ductilities of the structural elements, are low, and if the infill walls are relatively weak and brittle.
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 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 authors reviewed the strengthening and fracture mechanisms that operate in carbon and low-alloy carbon steels with martensitic microstructures tempered at low temperatures, between 150 °C and 200 °C.
Abstract: This article reviews the strengthening and fracture mechanisms that operate in carbon and low-alloy carbon steels with martensitic microstructures tempered at low temperatures, between 150 °C and 200 °C. The carbon-dependent strength of low-temperature-tempered (LTT) martensite is shown to be a function of the dynamic strain hardening of the dislocation and transition carbide substructure of martensite crystals. In steels containing up to 0.5 mass pct carbon, fracture occurs by ductile mechanisms of microvoid formation at dispersions of second-phase particles in the matrix of the strain-hardened tempered martensite. Steels containing more than 0.5 mass pct carbon with LTT martensitic microstructures are highly susceptible to brittle intergranular fracture at prior austenite grain boundaries. The mechanisms of the intergranular fracture are discussed, and approaches that have evolved to minimize such fracture and to utilize the high strength of high-carbon hardened steels are described.
TL;DR: In this article, simple relations for the onset of competing brittle and quasi-plastic damage modes in Hertzian contact are presented, expressed in terms of well-documented material parameters, elastic modulus, toughness, and hardness, enabling a priori predictions for given ceramics and indenter radii.
Abstract: Simple relations for the onset of competing brittle and quasi-plastic damage modes in Hertzian contact are presented. The formulations are expressed in terms of well-documented material parameters, elastic modulus, toughness, and hardness, enabling a priori predictions for given ceramics and indenter radii. Data from a range of selected ceramic (and other) materials are used to demonstrate the applicability of the critical load relations, and to evaluate coefficients in these relations. The results confirm that quasi plasticity is highly competitive with fracture in ceramics, over a sphere radius range 1–10 mm. Implications concerning the brittleness of ceramics in the context of indentation size effects are discussed.
01 Apr 2001-Metallurgical and Materials Transactions B-process Metallurgy and Materials Processing Science
TL;DR: In this paper, the authors reviewed the strengthening and fracture mechanisms that operate in carbon and low-alloy carbon steels with martensitic microstructures tempered at low temperatures, between 150 °C and 200 °C.
Abstract: This article reviews the strengthening and fracture mechanisms that operate in carbon and low-alloy carbon steels with martensitic microstructures tempered at low temperatures, between 150 °C and 200 °C. The carbon-dependent strength of low-temperature-tempered (LTT) martensite is shown to be a function of the dynamic strain hardening of the dislocation and transition carbide substructure of martensite crystals. In steels containing up to 0.5 mass pct carbon, fracture occurs by ductile mechanisms of microvoid formation at dispersions of second-phase particles in the matrix of the strain-hardened tempered martensite. Steels containing more than 0.5 mass pct carbon with LTT martensitic microstructures are highly susceptible to brittle intergranular fracture at prior austenite grain boundaries. The mechanisms of the intergranular fracture are discussed, and approaches that have evolved to minimize such fracture and to utilize the high strength of high-carbon hardened steels are described.
TL;DR: In this paper, the authors measured the failure strength of granitic and andesite samples under various conditions of strain rate and confining pressure both in the dry and wet states.
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 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 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.
30 Sep 2001-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this article, the creep of WE43-T6 was characterized at temperatures ranging from 423 to 523 K and stresses ranging from to 30 to 300 MPa, and the creep stress exponent as well as activation energy were measured.
Abstract: Creep of WE43-T6 was characterized at temperatures ranging from 423 to 523 K and stresses ranging from to 30 to 300 MPa. Creep stress exponent as well as activation energy were measured. There appears to be a softening in creep strength at temperatures above 473 K. Also, the applied stress level affects the creep mechanism. Microstructures of the alloy both before and after creep were examined. Two kinds of precipitates, metastable β′ and stable β phases, were observed in the microstructure. These precipitates readily transform and coarsen during creep. Dislocation–precipitate interaction was quite extensive. Fracture surface revealed that there is a gradual transition from brittle to ductile mode as testing temperature increases. A comparison of the creep properties among the present alloy and other rare earth-containing alloys were made.
TL;DR: In this paper, a more realistic and responsive shear strength criterion, involving only two different strength parameters, that remain practically constant with the applied stress, is advocated to represent non-linear response that minimizes the drudgery by conducting a number of triaxial tests on the material.
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 paper, the effect of mechanical pre-conditioning on the macroscopic deformation of a brittle polymer, notably polystyrene, is studied, and the effect observed is of a temporary nature, as, due to progressive ageing, the yield stress increases and intrinsic softening is restored on a time-scale of minutes.