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The author is involved in a wide-ranging research programme, the objective being to extend the fracture mechanics methodology for sharp cracks to blunt flaws, so as to take credit for the blunt flaw geometry. The approach is based on the cohesive process zone representation of the micro-mechanistic processes that are associated with fracture. An earlier paper has derived a blunt flaw fracture initiation relation which gives the critical elastic flaw-tip peak stress σpcr (a “signifier” of a critical condition in the process zone) in terms of the process zone material parameters, subject to the proviso that the process zone size s is small compared with the flaw depth (length) and any characteristic dimension other than the flaw root radius ρ. The relation has been derived using a “two-extremes” procedure, whereby the separate σpcr solutions for small and large s/ρ are blended together to give an all-embracing relation that is valid for all s/ρ. A key feature of the relation is that σpcr essentially depends on only one geometrical parameter: the flaw root radius ρ. Though the relation has evolved from a consideration of the characteristics of one model, i.e. that of an elliptical flaw in an infinite solid that is subjected to an applied tensile stress, it is anticipated that the relation can be applied equally well for a wide range of geometrical configurations involving different flaw shapes. It is against this background that the present paper demonstrates that the relation also applies to the behaviour of an intrusion type flaw in the surface of a semi-infinite solid subjected to an applied tensile stress. [ABSTRACT FROM AUTHOR]

In this paper, an accurate formula for calculating the thermal residual stress field in a particle-reinforced composite are presented. Numerical examples are given to show r-variations of the thermal residual stresses. The increase in fracture toughness of matrix predicted by the thermal residual stress field is compared well with the experimentally measured increase. [ABSTRACT FROM AUTHOR]

This paper aims to statistically estimate the dynamic fatigue strength in brittle materials under a wide range of stress rates. First, two probabilistic models were derived on the basis of the slow crack growth (SCG) concept in conjunction with two-parameter Weibull distribution. The first model, Model I, is a conventional probabilistic delayed-fracture model based on a concept wherein the length of the critical crack growth due to SCG is enough larger than the initial crack length. For the second model, Model II, a new probabilistic model is derived on the basis of a concept wherein the critical cracks have widely ranging lengths. Next, a four-point bending test using a wide range of stress rates was performed for soda glass and alumina ceramics. We constructed fracture probability-strength-time diagrams (F-S-T diagrams) with the experimental results of both materials using both models. The F-S-T diagrams described using Model II were in good agreement with plots of the fracture strength and the fracture time of both materials more so than Model I. [ABSTRACT FROM AUTHOR]

A high fidelity assessment of accumulative damage of woven fabric composite structures subjected to aggressive loadings is strongly reliant on the accurate characterization of the inherent multi-scale microstructures and the underlying deformation phenomena. Damage in composite sandwich and joint structures is characterized by the coexistence of discrete (delamination) and continuum damage (matrix cracking and intralaminar damage). A purely fracture mechanics-based or a purely continuum damage mechanics-based tool alone cannot effectively characterize the interaction between the discrete and continuum damage and their compounding effect that leads to the final rupture. In this paper, a hybrid discrete and continuum damage model is developed and numerically implemented within the LS-DYNA environment via a user-defined material model. The continuum damage progression and its associated stiffness degradation are predicted based on the constituent stress/strain and their associated failure criteria while the discrete delamination damage is captured via a cohesive interface model. A multi-scale computational framework is established to bridge the response and failure predictions at constituent, ply, and laminated composite level. The calculated constituent stress and strain are used in a mechanism-driven failure criterion to predict the failure mode, failure sequence, and the synergistic interaction that leads to global stiffness degradation and the final rupture. The use of the cohesive interface model can capture the complicated delamination zone without posing the self-similar crack growth condition. The unified depiction of the continuum and discrete damage via the damage mechanics theory provides a rational way to study the coupling effects between the in-plane and the out-of-plane failure modes. The applicability and accuracy of the damage models used in the hybrid dynamic failure prediction tool are demonstrated via its application to a circular plate and a composite hat stiffener subjected to shock and low velocity impact loading. The synergistic interaction between the continuum and discrete damage is explored via its application to a sandwich beam subjected to a low velocity impact. [ABSTRACT FROM AUTHOR]

The fracture process of composite laminates subjected to static or fatigue tensile loading involves sequential accumulation of intra- and interlaminar damage, in the form of transverse cracking, splitting and delamination, prior to catastrophic failure. Matrix cracking parallel to the fibres in the off-axis plies is the first damage mode observed. Since a damaged lamina within the laminate retains certain amount of its load-carrying capacity, it is important to predict accurately the stiffness properties of the laminate as a function of damage as well as progression of damage with the strain state. In this paper, theoretical modelling of matrix cracking in the off-axis plies of unbalanced symmetric composite laminates subjected to in-plane tensile loading is presented and discussed. A 2-D shear-lag analysis is used to determine ply stresses in a representative segment and the equivalent laminate concept is applied to derive expressions for Mode I, Mode II and the total strain energy release rate associated with off-axis ply cracking. Dependence of the degraded stiffness properties and strain energy release rates on the crack density and ply orientation angle is examined for glass/epoxy laminates. Suitability of a mixed mode fracture criterion to predict the cracking onset strain is also discussed. [ABSTRACT FROM AUTHOR]

In a bending load test for brittle materials, such as ceramics for spacecraft and aircraft, decreasing the number of test specimens required is a crucial problem. This paper discusses the effectiveness of using the information of both fracture stress and fracture location to decrease the number of specimens required to obtain the same precision as the Weibull estimator. The following results were obtained: It was found that by adding the fracture location information, the precision of the Weibull parameter estimation under the optimal design became 1.5–1.9 times better compared with the case of using only the fracture stresses. This means the number of samples necessary to attain the same precision becomes 1/1.5–1/1.9. Tables and figures which give information on the number of samples necessary to attain the required precision are given. [ABSTRACT FROM AUTHOR]

The moisture content is known to influence the properties of wood. While mechanical parameters like stiffness and strength decrease with increasing moisture content, investigations on the influence on fracture properties have been concentrated on linear elastic fracture mechanics. In this paper, the wedge splitting method is used to investigate also the non-linear fracture behaviour of sprucewood at different moisture contents. Complete load-displacement curves at four different moisture levels are evaluated. While the initial slope and the maximum stress state are decreasing with increasing moisture content the specific fracture energy is increasing. It is shown that this behaviour can be attributed to an increase in ductility which increases both the dissipated energy during crack initiation and the one in the crack propagation phase. [ABSTRACT FROM AUTHOR]

The radial direction of wood is reinforced by an additional tissue called rays. These rays are one of the reasons for the anisotropy of wood in the transverse plane. In this paper the influence of rays on the mechanical properties like tensile strength as well as on fracture mechanical parameters like critical stress intensity factor and specific fracture energy is shown. By investigating two deciduous tree species with a similar wood structure in general but a different ray characteristic in particular the importance of this radial reinforcement of the wood structure could be demonstrated. The relevance for the living tree is discussed. [ABSTRACT FROM AUTHOR]

The applicability of instrumented falling weight impact techniques in characterizing mechanically thermoplastic foams at relatively high strain rates is presented in this paper. In order to try simulating impact loading of foams against sharp elements, an instrumented dart having a hemispherical headstock was employed in the tests. Failure strength and toughness values were obtained from high-energy impact experiments, and the elastic modulus could be measured from both flexed plate and indentation low-energy impact tests. The results indicate a dependence of the failure strength, toughness, and the elastic modulus on the foam density, the foaming process, and the chemical composition. This influence was found to be similar to that of pure nonfoamed materials and also to that observed from low-rate compression tests. The results also indicate that the indentation low-energy impact tests were more accurate in obtaining right values of the elastic modulus than the flexed plate low-energy impact tests usually used to characterize rigid plastics. The foam indentation observed with this test configuration contributes to obtaining erroneous values of the elastic modulus if only a simple flexural analysis of plates is applied. [ABSTRACT FROM AUTHOR]

The fracture resistance of chemically modified crumb rubber asphalt (CMCRA) pavement was evaluated based on the J-integral concept. The chemical modification process used was developed by the Federal Highway Administration and patented in 1998. The results were compared to that of crumb rubber asphalt (CRA) and control asphalt pavement. Four semi-circular core specimens (76 mm radius and 57 mm thickness) were cut from each gyratory compacted cylinder (GCC) for the fracture resistance tests. Notches with different depth to radius ratios were introduced at the middle of the flat surface of each specimen. Three point bend loading was used to allow the separation of the two surfaces due to tensile stresses at the crack tip. It was found that the CMCRA pavement, had the highest residual strength, at all notch depths tested. The fracture resistance of the CMCRA pavement, based on Jc was found to be about twice that of the CRA and control pavements. The CRA pavement was found to have a slightly higher fracture resistance than that of the control pavement. Scanning Electron Microscopic examination of the fracture surface of each mixture revealed the microstructural origin of the improved fracture resistance of the CMCRA pavement in comparison with the control pavement. [ABSTRACT FROM AUTHOR]

We report quantitative nanocrack formation upon adjusting the mechanical tensile strain applied to a Pd thin film on an elastomeric polydimethylsiloxane (PDMS) substrate and its detection properties for H gas in air. Nanocrack formation in Pd/PDMS substrates was controlled through the application of tensile strains that varied in the range 30-120 % during mechanical stretching/compression. This method can be used to modulate nanocrack formation along both the x- and y-axes over a large area. Increasing the applied tensile strain to 90 % induced the appearance of horizontal cracks along the y-axis in addition to an increased number of vertical cracks along the x-axis. When the strain reached 120 %, ordered nanocracks abundantly propagated on the Pd surface in both the directions. Gas detection properties were dramatically enhanced, with a very low detection limit of 50 ppm H in air observed for 120 % strain. This was attributed to the large surface area in the Pd nanocrack pattern, which readily allowed for volume expansion. These results provide a simple mechanism for controlling the detection properties of H gas sensors with low detection limits in air. [ABSTRACT FROM AUTHOR]

Infrared thermography was used to study damage developing in woven fabrics. Two different experiments were performed, a ±45° tensile test and a rail shear test. These two different types of tests show different damage scenarios, even if the shear stress/strain curves are similar. The ±45° tension test shows matrix hardening and matrix cracking whereas the rail shear test shows only matrix hardening. The infrared thermography was used to perform an energy balance, which enabled the visualization of the portion of dissipated energy caused by matrix cracking. The results showed that when the resin is subjected to pure shear, a larger amount of energy is stored by the material, whereas when the resin is subjected to hydrostatic pressure, the main part of mechanical energy is dissipated as heat. [ABSTRACT FROM AUTHOR]

The tensile properties of high strength polyacrylonitrile-based (IM600) and high modulus pitch-based (K13D) hybrid carbon fibers-reinforced epoxy matrix composite (CFRP) were investigated. Fiber orientation of the hybrid CFRP specimen was set to [0/0]. The fiber volume fraction of the hybrid CFRP specimen was 55.7 vol% (IM600: 29.3 vol%, K13D: 26.4 vol%). The tensile stress-strain curve of the hybrid CFRP specimen shows a complicated shape (jagged trace). By the high modulus K13D CFRP layers, the hybrid CFRP specimen shows the intermediate modulus in the initial stage of loading. Subsequently, when the K13D CFRP layers begin to fail, the high strength IM600 CFRP layers would hold the load (strength) and the material continues to endure high load without instantaneous failure. Because higher strength fiber can help the load for a certain time after failure occur, the hybrid composite could be considered one example of a material possessing preventing instantaneous failure. The Weibull statistical distributions of the mono (IM600 and K13D) and the hybrid CFRP specimens were also examined. The Weibull modulus for the mono CFRP specimens was calculated to be 22.9 for the IM600 CFRP specimen and 14.4 for the K13D CFRP specimen, respectively. The Weibull modulus for the hybrid CFRP specimen was calculated to be 39.6 for the initial fracture strength and 20.6 for the tensile fracture strength, respectively. The Weibull modulus for the initial fracture strength is higher than that for the K13D CFRP specimen and the Weibull modulus for the tensile fracture strength is almost similar to that for the IM600 CFRP specimen. [ABSTRACT FROM AUTHOR]

Fracture toughness measurements on bcc iron (Armco-iron), which is subjected to severe plastic deformation (SPD), were performed. Through high pressure torsion, an ultrafine grain structure was obtained and with subsequent heat treatments the grain size varied between 300 nm and 5 μm. The combination of SPD and individual heat treatments allows for a systematic study of the ductile to brittle transition (DBT) in the fracture behaviour as a function of grain size. Additionally, the influence of different crack plane orientations was taken into account. The results show that the DBT moves for smaller grain sizes (<1 μm) to higher transition temperatures. Furthermore, large differences in the absolute toughness values for a given temperature for the different crack plane orientations and grain sizes were determined. The findings can be related to a change in the crack path from transcrystalline fracture for grain sizes larger than 1 μm to intercrystalline-dominated fracture for grain sizes smaller than 1 μm. [ABSTRACT FROM AUTHOR]

It is pointed out that proposed mechanisms of fatigue fracture should be consistent with Coffin’s Law for constant strain-amplitude cyclic deformation. Most proposed mechanisms are not. The fracture of dislocation cores along their glide planes is consistent with the fact that the law describes the behavior during the first ¼ cycle as well as that after a very large number of cycles. Experimental evidences of the relative weakness of edge dislocation cores is described. Analysis of this weakness by Bullough, and in this article, discussed. It is concluded that this effect plays an important role in fatigue fracture. [ABSTRACT FROM AUTHOR]

Microstructure and fracture behavior of brazed joint between commercially pure titanium and low carbon steel using silver (Ag–34Cu–2Ti) and copper (Cu–12Mn–2Ni ) based alloys have been characterized to determine the effect of brazing parameters and chemical composition on the strength of brazed joints. It is found that the shear strength of brazed joints strongly depends on the lap width. Furthermore, the fracture path and the value of shear strength significantly changed with the type of filler alloy. The two filler metals showed metallurgical interaction with steel and titanium forming different kinds of intermetallic compounds such as CuTi, Cu2Ti, and FeTi with silver based filler and Ti2Cu, FeTi and TiCuFe with copper based filler. [ABSTRACT FROM AUTHOR]

The breaking force during transversal loading of fibre-cement corrugated roofing sheets was measured on several test samples from a serial production. The results were statistically analyzed assuming the 2-parameter Weibull statistics. In addition, Monte Carlo statistical simulations were made by using a computerised built-in random-number generator. While smaller sample data groups, mostly containing up to 50 samples, were studied in the literature, we extended their size up to 400 samples. We showed that some trends in the evaluation of statistical parameters which hold for smaller data groups, apply well to larger data groups. In particular, we confirmed that the statistical distribution of the Weibull parameters obtained from repeated Monte Carlo simulations is log-normal. Furthermore, we considered the influence of the measurement uncertainty on the statistical parameters. [ABSTRACT FROM AUTHOR]

An accurate solution for a rotating functionally graded annular disk is presented. Material properties of the present annular disk are assumed to be graded in the radial direction according to a simple exponential-law distribution. The inner surface of the disk is pure metal whereas the outer surface of the disk is pure ceramic. The boundary condition of rigid casing is considered herein, that is the vanishing of the radial displacement at the outer surface. The boundary condition at the inner surface of the disk is taken to be vanishing either radial displacement or radial stress. Analytical solutions for the elastic deformation of the rotating functionally graded annular disks subjected to these boundary conditions are obtained. Numerical results for radial displacement, circumferential and radial stresses are presented. Comparisons between the different rotating homogeneous and functionally graded annular disks are made at the same angular velocity. The results show that distributions of stresses and displacement through the radial direction of the rotating annular disk vary with different parameters. [ABSTRACT FROM AUTHOR]

The tensile deformation and fracture behavior of electrodeposited nanocrystalline Ni–15% Fe alloy samples after annealing for 90 min at 250, 400 and 500 °C temperatures were investigated. The structure of the samples was studied using TEM and XRD techniques and the fracture surfaces were investigated employing SEM. The results of this study indicated that annealing at 250 °C modified grain size distribution slightly but resulted in a significant increase in the initial strain hardening rate. While the average grain size in the 400 °C sample was increased to 59 nm, its yield strength was comparable to the as-deposited alloy with a 9 nm grain size. The plastic tensile elongation of all annealed samples was lowered significantly to less than 1% from approximately 6% in the as-deposited state. These results are discussed in terms of the inhomogeneity of plastic deformation and the evolution of internal stresses in nanocrystalline materials. [ABSTRACT FROM AUTHOR]

The fracture behaviour of glass microsphere-filled polypropylene/poly(ethylene terephthalate-co-isophthalate) blend-matrix composites was investigated in comparison with that of the glass microsphere-filled PP composites. Depending on the deformability displayed by the composite, it was carried out through the linear-elastic fracture mechanics or by applying the J-integral concept. The matrix ductility was regulated in the composite through the glass bead surface treatment applied with different silane-coupling agents, as well as with the addition of maleated PP as polymer compatibilizer. Whereas all the composites failed in a brittle manner at moderate impact speed, quasi-brittle fracture behaviour was only observed at low strain rate in composites having high and medium interfacial adhesion level. Results showed that composites containing both aminosilane-treated glass microspheres and maleated PP showed the highest values of fracture toughness. In composites with low adhesion level between matrix and glass beads, the critical J-integral value diminished due to the presence of PET. [ABSTRACT FROM AUTHOR]

The tensile behaviors of the ultrafine-grained (UFG) pure copper prepared by cold rolling have been investigated. The UFG-Cu exhibited high strengths and low ductility. The ductile dimple-like fracture surfaces and persistent-slip-bands (PSBs)-like local shear bands in the tension-deformed UFG-Cu were observed, both of which typically spanned tens of grain sizes. This observation indicated that the fracture mechanisms operated at a larger scale than the grain size and eventually involved collective grain activities. Moreover, localized shear bands provided the experimental evidence to the localized plastic deformation, which was the one of the dominant reasons, causing the low ductility in the UFG-Cu. [ABSTRACT FROM AUTHOR]

The effect of hydrogen on the stress corrosion cracking (SCC) for X-70A and X-70B pipeline steels in near-neutral pH environments was studied by monotonic tensile tests on slow strain rate tests (SSRT) machine and cyclic tests on SSRT machine and fatigue machine. The results showed that SCC intensity increased at high cathodic protection potential. Hydrogen charged at high pressure enhanced SCC sensitivity. Cyclic tests under elastic strain condition could not take the specimens to fracture and the hydrogen content was very low. However, when the load was raised to produce plastic deformation, the specimens failed within limited time and there was brittle fracture appeared on the fracture surface. Moreover, the hydrogen content was higher than that under elastic strain condition. The SCC mechanism of X-70A and X-70B pipeline steels in near-neutral pH environments was related to hydrogen accumulation induced by plastic strain. [ABSTRACT FROM AUTHOR]

Topological defects can be formed in carbon nanotubes (CNTs) either during processing or during subsequent thermo-mechanical loading. When multiple defects are formed, the defects interact with each other depending upon the distance of separation between them. Earlier studies have shown that under mechanical loading, such interacting defects coalesce to form a larger defect, ultimately leading to complete failure. While defect coalescence is possible, it has also been observed that some defects may disappear (anneal) under certain thermo-mechanical conditions. In this molecular dynamics (MD) based simulation studies, we show that two 5-7-7-5 type defects (Stone-Wales) in close proximity when subjected to either pure mechanical loading (tensile strain of 10%) or pure thermal loading (temperature up to 3000 K) remain stable. On the other hand, the defects annihilate completely under a combination of both thermal (2800 K) and mechanical loading (under 5%) applied concurrently. It is hypothesized that vibrational oscillations due to thermal effects combined with atomic separation induced due to mechanical load together can cause the defects to annihilate while either of them acting alone cannot do so. [ABSTRACT FROM AUTHOR]

Three high chromium white cast irons were examined in the as-cast state to determine the effect of the carbon content on the fracture toughness. The plane strain fracture toughness KIc and the fracture strength were measured for each alloy. X-ray mapping was used to identify the phases on the fracture surfaces. Scanning electron fractography and optical microscopy were used to determine the volume fraction of each phase on the fracture surfaces. It was found that most fracture occurred in the eutectic carbides, but that for the alloys with a reduced volume fraction of eutectic carbides, a small amount of crack propagation occurred in the austenitic dendrites. This change in crack path correlated with an increase in fracture toughness. The Ritchie-Knott-Rice model of brittle fracture was applied. It was found to sensibly predict the critical length for fracture for each alloy. Deep etching was employed to examine the distribution of eutectic carbides. It was found that the eutectic carbides formed a continuous network in each case. [ABSTRACT FROM AUTHOR]

A series of three-point bend tests using single edge notched testpieces of pure polycrystalline ice have been performed at three different temperatures (-20°C, -30°C and -40°C). The displacement rate was varied from 1 mm/min to 100 mm/min, producing the crack tip strain rates from about 10-3 to 10-1 s-1. The results show that (a) the fracture toughness of pure polycrystalline ice given by the critical stress intensity factor (KIC) is much lower than that measured from the J—integral under identical conditions; (b) from the determination of KIC, the fracture toughness of pure polycrystalline ice decreases with increasing strain rate and there is good power law relationship between them; (c) from the measurement of the J—integral, a different tendency was appeared: when the crack tip strain rate exceeds a critical value of 6 × 10-3 s-1, the fracture toughness is almost constant but when the crack tip strain rate is less than this value, the fracture toughness increases with decreasing crack tip strain rate. Re-examination of the mechanisms of rate-dependent fracture toughness of pure polycrystalline ice shows that the effect of strain rate is related not only to the blunting of crack tips due to plasticity, creep and stress relaxation but also to the nucleation and growth of microcracks in the specimen. [ABSTRACT FROM AUTHOR]

The toughening behavior of short glass fiber reinforced toughened polymers was studied using fracture mechanics and microscopic techniques. The essential work of fracture (EWF) analysis shows that the inclusion of short glass fibers not only provided a stiffening effect but also a toughening influence. It was observed that rubber-related toughening and fiber-related toughening were competitive in nature for the reinforced, toughened nylon 6,6. When the matrix stress was substantially reduced by the presence of short fibers via the load-shedding mechanism, rubber toughening was severely curtailed. At higher fiber volume fractions, fiber pull-out work contributed significantly to the enhancement of the specific essential fracture work. Fiber-end plasticity was evident under microscopic examination. [ABSTRACT FROM AUTHOR]

The influence of fiber rupture, fiber pull-out and fiber tensile strength distribution on the post-cracking behavior of short-randomly-distributed fiber reinforced brittle-matrix composites has been analyzed using an approach based on the Weibull weakest-link statistics. The analysis led to the development of a predicting model for the composite bridging stress-crack opening displacement (σc − δ) law—a fundamental material property necessary for the analysis of steady-state cracking in the composites. The proposed σc − δ relationship can be used to relate the composite tensile and fracture properties to the microstructural parameters. The model revealed the importance of fiber strength distribution as described by the Weibull weakest-link statistics in governing the post-cracking response of the composite. The proposed model was able to reproduce the results of an earlier model for a limiting case where fiber tensile rupture was accounted for assuming a deterministic fiber tensile rupture strength. Model-predicted post-peak σc − δ curve was also in close agreement with those obtained from uniaxial tensile tests of a Kevlar fiber reinforced cementitious composite where fiber tensile rupture was reported. The model provided physical insights as to the micro-mechanisms controlling the post-cracking response of short-fiber reinforced brittle-matrix composites where fibers have a tensile strength distribution described by the Weibull weakest-link statistics. [ABSTRACT FROM AUTHOR]

The effect of sulfate ions on the crack growth rates (CGRs) of notched specimens (CT) of Inconel 600 under constant load has been investigated in chloride containing aqueous solutions at 250 °C. The intergranular stress corrosion crack growth rates increased in chloride solutions while its hinder with increasing sulfate concentrations. Stress intensity factor ( k) for stress corrosion cracking decreased with increasing of aggressivity of chloride ions while increased in the presence of sulfate solutions. At very low concentration of chloride (0.001 m), k-value retardation was observed. It is clear from the results that hydrogen in the aqueous solutions has a deleterious effect on crack propagation. The difference in crack growth rates in chloride ions and in chloride containing sulfate solution at high temperature can be recognized as caused by the difference in local environment conditions at a crack tip. The results indicate that the crack don't propagate under this conditions in the presence of sulfate ions. It is mainly due to a hinderance of chloride ions adsorption on active sites of the fracture surfaces and the formation of chromium oxide layer which is stable at higher temperature. [ABSTRACT FROM AUTHOR]

The fracture mechanisms of wood have often been interpreted on the scale of cell walls. Although this scale is important, the scale of growth rings needs to be considered in the same context. In the present study, the crack tip strain field of radial TR cracks at the scale of growth rings is measured by electronic speckle photography. The methodology is discussed in detail as well as the data reduction scheme. The tip is in the earlywood layer and the crack plane of the TR crack is perpendicular to the stiffer latewood layer. Increasing opening mode load is applied in-situ as the crack is observed by reflected light optical microscopy. Strains are measured on direct images of the microstucture. In contrast to some other methodologies, this allows direct correlation between strain field and microstructure. In the softer earlywood, tangential strains extend considerable distances in the tangential direction. Due to the stiff latewood, the strain is heavily constrained in the radial direction. This nature of the local strain field has been largely neglected, despite its obuius significance to TR crack growth mechanisms. [ABSTRACT FROM AUTHOR]

The effect of static and cyclic loading conditions on the stress corrosion cracking of unidirectional glass reinforced polymer (GRP) rods used in composite high voltage insulator has been investigated. A series of stress corrosion experiments have been performed on unidirectional E-glass/modified polyester composite rods. The rods have been subjected to mechanical tensile static and cyclic stresses in the presence of a nitric acid solution. The stress corrosion fracture process in the rods was monitored using acoustic emission techniques. The experimental loading conditions simulated possible in-service loadis for composite suspension insulators. The results obtained in this study showed that the brittle fracture process can be generated in the rods when subjected to relatively low tensile stresses in the presence of a nitric acid solution. The morphology of the experimentally generated brittle fracture cracks in the rods closely resemble those from in-service failed composite suspension. It has also been shown in this research that low frequency, low amplitude vibrations in tensile loads can significantly accelerate the fracture process. It appears that the brittle fracture cracks in the rods generated under cyclic loads are less planar in nature in comparison with the cracks formed under static conditions. It has also been found that the acoustic emission generated during the stress corrosion fracture process in the rods is sensitive to the placement of the transducers. However, reasonably good correlation between the stress corrosion crack growth rates and acoustic emission has been attained. [ABSTRACT FROM AUTHOR]

A 95wt%W 3.5wt%Ni 1.5wt%Fe alloy is strained to failure in uniaxial tension at temperatures in the range −100°C to 300°C. The fracture surfaces of the tensile specimens are examined and the contributions from the fracture mechanisms: tungsten-tungsten intergranular decohesion, tungsten-binder interface decohesion, binder rupture and tungsten cleavage, are identified as a function of temperature. The observed variations are explained in terms of the flow stress temperature dependence of the two phases within the alloy. The results are consistent with a changing mode of specimen rupture from one of a localized cascade of nucleation events at low temperatures to one of crack propagation through linking of cracks within the necked region of the specimens deformed at higher temperatures. [ABSTRACT FROM AUTHOR]

The tensile mechanical behavior of Ti-47at%Al-1.5at%Cr-0.5ar%Mn-2.8at%Nb in full lamellar microstructure has been studied in the strain rate range from 100 s−1 to 800 s−1 and the complete stress-strain curves were obtained. Results show that the alloy is extremely brittle at different strain rate, exhibiting near-zero ductility. Both UTS and fracture strain of material are strain rate sensitive, increasing with the strain rate at room temperature. Fractography analysis indicates that the alloy fractures in a mixed mode of predominant transgranular cleavage and minor intergranular cracking. On basis of the experiment results and Weibull distribution theory, a statistic model has been developed to describe mechanical behavior of TiAl(FL) at different strain rate. The statistical parameters for material and their relationships with strain rate are obtained from tensile impact experimental results. The simulated stress strain curves from the model are in good agreement with the test data. The theoretical model and test results show that both the scale parameter σ0 and the shape parameter β are rate dependent, and a linear dependence of σ0 and β on lg ɛ∈ has been found. [ABSTRACT FROM AUTHOR]

In this part of the series, the impact behaviour of the PBT and PC blends without impact modifier was studied. Failure mechanism of the blends under various conditions was discussed. It was found that the key toughening process, i.e. interfacial debonding-cavitation, was disabled when the blends were subjected to impact loading. Hence, the fracture of the thick PBT/PC specimens with strong interface occurred under plane-strain condition. Their impact toughness obeys the rule of mixtures and synergistic toughening could not be achieved. When thinner specimens were tested, the fracture took place under non-plane-strain condition. But, the toughness of the blends was much lower than the value predicted by the rule of mixtures. Negative blending effect was obtained. Study on the strain rate effect suggests that under impact loading, the PC domains in the blends are subjected to an additional plastic constraint imposed by the neighboring PBT matrix, which is more rigid at a higher strain rate. Since fracture of the PC is highly sensitive to the plastic constraint at the crack-tip, the PBT imposed high plastic constraint promotes brittle fracture of the PC, leading to a deteriorated impact resistance. Evidences from TEM, SEM and OM studies support the mechanism proposed. Based on this mechanism, some suggestions on the selection of polymer components and design of microstructure for rigid-rigid polymer blends are also given. [ABSTRACT FROM AUTHOR]

The addition of monodisperse size crosslinked polystyrene (PS) particles, synthesized by emulsifier-free emulsion polymerization, to polysulfide matrix enhanced mechanical properties of the cured rubbery composites. The modulus, fracture strength, and elongation at break increased with increasing filler volume fraction up to 30 wt % PS particles. The strength and elongation at break decreased with increasing particle diameter from 0.315 to 1.25 μm. The strength at break increased, but the extension decreased, as the particle crosslink density increased from 0 to 5 mol % DVB. Interparticle interactions are dominant and lead to the formation of clusters which form a network structure in PS particle filled composites. Since the number density, as well as the total surface area, of particles increase with decreasing particle diameter, interparticle attractions are enhanced, the tendency for cluster formation increased with decreasing particle size from 1.25 to 0.315 μm. As particle crosslink density was reduced, the porosity and surface roughness of particles increased. Then, the dispersion of particles in the matrix was enhanced and particle agglomeration reduced but more polymer matrix was adsorbed on the particles. These particles or clusters act as physical crosslinks, resulting in an increased total effective crosslink density in the filled composites. [ABSTRACT FROM AUTHOR]

Fracture in an AA 6061 based metal matrix composite (MMC) containing 20 vol % Al2O3 particles is modelled using an axisymmetrical finite element model and a statistical approach for calculating the strength of reinforcing ceramic particles via the Weibull model. Within this model, variables such as the volume fraction, particle size and matrix alloy properties can be varied. When modelling the fracture behaviour of one particle, it is assumed that the survival probability of the ceramic particle is governed by a Weibull distribution. Fracture statistics of the MMC is examined by plotting the survival probability of an Al2O3 particle vs. the macroscopic axial stress applied on the whole MMC. Based on initial calculations it can be concluded that the relation between the macroscopic applied stress on the MMC and the survival probability of the ceramic particle can be described by the Weibull modulus m, as long as the stress distribution in the matrix surrounding the particle is proportional to the applied load and that triaxial loading of the MMC results in a lower survival probability compared to uniaxial loading. Fracture behaviour of MMCs can well be described and a 'mastercurve' can be made for various characteristic stresses and matrix yield stresses at a specific hardening exponent for the matrix material. [ABSTRACT FROM AUTHOR]