Showing papers on "Ultimate tensile strength published in 2009"

Enhanced mechanical properties of nanocomposites at low graphene content.

Abstract: In this study, the mechanical properties of epoxy nanocomposites with graphene platelets, single-walled carbon nanotubes, and multi-walled carbon nanotube additives were compared at a nanofiller weight fraction of 0.1 ± 0.002%. The mechanical properties measured were the Young’s modulus, ultimate tensile strength, fracture toughness, fracture energy, and the material’s resistance to fatigue crack propagation. The results indicate that graphene platelets significantly out-perform carbon nanotube additives. The Young’s modulus of the graphene nanocomposite was ∼31% greater than the pristine epoxy as compared to ∼3% increase for single-walled carbon nanotubes. The tensile strength of the baseline epoxy was enhanced by ∼40% with graphene platelets compared to ∼14% improvement for multi-walled carbon nanotubes. The mode I fracture toughness of the nanocomposite with graphene platelets showed ∼53% increase over the epoxy compared to ∼20% improvement for multi-walled carbon nanotubes. The fatigue resistance resu...

Strength and barrier properties of MFC films

Abstract: The preparation of microfibrillar cellulose (MFC) films by filtration on a polyamide filter cloth, in a dynamic sheet former and as a surface layer on base paper is described. Experimental evidence of the high tensile strength, density and elongation of films formed by MFC is given. Typically, a MFC film with basis weight 35 g/m2 had tensile index 146 ± 18 Nm/g and elongation 8.6 ± 1.6%. The E modulus (17.5 ± 1.0 GPa) of a film composed of randomly oriented fibrils was comparable to values for cellulose fibres with a fibril angle of 50°. The strength of the films formed in the dynamic sheet former was comparable to the strength of the MFC films prepared by filtration. The use of MFC as surface layer (0–8% of total basis weight) on base paper increased the strength of the paper sheets significantly and reduced their air permeability dramatically. FEG-SEM images indicated that the MFC layer reduced sheet porosity, i.e. the dense structure formed by the fibrils resulted in superior barrier properties. Oxygen transmission rates (OTR) as low as 17 ml m−2 day−1 were obtained for films prepared from pure MFC. This result fulfils the requirements for oxygen transmission rate in modified atmosphere packaging.

Natural and man-made cellulose fibre-reinforced poly(lactic acid) (PLA) composites: An overview about mechanical characteristics and application areas

Abstract: The paper describes the production and the mechanical characteristics of composites made completely of renewable raw materials. Composites of different kinds of natural fibres like cotton, hemp, kenaf and man-made cellulose fibres (Lyocell) with various characteristics were processed with a fibre mass proportion of 40% and poly(lactic acid) (PLA) by compression moulding. Additionally, composites were made of fibre mixtures (hemp/kenaf, hemp/Lyocell). The composites were tested for tensile strength, elongation at break, Young’s modulus and Charpy impact strength. Their characteristics varied markedly depending on the characteristics of the raw fibres and fibre bundles and fibre mixtures used. While kenaf and hemp/PLA composites showed very high tensile strength and Young’s modulus values, cotton/PLA showed good impact characteristics. Lyocell/PLA composites combined both, high tensile strength and Young’s modulus with high impact strength. Thus, the composites could be applied in various fields, each meeting different requirements.

Ultrahigh strength single crystalline nanowhiskers grown by physical vapor deposition.

Abstract: The strength of metal crystals is reduced below the theoretical value by the presence of dislocations or by flaws that allow easy nucleation of dislocations. A straightforward method to minimize the number of defects and flaws and to presumably increase its strength is to increase the crystal quality or to reduce the crystal size. Here, we describe the successful fabrication of high aspect ratio nanowhiskers from a variety of face-centered cubic metals using a high temperature molecular beam epitaxy method. The presence of atomically smooth, faceted surfaces and absence of dislocations is confirmed using transmission electron microscopy investigations. Tensile tests performed in situ in a focused-ion beam scanning electron microscope on Cu nanowhiskers reveal strengths close to the theoretical upper limit and confirm that the properties of nanomaterials can be engineered by controlling defect and flaw densities.

Fabrication and properties of dispersed carbon nanotube–aluminum composites

Abstract: Powder metallurgy techniques have emerged as promising routes for the fabrication of carbon nanotube (CNT) reinforced metal matrix composites. In this work, planetary ball milling was used to disperse 2 wt% MWCNT in aluminum (Al) powder. Despite the success of ball milling in dispersing CNTs in Al powder, it is often accompanied with considerable strain hardening of the Al powder, which may have implications on the final properties of the composite. Both un-annealed and annealed Al–2 wt% CNT composites were investigated. It was found that, ball-milled and extruded (un-annealed) samples of Al–2 wt% CNT demonstrated high notch-sensitivity and consistently fractured outside the gauge length during tensile testing. In contrast, extruded samples annealed at 400 and at 500 °C for 10 h prior to testing, exhibited more ductile behavior and no notch sensitivity. Under the present investigated processing conditions, ball milling for 3 h followed by hot extrusion and annealing at 500 °C resulted in enhancements of around 21% in tensile strength compared with pure aluminum with the same process history. The ball-milling conditions used were found to result in the creation of a nanostructure in all samples produced, as shown by XRD and TEM analysis. Such nanostructure was retained after prolonged exposures to temperatures up to 500 °C. The tensile testing fracture surfaces showed uniform dispersion and alignment of the CNTs in the aluminum matrix but also showed CNTs acting as nucleation sites for void formation during tensile testing. This has contributed to the observation of CNT pull-out due to the poor bond between the CNTs and the matrix.

Studies of Standard Heat Treatment Effects on Microstructure and Mechanical Properties of Laser Net Shape Manufactured INCONEL 718

Abstract: Laser net shape manufacturing (LNSM) is a laser cladding/deposition based technology, which can fabricate and repair near-net-shape high-performance components directly from metal powders. Characterizing mechanical properties of the laser net shape manufactured components is prerequisite to the applications of LNSM in aircraft engine industrial productions. Nickel-based superalloys such as INCONEL 718 are the most commonly used metal materials in aircraft engine high-performance components. In this study, the laser deposition process is optimized through a set of designed experiments to reduce the porosity to less than 0.03 pct. It is found that the use of plasma rotating electrode processed (PREP) powder and a high energy input level greater than 80 J/mm are necessary conditions to minimize the porosity. Material microstructure and tensile properties of laser-deposited INCONEL 718 are studied and compared under heat treatment conditions of as deposited, direct aged, solution treatment and aging (STA), and full homogenization followed by STA. Tensile test results showed that the direct age heat treatment produces the highest tensile strength equivalent to the wrought material, which is followed by the STA-treated and the homogenization-treated tensile strengths, while the ductility exhibits the reverse trend. Finally, failure modes of the tensile specimens were analyzed with fractography.

Elastic and frictional properties of graphene

Abstract: We descnbe studies of the elastic properties and frictional characteristics of graphene samples of varying thickness using an atomic force microscope. For tensile testing, graphene is suspended over micron-sized circular holes and indented by atomic force microscope (AFM) tips. Fitting of the force-displacement curves yields the prestress and elastic stiffness, while comparison of the breaking force to simulation gives the ultimate strength, which is the highest measured for any material. Experiments on samples with 1-3 atomic layers yield similar values for the intrinsic stiffness and strength of a single sheet, but also reveal differences in mechanical behavior with thickness. The frictional force between an AFM tip and graphene decreases with thickness for samples from 1 to 4 layers, and does not depend on the presence of a substrate. High-resolution friction force imaging in stick-slip mode shows the same trend, and allows direct imaging of the crystal lattice.

Stacked dielectric elastomer actuator for tensile force transmission

Abstract: This paper presents a novel approach for active structures driven by soft dielectric electro-active polymers (EAPs), which can perform contractive displacements at external tensile load. The active structure is composed of an array of equal segments, where the dielectric films are arranged in a pile-up configuration. The proposed active structure has the capability of exhibiting uniaxial contractive deformations, while being exposed to external tensile forces. The serial arrangement of active segments has one contracting degree of freedom in the thickness direction of the dielectric EAP film layers. Due to the envisaged tension force transmission capability, special attention is paid to the electrode design which is of paramount importance with regard to functionality of the actuator. A compliant electrode system with anisotropic deformation properties is presented based on nano scale carbon powder. In experiments, the free deformation as well as the contractive motion under external tensile loading of several actuator configurations with different setups is characterized. These involve the study of various sizes and numbers of stacked film layers as well as different electrode designs.

An experimental and numerical investigation into the damage mechanisms in notched composites

Abstract: Investigations of the effect of size on the tensile strength of composite laminates containing circular holes show that there is a large difference both in failure stress and mechanism due to changes in test configuration. This is particularly true of the ply and laminate thickness, and hole diameter. Interrupted tests have been performed on open hole tensile specimens at different load levels to determine the progressive damage development, evaluated through non-destructive testing (X-ray and C-scanning). The tests were also analysed using a novel Finite Element Modelling technique. This was able to accurately predict the wide range of ultimate strengths measured with variation in test parameters, principally through incorporation of the sub-critical damage in the analysis. A significant damage mechanism was seen to be delamination at the hole edge which generally occurred at a lower stress for a smaller hole diameter to ply block thickness ratio. Delaminations allowed damage to join up through the thickness of the laminate and propagate. In ply-level scaled specimens, the delamination propagation was the ultimate failure mode of most of the specimens. In sub-laminate level scaled specimens, localised damage relieved stress in the 0° fibres at the hole edge, delaying the onset of fibre failure. Less damage was seen for larger holes, thus leading to a decreasing failure stress with increasing hole diameter.

Mechanical properties of PLA composites with man-made cellulose and abaca fibres

Abstract: PLA biocomposites with abaca and man-made cellulose fibres were processed by using combined moulding technology: two-step extrusion coating process and consecutively injection moulding. By adding 30 wt% of man-made cellulose, the Charpy impact strength at ambient temperature increased by factor 3.60, compared to unreinforced PLA. Tensile strength rose by factor 1.45 and stiffness by approx. 1.75. Reinforcing with abaca fibres (30 wt%) enhanced both E-Modulus and tensile strength by factor 2.40 and 1.20, respectively. The Charpy A-notch impact resistance of PLA/abaca could be improved by factor 2.4. SEM photographs show fibre pull-outs from the polymer matrix. The fibre orientation was analysed via optical microscopy. The after-process fibre length was significantly affected already during compounding process.

Mechanical Properties of Vapor−Liquid−Solid Synthesized Silicon Nanowires

Abstract: The Young’s modulus and fracture strength of silicon nanowires with diameters between 15 and 60 nm and lengths between 1.5 and 4.3 μm were measured. The nanowires, grown by the vapor−liquid−solid process, were subjected to tensile tests in situ inside a scanning electron microscope. The Young’s modulus decreased while the fracture strength increased up to 12.2 GPa, as the nanowire diameter decreased. The fracture strength also increased with the decrease of the side surface area; the increase rate for the chemically synthesized silicon nanowires was found to be much higher than that for the microfabricated silicon thin films. Repeated loading and unloading during tensile tests demonstrated that the nanowires are linear elastic until fracture without appreciable plasticity.

Effect of fiber surface modification on the mechanical and water absorption characteristics of sisal/polyester composites fabricated by resin transfer molding

Abstract: Sisal fibers were subjected to various chemical and physical modifications such as mercerization, heating at 100 °C, permanganate treatment, benzoylation and silanization to improve the interfacial bonding with matrix. Composites were prepared by these fibers as reinforcement, using resin transfer molding (RTM). The mechanical properties such as tensile, flexural and impact strength were examined. Mercerized fiber-reinforced composites showed 36% of increase in tensile strength and 53% in Young’s modulus while the permanganate treated fiber-reinforced composites performed 25% increase in flexural strength. However, in the case of impact strength, the treatment has been found to cause a reduction. The water absorption study of these composites at different temperature revealed that it is less for the treated fiber-reinforced composites at all temperatures compared to the untreated one. SEM studies have been used to complement the results emanated from the evaluation of mechanical properties.

Enhanced properties of Mg-based nano-composites reinforced with Al2O3 nano-particles

Abstract: In this study, 0.5, 1 and 2 wt.% of alumina nano-particles were added to pure Mg and AZ31 magnesium alloy via a stir-casting method. A uniform distribution of the Al2O3 nano-particles with an average diameter of 100 nm, refined the grain structure of the cast materials and decreased the coefficient of thermal expansion (CTE), thus improving the dimensional stability of both pure magnesium and AZ31 alloy. The addition of 2 wt.% nano-Al2O3 particles showed great potential in the reduction of CTE from 27.9 to 25.9 × 10−6 K−1 in pure Mg, and from 26.4 to 25.2 × 10−6 K−1 in AZ31. Some of the cast samples were hot rolled and annealed to investigate the pinning effect of nano-particles on the recrystallization and subsequent mechanical property behavior. Characterization of mechanical properties revealed that the presence of nano-particles significantly increased yield stress and tensile strength but decreased the ductility of both pure magnesium and AZ31. The yield stress and tensile strength both increased by 40 MPa in the Mg–2Al2O3 nano-composite, whereas this improvement was about 65 MPa for AZ31–2Al2O3. The yield strength improvement was mostly due to the CTE mismatch between the matrix and the particles, and to a lesser extent to the Orowan and Hall-Petch strengthening mechanisms. The contribution of each of these mechanisms was used in a modified shear lag model to predict the total composite-strengthening achieved. Examination of fracture surfaces showed that the relatively ductile fracture of the monolithic materials changed to a more brittle mode due to the presence of nano-Al2O3 particles.

Effect of glass fiber hybridization on properties of sisal fiber–polypropylene composites

Abstract: Natural fiber reinforced polymer composites became more attractive due to their light weight, high specific strength, and environmental concern. However, some limitations such as low modulus, poor moisture resistance were reported. This study aimed to investigate the effect of glass fiber hybridization on the physical properties of sisal–polypropylene composites. Polypropylene grafted with maleic anhydride (PP-g-MA) was used as a compatibilizer to enhance the compatibility between the fibers and polypropylene. Incorporating glass fiber into the sisal–polypropylene composites enhanced tensile, flexural, and impact strength without having significant effect on tensile and flexural moduli. In addition, adding glass fiber improved thermal properties and water resistance of the composites.

Effect of moisture absorption on the mechanical properties of randomly oriented natural fibers/polyester hybrid composite

Abstract: In this work, the variation of mechanical properties such as tensile, flexural, and impact strengths of roselle and sisal fibers hybrid polyester composite at dry and wet conditions were studied. The composites of roselle/sisal polyester-based hybrid composites with different weight% of fibers were prepared. Roselle and sisal fibers at a ratio of 1:1 had been incorporated in unsaturated polyester resin at various fiber lengths. When the fiber content and length of the roselle and sisal fibers were increased, the tensile and flexural strength of the composite increased. When the samples were subjected to moisture environment, decrease in tensile and flexural strength was observed. The maximum percentage of strength reductions in tensile and flexural strength were observed for the composites having the fiber length of 150 mm and 30 wt% fiber content. For impact strength, it was with the composites of 20 wt% and 150 mm at wet conditions compared to dry conditions. The percentage of strength reductions increased with fiber content and length in wet conditions. A scatter in the impact strength values was identified on both the conditions. The moisture absorption characteristics of the natural fibers are very important to produce the natural fiber hybrid composite materials with the positive hybrid effect. The experimental results are compared with theoretical and empirical or statistical results and found to be in good agreement.

Mechanical properties of poly(butylene succinate) (PBS) biocomposites reinforced with surface modified jute fibre

Abstract: A biocomposite was originally fabricated with biodegradable polymer PBS and jute fibre, and the effects of fibre surface modification on characteristics of jute fibre and mechanical properties of the biocomposite were evaluated in this paper. The experimental results show that surface modification can remove surface impurities and reduce diameter of jute fibres. Regarding the mechanical properties of biocomposites, it is observed that the biocomposites with jute fibres treated by 2% NaOH, 2 + 5% NaOH or coupling agent, respectively, an optimum in mechanical properties can obtain at fibre content of 20 wt.%, which exhibit an obvious enhancement in mechanical strength and modulus compared to the ones with untreated jute fibre. Furthermore, surface modification also exhibits less effects on flexural properties compared to tensile properties and more on flexural or tensile modulus than on the strength.

Surface Conditioning Influences Zirconia Ceramic Bonding

Abstract: Air-abrasion seems to be mandatory for durable resin bonding to zirconia ceramic. Air-abrasion might compromise the ceramic strength by creating surface defects. Therefore, omitting air-abrasion or using reduced air-pressure seems desirable. We tested the null hypotheses that omitting air-abrasion or using reduced air-pressure does not affect zirconia ceramic bonding independent of using primers. Three mechanical surface conditions (polished, air-abraded at 0.05 or at 0.25 MPa) and 4 priming conditions were tested. After different surface conditioning, zirconia ceramic specimens were bonded, and tensile bond strengths were evaluated after water storage for 3 days or for 150 days with additional 37,500 thermal cyclings for artificial aging. Omitting air-abrasion resulted in debonding during artificial aging independent of using primers. The combination of air-abrasion and priming improved long-term resin bonding to zirconia ceramic significantly. With low-pressure air-abrasion, surface roughness was reduced without affecting long-term bond strength, provided that adequate adhesive primers were applied.