Showing papers on "Tensile testing published in 2009"

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.

Analysis of the tensile behavior of a TWIP steel based on the texture and microstructure evolutions

Abstract: The texture and microstructure evolutions of a fine-grained TWIP steel subjected to tensile tests at room temperature were investigated in relation to the mechanical behavior. This steel combines both high ductility and strength owing to the TWIP effect. Also the steel exhibits a high strain hardening rate that evolves according to five stages, which are related to the microstructure and texture evolutions and characteristics. The formation of nano-twins in the initial stage of deformation leads to an increase in strain hardening rate. The development of the pronounced fiber in the tensile direction sustains mechanical twinning and maintains the strain hardening rate on a high level. The resulting microstructure exhibits several types of twin configurations and sub-boundaries with high misorientations due to intense activities of dislocation glide. The twin volume fraction was estimated to be 9% at the final stage of tensile deformation. The new orientations generated by mechanical twinning do not change considerably the final texture.

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.

The effect of intercritical heat treatment temperature on the tensile properties and work hardening behavior of ferrite–martensite dual phase steel sheets

Abstract: This paper aims to investigate tensile properties and work hardening behavior of dual phase (DP) steels A series of DP steels containing ferrite and martensite with different volume fractions of martensite ( V m ) were produced by intercritical heat treatment Microstructural investigations, hardness test and tensile test were carried out Hardness, yield strength, ultimate tensile strength, ductility and fracture energy were correlated to martensite volume fraction The experimental results showed that dual phase steels with an equal amount of ferrite and martensite have excellent mechanical properties in terms of tensile strength, ductility and fracture energy A further increase in V m was found to decrease tensile strengths and ductility The increasing and then decreasing trend in tensile strength is in contrast to the law of mixture These unusual behaviors are discussed and explained Work hardening behavior was analyzed in terms of Holloman analysis Results showed that in DP steels with less than 50% V m , the work hardening took place in one stage and the work hardening exponent increased with increasing V m By increasing the volume fraction of martensite ( V m > 50%) two work hardening stages were observed in the Hollomon analysis

Structure and properties of the nanocomposite films of chitosan reinforced with cellulose whiskers

Abstract: Bio-based nanocomposite films were successfully developed using cellulose whiskers as the reinforcing phase and chitosan as the matrix. Cellulose whiskers, with the lengths of 400 ± 92 nm and diameters of 24 ± 7.5 nm on average, were prepared by hydrolyzing cotton linter with sulfuric acid solution. The effects of whisker content on the structure, morphology and properties of the nanocomposite films were characterized by SEM, XRD, FTIR, UV-vis spectroscopy, DMA, TG, tensile testing, and swelling experiment. The results indicated that the nanocomposites exhibited good miscibility, and strong interactions occurred between the whiskers and the matrix. With increasing whisker content from 0 to 15–20 wt %, the tensile strength of the composite films in dry and wet states increased from 85 to 120 MPa and 9.9 to 17.3 MPa, respectively. Furthermore, the nanocomposite films displayed excellent thermal stability and water resistance with the incorporation of cellulose whiskers. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1069–1077, 2009

Self-Healing Capability of Fibre Reinforced Cementitious Composites

Abstract: In order to investigate the self-healing capability of fibre reinforced cementitious composites (FRCC), mechanical properties and surface morphology of crack in FRCC were studied. Three types of FRCC specimens containing (1) polyethylene (PE) fibre, (2) steel cord (SC) fibre, and (3) hybrid fibres composite (both of PE and SC) were prepared. These specimens, in which cracks were introduced by tension test, were retained in water for 28 days. The self-healing capability of the specimens was investigated by means of microscope observation, water permeability test, tension test and backscattered electron image analysis. It was found that many very fine fibres of PE were bridging over the crack and crystallization products became easy to be attached to a large number of PE fibres. As a result, water permeability coefficient decreased and tensile strength was improved significantly. Therefore amount of the PE fibre per volume was indicated to have a great influence on self-healing. Furthermore, by means of backscattered electron image analysis, it was also shown that the difference of hydration degree in each FRCC has only little influence on the self-healing capability in case of the employed test series.

Effects of strain rate, mixing ratio, and stress-strain definition on the mechanical behavior of the polydimethylsiloxane (PDMS) material as related to its biological applications.

Abstract: Tensile tests on Polydimethylsiloxane (PDMS) materials were conducted to illustrate the effects of mixing ratio, definition of the stress-strain curve, and the strain rate on the elastic modulus and stress-strain curve. PDMS specimens were prepared according to the ASTM standards for elastic materials. Our results indicate that the physiological elastic modulus depends strongly on the definition of the stress-strain curve, mixing ratio, and the strain rate. For various mixing ratios and strain rates, true stress-strain definition results in higher stress and elastic modulus compared with engineering stress-strain and true stress-engineering strain definitions. The elastic modulus increases as the mixing ratio increases up-to 9:1 ratio after which the elastic modulus begins to decrease even as the mixing ratio continues to increase. The results presented in this study will be helpful to assist the design of in vitro experiments to mimic blood flow in arteries and to understand the complex interaction between blood flow and the walls of arteries using PDMS elastomer.

Tensile Testing of Cellulose Based Natural Fibers for Structural Composite Applications

Abstract: A series of tensile tests were conducted on a Lloyd LRX tensile testing machine for numerous natural fibers deemed potential candidates for development in composite applications. The tensile tests were conducted on the fibers jute, kenaf, flax, abaca, sisal, hemp, and coir for samples exposed to moisture conditions of (1) room temperature and humidity, (2) 65% moisture content, (3) 90% moisture content, and (4) soaked fiber. These seven fibers were then tested for the four conditions and the mechanical properties of tensile strength, tensile strain to failure, and Young's modulus were calculated for the results. These results were then compared and verified with those from the literature, with some of the fibers showing distinctly promising potential. Additionally, a study on the effect of alkalization using 3% NaOH solution was carried out on flax, kenaf, abaca, and sisal to observe impact that this common fiber pre-treatment process has on fiber mechanical properties. The result of the investigation indicated that over treatment of natural fibers using NaOH could have a negative effect on the base fiber properties. It is consequently apparent that a treatment time of less than 10 min is sufficient to remove hemicelluloses and to give the optimum effect.

Micro- and nanoscale tensile testing of materials

Abstract: This article reviews concepts and techniques for performing instrumented tensile testing of materials at small dimensions. State-of-the-art methods to probe tensile behavior of micro- and nanoscaled materials span many orders of magnitudes of force and displacement, often requiring a custom solution for each new material discovery. We discuss the experimental opportunities, challenges, and pitfalls in concert with the scientific insights revealed from tensile investigations at length scales where conventional wisdom is challenged on how materials deform.

Influence of specimen dimensions and strain measurement methods on tensile stress–strain curves

Abstract: Miniature tensile specimens, having various sizes and geometries, are often used to measure the mechanical properties of bulk nanostructured materials. However, these samples are generally too small for use with conventional extensometers so that the strains are usually calculated from the crosshead displacements. This study uses experimental results and finite element modeling (FEM) to critically evaluate the influence of the specimen dimensions and strain measurement methods on the tensile curves obtained from miniature specimens. Using coarse-grained Cu as a model material, the results demonstrate that the values of strain obtained from the crosshead displacement are critically influenced by the specimen dimensions such that the uniform elongation and the post-necking elongation both increase with decreasing gauge length and increasing specimen thickness. The results provide guidance on the optimum procedures for the tensile testing of miniature specimens of both coarse-grained and nanostructured materials.

Deformation behavior of TRIP and DP steels in tension at different temperatures over a wide range of strain rates

Abstract: The dependence of the mechanical behavior of DP and TRIP steels on temperature and strain rate is still not completely understood. Therefore, the mechanical properties of a DP 600 and a TRIP 700 steel were characterized by tensile tests over a temperature range of −100 ≤ T ≤ 235 °C at strain rates ranging from 10−3 to 1250 s−1. The results show that the strain hardening behavior of the TRIP steel depends strongly on the initial test temperature, to which the DP steel seems to be relatively insensitive in the studied temperature interval. On the other hand, the tensile strength of the TRIP steel appears to be much less sensitive to strain rate than that of the DP steel. This is explained by the effects of deformation induced heating on the martensite transformation based hardening of the TRIP steel, partially offsetting the direct effects of strain rate.

Interfacial shearing strength and reinforcing mechanisms of an epoxy composite reinforced using a carbon nanotube/carbon fiber hybrid

Abstract: The performance of a composite material system depends critically on the interfacial characteristics of the reinforcement and the matrix material. In this study, the interfacial shearing strength (IFSS) of a composite with an epoxy matrix and a novel carbon nanotube/carbon fiber (CNT/CF) multi-scale reinforcement was determined by single fiber-microdroplet tensile test, and the interfacial reinforcing mechanisms of the composite were discussed. Results show that the IFSS of the epoxy composite reinforced by CNT/CF is as high as 106.55 MPa, which is 150% higher than that of the as-received T300 fiber composite. And the main interfacial reinforcing mechanisms of this novel composite could be interpreted as chemical bonding, Van der Waals binding, mechanical interlocking, and surface wetting.