Showing papers on "Fracture toughness published in 2010"

Fracture and fatigue in graphene nanocomposites

Abstract: Graphene, a single-atom-thick sheet of sp-bonded carbon atoms, has generatedmuch interest due to its high specific area and novel mechanical, electrical, and thermal properties. Recent advances in the production of bulk quantities of exfoliated graphene sheets from graphite have enabled the fabrication of graphene–polymer composites. Such composites show tremendous potential for mechanical-property enhancement due to their combination of high specific surface area, strong nanofiller–matrix adhesion and the outstanding mechanical properties of the sp carbon bonding network in graphene. Graphene fillers have been successfully dispersed in poly(styrene), poly(acrylonitrile) and poly(methyl methacrylate) matrices and the responses of their Young’s modulus, ultimate tensile strength, andglass-transition temperaturehave been characterized. However, to the best of our knowledge there is no report on the fracture toughness and fatigue properties of graphene–polymer composites. Fracture toughness describes the ability of a material containing a crack to resist fracture and it is a critically important material property for design applications. Fatigue involves dynamic propagation of cracks under cyclic loading and it is one of the primary causes of catastrophic failure in structural materials. Consequently, the material’s resistance to fracture and fatigue crack propagation are of paramount importance to prevent failure. Herein we report the fracture toughness, fracture energy, and fatigue properties of an epoxy polymer reinforced with various weight fractions of functionalized graphene sheets. Remarkably, only 0.125% weight of functionalized graphene sheets was observed to increase the fracture toughness of the pristine (unfilled) epoxy by 65% and the fracture energy by 115%.Toachievecomparableenhancement,carbonnanotube (CNT) and nanoparticle epoxy composites require one to two orders of magnitude larger weight fraction of nanofillers. Under fatigue conditions, incorporation of 0.125% weight of functionalized graphene sheets drastically reduced the rate of crack propagation in the epoxy 25-fold. Fractography analysis

Temperature and strain-rate dependent fracture strength of graphene

Abstract: We investigate the variation in fracture strength of graphene with temperature, strain rate, and crack length using molecular dynamics (MD) simulations, kinetic analysis of fracture with a nonlinear elastic relation, and the quantized fracture mechanics theory. Young’s modulus does not vary significantly with temperature until about 1200 K, beyond which the material becomes softer. Temperature plays a more important role in determining the fracture strength of graphene. Our studies suggest that graphene can be a strong material even, when subjected to variations in temperature, strain rate, and cracks.

“Electrochemical Shock” of Intercalation Electrodes: A Fracture Mechanics Analysis

Abstract: Fracture of electrode particles due to diffusion-induced stress has been implicated as a possible mechanism for capacity fade and impedance growth in lithium-ion batteries. In brittle materials, including many lithium intercalation materials, knowledge of the stress profile is necessary but insufficient to predict fracture events. We derive a fracture mechanics failure criterion for individual electrode particles and demonstrate its utility with a model system, galvanostatic charging of Li x Mn 2 O 4 . Fracture mechanics predicts a critical C-rate above which active particles fracture; this critical C-rate decreases with increasing particle size. We produce an electrochemical shock map, a graphical tool that shows regimes of failure depending on C-rate, particle size, and the material's inherent fracture toughness K Ic . Fracture dynamics are sensitive to the gradient of diffusion-induced stresses at the crack tip; as a consequence, small initial flaws grow unstably and are therefore potentially more damaging than larger initial flaws, which grow stably.

Effect of grain refinement to 1 μm on strength and toughness of dual-phase steels

Abstract: Large strain warm deformation at different temperatures and subsequent intercritical annealing has been applied to obtain fine grained (2.4m) and ultrafine grained (1.2m) ferrite/martensite dual-phase (DP) steels. Their mechanical properties were tested under tensile and impact conditions and compared to a hot deformed coarse grained (12.4m) reference material. Both yield strength and tensile strength follow a Hall–Petch type linear relationship, whereas uniform elongation and total elongation are hardly affected by grain refinement. The initial strain hardening rate as well as the post-uniform elongation increase with decreasing grain size. Ductile fracture mechanisms are considerably promoted due to grain refinement. Grain refinement further lowers the ductile-to-brittle transition temperature and leads to higher absorbed impact energies. Besides the common correlations with the ferrite grain size, these phenomena are explained in terms of the martensite particle size, shape and distribution and the more homogeneous dislocation distribution in ultrafine ferrite grains.

Shape Memory Bulk Metallic Glass Composites

Abstract: Bulk metallic glasses (BMGs) are being studied extensively as potential structural materials as they have a unique array of mechanical properties compared to traditional crystalline metals ( 1 – 4 ). Their amorphous microstructure and variable composition give BMGs ultrahigh-yield strengths, large elastic strain limits, high hardness, corrosion resistance, and the ability to be processed like a plastic. So far, however, BMGs have not found many structural applications because of their catastrophic failure under tension (tensile loading) and their typically low fracture toughness (resistance to cracking), both resulting from the same amorphous microstructure that differentiates them from crystalline metals. This shortcoming has been addressed in recent years with the development of BMG matrix composites (BMGMCs)—two-phase alloys consisting of soft, crystalline dendrites grown in situ in a glass-forming matrix ( 5 – 9 ). When designed and processed properly, BMGMCs retain the positive structural features exhibited by monolithic (single-phase) BMGs, but can also exhibit enhanced tensile ductility, fracture toughness, and fatigue endurance, which makes them desirable as engineering materials ( 5 , 10 , 11 ).

The effect of adding carbon nanotubes to glass/epoxy composites in the fibre sizing and/or the matrix

Abstract: Carbon nanotubes (CNTs) were integrated in glass fibres epoxy composites by either including CNTs in the fibre sizing formulation, in the matrix, or both. The effects of such controlled placement of CNTs on the thermophysical properties (glass transition temperature and coefficient of thermal expansion) and the Mode I interlaminar fracture toughness of the composites were studied. The present method of CNT-sizing of the glass fibres produces an increase of almost +10% in the glass transition temperature and a significant reduction of −31% in the coefficient of thermal expansion of the composites. Additionally, the presence of CNTs in the sizing resulted in an increased resistance of crack initiation fracture toughness by +10%, but a lowered crack propagation toughness of −53%. Similar trends were observed for both instances when CNTs were introduced only in the matrix and in combination of both matrix and sizing.

Toughening of Epoxies with Block Copolymer Micelles of Wormlike Morphology

Abstract: An amphiphilic poly(ethylene-alt-propylene)-b-poly(ethylene oxide) (PEP-PEO) block copolymer (BCP) was blended with a bisphenol A-based epoxy resin formulation and self-assembled into a wormlike micelle structure. With an incorporation of 5 wt % of the BCP material, the fracture toughness was improved by >100% over the neat epoxy. The morphology and mechanical properties of this BCP-modified epoxy were investigated using transmission electron microscopy, dynamic mechanical analysis, tensile tests, and fracture toughness measurements. Toughening mechanisms from the wormlike micelle-modified material were investigated using the double-notch four-point-bending technique, and the results are compared with data obtained from the same epoxy thermoset formulation containing a BCP that self-assembled into spherical micelles. Elongated cylindrical micelles produce improved toughness, which is interpreted on the basis of a combination of mechanisms including crack tip blunting, cavitation, particle debonding, limit...

Carbon nanotube toughened aluminium oxide nanocomposite

Abstract: This paper describes the mechanical properties of carbon nanotube-reinforced Al2O3 nanocomposites fabricated by hot-pressing. The results showed that compared with monolithic Al2O3 the fracture toughness, hardness and flexural strength of the nanocomposites were improved by 94%, 13% and 6.4% respectively, at 4 vol.% CNT additions. For 10 vol.% CNT additions, with the exception of the fracture toughness, which was improved by 66%, a decrease in mechanical properties was observed when compared with those for monolithic Al2O3. The toughening mechanism is discussed, which is due to the uniform dispersion of CNTs within the matrix, adequate densification, and proper CNT/matrix interfacial connections.

Materials Science and Engineering A

Abstract: Open-faced double cantilever beam (DCB) specimens of a toughened epoxy-aluminum adhesive system were degraded over a relatively wide range of temperature, relative humidity (RH) and exposure time, dried and tested to characterize the irreversible evolution of the mixed-mode fracture resistance curves (R-curves). The water diffusion properties of the bulk adhesive were modeled using an earlier sequential dual Fickian (SDF) model for the same adhesive in order to predict the adhesive water content. Three temporal stages of degradation possessing different R-curve and fracture surface characteristics were observed. In general, the steady-state critical strain energy release rate (Gcs), the rate of toughening (dGcr/da) and the length of the rising part of the R-curve decreased with increasing exposure temperature, RH and water concentration, while the initiation Gc (Gci) remained unchanged. It is hypothesized that crack initiation is governed by the properties of the epoxy matrix and that the toughening action of rubber particles does not become appreciable until after a certain amount of crack extension (more than about 50m in the present case). The irreversible degradation of fracture toughness was found to be insensitive to the phase angle, which simplifies the construction of the fracture toughness envelope for a given level of degradation. These effects were incorporated into a new R-curve degradation model which has an application in the R-curve prediction for closed joints having nonuniform degradation.

Toughened ZrB2-based ceramics through SiC whisker or SiC chopped fiber additions

Abstract: In order to improve the fracture toughness, SiC whiskers or SiC chopped fibers were added to a ZrB 2 matrix in volumetric fraction of 10 and 20 vol.%. The composites were hot-pressed between 1650 and 1730 °C and their final relative densities were higher than 95%. Even at the lowest sintering temperature, the whiskers showed an evident degradation. On the other hand, the fibers maintained their initial shape and a strong interface formed between matrix and reinforcement. The fracture toughness of the composites increased from 30 to 50% compared to the baseline material, with the fibers showing a slightly higher toughening effect. In the whiskers-reinforced composites, the room-temperature strength increased when 10 vol.% whiskers were added. In the fibers-reinforced composites, the room-temperature strength decreased regardless the amount of fibers added. The high-temperature strength of the composites was higher than that of the baseline material for both types of reinforcement.

Tribology of thermal sprayed WC–Co coatings

Abstract: This paper looks at the tribology of thermal sprayed WC–Co based coatings and covers the high energy air–sand erosion resistance and slurry jet impingement erosion performance, dry and wet sliding tribology of thermal spray WC–Co based coatings as well as the abrasion and abrasion–corrosion of these coatings. The tribological and tribo-corrosion performance of the coatings will be related to their mechanical and corrosion properties as well as deposition parameters, microstructure and actual composition. For example, the anisotropic microstructure of thermally sprayed WC–Co–Cr coatings, in particular the low fracture toughness in a direction parallel to the substrate, has been observed to affect the nature of crack formation under 200 μJ air–solid particle erosion conditions. Voids and occasionally other microstructural features (i.e., cobalt lakes, splat boundaries, interfacial inclusions) in the coating act as crack initiation sites. The erosion rate was dominated by cracks within 5 μm of the surface and was relatively insensitive to total length of cracks, showing a near-surface damage front controls the erosion rate and this region is coincident with the region of maximum shear stress induced by erodent impacts. Optimisation of the deposition parameters of HVOF 86WC–10Co–4Cr coatings show an improvement in erosion resistance of more than 50% over the conventional D-gun applied coating of identical nominal composition. The variation in the slurry erosion performance of the thermally sprayed coatings is also linked to directional fracture toughness and crack propagation paths which are influenced by the presence of pores, inhomogeneous carbide distributions and substrate grit blast remnants. The influence of slurry jet angle is more pronounced under 0.4 μJ energy conditions where maximum erosion occurred at 90° and the minimum at 30° in contrast to 7 μJ slurry erosion rates which were independent of jet angle. This reflects the lower levels of fluctuating stresses imparted to the coating during low energy slurry impacts leading to the impact angle having a greater effect on sub critical crack growth rate than for higher energy conditions. The abrasion resistance of these coatings was found comparable to sintered cermets of the same composition. The synergistic effects between micro and macro abrasion and corrosion for detonation gun (D-gun) sprayed WC–10Co–4Cr coatings are shown to be significant and depend on the environment. The size effect of the abradant relative to the microstructure and splat size is important as well as the propensity for the various phases to passivate to control corrosion levels. Comparisons between exposed and freshly polished coating surfaces in strong NaOH solutions (pH 11) show that significantly lower wear rates were seen for the exposed surface due to a negative wear–corrosion synergy due to selective phase removal and the effects of localised passivation. Dry and wet sliding wear resistance of these coatings is shown to be high (wear rates of 10 −16 –10 −18 m 3 /Nm) with modest coefficient of friction levels between 0.2 and 0.5. The presence of oxides on the binder phases appears to influence the friction and wear levels. Wear appears to be by carbide ejection and/or by tribo-chemical processes.

Fracture toughness of polycrystalline tungsten alloys

Abstract: Tungsten and tungsten alloys show the typical change in fracture behavior from brittle at low temperatures to ductile at high temperatures. In order to improve the understanding of the effect of microstructure the fracture toughness of pure tungsten, potassium doped tungsten, tungsten with 1 wt.% La 2 O 3 and tungsten rhenium alloys were investigated by means of 3-point bending, double cantilever beam and compact tension specimens. All these materials show the expected increase in fracture toughness with increasing temperature. The experiments demonstrate that grain size, texture, chemical composition, grain boundary segregation and dislocation density seem to have a large effect on fracture toughness below the DBTT. These influences can be seen in the fracture behavior and morphology, where two kinds of fracture occur: on the one hand transgranular and on the other hand intergranular fracture. Therefore, techniques like electron backscatter diffraction (EBSD), Auger electron spectroscopy (AES) and X-ray line profile analysis were used to improve the understanding of the parameters influencing fracture toughness.

A Study on the Effect of the Stress State on Ductile Fracture

Abstract: The main purpose of this paper is to demonstrate that besides the stress triaxiality parameter, the Lode angle, which can be related to the third invariant of the deviatoric stress tensor, also has an important effect on ductile fracture. This is achieved by conducting a series of micromechanics analyses of void-containing unit cells and experimental-numerical studies of carefully designed specimens experiencing a wide range of stress states. As a result, a fracture criterion is expressed in terms of the equivalent failure strain as a function of the stress triaxiality and the Lode angle (or the third invariant of the stress deviator) and this function is calibrated for a DH36 steel plate.

Improved fracture toughness of carbon fibre/epoxy composite laminates using dissolvable thermoplastic fibres

Abstract: This paper reports on a novel toughening concept based on dissolvable phenoxy fibres, which are added at the interlaminar region in a carbon fibre/epoxy composite. The composites were prepared by resin infusion of carbon fibre fabric with the phenoxy introduced as a chopped fibre interleaf between the carbon fibre plies. The thermoplastic phenoxy fibre dissolved in the epoxy during curing at elevated temperatures and a phase separated morphology with phenoxy-rich secondary phase was formed upon curing. It was found that the average Mode-I fracture toughness value, G1c increased tenfold with only 10 wt.% (with regard to the total matrix content) phenoxy fibre added. Other properties such as Young’s modulus, tensile strength and thermal stability were not adversely affected. The mechanical and thermal properties of the neat epoxy–phenoxy blends were also studied for comparison.

The fracture toughness of bulk metallic glasses

Abstract: Stiffness, strength, and toughness are the three primary attributes of a material, in terms of its mechanical properties. Bulk metallic glasses (BMGs) are known to exhibit elastic moduli at a fraction lower than crystalline alloys and have extraordinary strength. However, the reported values of fracture toughness of BMGs are highly variable; some BMGs such as the Zr-based ones have toughness values that are comparable to some high strength steels and titanium alloys, whereas there are also BMGs that are almost as brittle as silicate glasses. Invariably, monolithic BMGs exhibit no or low crack growth resistance and tend to become brittle upon structural relaxation. Despite its critical importance for the use of BMGs as structural materials, the fracture toughness of BMGs is relatively poorly understood. In this paper, we review the available literature to summarize the current understanding of the mechanics and micromechanisms of BMG toughness and highlight the needs for future research in this important area.

On the effect of x-ray irradiation on the deformation and fracture behavior of human cortical bone

Abstract: submitted to Bone On the effect of x‐ray irradiation on the deformation and fracture behavior of human cortical bone Holly D. Barth a,b,c,† , Maximilien E. Launey a,† , Alastair A. MacDowell b Joel W. Ager III a and Robert O. Ritchie a,c,  a Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA Experimental Systems Group, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA b c Department of Materials Science & Engineering, University of California, Berkeley, CA 94720, USA ABSTRACT In situ mechanical testing coupled with imaging using high‐energy synchrotron x‐ray diffraction or tomography imaging is gaining in popularity as a technique to investigate micrometer and even sub‐micrometer deformation and fracture mechanisms in mineralized tissues, such as bone and teeth. However, the role of the irradiation in affecting the nature and properties of the tissue is not always taken into account. Accordingly, we examine here the effect of x‐ray synchrotron‐source irradiation on the mechanistic aspects of deformation and fracture in human cortical bone. Specifically, the strength, ductility and fracture resistance (both work‐of‐fracture and resistance‐ curve fracture toughness) of human femoral bone in the transverse (breaking) orientation were evaluated following exposures to 0.05, 70, 210 and 630 kGy irradiation. Our results show that the radiation typically used in tomography imaging can have a major and deleterious impact on the strength, post‐yield behavior and fracture toughness of cortical bone, with the severity of the effect progressively increasing with higher doses of radiation. Plasticity was essentially suppressed after as little as 70 kGy of radiation; the fracture toughness was decreased by a factor of five after 210 kGy of radiation. Mechanistically, the irradiation was found to alter the salient toughening mechanisms, manifest by the progressive elimination of the bone’s capacity for plastic deformation which restricts the intrinsic toughening from the formation “plastic zones” around crack‐like defects. Deep‐ultraviolet Raman spectroscopy indicated that this behavior could be related to degradation in the collagen integrity. Keywords: Human cortical bone, x‐ray radiation, deformation, toughness, collagen Both authors contributed equally to this work Corresponding author. tel: +1‐510‐486‐5798; fax: +1‐510‐643‐5792. E‐mail address: RORitchie@lbl.gov (R.O. Ritchie)

A Semi-Circular Bend Technique for Determining Dynamic Fracture Toughness

Abstract: We propose and validate a fracture testing method using a notched core-based semi-circular bend (SCB) specimen loaded dynamically with a modified split Hopkinson pressure bar (SHPB) apparatus. An isotropic fine-grained granitic rock, Laurentian granite (LG) is tested to validate this dynamic SCB method. Strain gauges are mounted near the crack tip of the specimen to detect the fracture onset and a laser gap gauge (LGG) is employed to monitor the crack surface opening distance. We demonstrate that with dynamic force balance achieved by pulse shaping, the peak of the far-field load synchronizes with the specimen fracture time. Furthermore, the evolution of dynamic stress intensity factor (SIF) obtained from the dynamic finite element analysis agrees with that from quasi-static analysis. These results prove that with dynamic force balance in SHPB, the inertial effect is minimized even for samples with complex geometries like notched SCB disc. The dynamic force balance thus enables the regression of dynamic fracture toughness using quasi-static analysis. This dynamic SCB method provides an easy and cost-effective way to measure dynamic fracture toughness of rocks and other brittle materials.

Effect of densification on crack initiation under Vickers indentation test

Abstract: Crack initiation in various commercial glass compositions was investigated by measuring “crack resistance”, which is determined by a series of Vickers indentation and counting of cracks around the indentation. The crack resistance of glass does not have clear relationship with hardness, fracture toughness, nor “brittleness” which is a ratio of the hardness to the fracture toughness. However, the crack resistance has a strong relationship with densification. Glass experiencing larger densification around the indentation shows higher crack resistance. Densification is assumed to reduce residual stress around the indentation, resulting in an increase in the crack resistance.