Showing papers on "Tensile testing published in 2006"

Tailoring stacking fault energy for high ductility and high strength in ultrafine grained Cu and its alloy

Abstract: Bulk ultrafine grained (UFG) materials produced by severe plastic deformation often have low ductility. Here the authors report that simultaneous increases in ductility and strength can be achieved by tailoring the stacking fault energy (SFE) via alloying. Specifically, UFG bronze (Cu 10wt.% Zn) with a SFE of 35mJ∕m2 was found to have much higher strength and ductility than UFG copper with a SFE of 78mJ∕m2. Accumulations of both twins and dislocations during tensile testing play a significant role in enhancing the ductility of the UFG bronze. This work demonstrates a strategy for designing UFG alloys with superior mechanical properties.

The effect of crosslinking on the mechanical properties of polylactic acid/polycaprolactone blends

Abstract: The improvement of the brittle behavior of Polylactic acid (PLA) resin was studied by blending it with Polycaprolactone (PCL) resin. These materials were fabricated into the compressed films and injection moldings. The values of tensile modulus and strength were appropriate, judging from the rule of mixtures. However, the ultimate tensile strain was very small. Dicumyl peroxide (DCP) was added to this blend system to improve its ultimate tensile strain. It was found that the value of ultimate tensile strain peaked at low DCP concentration. The samples at low DCP contents show yield point and ductile behavior under tensile test. The impact strength of the optimum composition was 2.5 times superior to neat PLA, and ductile behavior such as plastic deformation was observed at its fracture surface. It was found that the carbonyl groups of the blend material with DCP were altered by using FTIR spectroscopy. Dynamic mechanical analysis data revealed the dual phase nature of PLA/PCL blend albeit with good interfacial adhesion, and the DCP enhanced the viscous property in PCL phase, which agreed with tensile ductility and impact strength. The mechanical properties of this blend are comparable to those of general purpose HIPS and ABS. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 1816–1825, 2006

An experimental investigation of humidity and temperature effects on the mechanical properties of perfluorosulfonic acid membrane

Abstract: The mechanical properties of a perfluorosulfonic acid (PFSA) membrane have been investigated at different humidities and temperatures in a custom-designed environmental chamber. Tensile tests were conducted to determine Young's modulus, the proportional limit stress (“yield strength”), break stress, and break strain. In-plane dimensional changes of the membrane at different temperature and humidities were also determined. The results indicate that Young's modulus and the proportional limit stress of the PFSA membrane decrease as humidity and temperature increase. Higher temperature leads to lower break stress and higher break strain. However, humidity has little effect on the break stress and break strain. A nonparametric statistical analysis, Kruskal–Wallis test, is applied to the experimental results, which shows that the effects of temperature and humidity on Young's modulus and proportional limit stress are statistically significant.

Symmetry-, time-, and temperature-dependent strength of carbon nanotubes

Abstract: Although the strength of carbon nanotubes has been of great interest, their ideal value has remained elusive both experimentally and theoretically. Here, we present a comprehensive analysis of underlying atomic mechanisms and evaluate the yield strain for arbitrary nanotubes at realistic conditions. For this purpose, we combine detailed quantum mechanical computations of failure nucleation and transition-state barriers with the probabilistic approach of the rate theory. The numerical results are then summarized in a concise set of equations for the breaking strain. We reveal a competition between two alternative routes of brittle bond breaking and plastic relaxation, determine the domains of their dominance, and map the nanotube strength as a function of chiral symmetry, tensile test time, and temperature.

Anisotropic constitutive equations and experimental tensile behavior of brain tissue

Abstract: The present study deals with the experimental analysis and mechanical modeling of tensile behavior of brain soft tissue. A transversely isotropic hyperelastic model recently proposed by Meaney (2003) is adopted and mathematically studied under uniaxial loading conditions. Material parameter estimates are obtained through tensile tests on porcine brain materials accounting for regional and directional differences. Attention is focused on the short-term response. An extrapolation of tensile test data to the compression range is performed theoretically, to study the effect of the heterogeneity in the tensile/compressive response on the material parameters. Experimental and numerical results highlight the sensitivity of the adopted model to the test direction.

Processing and properties of glass bead particulate-filled functionally graded Nylon-11 composites produced by selective laser sintering

Abstract: This article investigates the fabrication of functionally graded materials (FGMs) by selective laser sintering (SLS) of Nylon-11 composites filled with different volume fractions of glass beads (0–30%). The investigation involved a combination of experimental studies, theoretical modeling and numerical analysis. Optimal processing parameters for each composition were developed by design of experiments (DOE). These parameters were then compared with parameters predicted by numerical modeling. Specimens for tensile and compressive testing for each composition and for a 1D FGM composition were fabricated and tested. The experimentally measured tensile and compressive moduli were compared with moduli predicted by Halpin's theoretical model and were found to be in excellent agreement. The results showed that the tensile and compressive modulus increases while strain at break and strain at yield decreases as a function of glass bead volume fraction. A finite element model for the compressive properties of the 1D FGM specimen showed good agreement with experimentally measured values. Finally, to demonstrate the SLS-based FGM approach, two components exhibiting a one-dimensional functional gradient of particulate-filled polymer composites were fabricated.

Mechanical properties of cold-sprayed and thermally sprayed copper coatings

Abstract: The present investigation compares the mechanical properties of cold-sprayed and thermally sprayed copper coatings. The mechanical properties of the Cu-coatings are determined by in plane tensile test using micro-flat tensile specimen technique. A deeper view into the type of obtained defects, their stability and their influence on coating performance, is supplied by subsequent failure analyses and the comparison to annealed copper coatings. The results demonstrate that cold-sprayed coatings, processed with helium as propellant gas, show similar performance as highly deformed bulk copper sheets and respective changes in properties after annealing. In the as-sprayed condition, cold-sprayed coatings processed with nitrogen and thermally sprayed coatings show rather brittle behavior. Whereas subsequent annealing can improve the properties of the cold-sprayed coating, processed with nitrogen, such heat treatments have only minor influence on the tensile properties of thermally sprayed copper coatings. The investigation of failure modes for the as-sprayed states and after different heat treatments provided further information concerning particle–particle bonding and the effect of oxides on mechanical properties.

Fabrication and evaluation of carbon nano fiber filled carbon/epoxy composite

Abstract: In the present investigation, carbon nano fibers (CNFs) were infused into part-A of SC-15 epoxy (diglycidylether of bisphenol A) through a high intensity ultrasonic liquid processor and then mixed with part-B of SC-15 (cycloaliphatic amine hardener) using a high speed mechanical agitator. The trapped air and reaction volatiles were removed from the mixture using a high vacuum. DMA, TGA, and tensile tests were performed on unfilled, 1, 2, and 3 wt.% CNF filled SC-15 epoxy to identify the loading effect on thermal and mechanical properties of the matrix. The tensile results indicated that 2.0 wt.% CNF/epoxy resin showed the highest improvement in strength as compared to the neat systems. After that, the nanophased matrix with 2 wt.% CNF was then utilized in a vacuum assisted resin transfer molding (VARTM) set up with satin weave carbon preforms to fabricate laminated composites. The resulting structural composites have been tested under flexural loads to evaluate mechanical properties, and 22.3% improvement in flexural strength was observed in nanocomposite. Based on the experimental result, a linear damage model has been combined with the Weibull distribution function to establish a constitutive equation for neat and nanophased carbon/epoxy.

Elastic modulus of polystyrene film from near surface to bulk measured by nanoindentation using atomic force microscopy

Abstract: Using atomic force microscopy with a spherical indenter, we evaluated the surface and bulk elastic modulus of thick and thin polystyrene films. The elastic modulus of the thick films at penetration depths of more than 10nm was equivalent to that of bulk measured by a tensile test. In the thin films, the estimated values were greatly affected by the substrate material at penetration depths of more than 7nm. When the penetration depth was less than 5nm, however, the elastic modulus of both thin and thick films was slightly smaller than that of the bulk values.

Experimental Investigation of Progressive Damage and the Effect of Layup in Notched Tensile Tests

Abstract: This article presents results from a new approach to the finite element modeling of notched damage in composite materials using interface elements to model intra- and interply damage. The technique is used to examine and predict the failures observed in tensile, double edge-notched specimens using four different layups made up from glass/epoxy prepreg. Owing to the detailed modeling of the individual damage modes, their interaction is well characterized. The analytical results obtained compare well with detailed test observations, capturing delamination and intraply splitting. By including the subcritical damage that occurs at the notch tip in the model, the stress singularity is removed and failure criteria can be used to predict ultimate ply failures.

Tensile and fracture behavior of polymer foams

Abstract: Tensile and mode-I fracture behavior of cross-linked polyvinyl chloride (PVC) and rigid polyurethane (PUR) foams are examined. Tension tests are performed using prismatic bar specimens and mode-I fracture tests are performed using single edge notched bend (SENB) specimens under three-point bending. Test specimens are prepared from PVC foams with three densities and two different levels of cross-linking, and PUR foam with one density. Tension and quasi-static fracture tests are performed using a Zwick/Rowell test machine. Dynamic fracture tests are performed using a DYNATUP model 8210 instrumented drop-tower test set up at three different impact energy levels. Various parameters such as specimen size, loading rate, foam density, cross-linking, crack length, cell orientation (flow and rise-direction) and solid polymer material are studied. It is found that foam density and solid polymer material have a significant effect on tensile strength, modulus, and fracture toughness of polymer foams. Level of polymer cross-linking is also found to have a significant effect on fracture toughness. The presence of cracks in the rise- and flow direction as well as loading rate has minimal effect. Dynamic fracture behavior is found to be different as compared to quasi-static fracture behavior. Dynamic fracture toughness (Kd) increases with impact energy. Examination of fracture surfaces reveals that the fracture occurs in fairly brittle manner for all foam materials.

Poly(ε-caprolactone) - Clay nanocomposites: Structure and mechanical properties

Abstract: The modulus-volume fraction relationship for a poly(e-caprolactone)- montmorillonite nanocomposite follows composite materials theory provided the clay volume fraction is correctly calculated. Thus, for interpretation of mechanical properties, nanocomposites do not have to be treated as a separate class of material. The tensile modulus of biodegradable poly(e- caprolactone) was increased by 50% at 8 wt % clay addition (as corrected for surfactant), but the more dramatic improvement was in tensile elongation at break which increased from 165% to 550% for additions of up to 10 wt % clay. Poly(e-caprolactone) nanocomposites with various clay volume fractions were produced with two organo-modified montmorillonites. Untreated montmorillonite was used as an experimental control to compare the properties with a conventional composite over the same clay volume fraction range, The composites were confirmed and characterized by XRD, DSC, NMR, and TEM. © 2006 American Chemical Society.

Properties and morphology of segmented copoly(ether urea)s with uniform hard segments

Abstract: Block copoly(ether urea)s with uniform hard blocks consisting of two urea groups possess appealing elastomeric properties. The crystal structure of a model bisurea illustrates the formation of long stacks of hydrogen-bonded urea groups. Thermal analysis of these polymers demonstrates the reversible melting of the hard blocks, causing the material to flow. The low glass transition temperature ensures excellent low-temperature flexibility. The morphology of the material consists of long stacks of associated hard blocks embedded in the soft phase. Elongation of the materials demonstrates their highly elastic behavior, with a strain at break ranging from 1000 to 2100%. During tensile testing, irreversible deformation and reorganization of the hard blocks occur, resulting in a significant amount of tensile set. These well-defined polymers proved to be superior compared to a less-defined analogue having a polydisperse hard block.

Microstructural effects of AZ31 magnesium alloy on its tensile deformation and failure behaviors

Abstract: An Mg AZ31 alloy with modified microstructures was investigated to determine microstructural effects on room temperature mechanical properties acquired from low strain rate (∼10 −3 s −1 ) tensile testing to failure. Three distinct microstructures were generated via heat treatment, viz.: (1) single phase, fine equiaxed grains; (2) single phase, coarse grains with twins; (3) fine, equiaxed grains decorated with Mg 17 (Al,Zn) 12 grain-boundary precipitates. Each microstructure was separately characterized with optical microscopy, X-ray crystallography, fractography, and hardness measurement prior to tensile testing. Tensile coupons fabricated from each microstructure were then elongated to failure in a miniature stage, and true stress–true strain curves were computed with the digital image correlation (DIC) technique. Initial yield point, ultimate tensile strength and maximum elongation were also computed and examined within the context of key features of each microstructure to infer the mechanism of plastic deformation in tension and whether or not the potential for improved room temperature formability exists.

Creep behavior of carbon fiber/epoxy matrix composites

Abstract: The creep behavior of a carbon fiber/epoxy matrix composite was studied through tensile and flexural creep testing. No creep rupture failures were observed in short-term (less than 1600 h) room temperature tensile creep tests at loads up to 77% ultimate tensile strength (UTS). For elevated temperature flexural creep compliance data taken at isotherms between 30 and 75 °C, the principle of time–temperature superposition held. Master curves were generated by shifting the data by hand and also using the constant activation energy of the glass transition relaxation to estimate the shift factors. It was shown that the constant activation energy assumption worked fairly well, but only for temperatures below the onset T g of the material. Predictions were made concerning the creep levels at the end of a proposed 50-year design life.