Showing papers on "Tensile testing published in 2001"

Improving the mechanical properties of magnesium and a magnesium alloy through severe plastic deformation

Abstract: Pure Mg and Mg alloys generally exhibit only limited ductilities at ambient temperatures. Experiments were conducted to evaluate the potential for improving the mechanical properties of pure Mg and an Mg–0.9% Al alloy at room temperature by subjecting these materials to severe plastic deformation through the procedure of equal-channel angular pressing (ECAP). It is shown that ECAP may be applied successfully to these materials at elevated temperatures and this leads to grain refinement due to the occurrence of recrystallization during the pressing process and to significant improvements in the strength and ductility of these materials. Since these improvements are apparent after only a single pass through the ECAP die, it is concluded that the introduction of ECAP provides a simple and effective procedure for improving the ambient temperature mechanical properties of materials, such as hcp metals, where the measured ductilities are generally limited.

Mechanical behavior and microstructure of a thermally stable bulk nanostructured Al alloy

Abstract: A commercial aluminum alloy, 5083, was processed using a cryomilling synthesis approach to produce powders with a nanostructured grain size. The powders were subsequently degassed, hot isostatically pressed, and extruded. The grain size at each processing step was measured utilizing both X-ray diffraction and transmission electron microscopy (TEM). The mechanical properties of the n-5083 extruded material were determined utilizing ASTM E8-93, Standard Test Methods for Tension Testing of Metallic Materials. This processing technique was found to produce a thermally stable nanostructured aluminum alloy which maintained an average grain size of 30 to 35 nm through several processing steps up to 0.61 T mp . The thermal stability was attributed to Zener pinning of the grain boundaries by AIN and Al2O3 particles and solute drag of numerous atomic species. The nanostructured 5083 was found to have a 30 pct increase in yield strength and ultimate strength over the strongest commercially available form of 5083, with no corresponding decrease in elongation. The enhanced ductility is attributed to the presence of a few large, single-crystal aluminum grains acting as crack-blunting objects.

Distribution of tensile property and microstructure in friction stir weld of 6063 aluminum

Abstract: Dominant microstructural factors governing the global tensile properties of a friction-stir-welded joint of 6063 aluminum were examined by estimating distribution of local tensile properties corresponding to local microstructure and hardness. Yield and ultimate tensile strengths of the as-welded weld were significantly lower than those of the base material. Postweld aging and postweld solution heat-treatment and aging (SHTA) restored the strengths of the weld to the levels of the base material. Elongation was found to increase with increasing strength. Hardness tests showed that the as-welded weld was soft around the weld center and that the aged weld and the SHTA weld had relatively homogeneous distributions of high hardness. Hardness profiles of the welds were explained by precipitate distributions and precipitation sequences during the postweld heat treatments. The strengths of the welds were related to each minimum hardness value. In a weld having a heterogeneous hardness profile, the fracture occurred in the region with minimum hardness. When a weld had a homogeneous hardness profile, its fracture site depended on both crystallographic-orientation distribution of the matrix grains and strain tensor of the imposed deformation, i.e., it fractured in the region with a minimum average Taylor factor.

Fracture properties of bamboo

Abstract: Bamboo is a typical natural composite material, which is longitudinally reinforced by strong fibers. The fibers are distributed densely in the outer surface region, and sparsely in the inner surface region, and their volume fraction changes with respect to radius. The structure of bamboo has been characterized by tensile tests and its mechanical properties have been related to its structure. This paper presents the fracture toughness of bamboo culms and nodes. A notch is inserted into the culm and node specimens using a razor blade with a thickness of 0.4 mm. Tensile tests are carried out to evaluate fracture toughness. The average value obtained was 56.8 MPa m1/2, which is higher than that of Al-alloy. It was concluded that the fracture toughness of the bamboo culm depends on the volume fraction of fibers.

A high-strain-rate superplastic ceramic

Abstract: High-strain-rate superplasticity describes the ability of a material to sustain large plastic deformation in tension at high strain rates of the order of 10-2 to 10-1 s-1 and is of great technological interest for the shape-forming of engineering materials. High-strain-rate superplasticity has been observed in aluminium-based and magnesium-based alloys. But for ceramic materials, superplastic deformation has been restricted to low strain rates of the order of 10-5 to 10-4 s-1 for most oxides and nitrides with the presence of intergranular cavities leading to premature failure. Here we show that a composite ceramic material consisting of tetragonal zirconium oxide, magnesium aluminate spinel and alpha-alumina phases exhibits superplasticity at strain rates up to 1 s-1. The composite also exhibits a large tensile elongation, exceeding 1,050 per cent for a strain rate of 0.4 s-1. The tensile flow behaviour and deformed microstructure of the material indicate that superplasticity is due to a combination of limited grain growth in the constitutive phases and the intervention of dislocation-induced plasticity in the zirconium oxide phase. We suggest that the present results hold promise for the application of shape-forming technologies to ceramic materials.

Derivation of plastic stress–strain relationship from ball indentations: Examination of strain definition and pileup effect

Abstract: The ball indentation technique has the potential to be an excellent substitute for a standard tensile test, especially in the case of small specimens or property-gradient materials such as welds. In our study, the true stress–true strain relationships of steels with different work-hardening exponents (0.1–0.3) were derived from ball indentations. Four kinds of strain definitions in indentation were attempted: 0.2sinγ, 0.4hc/a, ln[2/(1 + cosγ)], and 0.1tanγ. Here, γ is the contact angle between the indenter and the specimen, hc is the contact depth, and a is the contact radius. Through comparison with the standard data measured by uniaxial tensile testing, the best strain definition was determined to be 0.1tanγ. This new definition of strain, in which tanγ means the shear strain at contact edge, reflected effectively the work-hardening characteristics. In addition, the effects of pileup or sink-in were considered in determining the real contact between the indenter and the specimen from the indentation load–depth curve. The work-hardening exponent was found to be a main factor affecting the pileup/sink-in phenomena of various steels. These phenomena influenced markedly the absolute values of strain and stress in indentation by making the simple traditional relationship Pm/σR ≈ ≈ 3 valid for the fully plastic regime.

Fatigue in thin films: lifetime and damage formation

Abstract: Two new techniques, developed for studying the fatigue behavior of thin metal films on substrates, are presented: The first technique involves deposition of Cu films onto elastic polyimide substrates. During cyclic tensile testing of the film–substrate composite, the film is subjected to tension–compression cycles, since it is plastically deformed, while the substrate undergoes only elastic deformation. Using this technique, it was found that, for 3 μm thick Cu films, the number of cycles to failure follows the phenomenological Coffin–Manson relationship. For the other technique, thin films, here Ag films with thicknesses ranging from 0.2 to 1.5 μm, are deposited onto micromachined SiO2 cantilever beams. The beams are then deflected with a frequency of 45 Hz using a nanoindentation system. A detailed investigation of the damage formation in both fatigued Cu and Ag films revealed surface roughening prior to failure. Extrusions and cracks are formed inside large grains and between small grains, respectively.

Mechanical properties and microstructure of gas tungsten arc welded magnesium AZ91D plates

Abstract: Gas tungsten arc (GTA) welds were performed on 4-mm thick plates, machined from as-cast magnesium AZ91D ingots. The microstructure and defect formation was investigated by optical and scanning electron microscopy. Mechanical properties were determined by standard tensile tests on small-scale specimens. A wide heat affected zone (HAZ) (>3 mm) was created adjacent to the fusion line that consisted of two regions: (1) a partially melted zone (PMZ), created near the fusion line; and (2) a wide region, which was heat affected without melting. It was found that, after resolidification, a continuous Al12Mg17 phase existed along the PMZ grain boundaries that markedly reduced the joint strength to below that of fully annealed Mg-AZ91D alloys. In contrast, specimens machined from welded metal exhibited improved strength and ductility that resulted from the microstructural refinement caused by the rapid cooling during resolidification of the fusion zone. When the creation of a large PMZ was prevented, as in electron beam welding, the joint strength was comparable to that of the base metal.

Raman scattering test of single-wall carbon nanotube composites

Abstract: Raman spectroscopy is used to infer elastic properties of single-wall carbon nanotubes (SWNTs) in composites. This letter presents strain-induced frequency shift of tangential Raman active modes of SWNTs embedded in epoxy resin subjected to bending. Epoxy curing and sample extension in the tensile strength test are found to create residual strains on the SWNT ropes. We demonstrate that specimen compression in combination with the Raman microprobe technique provides a means for determining of these strains and hence load transfer effectiveness.

Overview of fatigue properties of fine grain 5056 Al-Mg alloy processed by equal-channel angular pressing

Abstract: The fatigue performance of the fine-grain 5056 Al-Mg alloy processed by severe plastic deformation through equal-channel angular pressing (ECAP) is assessed in both stress- and plastic strain-controlled experiments. Compared to its conventional counterpart, the ECAP material exhibits a high tensile and low-cyclic fatigue strength under constant stress amplitude. However, its fatigue life under strain-controlled conditions is notably shorter than that of the O-temper specimens. Despite severe pre-straining of the specimen during ECA-pressing, cyclic softening was found to be rather small. It is shown that the mechanical characteristics obtained after ECAP can be significantly improved during short time annealing at moderate temperatures (150°C, 15 min) after fabrication. Such heat treatment is supposed to recover partially the grain boundary region, which has been most heavily distorted during processing. Mechanisms of fatigue in ECA-processed materials are discussed within a framework of a simple one-parametric model of dislocation kinetics.

Production of aluminium matrix composite components using conventional PM technology

Abstract: The machining difficulties and processing costs related to particle reinforced aluminium matrix composites have limited the application range of these advanced materials. The present investigation deals with the production of net shape SiC particle reinforced AA6061 aluminium alloy components by a conventional powder metallurgical processing technique, involving the uniaxial cold pressing and sintering of composite powders. Three aluminium alloy powder sizes were combined with three SiC particle sizes in various volume fractions of up to 20% SiC, and processed to produce net shape tensile test samples. The tensile test results and the morphological analysis of the fracture surfaces show that this process may be used to produce well sintered, small-to-medium sized net shape aluminium composite components exhibiting ultimate tensile strength (UTS) of up to 324 MPa. The successful production of these aluminium powder based components is affected strongly by the aluminium powder size, SiC volume fraction and the SiC particle size. It is also linked to the penetration of the aluminium powder oxide layer by the hard SiC particles, which occurs during cold compaction. Based on the present investigations, a number of processing improvements are suggested, which should further advance the usefulness of this technically uncomplicated net shape processing method for the production of aluminium matrix composites.

Tensile-mode fatigue testing of silicon films as structural materials for MEMS

Abstract: A new technique for the fatigue testing of thin-films under an approximately uniaxial tensile condition is presented. A perfect grip on both ends of the specimen was achieved by using a silicon device which integrated the specimen and loading system on the same chip. Low-cycle fatigue testing was carried out by controlling the strain applied to the specimen. Test materials were single-crystal-silicon specimens 50 μm long, 50 μm wide, and 5 μm thick with axes aligned in three different orientations. Differences between the number of cycles to failure were observed as differences in the amplitudes of the applied strains.

Microscale materials testing using MEMS actuators

Abstract: Small size scale and high resolutions in force and displacement measurements make MEMS actuators appropriate for micromechanical testing. In this paper, for the first time, we present methodologies for uniaxial tensile and cantilever bending testing of both micrometer- and submicrometer-scale freestanding specimens using MEMS actuators. We also introduce dry fabrication processes for the specimens. The methodologies allow freestanding single or multilayered thin-film specimens to be fabricated separately from the MEMS actuators. For the uniaxial tension test, tensile forces are applied by lateral comb drive actuators capable of generating a total load of 383 /spl mu/N at 40 V with resolutions on the order of 3 nN. A similar actuator is used in the bending test, with load resolution of 58 nN and spring constant of 0.78 N/m. The tensile testing methodology is demonstrated with the testing of a 110-nm-thick freestanding aluminum specimen. The cantilever bending experiment is performed on a 100-nm-thick aluminum specimen. The experimental setups can be mounted in a SEM (and also in a TEM after modifications for tensile testing) for in situ observation of materials behavior under different environmental conditions. Remarkable strengthening is observed in all the specimens tested compared to their bulk counterparts in both tensile and bending experiments. Experimental results highlight the potential of MEMS actuators as a new tool for materials research.

Hybrid effects on tensile properties of hybrid short-glass-fiber-and short-carbon-fiber-reinforced polypropylene composites

Abstract: Hybrid composites of polypropylene reinforced with short glass fibers and short carbon fibers were prepared using extrusion compounding and injection molding techniques The tensile properties of these composites were investigated taking into account the effect of the hybridization by these two types of short fibers It was noted that the tensile strength and modulus of the hybrid composites increase while the failure strain of the hybrid composites decreases with increasing the relative carbon fiber volume fraction in the mixture The hybrid effects for the tensile strength and modulus were studied by the rule of hybrid mixtures (RoHM) using the tensile strength and modulus of single-fiber composites, respectively It was observed that the strength shows a positive deviation from that predicted by the RoHM and hence exhibits a positive hybrid effect However, the values of the tensile modulus are close to those predicted by the RoHM and thus the modulus shows no existence of a hybrid effect Moreover, the failure strains of the hybrid composites were found to be higher than the failure strain of the single carbon fiber-reinforced composite, indicating that a positive hybrid effect exists Explanations for the hybrid effects on the tensile strength and failure strain were finally presented

Microsample tensile testing of LIGA nickel for MEMS applications

Abstract: Electro-deposited LIGA Ni components are being considered for use in a number of microelectromechanical systems (MEMS) and applications. The metrology of these components and their non-equilibrium microstructures play an important role in determining the mechanical response of these structures. Microsample testing has proven to be a reliable way of measuring the elastic and plastic tensile properties of these 100–200 μm thick films. Measured values of the in-plane Young's modulus (180±24 GPa) are significantly lower than the modulus for bulk coarse grained Ni (207 GPa), but can be explained and modeled in terms of the crystallographic texture in the as-deposited films. Variations in strength associated with post-processing heat treatments are highlighted and explained in terms of microstructural coarsening. The effect that coarsening and the related drop in strength have on the performance of LIGA Ni spring-like structures is discussed, and the need for further high temperature testing is highlighted.

Anisotropy of Tensile Strength of Root Dentin

Abstract: An effect of dentinal tubule orientation on mechanical properties of dentin has been difficult to demonstrate. We have tested the hypothesis that ultimate tensile strength (UTS) of dentin is affected by tubule (and hence collagen fibril) orientation. The UTS of human root dentin was investigated by direct tensile and diametral testing of specimens of known orientation prepared from extracted teeth. Dumbbell-shaped samples were machined for direct tensile testing (load parallel or perpendicular to tubule direction) and cylinders for diametral testing (load at 0 degrees, 45 degrees, 67.5 degrees, and 90 degrees). Fractured surfaces were examined by SEM. UTS was lowest when the tensile force was parallel to tubule orientation, and greatest at 90 degrees to tubule orientation (fracture parallel to tubule direction). SEM views of fractured surfaces suggested that microstructure contributes to fracture patterns. At least for properties involving disruption of the collagen matrix, root dentin shows a definite anisotropy.

Tensile properties of Argiope trifasciata drag line silk obtained from the spider's web

Abstract: The tensile properties of Argiope trifasciata (Argiopidae) drag line silk retrieved from mooring threads in the web were characterized. Scanning electron microscope images were used to determine the cross-sectional area of the samples, allowing force-displacement plots to be rescaled as stress–strain curves and to characterize fracture surfaces. Twenty-eight samples were tested to obtain statistically significant values of the mechanical parameters (elastic modulus, stress and strain at the proportional limit, and tensile strength). The tensile strength of the material was subjected to a Weibull analysis—the first time that this has been attempted with a spider silk. A low value of the Weibull modulus, m = 3.4, was obtained, demonstrating that drag line monofilament does not have a sufficiently reliable tensile strength to function as an engineering material on its own. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 2245–2251, 2001

New soybean oil‐styrene‐divinylbenzene thermosetting copolymers. III. Tensile stress–strain behavior

Abstract: The tensile stress–strain behavior and fracture properties of some new soybean oil based polymeric materials were investigated at room temperature. These materials were prepared by the cationic copolymerization of regular soybean oil, low saturation soybean oil (LoSatSoy oil), or conjugated LoSatSoy oil with styrene and the diene comonomers divinylbenzene, norbornadiene, or dicyclopentadiene in a process initiated by boron trifluoride diethyl etherate (BF3 · OEt2) or related modified initiators. These new polymeric materials exhibited tensile stress–strain behavior ranging from soft rubbers through ductile to relatively brittle plastics. The Young's moduli of these polymers varied from 3 to 615 MPa, the ultimate tensile strengths varied from 0.3 to 21 MPa, and the elongation at break varied from 1.6 to 300%. These properties are obviously related to their crosslink densities. The conjugated LoSatSoy oil polymers had higher mechanical properties than the corresponding LoSatSoy oil and regular soybean oil polymers with the same stoichiometry. Some conjugated LoSatSoy oil polymers with appropriate stoichiometries showed yielding behavior in the tensile test process. A variety of new polymer materials can thus be prepared by varying the stoichiometry, the type of soybean oil, and the crosslinking agent. These soybean oil based polymers possessed mechanical properties comparable to those of commercially available rubbery materials and conventional plastics and thus may serve as replacements in many applications. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 60–77, 2001