Showing papers on "Fracture toughness published in 2005"
TL;DR: The intrinsic plasticity or brittleness of crystalline metals correlates with the ratio of the elastic shear modulus to the bulk modulus, and when the ratio exceeds a critical value, the metal is brittle as mentioned in this paper.
Abstract: The intrinsic plasticity or brittleness of crystalline metals correlates with the ratio of the elastic shear modulus μ to the bulk modulus B; when the ratio μ/B exceeds a critical value, the metal is brittle. Sufficient data on elastic moduli and toughness are now available to permit an assessment for metallic glasses. We find a similar correlation, with the critical value of μ/B for metallic glasses (0.41–0.43) more sharply defined than for crystalline metals. This critical value applies also for annealing-induced embrittlement of metallic glasses. The clear correlation between mechanical behaviour (plasticity or brittleness) and μ/B assists in understanding flow and fracture mechanisms, and in guiding alloy design to alleviate brittleness of metallic glasses.
1,064 citations
TL;DR: In this article, the microstructures of the nanocomposites (epoxy/S-clays) were characterized by means of optical microscopy and transmission electron microscopy (TEM).
Abstract: Epoxy/clay nanocomposites with a better exfoliated morphology have been successfully prepared using a so-called “slurry-compounding” process. The microstructures of the nanocomposites (epoxy/S-clays) were characterized by means of optical microscopy and transmission electron microscopy (TEM). It was found that clay was highly exfoliated and uniformly dispersed in the resulting nanocomposite. Characterizations of mechanical and fracture behaviors revealed that Young's modulus increases monotonically with increasing the clay concentration while the fracture toughness shows a maximum at 2.5 wt % of clay. No R-curve behavior was observed in these nanocomposites. The microdeformation and fracture mechanisms were investigated by studying the microstructure of arrested crack tips and the damage zone using TEM and scanning electron microscopy (SEM). The initiation and development of microcracks are the dominant microdeformation and fracture mechanisms in the epoxy/S-clay nanocomposites. Most of the microcracks in...
533 citations
TL;DR: The results indicate that the fracture in brittle metallic glassy materials might also proceed through the local softening mechanism but at different length scales.
Abstract: We report a brittle Mg-based bulk metallic glass which approaches the ideal brittle behavior. However, a dimple structure is observed at the fracture surface by high resolution scanning electron microscopy, indicating some type of "ductile" fracture mechanism in this very brittle glass. We also show, from the available data, a clear correlation between the fracture toughness and plastic process zone size for various glasses. The results indicate that the fracture in brittle metallic glassy materials might also proceed through the local softening mechanism but at different length scales.
502 citations
TL;DR: In this article, an in situ self-healing epoxy matrix composite that incorporates microencapsulated dicyclopentadiene (DCPD) healing agent and Grubbs first generation Ru catalyst was used to arrest and retardation of fatigue cracks.
399 citations
TL;DR: In this article, the authors investigated the influence of the volumetric fraction of the fibers on the fracture toughness of geopolymeric cement concretes reinforced with basalt fibers, and found that the concretized concrete has better fracture properties than conventional Portland cement.
Abstract: The purpose of this work was to investigate the influence of the volumetric fraction of the fibers on the fracture toughness of geopolymeric cement concretes reinforced with basalt fibers. The values of fracture toughness, critical stress intensity factor and critical crack mouth opening displacement were measured on 18 notched beams tested by three-point bending. The a 0 / h (notch height/beam height) ratio was equal to 0.2 and the L 0 / h (distance between the supports/beam height) ratio was equal to 3. According to the experimental results, geopolymeric concretes have better fracture properties than conventional Portland cement. They are also less sensitive to the presence of cracks.
388 citations
TL;DR: The use of self-reinforcement by larger elongated grains in silicon nitride ceramics requires judicious control of the microstructure to achieve high steady-state toughness and high fracture strength as discussed by the authors.
Abstract: The use of self-reinforcement by larger elongated grains in silicon nitride ceramics requires judicious control of the microstructure to achieve high steady-state toughness and high fracture strength. With a distinct bimodal distribution of grain diameters, such as that achieved by the addition of 2% rodlike seeds, the fracture resistance rapidly rises with crack extension to steady-state values of up to 10 MPa{center_dot}m{sup 1/2} and is accompanied by fracture strengths in excess of 1 GPa. When the generation of elongated reinforcing grains is not regulated, a broad grain diameter distribution is typically generated. While some toughening is achieved, both the plateau (steady-state) toughness and the R-curve response suffer, and the fracture strength undergoes a substantial reduction. Unreinforced equiaxed silicon nitride exhibits the least R-curve response with a steady-state toughness of only 3.5 MPa{center_dot}m{sup 1/2} coupled with a reduced fracture strength.
383 citations
TL;DR: In an attempt to measure slower growth rates, it was found that the behavior switched to a regime dominated by time-dependent crack blunting, similar to that reported for dentin, which permitted subcritical crack growth to readily take place at higher stress intensities.
318 citations
TL;DR: In this article, the authors derived closed form expressions for the strain energy averaged in a finite size volume surrounding the root of blunt V-shaped notches under Mode I loading, which matches Williams and Creager-Paris' solutions in the particular cases of sharp V-notches or blunt cracks.
Abstract: The paper gives some closed form expressions for the strain energy averaged in a finite size volume surrounding the root of blunt V-shaped notches under Mode I loading. The control volume, reminiscent of Neuber’s concept of elementary structural volumes, is thought of as dependent on the ultimate tensile strength and the fracture toughness KIC in the case of brittle or quasi-brittle materials subjected to static loads. Expressions for strain energy density under plane strain conditions and Mode I loading have been derived from an analytical frame recently reported in the literature, which matches Williams and Creager-Paris’ solutions in the particular cases of plates weakened by sharp V-notches or blunt cracks (U-notches), respectively. In order to validate a local-strain-energy based approach, a well-documented set of experimental data recently reported in this journal by Gomez and Elices has been used. Data refer to blunt and sharp V-specimens of PMMA subjected to static tension loads and characterised by a large variability of notch root radius (from 0 to 4.0 mm) and notch angle (from 0° to 150°). Critical loads obtained experimentally have been compared with the theoretical ones, estimated here by keeping constant the mean value of the strain energy in a well-defined small size volume.
305 citations
TL;DR: In this paper, a reaction between epoxy groups and silanol groups present on the surface of the silica phase was detected, leading to an increased interfacial adhesion and a conspicuous increase of modulus and yield strength.
299 citations
TL;DR: In this article, the authors deal with the complex problem of a-however initiated-crack, that is subjected to a mixed-mode loading, and present the hypotheses and concepts, which describe the superposition of Mode I and Mode II (plane mixed mode) as well as the subgroup of all three modes (Mode I, II and III) for spatial loading conditions.
Abstract: In many practical cases, the crack growth leads to abrupt failure of components and structures. For reasons of a reliable quantification of the endangerment due to sudden fracture of a component, therefore, it is of enormous importance to know the threshold values, the crack paths and the growth rates for the fatigue crack growth as well as the limiting values for the beginning of unstable crack growth (fracture toughness). This contribution deals with the complex problem of a-however initiated-crack, that is subjected to a mixed-mode loading. It will present the hypotheses and concepts, which describe the superposition of Mode I and Mode II (plane mixed mode) as well as the superposition of all three modes (Mode I, II and III) for spatial loading conditions. Those concepts admit a quantitative appraisal of such crack situations and a characterization of possible crack paths.
293 citations
TL;DR: Aircraft grade epoxy-clay nanocomposites based on tetraglycidyl-4, 4′-diaminodiphenylmethane (TGDDM) cured with diaminodiphhenyl sulphone (DDS) were synthesized as discussed by the authors.
TL;DR: In this article, small, brittle particulate inclusions have been shown to cause crack-tip bridging at short distances behind the crack tip, and it is the latter that produces the very high fracture strength of the ceramic nanocomposite.
Abstract: Crack-tip bridging by particles is considered to be one of the primary strengthening mechanisms of ceramic nanocomposites. Small, brittle particulate inclusions have been shown to cause crack-tip bridging at short distances behind the crack tip. This mechanism leads to modest toughness but a very steep R-curve, and it is the latter that produces the very high fracture strength of the ceramic nanocomposite. Localized high residual stress around the particles (particularly in the case of silicon carbide-alumina material) causes the strengthening mechanism to operate effectively, even at a small volume fraction of 5%. The present study predicts the magnitude of the toughness increase and the extent of R-curve behavior for the nanocomposite.
TL;DR: In this paper, a new technique for measuring the toughness of coatings is described, using a precracked micro-beam, which was produced using focused ion beam machining, then imaged and loaded to fracture using a nanoindenter.
Abstract: Measuring the toughness of brittle coatings has always been a difficult task. Coatings are often too thin to easily prepare a freestanding sample of a defined geometry to use standard toughness measuring techniques. Using standard indentation techniques gives results influenced by the effect of the substrate. A new technique for measuring the toughness of coatings is described here. A precracked micro-beam was produced using focused ion beam (FIB) machining, then imaged and loaded to fracture using a nanoindenter.
TL;DR: In this paper, specific strength, moduli and fracture toughness of varied microstructures of syntactic foam was evaluated by using 3M Scotchlite ™ K15 and K46 glass bubbles, and Phenoset BJO-093 hollow phenolic microspheres.
TL;DR: In this article, a modification to the traditional Griffith energy balance as used in linear elastic fracture mechanics (LEFM) is described, which involves using a finite amount of crack extension instead of an infinitesimal extension (da) when calculating the energy release rate.
TL;DR: In this paper, a pin-on-disk sliding friction and wear experiments were conducted on two different titanium alloys (Ti-6Al-4V and Ti-8Al-2Sn-4Zr-2Mo) against fixed bearing balls composed of 440C stainless steel, silicon nitride, alumina, and polytetrafluoroethylene (PTFE) at two speeds: 0.3 and 1.0 m/s.
TL;DR: In this article, the authors considered the problem of hydraulic fracture in which an incompressible Newtonian fluid is injected at a constant rate to drive a fracture in a permeable, infinite, brittle elastic solid.
Abstract: This paper considers the problem of a hydraulic fracture in which an incompressible Newtonian fluid is injected at a constant rate to drive a fracture in a permeable, infinite, brittle elastic solid. The two cases of a plane strain and a penny-shaped fracture are considered. The fluid pressure is assumed to be uniform and thus the lag between the fracture front and the fluid is taken to be zero. The validity of these assumptions is shown to depend on a parameter, which has the physical interpretation of a dimensionless fluid viscosity. It is shown that when the dimensionless viscosity is negligibly small, the problem depends only on a single parameter, a dimensionless time. Small and large time asymptotic solutions are derived which correspond to regimes dominated by storage of fluid in the fracture and infiltration of fluid into the rock, respectively. Evolution from the small to the large time asymptotic solution is obtained using a fourth order Runge–Kutta method.
TL;DR: In this paper, the fracture resistance of self-reinforced silicon nitride ceramics has been improved by tailoring the chemistry of the intergranular amorphous phase.
Abstract: Significant improvements in the fracture resistance of self-reinforced silicon nitride ceramics have been obtained by tailoring the chemistry of the intergranular amorphous phase. First, the overall microstructure of the material was controlled by incorporation of a fixed amount of elongated s-Si3N4 seeds into the starting powder to regulate the size and fraction of the large reinforcing grains. With controlled microstructures, the interfacial debond strength between the reinforcement and the intergranular glass was optimized by varying the yttria-to-alumina ratio in the sintering additives. It was found that the steady-state fracture toughness value of these silicon nitrides increased with the Y:Al ratio of the oxide additives. The increased toughness was accompanied by a steeply rising R-curve and extensive interfacial debonding between the elongated s-Si3N4 grains and the intergranular glassy phase. Microstructural analyses indicate that the different fracture behavior is related to the Al (and O) content in the s´-SiAlON growth layer formed on the elongated s-Si3N4 grains during densification. The results imply that the interfacial bond strength is a function of the extent of Al and Si bonding with N and O in the adjoining phases with an abrupt structural/chemical interface achieved by reducing the Al concentration in both the intergranular phase and the s´-SiAlON growth layer. Analytical modeling revealed that the residual thermal expansion mismatch stress is not a dominant influence on the interfacial fracture behavior when a distinct s´-SiAlON growth layer forms. It is concluded that the fracture resistance of self-reinforced silicon nitrides can be improved by optimizing the sintering additives employed.
TL;DR: In this paper, the authors used a cohesive-zone approach to model the mode-I fracture of adhesive joints made from a polymer-matrix composite by matching numerical results to experimental observations.
TL;DR: In this paper, a two-parameter model with a characteristic toughness and a characteristic strength can be used to predict the fracture of notched or cracked specimens, which can be determined by comparing numerical predictions to experimental observations of a fracture test.
TL;DR: In this paper, the J-integral fracture toughness of polytetrafluoroethylene (PTFE) was investigated for a range of temperatures and loading rates employing the single specimen normalization technique, and it was found that crack propagation is strongly phase dependent with a brittle-to-ductile transition in the crack propagation behavior associated with the two room temperature phase transitions.
TL;DR: In this article, a review of the fracture properties of nanocomposites is presented, emphasizing the newly developed concept of material design for ceramics and several mechanisms proposed previously to explain these characteristics were reviewed.
TL;DR: In this article, a compilation of existing data shows that an increase in the volume fraction of the α-Mo phase increases the room-temperature fracture toughness at the expense of the oxidation resistance and the creep strength.
Abstract: Mo-Si-B intermetallics consisting of the phases Mo3Si and Mo5SiB2, and a molybdenum solid solution (“α-Mo”), have melting points on the order of 2000 °C. These alloys have potential as oxidation-resistant ultra-high-temperature structural materials. They can be designed with microstructures containing either individual α-Mo particles or a continuous α-Mo phase. A compilation of existing data shows that an increase in the volume fraction of the α-Mo phase increases the room-temperature fracture toughness at the expense of the oxidation resistance and the creep strength. If the α-Mo phase could be further ductilized, less α-Mo would be needed to achieve an adequate value of the fracture toughness, and the oxidation resistance would be improved. It is shown that microalloying of Mo-Si-B intermetallics with Zr and the addition of MgAl2O4 spinel particles to Mo both hold promise in this regard.
TL;DR: In this article, the ductile-brittle transition behavior of a French A508 Cl3 (16MND5) steel with Charpy V-notch test in this range of temperature was investigated.
TL;DR: The fracture toughness of extruded pure magnesium increased with grain refinement due to the effect of the plastic zone, which is a sensitive factor related to the mechanical properties of yield strength, elongation-to-failure and strain hardening exponent as discussed by the authors.
TL;DR: In this paper, a fracture mechanics approach is developed, by means of which cracks are modeled in the critical areas of the TBC system and assessed using the modified crack closure integral method for determining the mode-dependent crack loading.
Journal Article•
TL;DR: In this article, the authors highlight several aspects of the toughness of (8-nucleated polypropylene (PP) and focus on dynamic fracture properties, a topic which is largely documented in the literature.
Abstract: This review highlights several aspects of the toughness of (8-nucleated polypropylene (PP). The focus is on dynamic fracture properties, a topic which is largely documented in the literature. The role of intrinsic parameters like molecular weight, polydispersity and matrix randomization has been discussed, that of extrinsic factors like stress-state, processing conditions, test speed and temperature illustrated. Under defined conditions, the toughness is also defined by the content and spatial distribution of the β-nucleating agent. The increase in fracture resistance is more pronounced in PP homopolymers than in random or rubber-modified copolymers. In the case of sequential copolymers, the molecular architecture inhibits a maximization of the amount of β-phase; in that of heterophasic systems, the rubber phase mainly controls the fracture behavior. The performance of β-nucleated PP has been explained in terms of smaller spherulitic size, lower packing density and favorable lamellar arrangement of the β-modification which induce a higher mobility of both crystalline and amorphous phases. The damage process is accompanied by numerous microvoids, the development of which has been utilized for breathable films. Other interesting application segments are fibers, glass-fiber reinforced PP, thermoformable grades and piping systems.
TL;DR: In this article, the authors investigated the impact of rigid adhesives on adhesive joint fracture by modelling the adherents and a finite thickness adhesive layer in which a single row of cohesive zone elements representing the fracture process is embedded.
Abstract: Constraint effects in adhesive joint fracture are investigated by modelling the adherents as well as a finite thickness adhesive layer in which a single row of cohesive zone elements representing the fracture process is embedded. Both the adhesive and the adherents are elastic-plastic with strain hardening. The bond toughness Gamma(work per unit area) is equal to Gamma(0) + Gamma(p), where Gamma(0) is the intrinsic work of fracture associated with the embedded cohesive zone response and FP is the extra contribution to the bond toughness arising from plastic dissipation and stored elastic energy within the adhesive layer. The parameters of the model are identified from experiments on two different adhesives exhibiting very different fracture properties. Most of the tests were performed using the wedge-peel test method for a variety of adhesives, adherents and wedge thicknesses. The model captures the constraint effects resulting from the change in Gamma(p): (i) the plastic dissipation increases with increasing bond line thickness in the fully plastic regime and then decreases to reach a constant value for very thick adhesive layers; (ii) the plastic dissipation in the fully plastic regime increases drastically as the thickness of the adherent decreases. Finally, this model is used to assess a simpler approach which consists of simulating the full adhesive layer as a single row of cohesive elements. (c) 2005 Elsevier Ltd. All rights reserved.
TL;DR: In this paper, an Al6061-20%Al 2 O 3 powder metallurgy (PM) metal matrix composite (MMC) with a strongly clustered particle distribution is subjected to equal channel angular pressing (ECAP) at a temperature of 370°C.
TL;DR: In this article, a 2D model is proposed to describe the increase in indentation fracture toughness, which agrees with the experimental results, and nanoZrO2-HA composite powders could be considered for loadbearing orthopaedic implants because of its unique fracture toughness and high porosity.