Showing papers on "Tensile testing published in 1992"

Room-temperature tensile deformation of polysynthetically twinned (PST) crystals of TiAl

Abstract: Tensile deformation behavior of polysynthetically twinned (PST) crystals of TiAl with a nearly stoichiometric composition was investigated as a function of the angle o between the lamellar boundaries and tensile axis at room temperature. Tensile elongation to fracture strongly depends on the angle o but its dependence on the angle o is not symmetrical with respect to o = 45°. A tensile elongation as large as 20%, which is far larger than any other reported values on TiAl-based compounds, has been obtained for o = 31°. The yield stress also strongly depends on the angle o but any particularly significant tension-compression asymmetry in yield stress has not been observed. Fracture has been found to occur in a brittle manner without showing any local contraction even after deformation to more than 10%. When the tensile axis is perpendicular or inclined to the lamellar boundaries, fracture occurs in a cleavage-like mode with a habit plane parallel to the lamellar boundaries while fracture occurs in a zigzag across the lamellar boundaries when the tensile axis is parallel to the lamellar boundaries.

Video-controlled tensile testing of polymers and metals beyond the necking point

Abstract: A novel technique has been developed to record the intrinsic plastic behaviour of ductile materials by monitoring the effective strain and the effective stress in the mid-plane of hour-glass-shaped tensile specimens. The method utilizes a computer-aided video extensometer which analyses the sample profile in real time. The effective strain is computed automatically from the minimum diameter, and the effective stress is deduced from the applied load by taking into account the stress triaxiality corresponding to the local radius of curvature of the sample profile. Furthermore, a digital closed-loop system controls the ram speed of the hydraulic tensile testing machine in such a way that the local effective strain rate is maintained at a constant value. It is shown that most polymeric and metallic materials are entitled to be investigated by this method, which gives access in real time to the constitutive plastic equation, up to strains far beyond the necking point. The capabilities of the technique are illustrated and discussed critically, with more details for two polymers of different structures: polyethylene and polycarbonate.

Tensile and tensile fatigue behaviour of concrete; experiments, modelling and analyses

Abstract: A model based on nonlinear fracture mechanics is presented for the fatigue behavior of plain concrete. The tensile behavior of concrete for a monotonic increasing deformation is described. The softening relation is described and the way the fracture mechanics parameters are influenced by several variables is shown. The behavior of a softening material subjected to a uniaxial tensile test is explained. Based on experimental results, a constitutive model for the crack cyclic behavior of concrete is proposed. The adequacy of the model is shown by numerical simulation of notched beams under 4-point bending. It is shown by experiments, that localization and non-uniform crack opening occurs in tensile fatigue tests just as in monotonic loaded tensile tests.

The development of a measurement and analysis system to accurately determine asphalt concrete properties using the indirect tensile mode

Abstract: The objectives of this study were: (1) to briefly review existing laboratory testing modes and establish the relevance, adequacy, advantages, and disadvantages of using the indirect tensile mode to obtain properties of asphalt concrete mixtures; (2) to illustrate the serious errors that can be made in determining asphalt concrete properties when these are computed using the conventional vertical and horizontal deformation measurements obtained on the specimen's exterior; (3) to develop a measurement and analysis system to accurately determine asphalt concrete properties using the indirect tensile mode; and (4) to evaluate and compare the accuracy of the proposed measurement and analysis system to that of existing measurement and analysis systems.

Influence of microstructure on crack-tip micromechanics and fracture behaviors of a two-phase TiAl alloy

Abstract: The tensile deformation, crack-tip micromechanics, and fracture behaviors of a two-phase (γ + α2) gamma titanium aluminide alloy, Ti-47Al-2.6Nb-2(Cr+V), heat-treated for the microstructure of either fine duplex (gamma + lamellar) or predominantly lamellar microstructure were studied in the 25 °C to 800 °C range.In situ tensile and fracture toughness tests were performed in vacuum using a high-temperature loading stage in a scanning electron microscope (SEM), while conventional tensile tests were performed in air. The results revealed strong influences of microstructure on the crack-tip deformation, quasi-static crack growth, and the fracture initiation behaviors in the alloy. Intergranular fracture and cleavage were the dominant fracture mechanisms in the duplex microstructure material, whose fracture remained brittle at temperatures up to 600 °C. In contrast, the nearly fully lamellar microstructure resulted in a relatively high crack growth resistance in the 25 °C to 800 °C range, with interface delamination, translamellar fracture, and decohesion of colony boundaries being the main fracture processes. The higher fracture resistance exhibited by the lamellar microstructure can be attributed, at least partly, to toughening by shear ligaments formed as the result of mismatched crack planes in the process zone.

Simple constitutive equations for steel at high temperature

Abstract: This work develops and investigates simple unified constitutive equations to model the mechanical behavior of plain carbon steel in the austenite temperature region for use in finite element stress analysis of processes such as continuous casting. Four different forms of constitutive relations are considered: constant structure, time-hardening, strain-hardening, and simultaneous time- and strain-hardening models. Each relation is judged on its ability to reproduce experimental data from both tensile and creep tests and its ability to exhibit reasonable behavior under complex loading conditions. Three of the equations appear suitable for small strain monotonic loading conditions for a wide range of low strain rates (10−3 to 10−6 s−1), high temperatures (950 °C to 1400 °C), and varying carbon contents (0.005 to 1.54 wt pct C).

The effect of iron, gallium and molybdenum on the room temperature tensile ductility of NiAl

Abstract: The low density and high thermal conductivity of NiAl compared to nickel-base superalloys make NiAl alloys attractive materials for turbine airfoils. The lack of ductility at lower temperatures has been one of the barriers limiting the use of NiAl alloys. The authors identify microalloying additions to NiAl which have demonstrated very significant improvements in the room temperature tensile ductility. The purpose of this paper is to provide details of the experimental data and preliminary information on the potential mechanisms for the ductility improvement.

Correlation of deformation mechanisms with the tensile and compressive behavior of NiAl and NiAl(Zr) intermetallic alloys

Abstract: To identify the mechanisms controlling strength and ductility in powder-extruded NiAl and NiAl + 0.05 at. pct Zr, tensile and compressive testing was performed from 300 to 1300 K for several grain sizes. Grain size refinement significantly increased yield stress in both alloys and, in some cases, slightly lowered the ductile-to-brittle transition temperature (DBTT), although no room-temperature tensile ductility was observed even in the finest grain size specimens. The small Zr addition increased the DBTT and changed the low-temperature fracture mode from intergranular in NiAl to a combination of intergranular and transgranular in the Zr-doped alloy. Scanning electron microscopy (SEM) of compression specimens deformed at room temperature revealed the presence of grain-boundary cracks in both alloys. These cracks were due to the incompatibility of strain in the poly crystalline material, owing to the lack of five independent slip systems. The tendency to form grain-boundary cracks, in addition to the low fracture stress of these alloys, contributed to the lack of tensile ductility at low temperatures. The operative slip system, both below and above the DBTT, was {110} 〈001〉 for both alloys. The change from brittle to ductile behavior with increasing temperatures was associated with the onset of diffusional processes.

Concrete under impact tensile loading and lateral compression

Abstract: Abstract At the Delft University of Technology the dynamic material response of concrete is being investigated, using the split Hopkinson bar technique (SHB). To gain more insight into the influence of the initial stress conditions on the dynamic response, the SHB has been combined with a static compression loading device perpendicular to the SHB. This paper describes the test set-up, the monitoring techniques and the tests performed during the development of the biaxial loading device. Special attention is paid to the interaction of both loading devices in the biaxial set-up.

Sintering time and atmosphere influences on the microstructure and mechanical properties of tungsten heavy alloys

Abstract: The mechanical properties of tungsten heavy alloys are sensitive to the processing cycle and are adversely affected by residual porosity. Sintering times greater than 2 hours usually result in pore growth with degraded properties. The development of an optimized sintering atmosphere has allowed exploration of long sintering times without significant property degradation due to pore growth. The optimal cycle was used to sinter two heavy alloy compositions (88 and 95 wt pct W) for times up to 600 minutes at 1480 °C. The 88 pct W samples slumped, but the 95 pct W samples were fully densified and suitable for tensile testing. At long sintering times, the tungsten grains flattened and the tungsten contiguity decreased, indicating a transition to low-energy configurations for the solid-liquid interfaces. The cube of the mean grain size varied linearly with the isothermal sintering time. This allowed determination of grain size effects on mechanical properties, showing a decreasing yield strength with increasing time in agreement with the Hall-Petch behavior. The tensile strength and elongation were highest for sintering times from 30 to 90 minutes, reflecting a minimum in the residual porosity.

Studies on the thermal spraying of apatite bioceramics

Abstract: Hydroxylapatite (HA) coatings on metal substrates have been investigated for many years. These coatings have proved to be compatible with bone. The degree of crystallinity of HA changed, and sometimes dissociation was observed with respect to the plasma spray process. However, the plasma spray process hardly altered the crystallographic structure, with only line broadening visible. Thein vitro solubility is dependent on the degree of crystallinity of the coating. Tensile strength measurements on the strength of the coating-substrate interface using various adhesives revealed a significant difference between epoxy resin and methacrylate. The failure mode of this tensile test was dependent on the coating thickness and surface texture (polished versus nonpolished). In animal studies, the fixation of hydroxylapatite plasma- spray coated cylinders as well as noncoated Ti- 6A1- 4V cylinders (Ti) in cortical bone was evaluated using pushout tests. It appeared that HA- coated implants showed higher push- out strengths in the first months than the titanium implants, because of the earlier bone formation against the HA coating.

Characteristics of Deformation and Transformation in Ti44Ni47Nb9 Shape Memory Alloy

Abstract: The mechanical behavior and the effect of deformation on the martensitic transformation of Ti 44 Ni 47 Nb 9 alloy were investigated in detail by tensile tests and electrical resistance measurements. The tensile behavior at various temperatures, the effect of annealing temperature after cold work and the aging effect were studied. As a whole, the tensile behavior was found to be very similar to that of Ti-Ni binary alloys with low Ni content which shows no aging effect. Superelasticity was found for the first time in the present alloy. The rise of A s due to deformation was confirmed

Intercritically annealed and isothermally transformed 0.15 Pct C steels containing 1.2 Pct Si-1.5 Pct Mn and 4 Pct Ni: Part I. transformation, microstructure, and room-temperature mechanical properties

Abstract: Steels containing 0.15 pct C and 1.2 pct Si-1.5 pct Mn or 4 pct Ni were intercritically annealed and isothermally transformed between 300 °C and 500 °C for 1 to 60 minutes. The specimens were subjected to tensile testing at room temperature, and the microstructures were evaluated by light microscopy, scanning and transmission electron microscopy (SEM and TEM, respectively), and X-ray diffraction (XRD). The microstructures consist of dispersed regions of bainite, martensite, and austenite in a matrix of ferrite, and a maximum of 11.6 pct austenite is retained after isothermal holding at 450 °C in the Si-Mn steel. In specimens where austenite transforms to martensite during quenching after isothermal holding, the stress-strain curves show continuous yielding, high ultimate tensile strength (UTS), and relatively low ductility. In specimens where higher volume fractions of austenite transform to bainite during isothermal holding, the stress-strain curves show discontinuous yielding, low UTS, and high ductility.

The effect of strain rate and temperature on the tensile properties of NiAl

Abstract: Tensile testing of cast and extruded binary NiAl was performed from 300 to 900 K at strain rates of 1.4 × 10−4 to 1.4 × 10−1 × s−1. The brittle-to-ductile transition temperature (BDTT) was dependent on strain rate, with a three order of magnitude increase in strain rate resulting in approximately a 200 K increase in transition temperature. Regardless of strain rate, at temperatures just above the BDTT the fracture strength increased significantly and the fracture morphology changed from mostly intergranular to predominantly transgranular. It was also determined that the mechanism responsible for the brittle-to-ductile transition in NiAl had an apparent activation energy of approximately 118 kJ/mol. These results support the argument that the mechanism for the brittle-to-ductile transition in NiAl is associated with the onset of a thermally activated deformation process. This process is probably dislocation climb controlled by short circuit diffusion.

Thermally and mechanically induced residual strains in Al-SiC composites

Abstract: Neutron diffraction experiments were conducted on 15vol.% whisker and 20vol.% particulate reinforced aluminum/silicon carbide composites subjected to a rapid quench followed by various deformation histories. Corresponding numerical simulations were carried out using an axisymmetric unit cell model, with a phenomenological, isotropic hardening descriotion of matrix plasticity. Thermal expansion and the temperature dependence of material properties were accounted for. For the whisker reinforced matrix, quantitative agreement was generally found between the measured and calculated residual elastic strains. For the particulate reinforced matrix, the calculations tended to overestimate the magnitude of the residual strains parallel to the deformation axis, but very good agreement was obtained transverse to the deformation axis. For the silicon carbide reinforcement, both whisker and particulate, the variation of predicted residual elastic strains along the deformation axis was qualitatively consistent with the measurements, although quantitative agreement was often lacking. Measured and predicted residual strains perpendicular to the deformation axis for the silicon carbide typically were not in agreement. Parametric studies were carried out to ascertain the dependence of calculated flow strengths and residual strains on cell and reinforcement aspect ratio, and on reinforcement spacing and shape.

Scale effects in the response and failure of fiber reinforced composite laminates loaded in tension and in flexure

Abstract: The feasibility of using scale model testing for predicting the full-scale behavior of flat composite coupons loaded in tension and beam-columns loaded in flexure is examined. Classical laws of similitude are applied to fabricate and test replica model specimens to identify scaling effects in the load response, strength, and mode of failure. Experiments were performed on graphite-epoxy composite specimens having different laminate stacking sequences and a range of scaled sizes. From the experiments it was deduced that the elastic response of scaled composite specimens was independent of size. However, a significant scale effect in strength was observed. In addition, a transition in failure mode was observed among scaled specimens of certain laminate stacking sequences. A Weibull statistical model and a fracture mechanics based model were applied to predict the strength scale effect since standard failure criteria cannot account for the influence of absolute specimen size on strength.

Microdamage and interfacial adhesion in glass bead-filled high-density polyethylene

Abstract: The damage mechanism in tensile testing of glass bead-filled high-density polyethylene was studied byin situ scanning electron microscopy observation, acoustic emission monitoring and volume dilatation tests. The critical damage stress was measured by micromechanical tests. When the stress field of a multiparticle model has been calculated via Eshelby's method, the interfacial bonding strength can be determined.

Improvement of tensile ductility in high-purity alumina due to magnesia addition

Abstract: High-temperature plastic flow of high-purity A1 2 O 3 and MgO-doped A1 2 O 3 has been examined in a temperature range 1300 ∼ 1450°C. High-purity A1 2 O 3 with an initial grain size of about 0.9 μm is characterized by large strain-hardening rate and small tensile ductility up to 20%. The addition of 0.1 wt% MgO into A1 2 O 3 results in a decrease of strain-hardening rate and improves tensile ductility. The tensile elongation up to 70% is obtained in MgO-doped A1 2 O 3 . The difference in plastic flow behavior between pure and MgO-doped A1 2 O 3 is caused by the difference in grain growth during high-temperature deformation between the two materials. The grain growth inhibition caused by the MgO addition is very important to improve the tensile ductility. The tensile ductility of MgO-doped A1 2 O 3 is slightly but further improved in a temperature-increasing test. The role of grain size on tensile ductility is discussed in this paper.

Study on PVA hydrogel crosslinked by epichlorohydrin

Abstract: Poly(vinyl alcohol) hydrogel was prepared by using epichlorohydrin as a crosslinking agent. The molecular weight between crosslinks was calculated. The swelling behavior, as well as mechanical properties, of the hydrogels were examined by weighing and tensile testing, respectively. Values of equilibrium water content of more than 95% were reached while Young's modulus and tensile strength of the hydrogels were about 105 dyn/cm2. Crystallization was introduced by drying the hydrogels at 90°C. The dried hydrogel samples were used for swelling kinetic study, then reequilibrated with water and subjected again to tensile testing. Due to the formation of microcrystalline domains in the PVA hydrogel network after annealing, hydrogels with greatly enhanced Young's modulus, tensile strength, and elongation at break were obtained and their equilibrium water content values remained still high, i.e., 87 and 93%. © 1992 John Wiley & Sons, Inc.

Superplastic behavior of two-phase titanium aluminides

Abstract: A two-phase Ti(57 at. pct)-Al(43 at. pct) alloy with an initial lamellar microstructure was thermomechanically processed to form an equiaxed fine-grained structure. The fine-grained (- L = 5 μm) material was superplastic in the temperature range 1000 °C to 1100 °C, exhibiting a stress exponent of about 2 with a tensile ductility of 275 pct. The rate-controlling deformation mechanism is proposed to be grain boundary sliding accommodated by slip controlled by lattice diffusion in TiAl. At room temperature, the lamellar and fine-grained materials exhibit the same compressive yield stress. The compressive strain to failure, however, for the fine-grained material was about 28 pct compared to 6 pct for the lamellar material.

Effectiveness of hot tensile test in simulating straightening in continuous casting

Abstract: The extent to which the hot tensile test can be used to assess the relationship of steel composition to the incidence of transverse cracking in continuous casting has been examined. The influence of S, Ca, Ti, and Nb on the hot ductility of C–Mn–Al steels was investigated for the temperature range 700–1100°C and the results were compared with available commercial data on the effect of these elements on transverse cracking. Three different heat treatments that were carried out before straining have been examined: (i) reheating hot rolled plate to 1330°C; (ii) in situ casting of tensile samples; and (iii) as for (ii) followed by reheating to 1330°C. After these treatments, the samples were cooled directly to test temperature at a cooling rate of 100 K min−l and strained to failure at a strain rate of 2×10−3 s−l. All heat treatments showed that Nb gave the worst ductility in accordance with its known propensity to favour transverse cracking. Reheating hot rolled plate was found to be the simplest and...

The inhibition of corrosion and hydrogen embrittlement of AISI 410 stainless steel

Abstract: A study was carried out on the inhibition of corrosion and hydrogen embrittlement of AISI 410 stainless steel by two organic inhibitors, namely benzotriazole and benzonitrile. Tensile testing, scanning electron microscopy, weight loss measurements and potentiodynamic polarization were the techniques used for this study. Tensile tests showed that 410 steel is highly susceptible to hydrogen stress cracking. Scanning electron microscopic observations of fracture surfaces showed a brittle quasi-cleavage type of failure when the steel was hydrogen charged from 0.5m H2SO4. Both inhibitors reduced hydrogen induced ductility loss though the fracture mode was unaltered. They showed increasing inhibition efficiencies for corrosion as well as cathodic hydrogen evolution as their concentration in H2SO4 increased from 3.9×10−5m to 8.4×10−3m. Benzonitrile was found to be a more efficient inhibitor than benzotriazole for AISI 410 stainless steel exposed to 0.5m H2SO4.

Specimen size effect during tensile testing of an unreinforced polymer

Abstract: Various specimen sizes of an unreinforced polymer, Hercules 3501 -6 thermosetting epoxy, were subjected to a tensile test. The general specimen geometry was a rectangular dog-bone shape with constant gauge length, but with each specimen size having a different crosssectional area. These cross-sectional areas were obtained by varying the thickness of the epoxy during casting, and the gauge section width during grinding. The resulting failure surfaces of the specimens were observed and photographed using scanning electron microscopy. The results indicate that failure stress, dimensions of the critical flaw which caused failure, and a quantity which is proportional to the fracture toughness, are all correlated with specimen size.

A Large Tensile Elongation Induced by Crystallization in an Amorphous Al 88 Ni 10 Ce 2 Alloy

Abstract: An Al 88 Ni 10 Ce 2 amorphous alloy with the two-stage crystallization process of amorphous→amorphous plus fcc-Al→Al plus compounds exhibited a large elongation reaching 45% in the temperature range of 450 to 465 K. The temperature range agrees with that where the primary Al particles with a size of about 12 nm precipitate homogeneously at an interparticle spacing of about 6 nm in the amorphous matrix. It is therefore concluded that the appearance of the significant elongation is due to a crystallization-induced elongation phenomenon

Influence of Strain Rate and Yarn Number on Tensile Test Results

Abstract: The influence of strain rate and yarn number on tensile test results is investigated using a model to describe the tensile curve. A good correlation is obtained with the experimental stress-strain ...

Effect of environment on the thermal fatigue response of an SCS-6/Ti-24Al-11Nb composite

Abstract: A study has been conducted examining the thermal fatigue characteristics of an α2/SiC composite; in particular, SCS-6 reinforced Ti-24Al-11Nb (at. pct). The effort included the investigation of the effect of the environment by cycling coated and uncoated specimens in air and in an inert environment. Damage assessment was determined by postcycling room-temperature tension testing as well as by microstructural examination, including both optical microscopy and scanning electron microscopy (SEM). Significant reductions in postcycling tensile strength were observed for coated and uncoated specimens thermally cycled in air from 150 °C to 815 °C for 500 cycles, while no measurable loss of strength was found for specimens cycled in a low-pressure inert environment under otherwise identical conditions. The synergistic effect of residual stresses due to a coefficient of thermal expansion (CTE) mismatch and environment on the degradation of tensile properties of the thermally cycled composite is found to be the critical damage evolution mechanism for both coated and uncoated composites cycled in air. Residual stresses alone were found not to be critical in creating damage that could be tracked by a loss in residual strength.

Effect of the specimen size in predicting the mechanical properties of PbSn solder alloys

Abstract: The data obtained from bulk test specimens are often used in deformation models to predict the mechanical responses of parts that are very different in size and shape, such as solder joints in VLSI surface mount components. Such predictions are valid only if the size of the solder joints exceeds the representative deformation volume of the bulk specimens. In the present study, the measurement of the representative volume was performed for a single-phase solder alloy (98Pb-2Sn wt.%) deformed in both tension and cyclic loading. Deformation of 98Pb-2Sn alloy at room temperature was shown to be extremely inhomogeneous in both tensile and cyclic loading, and strain localization was observed at the grain boundaries. Strain distribution is more inhomogeneous in the specimen tested in fatigue than the specimen tested in uniaxial tension. This may be due to fatigue inhomogeneity in strain distribution or a small degree of buckling. The number of grains required for a representative volume of the 98Pb-2Sn alloy specimens tested is quite large. >