Showing papers on "Ultimate tensile strength published in 1997"

Seventeenth Canadian Geotechnical Colloquium: The effect of cohesion loss and stress path on brittle rock strength

Abstract: Stress-strain curves for brittle rocks show three characteristic stress levels: crack initiation, long-term strength, and peak strength. Damage-controlled testing at low confining stresses has shown that the long-term and peak strengths are sensitive to the amount of induced damage, i.e., the greater the amount of damage, the lower the long-term and peak strengths. These tests also showed that the brittle-failure process is characterized by a loss of cohesion as friction is mobilized. Excavation of a circular test tunnel in massive brittle rock resulted in failure around the tunnel. The back-calculated strength for the failed rock around the tunnel is approximately one-half of that measured in laboratory tests. Crack-induced damage of Lac du Bonnet granite, both in the laboratory and in situ, begins when the load exceeds approximately one-third of the unconfined compressive strength. However, the stress level associated with failure is a function of loading path. In the laboratory, where the loading path monotonically increases, the ultimate strength of an unconfined sample is 225 MPa. Numerical studies suggest that in situ the loading path around the tunnel is more complex, involving stress increase and decrease and stress rotation. For this loading path, failure initiates at a stress between 100 and 120 MPa. Conventional frictional failure criteria did not adequately predict the extent of brittle failure measured around the circular tunnel. The results from the damage-controlled laboratory tests and the microseismic monitoring carried out during tunnel construction indicate a constant-deviatoric-stress criterion is a reliable indicator for predicting the onset of damage. This criterion was also found to give a reasonable prediction for the maximum depth of failure around the test tunnel. The fundamental assumption in the constant-deviatoric-stress criterion is that at low confining stresses, such as those which occur around underground openings, the brittle-failure process is dominated by cohesion loss.

Highly Conductive PEO-like Polymer Electrolytes

Abstract: Polymer electrolyte membranes comprising poly(vinylidene fluoride)−hexafluoropropene (PVdF−HFP) copolymer plasticized with a solution of LiSO3CF3, LiN(SO2CF3)2, or LiPF6 in oligomeric poly(ethylene glycol) dimethyl ethers (PEGDME, Mw = 250, 400, and 500) were prepared by hot-melt-rolling or solvent-casting techniques. Since the electrolytes containing PEGDME400 and PEGDME500 are “dry” with essentially no volatile components up to 150 °C, we have dubbed them PEO-like. Their thermal stability, mechanical strength, conductivity, electrochemical stability window, and Li/electrolyte interface stability were characterized. Plasticizing PVdF−HFP with the PEGDME/LiX solutions disordered the polymer structure leading to polymer electrolytes having lower crystallinity than the polymer host itself. The mechanical strength of the electrolyte membranes varied depending on the PVdF content. Tensile strength (stress) as high as 420 psi at an elongation-at-break value (strain) of 75% was observed. The conductivities of t...

Mechanical properties of pineapple leaf fiber‐reinforced polyester composites

Abstract: Pineapple leaf fiber (PALF) which is rich in cellulose, relatively inexpen- sive, and abundantly available has the potential for polymer reinforcement. The present study investigated the tensile, flexural, and impact behavior of PALF-reinforced polyes- ter composites as a function of fiber loading, fiber length, and fiber surface modification. The tensile strength and Young's modulus of the composites were found to increase with fiber content in accordance with the rule of mixtures. The elongation at break of the composites exhibits an increase by the introduction of fiber. The mechanical proper- ties are optimum at a fiber length of 30 mm. The flexural stiffness and flexural strength of the composites with a 30% fiber weight fraction are 2.76 GPa and 80.2 MPa, respec- tively. The specific flexural stiffness of the composite is about 2.3 times greater than that of neat polyester resin. The work of fracture (impact strength) of the composite with 30% fiber content was found to be 24 kJ m 02 . Significant improvement in the tensile strength was observed for composites with silane A172-treated fibers. Scanning electron microscopic studies were carried out to understand the fiber-matrix adhesion, fiber breakage, and failure topography. The PALF polyester composites possess superior mechanical properties compared to other cellulose-based natural fiber composites. q 1997 John Wiley & Sons, Inc. J Appl Polym Sci 64: 1739-1748, 1997

Mechanical properties of steel fiber-reinforced, high-strength, lightweight concrete

Abstract: This paper presents basic information on the mechanical properties of steel fiber-reinforced, high-strength, lightweight concrete with compressive and flexural strengths up to 85.4 MPa and 11.8 MPa, respectively. The influence of steel fiber on modulus of elasticity and Poisson's ratio of concrete are investigated, and flexural fracture toughness is calculated. Test results show that the effect of fiber volume fraction (Vf) and aspect ratio (lfdf) on flexural strength and fracture toughness is extremely prominent, compressive strength is only slightly improved, and tensile/compressive strength ratio is obviously enhanced. It is observed that the flexural deflection corresponded to ultimate load increased with the increase of Vf and lfdf, and due to fiber arresting cracking, the shape of the descending branch of load-deflection tends towards gently.

Preparation and properties of polypropylene composites reinforced with wheat and flax straw fibres: Part II Analysis of composite microstructure and mechanical properties

Abstract: The microstructure and mechanical properties of polypropylene composites containing flax and wheat straw fibres are discussed. Particular emphasis has been given to determining the nature and consequences of fibre damage induced during melt-processing operations, fibre orientation occurring in mouldings, and possible interfacial adhesion between the matrix and fibres. Compared to unfilled polypropylene, addition of flax and wheat straw caused a significant increase in tensile modulus, particularly, in the case of flax fibres, which also gave higher tensile yield strength and Charpy toughness, despite a lack of interfacial bonding. Tensile strength was increased further through inclusion of 5% by weight of maleic anhydride-modified polypropylene, which was shown to promote adhesion between fibres and matrix.

Shear and Flexural Strengthening of R/C Beams with Carbon Fiber Sheets

Abstract: The results of an experimental and analytical study of the behavior of damaged or understrength concrete beams retrofitted with thin carbon fiber reinforced plastic (CFRP) sheets are presented. The CFRP sheets are epoxy bonded to the tension face and web of concrete beams to enhance their flexural and shear strengths. The effect of CFRP sheets on strength and stiffness of the beams is considered for various orientations of the fibers with respect to the axis of the beam. Nineteen beams were fabricated, loaded beyond concrete cracking strength, and retrofitted with three different CFRP systems. The beams were subsequently loaded to failure. Different modes of failure and gain in the ultimate strength were observed, depending on the orientation of the fibers.

Short Banana Fiber Reinforced Polyester Composites: Mechanical, Failure and Aging Characteristics

Abstract: This paper describes the tensile, impact, flexural properties and aging behavior of short banana fiber reinforced polyester composites with special reference to the effect of fiber length and fiber content. Maximum tensile strength was observed at 30 mm fiber length while impact strength gave the maximum value for 40 mm fiber length. Incorporation of 40% untreated fibers gave a 20% increase in the tensile strength and a 341% increase in impact strength. On treatment with silane coupling agent, composites showed a 28% increase in tensile strength and a 13% increase in flexural strength. Aging studies showed a decrease in tensile strength of the composites. The experimental tensile strength values were compared with theoretical predictions according to Piggot equation. Scanning electron microscopy studies were carried out to understand the morphology of the fiber surface, fiber pullout and interface bonding. Water absorption studies showed an increase in water uptake with increase in fiber content. Finally,...

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.

Theoretical modelling of tensile properties of short sisal fibre-reinforced low-density polyethylene composites

Abstract: The experimentally observed tensile properties (tensile strength and modulus) of short sisal fibre-reinforced LDPE with different fibre loading have been compared with the existing theories of reinforcement The macroscopic behaviour of fibre-filled composites is affected by fibre loading, orientation and length of the fibres in the continuous medium The interfacial adhesion between fibre and matrix also plays a major role in controlling the mechanical properties of the fibre-filled composites In this study, a comparison is made between experimental data and different theoretical models Composite models, such as parallel and series, Hirsch, Cox, Halpin–Tsai, modified Halpin–Tsai and modified Bowyer and Bader, have been tried to fit the experimental data

Effect of combinations of surface treatments and bonding agents on the bond strength of repaired composites.

Abstract: Statement of problem. Enhancement of bond strength between new and old composite usually requires increasing the surface roughness to promote mechanical interlocking and coating of old composite with unfilled resin bonding agents to advance surface wetting and chemical bonding. Purpose. The purpose of this study was to evaluate the effect of combinations of surface treatments and bonding agents on the shear bond strength between new and old composite. Material and methods. Six surface treatments, two bonding agents, and an untreated control comprised 18 different subgroups. Results. The use of unfilled resin, alone or combined with silane, was the most effective procedure to enhance the shear bond strength of the repaired composite specimens, irrespective of the surface pretreatment processes. Silanation and unfilled resin slightly but not significantly improved the repair strength compared with unfilled resin alone. Conclusions. Different combinations of surface treatments and bonding agents affect shear bond strength differently. The highest shear bond strength values were achieved by grinding the surface with green Carborundum stone or sandblasting, whereas the lowest values were obtained with hydrofluoric acid as the surface treatment agent. (J Prosthet Dent 1997;77:122-26.)

Tissue Engineering of Bone

Abstract: Bone is a dynamic, highly vascularized tissue with the unique capacity to heal and to remodel depending on line of stress (Buckwalter et al, 1995ab). It exhibits the unlikely combination of high compressive strength and tensile strength due to the composite of calcium phosphate salts (hydroxyapatite) and collagen, respectively (Yaszemski et al, 1996a). It is difficult to find materials to mimic such a complex system when filling bone defects. However, current research capitalizes on the dynamic properties of bone by providing a biodegradable scaffold to guide healing.

Nickel filament polymer-matrix composites with low surface impedance and high electromagnetic interference shielding effectiveness

Abstract: The processing of nickel filaments of 0.4 Μm diameter gives polyethersulfone-matrix composites with high electromagnetic interference shielding effectiveness, high reflection coefficient and low surface impedance at 1-2 GHz. With 7 vol.% nickel filaments, the composite exhibited shielding effectiveness 87 dB (compared to 90 dB for solid copper), surface impedance 1.2 Ω (same as for solid copper), tensile strength 52 MPa, modulus 5 GPa, ductility 1.0%, and density 1.87 g/cm3.

Small‐scale polygonal features on Mars: Seasonal thermal contraction cracks in permafrost

Abstract: A time-dependent viscoelastic model of thermal stress in Martian ice-rich permafrost is developed to test the hypothesis that small-scale polygonal features observed from orbit and by the Viking Lander 2 are the result of thermal contraction cracking, as commonly occurs in terrestrial permafrost. Results indicate that significant tensile stress occurs in Martian ice-rich permafrost as a result of seasonal cycles in the ground temperature. Using conservative rheological parameters appropriate for ice at low temperatures, tensile stresses poleward of about 20° to 30° latitude easily exceed the tensile strength (assumed to be 2 to 3 MPa) and fractures should readily form. In the equatorial regions, special conditions may allow tensile stresses to approach the tensile strength. These results support a thermal contraction origin of observed small-scale polygonal features and emphasize the utility of these features as valuable morphological indicators of ground ice.

Biodegradation behavior and material properties of aliphatic/aromatic polyesters of commercial importance

Abstract: Copolyesters of aliphatic monomers with a defined amount of terephthalic acid recently have been shown to be biodegradable. This group of plastic materials exhibits very interesting material properties with regard to their technical application potential. A tensile strength of 25 N/mm2 combined with elongations at break up to 1500% was achieved for BTA materials. Melting points varied from 80 to 140‡C. Biodegradation rate under compositing conditions were determined, showing typical erosion rates of films, in the range of 5 to 10 Μm/week. The material properties and the degradation rate as well can be adjusted by the copolymer composition. Stretching of the polymer in the cold state leads to 10-fold higher mechanical strength of the material. The polyester chain can be extended to high molar masses, resulting in melt viscosities suitable, e.g., for melt below extrusion.

Effects of hydroxypropyl methylcellulose and other gelling agents on the handling properties of calcium phosphate cement

Abstract: The calcium phosphate cement (CPC) used in this study was formed by combining equimolar amounts of tetracalcium phosphate (TTCP) and dicalcium phosphate anhydrous (DCPA). This powder, when mixed with water, sets to a hard cement in about 30 min. However, the water-based CPC paste is not highly cohesive and is vulnerable to washout until hardening occurs. The objectives of this study were to investigate the effects on handling properties, washout resistance, cement hardening behavior, and mechanical properties of adding several gelling agents to CPC paste. Aqueous solutions that contained a mass fraction of 2-4% of hydroxypropyl methylcellulose (HPMC), carboxyl methylcellulose (CMC), chitosan acetate, and chitosan lactate were used as cement liquids. Hardening time was measured by the Gilmore needle test; resistance to washout was evaluated by the disintegration of the cement specimen in water with agitation; and mechanical strength was evaluated by the measurement of diametral tensile strength and compressive strength. Handling properties were greatly improved by the addition of HPMC, CMC, chitosan acetate, and chitosan lactate. Hardening time was retarded by the additions of HPMC and CMC, and mechanical strength was weakened by the addition of either the chitosan lactate or the chitosan acetate.

Statistical tensile strength of Nextel TM 610 and Nextel TM 720 fibres

Abstract: The properties of fibre-reinforced composites are dependent not only on the strength of the reinforcement fibre but also the distribution of fibre strength. In this study, the single filament strength of several lots of NextelTM 610 and NextelTM 720 ceramic fibres was measured. Fracture statistics were correlated with the effects of gauge length and diameter variation, and the Weibull modulus was calculated using several different techniques. It was found that the measured Weibull modulus at a single gauge length did not accurately predict either the gauge length or diameter dependence of tensile strength.

Thermal stability of a PCS-derived SiC fibre with a low oxygen content (Hi-Nicalon)

Abstract: The oxygen free Si–C fibre (Hi-Nicalon) consists of β-SiC nanocrystals (≈5nm) and stacked carbon layers of 2–3nm in extension, in the form of carbon network along the fibre. This microstructure gives rise to a high density, tensile strength, stiffness and electrical conductivity. With respect to a Si–C–O fibre (Nicalon NL202), the Si–C fibres have a much greater thermal stability owing to the absence of the unstable SiOxCy phase. Despite its high chemical stability, it is nevertheless subject to a slight structural evolution at high temperatures of both SiC and free carbon phases, beginning at pyrolysis temperatures in the range 1200–1400°C and improving with increasing pyrolysis temperature and annealing time. A moderate superficial decomposition is also observed beyond 1400°C, in the form of a carbon enriched layer whose thickness increases as the pyrolysis temperature and annealing time are raised. The strength reduction at ambient for pyrolysis temperatures below 1600°C could be caused by SiC coarsening or superficial degradation. Si–C fibres have a good oxidation resistance up to 1400°C, due to the formation of a protective silica layer.

Phase Transformations and Mechanical Properties of Fe-Mn-Si-Al TRIP-Steels

Abstract: Deformation twinning, martensitic phase transformation and mechanical properties of austenitic Fe-(15-30)wt%Mn alloys with additions of aluminium and silicon have been investigated. Tensile tests were carried out at different strain rates and temperatures. The formation of twins, α ' - and £-martensite during plastic deformation was analysed by optical microscopy, X-ray diffraction, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The stacking fault energy γ fcc and the free energy ΔG γ-e for the γ→e phase transformation were calculated using the regular solution model. It is known that additions of aluminium increase γ fcc and therefore strongly suppress the γ→e transformation while silicon decrease γ fcc and sustains the γ-e transformation. The γ→e phase transformation takes place in alloys with γ fcc 20 mJ/m 2 . The stacking fault energy of the Fe-25Mn-3Si-3Al alloy was calculated as a function of temperature and related with microstructural changes of the strained sample at different temperatures. These steels with reduced density of about 7,3 g/cm -3 combine high tensile ductility up to 80 % at high strain rates with true tensile strength of about 1000 MPa. The excellent plasticity induced by twinning and additional phase transformation up to extremely high strain rates of about e = 10 3 s -1 results in an extraordinary shock resistence and enables deep drawing and backward extrusion operations of parts with complex shapes and high production rates.