Showing papers on "Tensile testing published in 2007"

Twinning and the ductility of magnesium alloys Part I: “Tension” twins

Abstract: Magnesium and its alloys do not in general undergo the same extended range of plasticity as their competitor structural metals. The present work is part I of a study that examines some of the roles deformation twinning might play in the phenomenon. A series of tensile test results are reported for the common wrought alloy AZ31. These data are employed in conjunction with a simple constitutive model to argue that View the MathML source twinning (which gives extension along the c-axis) can increase the uniform elongation in tensile tests. This effect appears to be similar to that seen in Ti, Zr and Cu–Si and in the so called TWIP phenomenon in steel.

Ductile crystalline–amorphous nanolaminates

Abstract: It is known that the room-temperature plastic deformation of bulk metallic glasses is compromised by strain softening and shear localization, resulting in near-zero tensile ductility. The incorporation of metallic glasses into engineering materials, therefore, is often accompanied by complete brittleness or an apparent loss of useful tensile ductility. Here we report the observation of an exceptional tensile ductility in crystalline copper/copper–zirconium glass nanolaminates. These nanocrystalline–amorphous nanolaminates exhibit a high flow stress of 1.09 ± 0.02 GPa, a nearly elastic-perfectly plastic behavior without necking, and a tensile elongation to failure of 13.8 ± 1.7%, which is six to eight times higher than that typically observed in conventional crystalline–crystalline nanolaminates (<2%) and most other nanocrystalline materials. Transmission electron microscopy and atomistic simulations demonstrate that shear banding instability no longer afflicts the 5- to 10-nm-thick nanolaminate glassy layers during tensile deformation, which also act as high-capacity sinks for dislocations, enabling absorption of free volume and free energy transported by the dislocations; the amorphous–crystal interfaces exhibit unique inelastic shear (slip) transfer characteristics, fundamentally different from those of grain boundaries. Nanoscale metallic glass layers therefore may offer great benefits in engineering the plasticity of crystalline materials and opening new avenues for improving their strength and ductility.

Proposed classification of HPFRC composites based on their tensile response

Abstract: The tensile response of fiber reinforced cement (FRC) composites can be generally classified in two distinct categories depending on their behavior after first cracking, namely, either strain-hardening or strain-softening. Within the strain-softening category, one can distinguish between deflection-hardening and deflection-softening behavior. Several standard tests (ASTM, JCI, RILEM) are available to assess the characteristics of mostly strain-softening FRC composites through bending tests, but no standard test is currently available to characterize strain-hardening response in tension. Such composites have been described as high performance FRC or HPFRC composites. In this paper a proposal is put forth to characterize the response of strain-hardening FRC composites based on the results of direct tensile tests. The classification is based on several parameters which include a minimum value of elastic modulus, a minimum value of peak strain after first cracking, and the tensile strength level. While one of the key obstacles remains, that is, to develop a realistic and meaningful tensile test standard, some requirements on minimum specimen size, fiber size and aggregate size are also suggested.

Optimising industrial hemp fibre for composites

Abstract: The optimisation of New Zealand grown hemp fibre for inclusion in composites has been investigated. The optimum growing period was found to be 114 days, producing fibres with an average tensile strength of 857 MPa and a Young’s modulus of 58 GPa. An alkali treatment with 10 wt% NaOH solution at a maximum processing temperature of 160 °C with a hold time of 45 min was found to produce strong fibres with a low lignin content and good fibre separation. Although a good fit with the Weibull distribution function was obtained for single fibre strength, this did not allow for accurate scaling to strengths at different lengths. Alkali treated fibres, polypropylene and a maleated polypropylene (MAPP) coupling agent were compounded in a twin-screw extruder, and injection moulded into composite tensile test specimens. The strongest composite consisted of polypropylene with 40 wt% fibre and 3 wt% MAPP, and had a tensile strength of 47.2 MPa, and a Young’s modulus of 4.88 GPa.

Development and effect of alkali treatment on tensile properties of curaua fiber green composites

Abstract: This paper describes development and improvement of mechanical properties of a so-called green composite that was fabricated by reinforcing a cornstarch-based biodegradable resin with high-strength natural fibers extracted from a plant named curaua. Two fabrication methods are proposed, in which stretched slivers of curaua fibers are prepared as reinforcement to increase the composite strength. Moreover, highly concentrated alkali treatment was applied to curaua fibers to improve mechanical properties of green composites. Tensile test results showed that alkali-treated fiber composites increased in fracture strain twice to three times more than untreated fiber composites, without a considerable decrease in strength. This result proves that appropriate alkali treatment is a key technology for improving mechanical properties of cellulose-based fiber composites.

A concise interface constitutive law for analysis of delamination and splitting in composite materials and its application to scaled notched tensile specimens

Abstract: A concise constitutive law for cohesive interfaces is proposed in this paper. A new state variable is introduced to track the extent of damage accumulated at the interface. The constitutive equations not only account for mixed-mode delamination propagation in composite materials, but also satisfactorily deal with mode ratio change during the debonding process. The interface model is implemented in the LS-DYNA explicit finite element code. The model has been applied to scaled open hole tension tests on laminated composite material. Comparison between numerical results and experiments shows good correlation for failure modes and strengths for a range of different specimen sizes. Copyright © 2006 John Wiley & Sons, Ltd.

Investigating laser rapid manufacturing for Inconel-625 components

Abstract: This paper presents an investigation of laser rapid manufacturing (LRM) for Inconel-625 components. LRM is an upcoming rapid manufacturing technology, it is similar to laser cladding at process level with different end applications. In general, laser-cladding technique is used to deposit materials on the substrate either to improve the surface properties or to refurbish the worn out parts, while LRM is capable of near-net shaping the components by layer-by-layer deposition of the material directly from CAD model. In the present study, a high-power continuous wave (CW) CO 2 laser system, integrated with a co-axial powder-feeding system and a three-axis workstation were used. The effect of processing parameters during LRM of Inconel-625 was studied and the optimum set of parameters for the maximum deposition rate was established employing Orthogonal L9 array of Taguchi technique. Results indicated that the powder feed rate and the scan speed contributed about 56% and 26%, respectively to the deposition rate, while the influence of laser power was limited to 10% only. Fabricated components were subjected to non-destructive testing (like—ultrasonic testing, dye-penetrant testing), tensile testing, impact testing, metallographic examinations and micro-hardness measurement. The test results revealed defect-free material deposition with improved mechanical strength without sacrificing the ductility.

Enhanced mechanical behavior of a nanocrystallised stainless steel and its thermal stability

Abstract: This paper discusses the mechanical properties of a nanocrystallised stainless steel obtained using surface mechanical attrition treatment (SMAT) and the underlying grain refinement mechanism using transmission electron microscopy (TEM). It was shown that grain refinement down to the nanometer range has the potential to significantly improve the mechanical properties of a 316L stainless steel which becomes comparable in strength to titanium alloys. Hence, promising structural applications could be considered for such a material. At the same time, the thermal stability of this nanocrystallised material was studied in the temperature range from 100 to 800 °C. The results show that the nanometer scaled microstructure is retained up to 600 °C and that a controlled annealing treatment could even lead to enhancement of both strength and ductility of this material. All these results are explained in terms of microstructural investigations, X-ray diffraction measurements, tensile and bending tests as well as microhardness measurements.

Design and Operation of a MEMS-Based Material Testing System for Nanomechanical Characterization

Abstract: In situ mechanical characterization of nanostructures, such as carbon nanotubes and metallic nanowires, in scanning and transmission electron microscopes is essential for the understanding of material behavior at the nanoscale. This paper describes the design, fabrication, and operation of a novel microelectromechanical-systems (MEMS)-based material testing system used for in situ tensile testing of nanostructures. The device consists of an actuator and a load sensor with a specimen in between. Two types of actuators, in-plane thermal and comb drive actuators, are used to pull the specimens in displacement control and force control modes, respectively. The load sensor works based on differential capacitive sensing, from which the sensor displacement is recorded. By determining sensor stiffness from mechanical resonance measurements, the load on the specimen is obtained. Load sensors with different stiffness were fabricated. The best resolutions were achieved with load sensors that are designed for testing nanotubes, reaching 0.05 fF in capacitance, 1 nm in displacement, and 12 nN in load. For the first time, this MEMS-based material testing scheme offers the possibility of continuous observation of the specimen deformation and fracture with subnanometer resolution, while simultaneously measuring the applied load electronically with nano-Newton resolution. The overall device performance is demonstrated by testing freestanding cofabricated polysilicon films and multiwalled carbon nanotubes.

Crosslinked poly(vinyl alcohol) and starch composite films. II. Physicomechanical, thermal properties and swelling studies

Abstract: Poly(vinyl alcohol) (PVA) was blended with 10, 20, 30, 40, and 50 wt % of starch with and without crosslinking by solution casting process. The solution-casted films were dried and tested for physicomechanical properties like tensile strength, tensile elongation, tensile modulus, tear and burst strengths, density, and thermal analysis by differential scanning calorimetry (DSC). These PVA/starch films were further characterized for moisture content; solubility resistance in water, 5% acetic acid, 50% ethanol, and sunflower oil; and swelling characteristics in 50% ethanol and sunflower oil. The crosslinked PVA/starch composite films show significant improvement in tensile strength, tensile modulus, tear and burst strengths, and solubility resistance over the uncrosslinked films. Between the crosslinked and uncrosslinked films, the uncrosslinked films have higher tensile elongation, moisture content, moisture absorption, and swelling over the crosslinked films. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 909–916, 2007

On the influence of the grain size and solute content on the AE response of magnesium alloys tested in tension and compression

Abstract: Tensile and compression tests were conducted for AZ31, AZ61 and AZ80 alloys at room temperature. The distinctive tension/compression asymmetry in the yield behaviour was analysed for textured samples from extruded bars with various grain sizes. Parallel measurements of the acoustic emission were carried out to gather information about the relative activity of twinning and dislocation glide during deformation. The acoustic emission data are used to elaborate on the possible roles of grain size and aluminium content on the deformation behaviour.

Characterization and properties of sepiolite/polyurethane nanocomposites

Abstract: In situ polymerization method is employed to prepare sepiolite/polyurethane nanocomposite. The morphology and the dispersion of sepiolite in polyurethane have been characterized by X-ray diffraction, scanning electron microscope, transmission electron microscope and Fourier transform infrared spectroscopy. The results show the sepiolite is dispersed homogeneously in the polyurethane matrix at a nanometer scale. The tensile test shows that the tensile strength and elongation at break for the nanocomposites increase with the addition of sepiolite as compared to those of the pure polyurethane. The TGA analysis reveals that the addition of nanofillers results in the higher thermal stability.

Experimental investigation of cold-formed steel material at elevated temperatures

Abstract: This paper presents the mechanical properties data for cold-formed steel at elevated temperatures. The deterioration of the mechanical properties of yield strength (0.2% proof stress) and elastic modulus are the primary properties in the design and analysis of cold-formed steel structures under fire. However, values of these properties at different temperatures are not well reported. Therefore, both steady and transient tensile coupon tests were conducted at different temperatures ranged approximately from 20 to 1000 °C for obtaining the mechanical properties of cold-formed steel structural material. This study included cold-formed steel grades G550 and G450 with plate thickness of 1.0 and 1.9 mm, respectively. Curves of elastic modulus, yield strength obtained at different strain levels, ultimate strength, ultimate strain and thermal elongation versus different temperatures are plotted and compared with the results obtained from the Australian, British, European standards and the test results predicted by other researchers. A unified equation for yield strength, elastic modulus, ultimate strength and ultimate strain of cold-formed steel at elevated temperatures is proposed in this paper. A full strain range expression up to the ultimate tensile strain for the stress–strain curves of cold-formed carbon steel at elevated temperatures is also proposed in this paper. It is shown that the proposed equation accurately predicted the test results.

Hall–Petch Behavior in Ultra-Fine-Grained AISI 301LN Stainless Steel

Abstract: An ultra-fine-grained AISI 301LN austenitic stainless steel has been achieved by heavy cold rolling, to induce the formation of martensite, and subsequent annealing at 800 °C, 900 °C, and 1000 °C, from 1 to 100 seconds. The microstructural evolution was analyzed using transmission electron microscopy and the yield strength determined by tension testing. Ultra-fine austenite grains, as small as ∼0.54 μm, were obtained in samples annealed at 800 °C for 1 second. For these samples, tensile tests revealed a very high yield strength of ∼700 MPa, which is twice the typical yield strength of conventional fully annealed AISI 301LN stainless steels. An analysis of the relationship between yield strength and grain size in these submicron-grained stainless steels indicates a classical Hall–Petch behavior. Furthermore, when the yield dependence on annealing temperature is considered, the results show that the Hall–Petch relation is due to an interplay between fine-grained austenite, solid solution strengthening, precipitate hardening, and strain hardening.

Multiple Passes of Friction Stir Processing for the Creation of Superplastic 7075 Aluminum

Abstract: A staggered pass sample of friction stir processed (FSP) 7075 aluminum was created to make samples with one through four passes of FSP under identical conditions. The tensile testing temperatures ranged from 673 to 763 K with initial strain rates ranging from 1 × 10−3 to 1 × 10−1 s−1. Materials processed by single as well as multiple passes exhibited superplasticity across various testing temperatures and strain rates while the as received materials exhibited elongations below 200%. This study demonstrated the effectiveness of four consecutive FSP passes in creating large areas of superplastic material. However, the largest elongations were observed for the single pass material.

Interfacial microstructure and strength of steel/aluminum alloy lap joint fabricated by magnetic pressure seam welding

Abstract: Lap joining of low carbon steel (SPCC)/A6111 aluminum alloy was carried out using the magnetic pressure seam welding method. Interfacial microstructure, in particular, an intermediate layer formed at the weld interface was precisely examined using TEM. Tensile tests were also performed for the lap joints. Lap joining was successfully attained in several microseconds with no temperature increase. Weld interface of the lap joint showed wavy morphology and the intermediate layer was observed along the wavy interface. These microstructures are similar to that of the explosive weld lap joint. TEM observation revealed that the intermediate layers consist of fine aluminum grains (around 100 nm) and more finely dispersed intermetallic particles. A6111 matrix close to the weld interface also exhibited extremely refined grain structure. The bonding strength of the joint was quite high and it failed at the parent plate. The multi-phase intermediate layer and grain-refined aluminum layer are considered to be the origin of high interfacial bonding strength of the lap joint.

Fracture strength and Young's modulus of ZnO nanowires

Abstract: The fracture strength of ZnO nanowires vertically grown on sapphire substrates was measured in tensile and bending experiments. Nanowires with diameters between 60 and 310 nm and a typical length of 2 μm were manipulated with an atomic force microscopy tip mounted on a nanomanipulator inside a scanning electron microscope. The fracture strain of (7.7 ± 0.8)% measured in the bending test was found to be close to the theoretical limit of 10% and revealed a strength about twice as high as in the tensile test. From the tensile experiments, the Young's modulus could be measured to be within 30% of that of bulk ZnO, contrary to the lower values found in the literature. © 2007 IOP Publishing Ltd.

Generation of aluminium–steel joints with laser-induced reactive wetting

Abstract: A new mean of assembling steel to aluminium was developed, following previous work by German workers [1] . In this new method, a laser-induced aluminium melt pool spreads and wets a solid steel, to generate, after solidification a sound and resistant interface layer. Joint properties were investigated, in terms of surface aspects, interface microstructures and mechanical resistances under tensile testing, for non-galvanized and galvanized DC04 steels. Thermal and diffusional finite element (FE) simulations were also carried out to calculate temperature history at interfaces, and reaction layer thickness. The 2–20 μm thick reaction layers formed all along the interface were found to be mostly composed of Fe 2 Al 5 intermetallic compound with a high hardness (1200 HV) and rather low ductility (presence of solidification cracks). The presence of a 10 μm thick Zn layer on the steel was shown to have a beneficial influence on the wetting characteristics of the joint, despite the formation of occluded pores in the melt pool due to Zn vaporisation. FE thermal modelling evidenced 760–1020 °C wetting temperatures at the interface between DC04 low carbon steel and 6016 aluminium sheets, with time maintains of the melt pool in the 0.2–0.5 s range, resulting in high-speed reaction kinetics. Using these temperature data, diffusion calculations were shown to provide a rather good prediction of intermetallic thicknesses. Tensile tests were considered on aluminium–steel lap joints and evidenced higher mechanical resistances (220 N/mm linear tensile strength) on galvanized steels, provided that fluxing of the steel surface was carried out prior to welding to avoid zinc vaporisation. Comparatively, non-galvanized assemblies exhibited much lower mechanical resistances (170 N/mm resulting in a 90 MPa interfacial shear strength). It was concluded that the laser-induced wetting technique is a rather effective way for generating Al-steel joints without filler material, and that it should be considered as a competitive technique versus solid assembly modes (friction stir welding …).

Strain Measurements of Silicon Dioxide Microspecimens by Digital Imaging Processing

Abstract: Silicon dioxide thin film is a common component in electronic devices and in MEMS, but its mechanical properties have rarely been studied. Techniques have been adapted and developed to conduct tensile tests on 1.0 μm thick silicon dioxide specimens that are 100, 150, and 200 μm wide and either 1 or 2 mm long. One end of the specimen remains fastened to the substrate, and the other is glued to a silicon carbide fiber attached to a 30 g load cell mounted on a piezoelectric translation stage. Strain is measured by digital imaging of two gold lines applied to the gage section of the transparent specimen. Twenty-five tests yield a Young’s modulus of 60.1 ± 3.4 GPa and a fracture strength of 364 ± 57 MPa.

Modulus, Fracture Strength, and Brittle vs. Plastic Response of the Outer Shell of Arc-grown Multi-walled Carbon Nanotubes

Abstract: The fracture strengths and elastic moduli of arc-grown multi-walled carbon nanotubes (MWCNTs) were measured by tensile loading inside of a scanning electron microscope (SEM). Eighteen tensile tests were performed on 14 MWCNTs with three of them being tested multiple times (3×, 2×, and 2×, respectively). All the MWCNTs fractured in the “sword-in-sheath” mode. The diameters of the MWCNTs were measured in a transmission electron microscope (TEM), and the outer diameter with an assumed 0.34 nm shell thickness was used to convert measured load-displacement data to stress and strain values. An unusual yielding before fracture was observed in two tensile loading experiments. The 18 outer shell fracture strength values ranged from 10 to 66 GPa, and the 18 Young's modulus values, obtained from a linear fit of the stress–strain data, ranged from 620 to 1,200 GPa, with a mean of 940 GPa. The possible influence of stress concentration at the clamps is discussed.