Showing papers on "Tensile testing published in 1996"

Preparation and Characterization of PMMA-Clay Hybrid Composite by Emulsion Polymerization

Abstract: Nanocomposites are prepared by a simple technique of emulsion polymerization using MMA monomer and Na+-montmorillonite. The products are purified by hot toluene extraction and characterized by FT-IR, X-ray diffraction, TGA, DSC, and tensile testing. The structural investigation confirms that the products are intercalated with PMMA chain molecules oriented parallel to the direction of lamellar layers whose separation is consequently more enlarged than in the polymer-free clay. DSC traces also corroborate the confinement of the polymer in the inorganic layer by exhibiting no observable transition in the thermogram. Both the thermal stability and tensile properties of the products appear to be substantially enhanced. The ion-dipole bonding is believed to be the driving force for the introduction and fixation of the organic polymer to the interfaces of montmorillonite. © 1996 John Wiley & Sons, Inc.

Degree of hydration-based description of mechanical properties of early age concrete

Abstract: For the evaluation of the risk of thermal cracking in hardening massive concrete elements, knowledge of the development of strength and deformability of early-age concrete is extremely important. Based on an extensive experimental research program on hardening concrete elements, a degree of hydration-based description for the compressive strength, Young's modulus, the uniaxial tensile strength, the splitting tensile strength, the flexural tensile strength, Poisson's ratio and the peak strain are all worked out. An extension of the formulation of Sargin for the stress-strain relation for short-term compressive loading leads to a degree of hydration-based stress-strain relation for hardening concrete. Good agreement with experimental results is reported.

Casting defects and the tensile properties of an AlSiMg alloy

Abstract: Samples containing either entrapped dross and oxide films, gas porosity or small drilled holes have been used to study the effect of different types of defects on the tensile behaviour of an Al 7Si 0.4Mg casting alloy. The tensile properties show little or no correlation with the bulk porosity content, especially in the case of samples containing dross and oxide films. In contrast, the decrease in tensile ductility and strength correlates with the area fraction of defects in the fracture surface of the samples. The experimental results are in agreement with the predictions of a simple analysis based on models for the growth of a plastic instability in a tensile sample.

Effects of β-α transformation on the static and dynamic tensile behavior of isotactic polypropylene

Abstract: It was demonstrated that the mechanical stress-induced βα-transformation in isotactic polypropylene (iPP) is associated with considerable toughness enhancement. This toughness improvement depends on the test conditions (loading frequency). The toughness of β-iPP was superior to the α-iPP by 13% under static (characterized by a load frequency of ca. 5 × 10−4 Hz) and 70% under dynamic (tensile impact with a loading frequency in the range of ca. 3 × 102…103 Hz) conditions, respectively. By applying the essential work of fracture (EWF) concept to single-edge notched tensile (SEN-T) specimens it was shown that for the toughness upgrading observed, energy dissipation in the enlarged plastic zone is responsible. The occurrence of the βα-transformation was evidenced by differential scanning calorimetry (DSC). Based on DSC measurements it was found that the degree of βα-trans-formation depends on the local strain. At high strain values the βα-conversion is complete (at elogation at break in uniaxial static tensile test), while this transformation is only partial at lower strains (at tensile impact). In addition, in the plastic (or deformation) zone the βα-conversion changed locally, and can be used for mapping of this region. © 1996 John Wiley & Sons, Inc.

Fundamentals of metal forming

Abstract: The Tensile Test and Basic Material Behavior. Tensors, Matrices, Notation. Stress. Strain. Standard Mechanical Principles. Elasticity. Plasticity. Crystal--Based Plasticity. Friction. Classical Forming Analysis. Index.

The effect of stress triaxiality and strain-rate on the fracture characteristics of ductile metals

Abstract: Notched tensile tests have been carried out on three common metals (pure iron, mild steel and aluminium alloy BS1474) over a wide range of strain-rates (10−3 to 104 s−1) and the strain-to-failure measured. The ductility of all three materials was found to be strongly dependent on the level of stress triaxiality in the specimen, this dependency being greatest for the ferrous materials and least for the aluminium alloy. No significant effect of strain-rate could be ascertained from the experimental results provided fracture remained fully ductile. However, for mild steel, a transition to a brittle fracture mode was observed for a given level of stress triaxiality as the strain-rate was increased. Numerical simulations of the experiments have been used to derive constants of a semi-empirical fracture model from the measured results. This model was found to give reasonable predictions of fracture over the range of conditions investigated.

Tensile Behavior of Cement‐Based Composites with Random Discontinuous Steel Fibers

Abstract: In this paper, the tensile properties of cement-based composites containing random discontinuous steel fibers are reported. Direct tensile tests were performed to study the effects of fiber length (hence fiber aspect ratio), interfacial bonding, and processing conditions on composite properties. Composite tensile strength and ductility are highlighted and discussed.

New test structures and techniques for measurement of mechanical properties of MEMS materials

Abstract: This paper presents techniques and procedures for addressing the three major problems of mechanical testing of the thin films used in surface micromachined microelectromechanical systems--specimen handling, friction, and strain measurement. The polysilicon tensile specimens are fabricated with two supporting side strips on silicon wafers at the Microelectronic Center of North Carolina. The tensile specimen is released by etching away the wafer, and the two support strips are cut after the specimen is glued in the test machine. Friction is reduced by a linear air bearing in the load train, and strain is measured with a noncontacting technique based on laser interferometry between two gold lines on the tensile specimen. The Young's modulus of polysilicon is 170 +/- 7 GPa and the strength is 1.21 +/- 0.16 GPa from a series of 29 tests. preliminary measurements have been made of Poisson's ratio and the fatigue behavior, and an attempt is underway to measure the fracture toughness.

Characterization of superplastic deformation behavior of a fine grain 5083 Al alloy sheet

Abstract: Superplastic deformation behavior of a fine grain 5083 Al sheet (Al-4.2 pct Mg-0.7 pct Mn, trade name FORMALL 545) has been investigated under uniaxial tension over the temperature range of 500 °C to 565 °C. Strain rate sensitivity values >0.3 were observed over a strain rate range of 3 × 10−5 s−1 to 1 × 10−2 s−1, with a maximum value of 0.65 at 5 × 10−4 s−1 and 565 °C. Tensile elongations at constant strain rate exceeded 400 pct; elongations in the range of 500 to 600 pct were obtained under constant crosshead speed and variable strain rates. A short but rapid prestraining step, prior to a slower superplastic strain rate, provided enhanced tensile elongation at all temperatures. Under the two-step schedule, a maximum tensile elongation of 600 pct was obtained at 550 °C, which was regarded as the optimum superplastic temperature under this condition. Dynamic and static grain growth were examined as functions of time and strain rate. It was observed that the dynamic grain growth rate was appreciably higher than the static growth rate and that the dynamic growth rate based on time was more rapid at the higher strain rate. Cavitation occurred during superplastic flow in this alloy and was a strong function of strain rate and temperature. The degree of cavitation was minimized by superimposition of a 5.5 MPa hydrostatic pressure during deformation, which produced a tensile elongation of 671 pct at 525 °C.

A technique to measure strain distributions in single wood pulp fibers

Abstract: Environmental scanning electron microscopy (ESEM) and digital image correlation (DIC) were used to measure microstrain distributions on the surface of wood pulp fibers. A loading stage incorporating a fiber gripping system was designed and built by the authors. Fitted to the tensile substage of an ESEM or a Polymer Laboratories MINIMAT tester, it provided a reliable fiber straining mechanism. Black spruce latewood fibers ( Picea mariana (Mill) B.S.P.) of a near-zero microfibril angle displayed a characteristically linear load elongation form. ESEM was able to provide real-time, high magnification images of straining fibers, crack growth, and complex single fiber failure mechanisms. Digital images of single fibers were also captured and used for subsequent DIC-based strain analysis. Surface displacement and strain maps revealed nonuniform strain distributions in seemingly defect-free fiber regions. Applied tensile displacements resulted in a strain band phenomenon. Peak strain (concentration) values within the bands ranged from 0.9% to 8.8%. It is hypothesized that this common pattern is due to a combination of factors including the action of microcompressive defects and straining of amorphous cell-wall polymeric components. Strain concentrations also corresponded well to locations of obvious strain risers such as visible cell-wall defects. Results suggest that the ESEM-based DIC system is a useful and accurate method to assess and, for the first time, measure fiber micro-mechanical properties.

Mechanical properties of injection moulded blends of polypropylene with thermotropic liquid crystalline polymer

Abstract: Blends of a thermotropic liquid crystalline polymer (LCP) with polypropylene (PP) were injection moulded. The LCP exhibited a higher viscosity than that of PP. Static and dynamic mechanical measurements, Izod impact tests, scanning electron microscopy (SEM), and thermogravimetric analysis (TGA) were performed on these blends. The static tensile tests show that the tensile modulus and strength of PP are improved with the addition of LCP. The improvement in mechanical properties is associated with the formation of LCP fibrils as evidenced by SEM observations. Dynamic studies on these blends show an increase in the storage modulus but a decrease in loss factor with the addition of LCP. Furthermore, TGA measurements show that the thermal stability of PP is improved substantially with the addition of LCP.

Mechanical behavior of cast SiCp-reinforced Al-4.5%Cu-1.5%Mg alloy

Abstract: SiC p -reinforced Al-4.5%Cu-1.5%Mg composite specimens were processed by vigorous stirring of the carbide in a semi-solid alloy slurry, followed by remelting and casting (stir-casting). The tensile and fatigue properties were evaluated in the as-cast and in the heat-treated conditions. In monotonic tensile testing, reinforcement with SiC p produced a substantial increase in the work hardening of the material. This increase became more significant with increasing volume fraction of carbide. The yield and ultimate tensile strength, and the elastic modulus of the material, increased with heat-treatment and volume fraction of carbide at the expense of ductility. These properties are inferior to those of other reinforced, more complex aluminum alloys processed by other methods. In stress-controlled fatigue tests under fully reversed ( R = −1) bending conditions, the fatigue life of the composite was longer than that of the unreinforced specimen at intermediate and lower stress levels. At higher stress levels the improvement was negligible. In heat-treated reinforced alloy specimens the fatigue strength at 1 × 10 7 cycles decreased with increasing carbide particle size. With solid solution and precipitation strengthening, as well as carbide dispersion strengthening of the alloy, the crack growth threshold stress intensity factor K th , increased, as did the crack initiation time and the crack growth rate.

Influence of copper, cadmium and tin additions in the galvanizing bath on the structure, thickness and cracking behaviour of the galvanized coatings

Abstract: The influence of copper, cadmium and tin additions in the galvanizing bath on the morphology and thickness of galvanized coatings has been studied using X-ray diffraction and optical and scanning electron microscopy associated with energy-dispersive spectroscopy microanalysis. Copper promotes the δ1 phase and hinders the ζ phase growth. In the opposite, cadmium promotes the ζ and Γ phase formation and hinders the growth of the δ1 phase. No change in the morphology of galvanized coatings has been observed with the addition of tin. Mechanisms have been proposed in order to explain the effect of the above elements on the formation of intermetallics. The cracking of the coatings after tensile testing in the plastic region of the specimens has been examined. The δ1 phase is responsible for the nucleation and propagation of transverse cracks parallel to the steel substrate. Cadmium hinders the nucleation and propagation of the transverse cracks during loading up to the yield point of the steel; however, the columnar growth of the ζ phase results in the complete flaking of the coating during loading up to the ultimate tensile strength of steel. The Γ phase remains always adhered to the steel substrate.

Tensile Testing of Polycrystalline Silicon Thin Films Using Electrostatic Force Grip.

Abstract: In this paper, a new tensile tester for thin films is presented. This tensile tester has a grip that fixes a thin film specimen using electrostatic force. The tester was constructed in a scanning electron microscope (SEM) chamber. Using this tester, the tensile strengths of polycrystalline silicon (poly-Si) thin films have been measured. The tested part of the specimen is 30-300μm long, 5μm wide and 2μm thick. The fracture of the poly-Si thin film was brittle. The mean tensile strength was 2.0-2.6GPa, depending on the length of the tested part. The size of the critical flaw that initiates fracture of the poly-Si thin film is 28-47nm, rather small than the grain size of the poly-Si thin film.

Plastic zone and pileup around large indentations

Abstract: Mechanical properties of cold-worked molybdenum, grade 4 titanium, and an α-β titanium alloy are measured with tensile tests and by indentations using conical indenters with 105, 120, and 137 deg included angles. The extent of plastic deformation and pileup around an indentation is measured using profilometry. Various models predicting the extent of plastic deformation and pileup are compared to the actual measured values. As inferred from indentation, the calculated yield strength of the material from the mean pressure does not correlate well to the yield strength measured by tensile testing. The plastic zone size surrounding an indentation can also be used to determine the yield strength of the material, and this does correlate to the yield strength measured by tensile tests. Furthermore, the extent of plastic deformation is relatively independent of the included angle of the indenter for the range of materials used in this system. Models predicting the amount of pileup at the edges of the indentation appear to approach but overestimate the actual amount of pileup in the materials tested.