Showing papers on "Tensile testing published in 1997"

Measurements of Young's modulus, Poisson's ratio, and tensile strength of polysilicon

Abstract: New techniques and procedures are described that enable one to measure the mechanical properties of polysilicon films that are 3.5 /spl mu/m thick. Polysilicon is deposited onto a silicon substrate which is then etched away to leave a tensile specimen in the middle of the die. The grip ends of the structure are glued to the grips of a linear air bearing attached to a piezoelectrically actuated loading system. Strain is measured directly on the specimen with laser interferometry. The specimens are fabricated at the Microelectronics Center of North Carolina with their MUMPs process. The results of 48 tests on five different sets of MUMPs specimens yield the following material properties: Young's modulus=169/spl plusmn/6.15 GPa, Poisson's ratio=0.22/spl plusmn/0.011, and tensile strength=1.20/spl plusmn/0.15 GPa These values have a reasonably low coefficient of variation which demonstrates the consistency of both the processing and the measurement techniques.

Effects of environmental exposure on fiber-reinforced plastic (FRP) materials used in construction

Abstract: A major hindrance to the acceptance of polymer composites in civil engineering applications is the susceptibility of the polymeric matrix to weathering. The polymer matrix is prone to degradation initiated by ultraviolet (UV) radiation, moisture, temperature, and high pH environments. The objective of this study was to characterize chemical and physical changes in polymeric matrix resins following exposure to these environments. The ultimate goal is to identify factors that contribute to matrix resin degradation under environmental and mechanical stresses. Resin systems studied included vinylester and isophthalic polyester, both of which are commonly used in construction applications. Neat polymer films were exposed to UV radiation, moisture, alkaline, and saline environments. Diffusion of water, alkali, and saline solution into the polymers was calculated from gravimetric measurements. Changes in strength, viscoelastic response, and thermal properties were evaluated through tensile testing, dynamic mechanical thermal analysis (DMTA), and differential scanning calorimetry (DSC). Atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) were also used for detecting changes in the polymer surface following UV exposure.

Significantly improved mechanical properties of Bi-Sn solder alloys by ag-doping

Abstract: The addition of small amounts of Ag (less than ~;0.5 wt. %) is found to significantly improve the ductility of the binary Bi-Sn eutectic solder. The ductility improvement, more than a threefold increase in tensile elongation, is observed even at a relatively high strain rate (0.01 s-1). As the Bi-Sn binary eutectic alloy tends to fail catastrophically by brittle fracture at high strain rates, the reduced strain-rate sensitivity in the Ag-containing alloy is beneficial for improving solder reliability on sudden impacting as might be encountered during device assembly, shipping, or thermal shock/cycling. The observed increase in alloy ductility by Ag additions is attributed to a substantial refinement of the solidification microstructure.

Cumulative damage models for system failure with application to carbon fibers and composites

Abstract: To estimate the probability distribution of the tensile strength of materials, specimens tested are typically of a variety of sizes, or “gauge lengths.” Here, a cumulative damage argument is used to develop new models for the strength of a general system that incorporate the size as a variable and are strong competitors to the commonly used Weibull model. These strength distributions can be represented as three-parameter versions of the Birnbaum-Saunders distribution and the inverse Gaussian distribution. Estimation is discussed and illustrations are presented for experimental strength data for carbon fibers and small composite specimens.

Mechanical and morphologic investigation of the tensile strength of a bone-hydroxyapatite interface.

Abstract: For load-bearing calcium-phosphate biomaterials, it is important to understand the relative contributions of direct physical-chemical bonding vs. mechanical interlocking to interfacial strength. In the limit of a perfectly smooth hydroxyapatite (HA) surface, a tensile test of the bone-HA interface affords an opportunity to isolate the bonding contribution related to HA surface chemistry alone. This study measured the bone-HA interfacial tensile strength for highly polished (approximately 0.05 micron alumina) dense HA disks (5.25 mm in diameter, 1.3 in mm thickness) in rabbit tibiae. Each of five rabbits received four HA disks, two per proximal tibia. Pull-off loads ranged from 3.14 +/- 2.38N at 55 days after implantation to 18.35 +/- 11.9N at 88 days; nominal interfacial tensile strengths were 0.15 +/- 0.11 MPa and 0.85 +/- 0.55 MPa, respectively. SEM of failed interfaces revealed failures between HA and bone, within the HA itself and within adjacent bone. Tissue remnants on HA were identified as mineralized bone with either a lamellar or trabecular structure. Oriented collagen fibers in the bone intricately interdigitated with the HA surface, which frequently showed breakdown at material grain boundaries and a rougher surface than originally implanted. Mechanical interlocking could not be eliminated as a mode of tissue attachment and contribution to bone-HA bonding, even after implanting an extremely smooth HA surface.

Evolution of anisotropy under plane stress

Abstract: Yield loci have been measured for cold rolled steel sheets prestrained by two stage loading. During the first loading, the sheets have been stretched by 3 and 6% tensile strain in the rolling direction. The second loading was at angles to the rolling direction with varying amounts of tensile strain. Then a set of tensile test specimens has been prepared from each of the prestrained sheets. From tensile tests, effects of the two-stage prestrains on the subsequent yielding has been investigated. Experiments show that initial orthotropic symmetry is maintained and that the orientations of orthotropy axes change continuously throughout the prestraining process. A simple phenomenological rule for the rotation of orthotropy axes is suggested.

Tensile Behavior of Free-Standing Gold Films

Abstract: A method for tensile testing thin gold films is presented. Free-standing tensile specimens were prepared by evaporating 0.8 μm of gold onto a patterned oxidized silicon wafer. Using common microelectronic fabrication techniques, free-standing thin film specimens were produced that span rectangular windows in the wafer. The wafer was diced into individual tensile specimens composed of a thin film surrounded by a silicon frame. The final step before testing was to cleave the silicon frame so that the load was completely carried by the metal film. The ultimate tensile strength of the films was found to be approximately 150% greater than that of annealed bulk gold. In contrast, the measured elastic modulus for the thin film specimens was approximately the same as that documented for bulk gold.

Microstructure, tensile deformation and fracture behaviour of aluminium alloy 7055

Abstract: The microstructure, tensile deformation and fracture behaviour of aluminium alloy 7055 were studied. Detailed optical and electron microscopy observations were made to analyse the as-received microstructure of the alloy. Detailed transmission electron microscopy observations revealed the principal strengthening precipitates to be the hexagonal disc-shaped η′ phase of size 2 mm×20 mm and fully coherent with the aluminium alloy matrix, the presence of spheroidal dispersoids, equilibrium grain-boundary η precipitates and narrow precipitate-free zones adjacent to grain-boundary regions. It is shown that microstructural characteristics have a profound influence on tensile deformation and fracture behaviour. Tensile test results reveal the alloy to have uniform strength and ductility in the longitudinal and transverse orientations. Strength marginally decreased with an increase in test temperature but with a concomitant improvement in elongation and reduction in area. No change in macroscopic fracture mode was observed with sample orientation. Fracture, on a microscopic scale, was predominantly ductile comprising microvoid nucleation, growth and coalescence. The tensile deformation and fracture process are discussed in the light of the competing influences of intrinsic microstructural effects, matrix deformation characteristics, test temperature and grain-boundary failure.

Fabrication, microstructures, and tensile properties of magnesium alloy AZ91/SiCp composites produced by powder metallurgy

Abstract: The tensile properties and microstructural evolution of hot extruded AZ91 magnesium alloy with and without reinforcement of SiC particles have been investigated in terms of extrusion parameters, such as extrusion ratio and extrusion temperature. Also, the effect of SiC particles on the grain size of the matrix in the composites was evaluated using the Hall-Petch equation. The AZ91 magnesium alloy powders prepared by wet attrition milling from magnesium machined chips were hot pressed with and without SiC particles, hot extruded, and then solution treated. Microstructural observation revealed that both the composites and the magnesium alloy have fine equiaxed grains due to the dynamic recrystallisation during hot extrusion. The tensile strength of both materials increased with increasing extrusion ratio, and the strengths of the composites were higher than that of the magnesium alloy without reinforcement. It was found that the tensile strength of both the materials decreased after solution treatme...

Mechanical properties of LIGA-deposited nickel for MEMS transducers

Abstract: This paper presents a new technique for measuring the stress-strain curve of nickel produced by the LIGA method. The results of nine tests on 200 /spl mu/m thick specimens produced on the 3rd LIGAMUMPS run show a Young's modulus similar to that of pure nickel, but strengths that are considerably higher.

Specimen size effect on tensile strength of surface micromachined polycrystalline silicon thin films

Abstract: A new tensile tester using electrostatic force grip was developed to evaluate the tensile strength and the reliability of thin film materials. The tester was constructed in a SEM chamber for in-situ observation, and was applied for tensile testing of polycrystalline silicon (poly-Si) thin films with dimensions of 30-300 /spl mu/m long, 2-5 /spl mu/m wide and 2 /spl mu/m thick. It was found that the mean tensile strength is 2.0-2.7 GPa depending on the length of the specimens, irrespective of the specimen width. Statistical analysis of these size effects on the tensile strength predicted that the location of the fracture origin is on the edge of the specimen, which is identified by the SEM observation of the fracture surface of the thin films.

The effect of shear controlled orientation in injection moulding on the mechanical properties of an aliphatic polyketone

Abstract: This article relates to an investigation of injection moulding a new commercial polymer, an aliphatic polyketone (PK). A terpolymer and a 30% glass-filled grade were used as study materials together with an isotactic polypropylene that was used as a basis for comparison. Both conventional injection moulding and shear-controlled orientation injection moulding (SCORIM) were employed in processing. Tensile testing was carried out at 80°C as well as at room temperature. Polarized light microscopy and wide-angle x-ray diffraction were used in the characterization of the mouldings. An increase of up to 30% in Young's modulus and 35% in ultimate tensile strength, and a 70-90% increase in strain at peak were gained for the terpolymer (PK) at room temperature, as a result of SCORIM processing. A substantial improvement at 80°C was also recorded for unfilled SCORIM PK mouldings, and is attributed to the pronounced molecular alignment that was induced in SCORIM mouldings, as shown by Debye patterns. It is notable that the SCORIM mouldings of PK exhibit a greater tensile strength at 80°C than the SCORIM mouldings of isotactic polypropylene at 23°C.

Fracture Criterion for Notched Thin Composite Laminates

Abstract: Fracture behavior of fiber-dominated center-notched AS4/3501-6 graphite-epoxy laminates is investigated in this study. Nine laminate configurations are studied to examine flaw size effects, crack tip damage mechanisms, and failure modes under uniaxial tensile loading. Results indicate that a constant value of fracture toughness K Q is a laminate material property. A layup independent failure criterion is proposed, which relates laminate fracture toughness to the fracture toughness K Q 0 of the principal load bearing ply. K Q 0 characterizes the in situ fracture toughness of a notched 0-deg layer in the event of fiber breakage along the plane of the notch. Once its value is estimated from preliminary tests, this parameter can be used to predict fracture toughness, and hence residual strength, of other fiber-dominated laminates of the same material system. The model predictions agree well with current experimental results, as well as with data published by other researchers. The model is further extended to predict residual strength of laminates with inclined cracks (mixed-mode loading). It is demonstrated that the normal projection of the crack to the applied load can be considered as the equivalent crack and governs laminate residual strength.

Transition between Fatigue and Ductile Fracture in Steel

Abstract: Cyclic tension and compression testing has revealed that cracking behavior in structural steel subjected to plastic strain reversals is transitional from a fatigue to a ductile mode with the elevation of strain amplitude. The transition can be identified by the plastic strain accumulation pattern, the fractographic ap­ pearance, the crack penetrating profile, and the convex feature of the Manson-Coffin' s curve. Since the strain amplitude induced by a severe earthquake is large enough to cause a crack in a few or several cycles of plastic strain, the crack is not a fatigue crack, but a ductile crack. Thus, the crack onset under cyclic seismic overloading can be predicted from the crack initiation strain under monotonic tensile testing. Because the monotonic tensile test is easily carried out in practice, it can be used to qualify steels and welds for seismic applications.

Tensile strength and fracture toughness of surface micromachined polycrystalline silicon thin films prepared under various conditions

Abstract: A new tensile tester for thin films was developed to evaluate the reliability of the microelectro- mechanical devices. This tester uses the grip that fixes a thin film specimen to a probe by electrostatic force. We applied this tester for polycrystalline silicon (poly-Si) thin films prepared under various conditions. The microstructure of the film is controlled by the crystallizing temperature. The process conditions and the microstructures that contribute to the strength of poly-Si film are identified by the tensile strength and the fracture toughness. The mean tensile strength of each specimen size ranges from 1.8 to 3.7 GPa, and the fracture toughness calculated from the strength of the notched specimen ranges from 1.9 to 4.5 MN/m3/2. The 1000°C annealed film has higher strength and toughness than the other films because of the high annealing temperature and the small grain size. The contributions to the strength are evaluated by the additional annealing at 1000°C for the low temperature annealed films.

Analysis of adhesion and interface debonding in laminated safety glass

Abstract: Abstraet-A tension adhesion test is developed and analyzed to characterize polymer/glass adhesion in laminated safety glass. Laminated safety glass typically consists of two sheets of glass bonded by a polymer adhesive interlayer, usually plasticized poly(vinyl butyral). In the tension test, a rectangular laminate specimen is cracked on both glass sides, and loaded under a remote tension P at a constant displacement rate. The load on the specimen typically rises to a peak or deflection point load, P*, and the value of P* depends on the adhesion between the polymer interlayer and the glass. The peak load P* can be used as a characterization of the adhesion strength of the interface, but does not give a sample geometry-independent value for the adhesion. However, using a micromechanical model of debonding and a detailed finite element computation, the experimental data can be analyzed to determine the fracture toughness, and shear and normal strengths of the interface. The relationship between the peak load...