Showing papers on "Young's modulus published in 2008"

Measurement of the Elastic Properties and Intrinsic Strength of Monolayer Graphene

Abstract: We measured the elastic properties and intrinsic breaking strength of free-standing monolayer graphene membranes by nanoindentation in an atomic force microscope. The force-displacement behavior is interpreted within a framework of nonlinear elastic stress-strain response, and yields second- and third-order elastic stiffnesses of 340 newtons per meter (N m(-1)) and -690 Nm(-1), respectively. The breaking strength is 42 N m(-1) and represents the intrinsic strength of a defect-free sheet. These quantities correspond to a Young's modulus of E = 1.0 terapascals, third-order elastic stiffness of D = -2.0 terapascals, and intrinsic strength of sigma(int) = 130 gigapascals for bulk graphite. These experiments establish graphene as the strongest material ever measured, and show that atomically perfect nanoscale materials can be mechanically tested to deformations well beyond the linear regime.

Stimuli-Responsive Polymer Nanocomposites Inspired by the Sea Cucumber Dermis

Abstract: Sea cucumbers, like other echinoderms, have the ability to rapidly and reversibly alter the stiffness of their inner dermis. It has been proposed that the modulus of this tissue is controlled by regulating the interactions among collagen fibrils, which reinforce a low-modulus matrix. We report on a family of polymer nanocomposites, which mimic this architecture and display similar chemoresponsive mechanic adaptability. Materials based on a rubbery host polymer and rigid cellulose nanofibers exhibit a reversible reduction by a factor of 40 of the tensile modulus, for example, from 800 to 20 megapascals (MPa), upon exposure to a chemical regulator that mediates nanofiber interactions. Using a host polymer with a thermal transition in the regime of interest, we demonstrated even larger modulus changes (4200 to 1.6 MPa) upon exposure to emulated physiological conditions.

Elasticity size effects in ZnO nanowires--a combined experimental-computational approach.

Abstract: Understanding the mechanical properties of nanowires made of semiconducting materials is central to their application in nano devices. This work presents an experimental and computational approach to unambiguously quantify size effects on the Young’s modulus, E, of ZnO nanowires and interpret the origin of the scaling. A micromechanical system (MEMS) based nanoscale material testing system is used in situ a transmission electron microscope to measure the Young’s modulus of [0001] oriented ZnO nanowires as a function of wire diameter. It is found that E increases from ∼140 to 160 GPa as the nanowire diameter decreases from 80 to 20 nm. For larger wires, a Young’s modulus of ∼140 GPa, consistent with the modulus of bulk ZnO, is observed. Molecular dynamics simulations are carried out to model ZnO nanowires of diameters up to 20 nm. The computational results demonstrate similar size dependence, complementing the experimental findings, and reveal that the observed size effect is an outcome of surface reconstr...

An estimation of the Young's modulus of bacterial cellulose filaments

Abstract: An estimation, using a Raman spectroscopic technique, of the Young’s modulus of a single filament of bacterial cellulose is presented. This technique is used to determine the local molecular deformation of the bacterial cellulose via a shift in the central position of the 1095 cm–1 Raman band, which corresponds to the stretching of the glycosidic bond in the backbone of the cellulose structure. By calculating the shift rate with respect to the applied strain it is shown that the stiffness of a single fibril of bacterial cellulose can be estimated. In order to perform this estimation, networks of fibres are rotated through 360° and the intensity of the 1095 cm−1 Raman band is recorded. It is shown that the intensity of this band is largely independent of the angle of rotation, which suggests that the networks are randomly distributed. The modulus is predicted from a calibration of Raman band shift against modulus, using previously published data, and by using Krenchel analysis to back-calculate the modulus of a single fibril. The value obtained (114 GPa) is higher than previously reported values for this parameter, but lower than estimates of the crystal modulus of cellulose-I (130–145 GPa). Reasons for these discrepancies are given in terms of the crystallinity and structural composition of the samples.

Elastic properties of mono- and polycrystalline hexagonal AlB2-like diborides of s, p and d metals from first-principles calculations

Abstract: We have performed accurate ab initio total energy calculations using the full-potential linearized augmented plane-wave (FP-LAPW) method with the generalized gradient approximation (GGA) for the exchange–correlation potential to systematically investigate elastic properties of 18 stable, metastable and hypothetical hexagonal (AlB2-like) metal diborides MB2, where M = Na, Be, Mg, Ca, Al, Sc, Y, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Ag and Au. For monocrystalline MB2, the optimized lattice parameters, independent elastic constants (Cij), bulk moduli (B) and shear moduli (G) are obtained and analyzed in comparison with the available theoretical and experimental data. For the first time, numerical estimates of a set of elastic parameters of the polycrystalline MB2 ceramics (in the framework of the Voigt–Reuss–Hill approximation), namely bulk and shear moduli, compressibility (β), Young's modulus (Y), Poisson's ratio (ν) and Lame coefficients (μ, λ), are performed.

Sign Change of Poisson's Ratio for Carbon Nanotube Sheets

Abstract: Most materials shrink laterally like a rubber band when stretched, so their Poisson's ratios are positive. Likewise, most materials contract in all directions when hydrostatically compressed and decrease density when stretched, so they have positive linear compressibilities. We found that the in-plane Poisson's ratio of carbon nanotube sheets (buckypaper) can be tuned from positive to negative by mixing single-walled and multiwalled nanotubes. Density-normalized sheet toughness, strength, and modulus were substantially increased by this mixing. A simple model predicts the sign and magnitude of Poisson's ratio for buckypaper from the relative ease of nanofiber bending and stretch, and explains why the Poisson's ratios of ordinary writing paper are positive and much larger. Theory also explains why the negative in-plane Poisson's ratio is associated with a large positive Poisson's ratio for the sheet thickness, and predicts that hydrostatic compression can produce biaxial sheet expansion. This tunability of Poisson's ratio can be exploited in the design of sheet-derived composites, artificial muscles, gaskets, and chemical and mechanical sensors.

Fabrication and characterization of TiO2–epoxy nanocomposite

Abstract: A systematic study has been conducted to investigate the matrix properties by introducing nanosize TiO2 (5–40 nm, 0.5–2% by weight) fillers into an epoxy resin. Ultrasonic mixing process, via sonic cavitations, was employed to disperse the particles into the resin system. The thermal, mechanical, morphology and the viscoelastic properties of the nanocomposite and the neat resin were measured with TGA, DMA, TEM and Instron. The nano-particles are dispersed evenly throughout the entire volume of the resin. The nanofiller infusion improves the thermal, mechanical and viscoelastic properties of the epoxy resin. The nanocomposite shows increase in storage modulus, glass transition temperature, tensile modulus, flexural modulus and short beam shear strength from neat epoxy resin. The mechanical performance and thermal stability of the epoxy nanocomposites are depending on with the dispersion state of the TiO2 in the epoxy matrix and are correlated with loading (0.0015–0.006% by volume). In addition, the nanocomposite shows enhanced flexural strength. Several reasons to explain these effects in terms of reinforcing mechanisms were discussed.

On the modulus of elasticity and strain capacity of Self-Compacting Concrete incorporating rubber aggregates

Abstract: Cement-based materials suffer from low tensile strength and poor strain capacity. They are brittle and highly sensitive to cracking, notably to shrinkage cracking, which is particularly detrimental for large surface areas. This paper focuses on the properties of a Self-Compacting Concrete (SCC) incorporating rubber aggregates, obtained by grinding end-of-life tyres, as a partial replacement for natural aggregates. Results show that the new cementitious material goes against some governing principles of mechanical behaviour of ordinary cement-based concrete. In particular, the modulus of elasticity of rubberized SCC is reduced and its variation with rubber aggregate content does not obey the conventional empirical relationship of modulus of elasticity with compressive strength. The strain capacity of SCC was quantified through flexural bending tests, which demonstrated that strain capacity increased when rubber aggregates were incorporated in concrete. This response is interpreted as a result of the ability of rubber aggregates to reduce the stress singularity at the first crack tips running into the rubber/cement–matrix interface, a mechanism slowing the cracking kinetics and delaying macrocrack localization. In such conditions, rubberized SCC is expected to be suitable when resistance to the cracking due to imposed deformation is a priority. This type of composite with low modulus of elasticity is also suitable for Controlled Modulus Columns (CMC) foundations, the ultimate solution for improving very soft soils subjected to settlement or stability problems caused by insufficient bearing capacity. Incidentally, the use of rubber aggregates in SCC provides an opportunity to recycle non-reusable end-of-life tyres.

The effect of fiber content on the mechanical and thermal expansion properties of biocomposites based on microfibrillated cellulose

Abstract: A new method to obtain composites of phenolic resin reinforced with microfibrillated cellulose with a wide fiber content was established and the mechanical properties were evaluated by tensile test A linear increase in Young’s modulus was observed at fiber contents up to 40 wt%, with a stabilizing tendency for higher fiber percentages These results were ratified by measurements of the coefficient of thermal expansion (CTE) relative to fiber content, which indicated a strong thermal expansion restriction rate below 60 wt% fiber content, indicating the effective reinforcement attained by the cellulose microfibrils The low CTE achieved of 10 ppm/K is one of the important properties of cellulose composites

The Effect of Strain Rate on the Mechanical Properties of Human Cortical Bone

Abstract: Bone mechanical properties are typically evaluated at relatively low strain rates. However, the strain rate related to traumatic failure is likely to be orders of magnitude higher and this higher strain rate is likely to affect the mechanical properties. Previous work reporting on the effect of strain rate on the mechanical properties of bone predominantly used nonhuman bone. In the work reported here, the effect of strain rate on the tensile and compressive properties of human bone was investigated. Human femoral cortical bone was tested longitudinally at strain rates ranging between 0.14-29.1 s(-1) in compression and 0.08-17 s(-1) in tension. Young's modulus generally increased, across this strain rate range, for both tension and compression. Strength and strain (at maximum load) increased slightly in compression and decreased (for strain rates beyond 1 s(-1)) in tension. Stress and strain at yield decreased (for strain rates beyond 1 s(-1)) for both tension and compression. In general, there seemed to be a relatively simple linear relationship between yield properties and strain rate, but the relationships between postyield properties and strain rate were more complicated and indicated that strain rate has a stronger effect on postyield deformation than on initiation of yielding. The behavior seen in compression is broadly in agreement with past literature, while the behavior observed in tension may be explained by a ductile to brittle transition of bone at moderate to high strain rates.

Strength, Modulus of Elasticity, and Brittleness Index of Rubberized Concrete

Abstract: This paper presents a study of rubberized concretes designed by replacing coarse aggregate in normal concrete with ground and crushed scrap tire rubber in various volume ratios. The objective of the study was to investigate the effect of rubber types and rubber content on strength and deformation properties. The compressive strength, static, and dynamic modulus of elasticity of rubberized concrete were tested and studied. The stress-strain hysteresis loops were obtained by loading, unloading, and reloading on specimens. Brittleness index values were calculated based on the hysteretic loops. The experiments revealed that strength and modulus elasticity of rubberized concrete decreased with the increasing amount of rubber content. Compressive strength and modulus of elasticity of crushed rubberized concrete were lower than that of ground rubberized concrete. An American Concrete Institute equation could reasonably predict modulus of elasticity of rubberized concrete. Brittleness index values of rubberized concrete were lower than that of normal concrete, which means that rubberized concrete had higher ductility performance than that of normal concrete.

Effect of capillary pressure and surface tension on the deformation of elastic surfaces by sessile liquid microdrops: an experimental investigation.

Abstract: Sessile liquid drops are predicted to deform an elastic surface onto which they are placed because of the combined action of the liquid surface tension at the periphery of the drop and the capillary pressure inside the drop. Here, we show for the first time the in situ experimental confirmation of the effect of capillary pressure on this deformation. We demonstrate micrometer-scale deformations made possible by using a low Young's modulus material as an elastic surface. The experimental profiles of the deformed surfaces fit well the theoretical predictions for surfaces with a Young's modulus between 25 and 340 kPa.

Correlation between hardness and elastic moduli of the ultraincompressible transition metal diborides RuB2, OsB2, and ReB2

Abstract: The ultraincompressible transition metal diborides RuB 2 , OsB 2 , and ReB 2 were synthesized by arc melting the elemental metals and boron under an argon atmosphere at ambient pressure The hardness and Young’s modulus were measured using nanoindentation with a Berkovich diamond indenter The bulk modulus and shear modulus were derived based on an isotropic model and then plotted as a function of hardness A strong correlation is observed between the hardness and shear modulus for these transition metal diborides © 2008 American Institute of Physics DOI: 101063/12946665

Surface stress effect on bending resonance of nanowires with different boundary conditions

Abstract: The influence of surface stress on the resonance frequencies of bending nanowires was studied by incorporating the generalized Young–Laplace equation into Euler–Bernoulli beam theory Theoretical solutions are presented for three different boundary conditions The overall Young’s modulus was used to study the surface stress influenced mechanical behavior of bending nanowires and a comparison was made for the overall Young’s modulus calculated from nanowires in resonance and static bending It was found that the overall Young’s modulus can be simply related to a nondimensional surface effect factor via empirical formulae

Polyethylene/boron nitride composites for space radiation shielding

Abstract: Composites made with boron might be absorbers of low energy neutrons, and could be used for structural materials for spacecraft. Polyethylene/boron nitride composites were fabricated using conventional polymer processing techniques, and were evaluated for mechanical and radiation shielding properties. The boron nitride powder surfaces were also functionalized to improve interfacial adhesion. Addition of neat boron nitride to an injection molding grade HDPE increased the tensile modulus from 588 to 735 MPa with 15 vol % filler. The bonding of a trifunctional alkoxysilane to the powder surface prior to processing increases the composite modulus to 856 MPa at the same loading. Scanning electron microscopy of fracture surfaces verified that the silane-treated powders had improved adhesion at the filler/polymer interface. Radiation shielding measurements of a 2 wt % boron nitride composite were improved over those of the neat polyethylene. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008

Polylactide-pine wood flour composites

Abstract: Biobased and biodegradable polylactide (PLA)-pine wood flour (PWF) composites were investigated as a means to reduce the overall material cost and tailor the material properties. The composites were prepared using a kinetic-mixer and an injection molding machine. The tensile modulus of the PLA-PWF composites increased with the PWF content whereas the toughness and strain-at-break decreased. The tensile strength remained the same irrespective of the PWF content (up to 40%). The storage modulus also increased with the PWF content. Additionally, composites containing PWF treated with silane showed higher storage modulus than those without the silane treatment. The area integration underneath the tan δ peaks decreased with increasing PWF, indicating that the PLA-PWF composites exhibited more elastic behavior with increasing PWF. The degree of crystallinity of the PLA-PWF composites increased significantly with the PWF content. Furthermore, the treatment of PWF with silane had a positive effect on its nucleating ability, as treated PLA-PWF composites showed higher crystallinity compared with their untreated counterparts. The morphology of the fracture surfaces were studied using a scanning electron microscope. Finally, a Halpin-Tsai analytical model to predict Young's modulus of PLA-PWF composites was presented to compare the theoretical results with that of experimental results. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers

Cement bonded composites – a mechanical review

Abstract: Over the last years promising cement bonded wood composites for structural purposes have evolved. Durability, toughness, high dimen-sional stability, resistance against environmental influences such as biodegradation or weathering but also availability of the raw material as well as economic factors are features which can make cement-bonded composites superior to conventionally bonded composites. This paper reviews the relationship of diverse parameters, including density and particle size on mechanical and physical properties of cement bonded composites, based on published sources from the last 60 years. For general and recent information about bonding mechanisms, compatibility and setting problems, determination and improvement of compatibility, the used raw materials as well as accelerators are discussed. The main part deals with failure mechanisms in connection with several production parameters. Furthermore, the influence of particle size and geometry, orientation of the particles, cement-wood ratio and the effect of accelerators and treatment of the particles on modulus of elasticity, modulus of rupture as well as thickness swelling are discussed.

Young's modulus of some SOFCs materials as a function of temperature

Abstract: Solid oxide fuel cells (SOFCs) have been under great consideration during this last decade and much effort has been dedicated to model their thermo-mechanical behavior, trying to take into account the different properties of the materials, such as their elasticity. In this paper, we report the elastic behavior of three classical materials used in SOFCs as a function of temperature: Yttria stabilized zirconia (YSZ), La0.8Sr0.2MnO3 (LSM) and Ni-YSZ. Both YSZ and LSM present unusual behaviors. The elastic modulus of YSZ first decreases slowly up to 150 °C, then dramatically up to 550 °C (certainly due to atomic motion) and finally increases probably because of an order–disorder transition (oxygen vacancies). The state of the art on zirconia was reviewed. For the LSM material, Young's modulus could not be determined below 600 °C. Above this temperature, samples with totally closed porosity present a continuously increasing modulus, while the other samples have a quite constant modulus.