Showing papers on "Fracture toughness published in 2011"

Toughening in graphene ceramic composites.

Abstract: The majority of work in graphene nanocomposites has focused on polymer matrices. Here we report for the first time the use of graphene to enhance the toughness of bulk silicon nitride ceramics. Ceramics are ideally suited for high-temperature applications but suffer from poor toughness. Our approach uses graphene platelets (GPL) that are homogeneously dispersed with silicon nitride particles and densified, at ∼1650 °C, using spark plasma sintering. The sintering parameters are selected to enable the GPL to survive the harsh processing environment, as confirmed by Raman spectroscopy. We find that the ceramic's fracture toughness increases by up to ∼235% (from ∼2.8 to ∼6.6 MPa·m(1/2)) at ∼1.5% GPL volume fraction. Most interestingly, novel toughening mechanisms were observed that show GPL wrapping and anchoring themselves around individual ceramic grains to resist sheet pullout. The resulting cage-like graphene structures that encapsulate the individual grains were observed to deflect propagating cracks in not just two but three dimensions.

A damage-tolerant glass.

Abstract: Owing to a lack of microstructure, glassy materials are inherently strong but brittle, and often demonstrate extreme sensitivity to flaws. Accordingly, their macroscopic failure is often not initiated by plastic yielding, and almost always terminated by brittle fracture. Unlike conventional brittle glasses, metallic glasses are generally capable of limited plastic yielding by shear-band sliding in the presence of a flaw, and thus exhibit toughness–strength relationships that lie between those of brittle ceramics and marginally tough metals. Here, a bulk glassy palladium alloy is introduced, demonstrating an unusual capacity for shielding an opening crack accommodated by an extensive shear-band sliding process, which promotes a fracture toughness comparable to those of the toughest materials known. This result demonstrates that the combination of toughness and strength (that is, damage tolerance) accessible to amorphous materials extends beyond the benchmark ranges established by the toughest and strongest materials known, thereby pushing the envelope of damage tolerance accessible to a structural metal.

Carbon Nanotubes and Carbon Nanofibers for Enhancing the Mechanical Properties of Nanocomposite Cementitious Materials

Abstract: Carbon nanotubes (CNTs) and carbon nanofibers (CNFs) are quickly becoming two of the most promising nanomaterials because of their unique mechanical properties. The size and aspect ratio of CNFs and CNTs mean that they can be distributed on a much finer scale than commonly used microreinforcing fibers. As a result, microcracks are interrupted much more quickly during propagation in a nano- reinforced matrix, producing much smaller crack widths at the point of first contact between the moving crack front and the reinforcement. In this study, untreated CNTs and CNFs are added to cement matrix composites in concentrations of 0.1 and 0.2% by weight of cement. The nanofilaments are dispersed by using an ultrasonic mixer and then cast into molds. Each specimen is tested in a custom-made three-point flexural test fixture to record its mechanical properties; namely, the Young's modulus, flexural strength, ultimate strain capacity, and fracture toughness, at 7, 14, and 28 days. A scanning electron microscope (SEM) is used to discern the difference between crack bridging and fiber pullout. Test results show that the strength, ductility, and fracture toughness can be improved with the addition of low concentrations of either CNTs or CNFs. DOI: 10.1061/(ASCE)MT.1943-5533.0000266. © 2011 American Society of Civil Engineers.

High mechanical performance of fibre reinforced cementitious composites: the role of “casting-flow induced” fibre orientation

Abstract: Governing the dispersion and the orientation of fibres in concrete through a suitably balanced set of fresh state properties and a carefully designed casting procedure, is a feasible and cost-effective way to achieve a superior mechanical performance of fibre reinforced cementitious composites, which may be required by the intended application, even keeping the fibre content at relatively low values (e.g. around 1% by volume). In this paper the possibility of pursuing the above said “integrated” approach has been addressed in the framework of larger project focused on developing a deflection-hardening FRCC (DHFRCC), reinforced with 100 kg/m3 (1.27% by volume) of short steel fibres (13 mm long and 0.16 mm in diameter). The material has to be employed to manufacture thin (30 mm) roof elements, without any kind of conventional reinforcement, which have been anticipated to work, as simply supported beams, over a 2.5 m span. The study hence paves the way to the possibility of exploiting at an industrial level the correlation among fresh state performance, fibre dispersion and hardened state properties of self consolidating steel fibre reinforced concrete to achieve enhanced structural performance tailored to the specific application.

The effect of carbon nanotubes on the fracture toughness and fatigue performance of a thermosetting epoxy polymer

Abstract: Multi-walled carbon nanotubes, with a typical length of 140 μm and a diameter of 120 nm, have been used to modify an anhydride-cured epoxy polymer. The modulus, fracture energy and the fatigue performance of the modified polymers have been investigated. Microscopy showed that these long nanotubes were agglomerated, and that increasing the nanotube content increased the severity of the agglomeration. The addition of nanotubes increased the modulus of the epoxy, but the glass transition temperature was unaffected. The measured fracture energy was also increased, from 133 to 223 J/m2 with the addition of 0.5 wt% of nanotubes. The addition of the carbon nanotubes also resulted in an increase in the fatigue performance. The threshold strain-energy release-rate, G th, increased from 24 J/m2 for the unmodified material to 73 J/m2 for the epoxy with 0.5 wt% of nanotubes. Electron microscopy of the fracture surfaces showed clear evidence of nanotube debonding and pull-out, plus void growth around the nanotubes, in both the fracture and fatigue tests. The modelling study showed that the modified Halpin–Tsai equation can fit very well with the measured values of the Young’s modulus, when the orientation and agglomeration of the nanotubes are considered. The fracture energy of the nanotube-modified epoxies was predicted, by considering the contributions of the toughening mechanisms of nanotube debonding, nanotube pull-out and plastic void growth of the epoxy. This indicated that debonding and pull-out contribute to the toughening effect, but the contribution of void growth is not significant. There was excellent agreement between the predictions and the experimental results.

Toughness amplification in natural composites

Abstract: Natural structural materials such as bone and seashells are made of relatively weak building blocks, yet they exhibit remarkable combinations of stiffness, strength and toughness. This performance can be largely explained by their “staggered microstructure”: stiff inclusions of high aspect ratio are laid parallel to each other with some overlap, and bonded by a softer matrix. While stiffness and strength are now well understood for staggered composites, the mechanisms involved in fracture are still largely unknown. This is a significant lack since the amplification of toughness with respect to their components is by far the most impressive feature in natural staggered composites such as nacre or bone. Here a model capturing the salient mechanisms involved in the cracking of a staggered structure is presented. We show that the pullout of inclusions and large process zones lead to tremendous toughness by far exceeding that of individual components. The model also suggests that a material like nacre cannot reach steady state cracking, with the implication that the toughness increases indefinitely with crack advance. These findings agree well with existing fracture data, and for the first time relate microstructural parameters with overall toughness. These insights will prove useful in the design of biomimetic materials, and provide clues on how bone fractures at the nano and microscales.

On fracture toughness of nano-particle modified epoxy

Abstract: A systematic study on the effects of silica and rubber nano-particles on the fracture toughness behavior of epoxy was conducted. Mode I fracture toughness ( G IC ) of binary silica/epoxy, binary rubber/epoxy and ternary silica/rubber/epoxy nanocomposites with different particle weight fractions was obtained by compact tension tests. It is found that G IC of epoxy can be significantly increased by incorporating either rubber or silica nano-particles. However, hybrid nanocomposites do not display any “synergistic” effect on toughness. Microstructures before and after fracture testing were examined to understand the role of nano-particles on the toughening mechanisms.

Critical tensile and compressive strains for cracking of Al2O3 films grown by atomic layer deposition

Abstract: Al2O3 atomic layer deposition (ALD) is a model ALD system and Al2O3 ALD films are excellent gas diffusion barrier on polymers. However, little is known about the response of Al2O3 ALD films to strain and the potential film cracking that would restrict the utility of gas diffusion barrier films. To understand the mechanical limitations of Al2O3 ALD films, the critical strains at which the Al2O3 ALD films will crack were determined for both tensile and compressive strains. The tensile strain measurements were obtained using a fluorescent tagging technique to image the cracks. The results showed that the critical tensile strain is higher for thinner thicknesses of the Al2O3 ALD film on heat-stabilized polyethylene naphthalate (HSPEN) substrates. A low critical tensile strain of 0.52% was measured for a film thickness of 80 nm. The critical tensile strain increased to 2.4% at a film thickness of 5 nm. In accordance with fracture mechanics modeling, the critical tensile strains and the saturation crack densiti...

Boron nitride nanotube reinforced hydroxyapatite composite: mechanical and tribological performance and in-vitro biocompatibility to osteoblasts.

Abstract: This study proposes boron nitride nanotube (BNNT) reinforced hydroxyapatite (HA) as a novel composite material for orthopedic implant applications. The spark plasma sintered (SPS) composite structure shows higher density compared to HA. Minimal lattice mismatch between HA and BNNT leads to coherent bonding and strong interface. HA-4 wt% BNNT composite offers excellent mechanical properties-120% increment in elastic modulus, 129% higher hardness and 86% more fracture toughness, as compared to HA. Improvements in the hardness and fracture toughness are related to grain refinement and crack bridging by BNNTs. HA-BNNT composite also shows 75% improvement in the wear resistance. The wear morphology suggests localized plastic deformation supported by the sliding of outer walls of BNNT. Osteoblast proliferation and cell viability show no adverse effect of BNNT addition. HA-BNNT composite is, thus, envisioned as a potential material for stronger orthopedic implants.

A Tough Silicon Nitride Ceramic with High Thermal Conductivity

Abstract: IO N The world is shifting energy sources from fossil fuel to electric power in order to cope with the energy and environmental problems. Driven by the demand for effi cient control and conversion of electric power, power electronic device technology is advancing toward higher voltage, larger current, greater power density, and smaller size, and this trend is poised to be accelerated with the replacement of Si by the wide-bandgap semiconductors (SiC and GaN) in the near future. [ 1 , 2 ] However, the high power will induce large thermal stresses in the devices, which pose great challenges for the assembly of the devices and the packaging materials, in particular the brittle ceramic substrates that provide functions of electrical insulation and heat dissipation. In many occasions, even the two high-grade ceramic substrate materials, AlN and Si 3 N 4 , become cracked due to low mechanical strength and fracture toughness (for AlN) or insuffi cient thermal conductivity (for Si 3 N 4 ). [ 3 , 4 ] The reliability problem caused by the ceramic substrates has become a bottleneck hindering the advancement of power device technology. Search for new ceramic materials with better thermomechanical properties is an urgent issue. Here, we show a new Si 3 N 4 ceramic that possesses a very high thermal conductivity (177 W m − 1 K − 1 ) along with a high fracture toughness (11.2 MPa m 1/2 ) and a high fracture strength (460 MPa). We expect this Si 3 N 4 will be used as the next-generation insulating substrate material for high-power electronic devices. Silicon nitride mainly exists in two hexagonal polymorphs, namely α and β -Si 3 N 4 , which are generally regarded as lowand high-temperature crystal forms, respectively. [ 5 , 6 ] As a highly covalent compound, Si 3 N 4 transports heat primarily by phonons at room temperature and below. In 1995, Haggerty and Lightfoot predicted that the intrinsic thermal conductivity of Si 3 N 4 might be 200 to 320 W m − 1 K − 1 at room temperature. [ 7 ] Later, Hirosaki et al. estimated that the intrinsic thermal conductivities of a β -Si 3 N 4 crystal were 170 and 450 W m − 1 K − 1 along the a -axis and c -axis, respectively. [ 8 ] However, the thermal conductivity of Si 3 N 4 ceramics is much lower than the intrinsic values. Si 3 N 4 ceramics are polycrystalline materials consolidated by liquid-phase sintering. During sintering, Si 3 N 4 raw powder, which is usually α phase, converts to the more stable β phase. In the microstructure of Si 3 N 4 ceramics,

Atomic scale fluctuations govern brittle fracture and cavitation behavior in metallic glasses

Abstract: We perform atomistic simulations on the fracture behavior of two typical metallic glasses, one brittle (FeP) and the other ductile (CuZr), and show that brittle fracture in the FeP glass is governed by an intrinsic cavitation mechanism near crack tips in contrast to extensive shear banding in the ductile CuZr glass. We show that a high degree of atomic scale spatial fluctuations in the local properties is the main reason for the observed cavitation behavior in the brittle metallic glass. Our study corroborates with recent experimental observations of nanoscale cavity nucleation found on the brittle fracture surfaces of metallic glasses and provides important insights into the root cause of the ductile versus brittle behavior in such materials.