Showing papers on "Tensile testing published in 2000"

Equal-channel angular pressing of commercial aluminum alloys: Grain refinement, thermal stability and tensile properties

Abstract: Using equal-channel angular (ECA) pressing at room temperature, the grain sizes of six different commercial aluminum-based alloys (1100, 2024, 3004, 5083, 6061, and 7075) were reduced to within the submicrometer range. These grains were reasonably stable up to annealing temperatures of ∼200 °C and the submicrometer grains were retained in the 2024 and 7075 alloys to annealing temperatures of 300 °C. Tensile testing after ECA pressing through a single pass, equivalent to the introduction of a strain of ∼1, showed there is a significant increase in the values of the 0.2 pct proof stress and the ultimate tensile stress (UTS) for each alloy with a corresponding reduction in the elongations to failure. It is demonstrated that the magnitudes of these stresses scale with the square root of the Mg content in each alloy. Similar values for the proof stresses and the UTS were attained at the same equivalent strains in samples subjected to cold rolling, but the elongations to failure were higher after ECA pressing to equivalent strains >1 because of the introduction of a very small grain size. Detailed results for the 1100 and 3004 alloys show good agreement with the standard Hall-Petch relationship.

Microsample tensile testing of nanocrystalline metals

Abstract: A novel non-contact strain measurement technique has been employed to measure the tensile properties of extremely small ‘microsamples’ of pure high-density ultrafine-grained Al (ufg-Al) nanocrystalline Cu (n-Cu) and nanocrystalline Ni (n-Ni). These microsample tests confirmed the absence of Young's modulus variations for metals with grain sizes approaching 25 nm. Significant strength enhancements were associated with the nanocrystalline specimens; the tensile stresses achieved in these microsample tests were measured to be an appreciable fraction of the theoretical shear strength for these metals. The ufg-Al samples (diameter, 250 nm) exhibited extensive plasticity while deformation in the n-Ni (diameter, 28 nm) remained almost entirely elastic up to failure at 1500MPa. The n-Cu samples were found to have a multiscale grain structure that produced an attractive balance of strength and ductility. Transmission electron microscopy investigations of deformed n-Ni failed to produce any evidence of dis...

Numerical and experimental uniaxial loading on in-plane auxetic honeycombs

Abstract: Auxetic honeycombs show in-plane negative Poisson's ratio properties; they expand in all directions when pulled in only one, and contract when compressed. This characteristic is due to the reentrant shape of the honeycomb unit cell. The cell convoluteness gives a geometric stiffening effect that affects the linear elastic properties of the whole cellular solid. In this paper finite element simulations are carried out to calculate the in-plane Poisson's ratio and Young's moduli of re-entrant cell honeycombs for different geometric layout combinations (side cell aspect ratio, relative thickness and internal cell angle) subjected to uniaxial loading. The results show a high sensitivity of the mechanical properties for particular ranges of the geometric cell parameters. An image data detection technique is used to extract displacements and strains from an aramid paper re-entrant honeycomb sample in a tensile test. The comparison between numerical and experimental results shows good agreement.

Deformation and fracture of aluminium foams

Abstract: The tensile and compressive properties and the fracture resistance of two aluminium alloy foams have been measured. The yield strength, unloading modulus and toughness increase with relative density in such a manner that the closed cell foams of this study behave as open cell foams. These relationships can be described adequately by power law fits. Experimental results, when compared with theoretical models based on idealised foam structures, reveal unexpected discrepancies. We conclude that they are caused by morphological defects in the microstructures of the foams, the effects of which were not included in the models. Tests on samples with deep sharp notches show that the tensile and compressive strengths are notch-insensitive. Fracture toughness measurements show an R-curve behaviour. This is analysed in terms of the underlying microstructure — the major cause of the R-curve was observed to be the development of crack bridging ligaments behind the crack tip. The compact tension specimens employed were sufficiently small for the uncracked ligaments to suffer plastic yielding during the fracture tests. The crack bridging response was quantified in terms of the normal traction versus plastic displacement curve; the area under this curve for a deep double edge-notched specimen is approximately equal to the measured steady state toughness. The accuracy of an existing micromechanical model for the fracture toughness of brittle open cell foams is assessed, and a new toughness model for ductile foams is derived.

Thermal stability and mechanical properties of ultrafine grained low carbon steel

Abstract: Ultrafine grained low carbon steel manufactured by equal channel angular pressing was annealed at 753 K, where negligible grain growth occurred, up to 72 h and the microstructural change and the mechanical properties were examined. This investigation was aimed at providing the guiding information for the effective use of ultrafine grained low carbon steel manufactured by severe plastic deformation processes. Under the present annealing conditions, the microstructural change was dominated by recovery. The tensile behavior of annealed ultrafine grained steel was characterized by much higher strength and the absence of strain hardening compared with that of large grained steel. In addition, the present ultrafine grained steel became mechanically stable by 24 h annealing treatment although recovery was in progress. The microstructure of the deformed sample of annealed ultrafine grained steel exhibited the elongated grains and dislocations distributed densely in the vicinity of grain boundaries. This finding indicated that dynamic recovery during deformation was associated with the absorption of dislocation by grain boundaries. The mechanical behavior of the present ultrafine grained low carbon steel was discussed in light of the recent development explaining that of nanocrystalline materials, i.e. the dislocation bow-out mechanism for high strength and the spreading kinetics of trapped lattice dislocation into grain boundary for the absence of strain hardening.

Microscale material testing of single crystalline silicon: process effects on surface morphology and tensile strength

Abstract: The mechanical properties of single-crystalline silicon are measured by uniaxial tension tests from microscale beam specimens patterned by four different common silicon etchants — KOH, EDP, TMAH and XeF2. SOI wafers are used to prepare test samples, which are 3–5 μm thick, 20–100 μm wide, and 6 mm long beam specimens; these are monolithically mounted on a temporary frame. A uniaxial tension test has been designed to accommodate microscale test requirements such as sample handling, sample alignment, and friction elimination. Stress and strain are measured using a commercial load cell and a laser interferometry system, respectively. Young's modulus of silicon in the 〈110〉 direction is measured to be 169.2±3.5 GPa, very close to the widely accepted value of 168.9 GPa obtained from a macroscale sample by an ultrasonic method. The fracture strength in the 〈110〉 direction is measured to vary from 0.6 to 1.2 GPa, and is apparently affected by the etching process employed to make the microscale specimen. As surface defects are expected to be the main factor determining the strength of the specimen, surface morphology is examined not only as a function of etchants but also as a function of mask-to-crystal direction misalignment after KOH etching. In the case of samples prepared by KOH etching, measured fracture strengths are 0.94 and 0.72 GPa from samples with 0° and 2° misalignments, respectively.

Processing nanocrystalline Ti and its nanocomposites from micrometer-sized Ti powder using high pressure torsion

Abstract: Nanocrystalline Ti and Ti–TiO 2 nanocomposites were produced by high pressure torsion (HPT) of precompacts of Ti powder (21 μm) and its mixture with TiO 2 powder (36 nm). Effects of processing temperature and pressure on material density and microhardness were systematically studied. The HPT process simultaneously consolidated the Ti and Ti–TiO 2 powders and refined the grains to nanometer size. The microstructure of as-processed samples contained high dislocation density, high internal stress, high angle, non-equilibrium grain boundaries, and texture. Mechanical properties such as microhardness increased with increasing density. Tensile testing showed that the as-processed materials were very brittle. High pressure torsion was found to be a promising technique for producing nanocrystalline materials from micrometer-sized metallic powders.

Effect of microvoid formation on the tensile properties of dual-phase steel

Abstract: A steel containing 0.32 wt.% C, 0.88 wt.% Mn, 0.99 wt.% Si, 0.9 wt.% Ni, and 0.9 wt.% Cr was intercritically annealed at different temperatures from 775 to 870 °C and quenched in oil to produce dual-phase steel microstructure. Tensile testing of these samples gave a series of strengths and ductilities. The tensile strength increased with the increased annealing temperatures and the martensite percentage increased with a reduction in ductility. Microvoids were formed near the fracture surfaces. The morphology of the microvoids changed with the martensite percentage from decohesion of the martensite particles to the intergranular and transgranular cracks, which defined the ultimate fracture mode of the specimens. The change in the morphology of microvoids may be due to a high percentage of carbon in the steel, which produced stresses in the matrix (ferrite) during phase transformation.

X-ray diffraction as a tool to study the mechanical behaviour of thin films

Abstract: In this paper, we present results of in-situ X-ray strain measurements on Al and Cu thin films, which were mechanically strained by (i) thermal cycling or (ii) a dedicated micro-tensile tester In the first case, continuous films or patterned lines were deposited on Si substrates and heated up to 500°C and strained as a result of thermal mismatch For the tensile testing, films were deposited onto compliant polymer substrates Then, film and substrate were strained simultaneously, while the film stress was determined by X-ray measurements Both types of experiments show that, roughly speaking, the film strength increases with decreasing film thickness A more detailed evaluation of the experimental results also provides information about effects, such as strain hardening and plastic anisotropy Based on these observations, implications for a theoretical description of deformation in thin films are critically discussed

Tensile Stress-Strain Modeling of Pseudostrain Hardening Cementitious Composites

Abstract: This study proposes a new theoretical approach for predicting the tensile stress-strain relation of random short-fiber-reinforced cement composites showing pseudostrain hardening. This approach is grounded on the solid basis of micromechanics, which describes the pseudostrain hardening phenomenon in terms of constitutive properties of the fiber, matrix, and fiber/matrix interface. The proposed modeling requires theoretical treatment of an inelastic strain due to multiple cracking. This modeling is achieved by employing a probabilistic description of initial flaw size distribution, which should be known for predicting the stress-strain relation. This study proposes a practical method for this identification using the tensile test result of a reference composite. A comparison with the test data indicates that the proposed model is capable of reasonably reproducing the stress- strain relation of ''similar'' composites. Such composites have a configuration similar to the reference configu- ration but different in fiber volume fraction and fiber length. Finally, the proposed theory is a potentially powerful tool for tailoring composites to satisfy targeted structural performance.

Section capacity of very high strength (VHS) circular tubes under compression

Abstract: This paper investigates the section capacity of very high strength (VHS®) circular tubes under compression. Full-section tensile tests were performed to determine the Young's modulus of elasticity, tensile yield stress and ultimate tensile strength. The modulus of elasticity was found around 200,000 MPa, the tensile yield stress (0.2% proof stress) was around 1350 MPa and the ultimate tensile strength was around 1500 MPa. Twelve stub column tests were carried out. The plate element slenderness varies from 86 to 130. It has been demonstrated that the limits on local buckling in most existing design standards are conservative when applied to VHS circular tubes. The reliability of section capacity based on the full effective section model has been verified using a FOSM (first order second moment) approach.

Mechanical characterization of brushite cements: a Mohr circles'approach

Abstract: Dry and wet brushite cements with various solid/liquid ratios were tested in compression and tension. Two different testing techniques were used to determine tensile strength: Direct Tensile test (DT) and Diametral Compression test (DC) (Brazilian test), which is an indirect way of measuring tensile strength on brittle materials. Statistical analysis of the results obtained on dry cements points out a constant ratio between the values measured by DT and Brazilian tests (DC/DT = 85%). The Mohr's circles representation allows us to understand that, for a material like our cement, ultimate stress measured with the Brazilian test can only underestimate tensile strength, because the compressive/tensile strength ratio is lower than 8. The second consequence of this low ratio is that, in the Brazilian test, the plane along which fracture initiates undergoes not only a normal tensile stress, but also a tangential stress component. Thus, the state of stress on the fracture plane differs from the one taking place in the direct tensile test. Consequently, with such a material (sigma(c)/sigma(t) < 8), the Brazilian test does not estimate the true tensile strength.

Microstructure and properties of titanium boride dispersed Cu alloys fabricated by spray forming

Abstract: Dispersion strengthened Cu alloys have been manufactured by conventional spray forming and also by reactive spray forming, followed by hot extrusion of the spray deposited billets. In this work, we have systematically investigated the relationship between the microstructure and mechanical properties of the Cu alloys fabricated through two techniques by means of optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and tensile testing. The size of dispersed particles in the reactive spray formed alloy was much finer than that in the conventionally spray formed alloy. That was because the dominant chemical reaction between Ti and B had occurred in Cu–Ti–B alloy melt in conventional spray forming while it had occurred after deposition of droplets in reactive spray forming. The yield strength of the reactive spray formed alloy was greater than that of the conventional spray formed alloy in spite of a lower volume fraction of reinforced particles. To understand the mechanism responsible for this experimental observation, the yield strength of the two Cu alloys were analyzed using the dislocation pile-up model and Orowan mechanism, which were fairly consistent with the experimental results. Increase in yield strength of reactive spray formed alloy compared with that of conventional spray formed alloy was largely attributed to nanoscale TiB dispersoids. This indicates that refining the reinforced particle size to nanoscale is of special importance for the development of high strength Cu alloys since the yield strength predominantly depends on the size and volume fraction of the reinforcement. From this point of view, reactive spray forming can be considered as a new promising process to develop high strength Cu alloys.

Effect of serrated flow on deformation behaviour of AISI 403 stainless steel

Abstract: AISI 403, a martensitic stainless steel in the quenched and tempered condition has been subjected to uni-axial tension test at a range of strain rates (2×10−5–10−2 s−1) and a range of temperatures (25–500°C) to investigate the effect of temperature and strain rate on its mechanical properties. Serrated flow behaviour, predominately of type A and B, has been observed in the temperature range of 250–375°C and at strain rates lower than 10−3 s−1, which suggests the presence of dynamic strain aging (DSA) in this parametric domain. In this domain, while an increase in ultimate tensile strength and work hardening have been observed, the yield strength has been found to be nearly independent of temperature. A loss in ductility, due to the presence of DSA has also been seen in this material. The solute responsible for serrated flow has been identified to be chromium, which is the major alloying addition in this material. McCormick’s method has been employed to determine the activation energy from the knowledge of critical strain for the appearance of serrations as a function of strain rate and temperature. The magnitude of activation energy is approximately 50% of that for bulk diffusion of chromium in body centered cubic iron suggesting that a mechanism other than volume diffusion is involved.

Effects of thermal treatments on microstructure and mechanical properties in a thixocast 319 aluminum alloy

Abstract: Among cast aluminium alloys, 319 ranks as one of the commercially important alloys used in automotive applications, on account of its excellent casting characteristics and good mechanical properties. It has become one of the candidates for shaping the aluminium alloys in the semisolid state or thixocasting. In this study, thixocast bars of 319 aluminium alloy were heat treated in T4, T5 and T6 conditions, and the microstructural evolution was followed by optical and scanning electron microscopy. Electrical conductivity and hardness measurements were also performed on aged samples to follow the precipitation process. After aging, samples were prepared for tensile testing at room temperature, to study the effect of heat treatment on the mechanical properties. Longitudinal sections of tensile-tested samples were examined to identify the failure mechanism. The rupture propagates in the eutectic region or where Si particles are present, leading to a fracture of the particles themselves. The mechanical properties of the thixocast samples are, in some cases, higher than those obtained from traditionally cast 319 alloys.

Chemical, structural, and mechanical properties of the LTCC tapes

Abstract: The chemical, structural and mechanical properties of six commercially available low temperature cofire ceramic (LTCC) systems were evaluated using X-ray diffraction, transmission electron microscopy, energy dispersive X-ray Spectroscopy (EDS), 3 and 4 point bend test, wetting angle and tensile testing (on unfired tape). Each of the systems (by manufacturer) was based on a different devitrifying glass-ceramic system, including Wollenstite, Anorthite and Celsian phases, plus expansion modifiers of alumina. For mechanical properties, both the fired strength and performance of the green, unfired tape were evaluated, including the viscoelastic response of unfired tape.

Free-form laser consolidation for producing metallurgically sound and functional components

Abstract: Functional metallic components can be built layer by layer from a computer-aided design model, by using an optical fiber coupled Nd:YAG laser beam along with the simultaneous delivery of desired metal/alloy powders through a nozzle into the molten pool. Building of shapes using various alloys, including 316 L stainless steel, Ni-base IN-625 superalloy, and M4 tool steel, have been investigated. The components built using the free-form laser consolidation are metallurgically sound, free of cracks and porosity. Surface finish on the order of 1–2 μm (Ra) can be obtained on the consolidated samples. The microstructure of the laser-consolidated samples is similar to the rapidly solidified materials. Under certain conditions, directionally solidified microstructure can also be obtained. X-ray diffraction analysis reveals that laser-consolidated material maintains the same phase structure as the original powder. The tensile properties of the laser-consolidated IN-625 alloy and 316 L stainless steel are comparabl...