Showing papers on "Ultimate tensile strength published in 1992"

Intrinsic stress in sputter-deposited thin films

Abstract: A review of the sputtered film stress literature shows that the intrinsic stress can be tensile or compressive depending on the energetics of the deposition process. Modeling studies of film growth and extensive experimental evidence show a direct link between the energetics of the deposition process and film microstructure, which in turn determines the nature and magnitude of the stress. The fundamental quantities are the particle flux and energy striking the condensing film, which are a function of many process parameters such as pressure (discharge voltage), target/sputtering gas mass ratio, cathode shape, bias voltage, and substrate orientation. Tensile stress is generally observed in zone 1-type, porous films and is explained in terms of the grain boundary relaxation model, whereas compressive stress, observed in zone T-type, dense films, is interpreted in terms of the atomic peening mechanism. Modeling of the atomic peening mechanism and experimental data indicate that the normalized moment...

Microstructural design of fiber composites

Abstract: The optimum performance design of composite microstructures is discussed. The forces driving progress in fiber composites are examined, and recent developments in the mechanics of laminated composites are surveyed, emphasizing thick laminates, hygrothermal effects, and thermal transient effects. The strength of continuous-fiber composites is discussed, presenting analyses of local load redistribution due to fiber breakages and treatments of statistical tensile strength theories. Modes of failure of laminated composites are examined. Elastic, physical, and viscoelastic properties as well as the strength and fracture behavior of short-fiber composites are studied, and it is shown how the performance of composites can be controlled by selecting material systems and their geometric distributions. 2D textile structural composites based on woven, knitted, and braided preforms are considered, and techniques for analyzing and modeling the thermomechanical behavior of 2D textile composites are presented. Recent developments in the processing of 3D textile preforms are introduced and the processing-microstructure relationship is demonstrated. Finite elastic deformation of flexible composites is addressed.

Cracking in drying soils

Abstract: Cracking in soils that are undergoing drying is controlled by soil suctions and by soil properties such as compression modulus, Poisson's ratio, shear strength, tensile strength, and specific surfa...

Mg–Cu–Y Bulk Amorphous Alloys with High Tensile Strength Produced by a High-Pressure Die Casting Method

Abstract: Amorphous Mg-Cu-Y alloys with diameters (or thicknesses) ranging from 1 to 7mm were formed in cylinder and sheet forms by using a high-pressure die casting process. The maximum diameter (or thickness) for formation of an amorphous phase was strongly dependent on composition and the largest value was obtained for Mg 65 Cu 25 Y 10 . There is a tendency for the maximum diameter to increase with an increase in the supercooled liquid region. The glass transition temperature, crystallization temperature and heat of crystallization are independent of sample diameter in the range below 7 mm while the heat of relaxation decreases with increasing sample diameter, indicative of the formation of a more relaxed atomic configuration in the thicker samples

Room-temperature tensile deformation of polysynthetically twinned (PST) crystals of TiAl

Abstract: Tensile deformation behavior of polysynthetically twinned (PST) crystals of TiAl with a nearly stoichiometric composition was investigated as a function of the angle o between the lamellar boundaries and tensile axis at room temperature. Tensile elongation to fracture strongly depends on the angle o but its dependence on the angle o is not symmetrical with respect to o = 45°. A tensile elongation as large as 20%, which is far larger than any other reported values on TiAl-based compounds, has been obtained for o = 31°. The yield stress also strongly depends on the angle o but any particularly significant tension-compression asymmetry in yield stress has not been observed. Fracture has been found to occur in a brittle manner without showing any local contraction even after deformation to more than 10%. When the tensile axis is perpendicular or inclined to the lamellar boundaries, fracture occurs in a cleavage-like mode with a habit plane parallel to the lamellar boundaries while fracture occurs in a zigzag across the lamellar boundaries when the tensile axis is parallel to the lamellar boundaries.

Video-controlled tensile testing of polymers and metals beyond the necking point

Abstract: A novel technique has been developed to record the intrinsic plastic behaviour of ductile materials by monitoring the effective strain and the effective stress in the mid-plane of hour-glass-shaped tensile specimens. The method utilizes a computer-aided video extensometer which analyses the sample profile in real time. The effective strain is computed automatically from the minimum diameter, and the effective stress is deduced from the applied load by taking into account the stress triaxiality corresponding to the local radius of curvature of the sample profile. Furthermore, a digital closed-loop system controls the ram speed of the hydraulic tensile testing machine in such a way that the local effective strain rate is maintained at a constant value. It is shown that most polymeric and metallic materials are entitled to be investigated by this method, which gives access in real time to the constitutive plastic equation, up to strains far beyond the necking point. The capabilities of the technique are illustrated and discussed critically, with more details for two polymers of different structures: polyethylene and polycarbonate.

Tensile and tensile fatigue behaviour of concrete; experiments, modelling and analyses

Abstract: A model based on nonlinear fracture mechanics is presented for the fatigue behavior of plain concrete. The tensile behavior of concrete for a monotonic increasing deformation is described. The softening relation is described and the way the fracture mechanics parameters are influenced by several variables is shown. The behavior of a softening material subjected to a uniaxial tensile test is explained. Based on experimental results, a constitutive model for the crack cyclic behavior of concrete is proposed. The adequacy of the model is shown by numerical simulation of notched beams under 4-point bending. It is shown by experiments, that localization and non-uniform crack opening occurs in tensile fatigue tests just as in monotonic loaded tensile tests.

Strength of Concrete‐Filled Thin‐Walled Steel Box Columns: Experiment

Abstract: An experimental study on the strength and deformation of concrete‐filled square box stub‐columns is presented. Six specimens of concrete‐filled composite columns were tested under cyclic compressive loads. For comparison, four specimens of steel columns were also loaded to failure. The ultimate strength, ductility, and collapse behavior of the two types of columns were compared. In the comparison, the effect of width‐thickness ratio and stiffener rigidity on the behavior of the columns was examined. The study showed that high strength and high ductility can be expected from the concrete‐filled composite column. In the case of concrete‐filled composite columns, an empirical reduction factor that accounts for the effect of the size of the filled‐in concrete prism and the concrete strength class was introduced in evaluating the compressive strength of the concrete, and the local buckling strength of the plate panel in composite columns was then compared with available empirical design formulas for a thin‐wal...

Compression Failure Mechanisms in Unidirectional Composites

Abstract: Compression failure mechanisms in unidirectional composites were examined. Possible failure modes of constituent materials are summarized and analytical models for fiber microbuckling are reviewed from a unified viewpoint. Due to deficiencies in available models, a failure model based on nonlinear properties and initial fiber curvature is proposed. The effect of constituent properties on composite compression behavior was experimentally investigated using two different graphite fibers and four different epoxy resins. The predominant microscopic scale failure mode was found to be shear crippling. In a soft resin, shear crippling was in the form of buckling of fibers on a microscopic scale. However, stiff resins failure was characterized by the formation of a kink band. For unidirectional laminates, compressive strength, and compressive modulus to a less extent, were found to increase with increasing magnitude of resin modulus. The change in compressive strength with resin modulus was predicted using the proposed nonlinear model.

Improving bond strength through acid etching of dentin and bonding to wet dentin surfaces.

Abstract: A bonding system using moisture on the tooth surface can be an enormous benefit as obtaining dentin dryness in the mouth is nearly impossible. The author describes a system that bonds to both wet and dry surfaces.

Effects of particle size on mechanical and impact properties of epoxy resin filled with spherical silica

Abstract: Effects of particle size on the mechanical and impact properties of cured epoxy resins are studied. This resin was filled with spherical silica particles prepared by hydrolysis of silicon tetrachloride. Particles were sorted into five kinds of different mean sizes in the range from 6–42 μm. A static flexural and tensile tests and an instrumented Charpy type impact test were carried out. Flexural strength, tensile strength, and impact-absorbed energy increased with a decrease in the particle size. Fractured surfaces were observed using a scanning electron microscope to clarify the initiation point of fracture.

Heat-pressed ceramics: technology and strength.

Abstract: The flexural strength of a new heat-pressed ceramic material (IPS-Empress) was measured before and after pressing and/or simulated firing treatments (eg, veneering, surface coloring, glazing). Heat pressing the material significantly improved its flexure strength whereas heat treating the material alone did not. Additional firings (heat treatments) after heat pressing further increased material strength. The final strength values ranged between 160 and 180 MPa and were comparable to some other all-ceramic systems. No clinical implications were drawn from these data.

Evaluation of hydroxylapatite/poly(L-lactide) composites: mechanical behavior

Abstract: By mixing hydroxylapatite (HA) into L(-)-dilactide monomer, prior to polymerization to poly(L-lactide) (PLLA), hydroxylapatite filled poly(L-lactide) composites were obtained. This study reports about the mechanical properties of these composites compared with unfilled PLLA. It was concluded that a 30 wt% HA/PLLA composite has better compressive and tensile strengths, higher stiffness and Vickers hardness number than unfilled PLLA (Mv: 125-150,000). Gas sterilization (ethylene oxide) affects molecular weight and flexural strength significantly. Implantation studies revealed loss of 50% of initial flexural strength within 3 weeks, and a faster decline of flexural strength was observed in phosphate buffered saline than in the subcutis of goats. From a mechanical point of view storage at -20 degrees C proved to be a safe method. In its current state HA/PLLA composites can not be used as implant materials that have to resist major forces. However, such composites might be useful in non-loadbearing applications in orthopedic or maxillofacial surgery.

Properties of Sisal-CNSL Composites

Abstract: Cashew nut shell liquid (CNSL) is a natural monomer blend that has been condensation poylmerized with formaldehyde in the presence of an alkaline catalyst to produce a thermosetting resin. Plain woven mats of mercerized sisal fibre have been impregnated with CNSL-formaldehyde resin to produce plain and corrugated laminated composites that have a mean tensile strength of 24.5 MPa and Young's modulus of 8.8 GPa. Bending tests have demonstrated that the corrugated composites have adequate strength for roofing applications. Dynamic mechanical thermal analysis has been used to assess the effect of simulated sunlight on composites as a function of time. After long irradiation times it has been deduced that the resin component of the composite undergoes further cross-linking whilst the reinforcing cellulosic sisal fibres suffer some degradation.

Grain size effects in nanocrystalline materials

Abstract: Nanocrystalline materials have a grain size of only a few nanometers and are expected to possess very high hardness and strength values. Even though the hardness/strength is expected to increase with a decrease in grain size, recent observations have indicated that the hardness increases in some cases and decreases in other cases. A careful analysis of the available results on the basis of existing models suggests that there is a critical grain size below which the triple junction volume fraction increases considerably over the grain boundary volume fraction and this is suggested to be responsible for the observed softening at small grain sizes.

Damage and Failure in Unidirectional Ceramic–Matrix Composites

Abstract: A study of the mechanical characteristics of a unidirectional fiber–reinforced calcium aluminosilicate matrix composite has been conducted. The properties have been related to the individual properties of the matrix, the fibers, and the interfaces, as well as the residual stress, using available models of matrix cracking and fiber fracture. Comparisons have been made with lithium aluminosilicate matrix composites. Predictions of initial matrix cracking and of ultimate strength using the models are found to correlate well with the measured values. However, deficiencies have been noted in the ability of the models to predict the evolution of matrix cracks, plus associated changes in the modulus.

The relationship between deposition conditions, the beta to alpha phase transformation, and stress relaxation in tantalum thin films

Abstract: We demonstrate that the high temperature polymorphic tantalum phase transition from the tetragonal beta phase to the cubic alpha phase causes a large decrease in the resistance of thin films and a complete stress relaxation in films that were intrinsically compressively stressed. 100 nm beta tantalum thin films with intrinsic stresses of 2.0×1010 dynes/cm2 (tensile) to −2.3×1010 dynes/cm2 (compressive) were deposited onto thermally oxidized (100) silicon wafers by evaporation or dc magnetron sputtering with argon. In situ stress and resistance at temperature were measured at 10 °C/min up to 850 °C in purified helium. Upon heating, the main stress mechanisms were elastic deformation at low temperature, plastic deformation at moderate temperatures and stress relief because of the beta‐to‐alpha phase transition at high temperatures. The temperature ranges over which the elastic and plastic deformation and the beta‐to‐alpha phase transition occurred varied with deposition pressure and substrate biasing. Incomplete compressive stress relaxation at high temperatures was observed if the film was initially deposited in the alpha phase or if the beta phase did not completely transform into alpha by 800 °C due to substrate biasing during the deposition. We conclude that the main stress relief mechanism for tantalum films with intrinsic compressive stresses to completely relax their stress is the beta‐to‐alpha phase transition, while for intrinsically tensile films, this transformation has a much smaller effect on the stress.

Measurement of interface strength by a laser spallation technique

Abstract: A laser spallation experiment has been developed to measure the strength of planar interfaces between a substrate and a thin coating (in the thickness range of 0.3–3 μm). In this technique a laser pulse of a high enough energy and a pre-determined duration is converted into a pressure pulse of a critical amplitude and width that is sent through the substrate toward the free surface with the coating. The reflected tensile wave from the free surface of the coating pries-off the coating. The critical stress amplitude that accomplishes the removal of the coating is determined from a computer simulation of the process. The simulation itself is verified by means of a piezo-electric crystal probe that is capable of mapping out the profile of the stress pulse generated by the laser pulse. Interface strength values ranging from 3.7 to 10.5 GPa were determined for the Si/SiC system. For the interfaces between pyrolytic graphite and SiC coatings an average strength of 7.2 GPA was measured, while the corresponding interface strength between a Pitch-55 type ribbon with a fiber-like morphology and SiC coatings was found to be 0.23 GPa. Intrinsic strengths of SiC coatings and Si crystal were also determined using this technique. These were, on the average, 8.6 GPa for Si crystals and 11.9 GPa for a SiC coating. Furthermore, the potential of the laser technique to determine the interface toughness was also demonstrated, provided well-characterizable flaws can be planted on the interface.

Effect of starch granule size on physical properties of starch-filled polyethylene film

Abstract: Physical properties of blown films (25-60-μm thickness) from compounded mixtures of linear low-density polyethylene (LLDPE) and starch were investigated. As starch content increased, tensile strength, percent elongation, and light transmittance decreased and film thickness increased. Among the tested films, small-particle corn starch (2-μm average diameter) film had the highest elongation rate and tensile and yield strength (560%, 3.15 kg/mm2, and 1.07 kg/mm2, respectively, at 15% starch content). Potato starch (35-μm average diameter) film had the lowest values (508%, 1.52 kg/mm2, and 0.55 kg/mm2, respectively, at 15% starch content). Potato starch-LLDPE film had the highest light transmittance and film thickness; small-particle corn starch had the lowest. Tensile and yield strength of the films had strong negative correlations with average starch granule diameter (R=0.99 and-0.94, respectively). Film thickness and light transmittance were linearly correlated with starch granule size (R=0.93 and 0.87, respectively). Using small-particle corn starch substantially increased incorporated starch level in the film while maintaining the film quality.

Studies on polypropylene composites filled with talc particles

Abstract: Tensile and impact properties of talc-filled isotactic polypropylene composites are investigated at 0–60 wt% filler contents. Tensile modulus registered an increase whereas tensile yield strength and strain-at-break decreased with increasing filler content. Mechanical restraint imposed by the talc particles on the molecular mobility or deformability of polypropylene explained the increase in modulus and decrease in strain-at-break while decrease in tensile yield strength was attributed to decreased crystallinity and formation of stress concentration points around the filler particles. Izod impact strength decreased with increased talc content. Surface modification of talc with a titanate coupling agent LICA 38 enhanced the filler-polymer interaction, further modifying the composite properties consequent upon significant decrease in the stress concentration. Scanning electron microscopic studies revealed better dispersion of surface-modified filler particles in the polymer matrix.

Method of increasing the tensile strength of a dilatation balloon

Abstract: A method of making a dilatation balloon with a high percentage of the maximum tensile strength of the balloon material from a thin wall parison of a biaxially orientable polymer, such as polyethylene terephthalate (PET). A reverse temperature gradient (decreasing going from the inner to outer diameters) is applied across the sidewall of the parison by flowing a heated fluid through the parison and then sealing one end of the parison and expanding with a heated expansion fluid. Decreases in wall thickness and/or increases in burst strength across the wall can be achieved.

Comparison of bond strength of six soft denture liners to denture base resin

Abstract: The bond strength of six commercial soft denture liners was evaluated using a modified tensile test. The soft denture liners investigated were Prolastic, VinaSoft, Flexor, Molloplast-B, Novus, and Super-Soft. The samples were processed according to the manufacturers' instructions to cured denture base resin (polymethyl methacrylate; PMMA). The soft denture liners were 10 x 10 x 3 mm and were processed between two PMMA blocks. The samples were placed in tension until failure. The mode of failure, cohesive or adhesive, was also recorded. The results of this study showed that the bond strength is related to the components of the materials. Prolastic, VinaSoft, and Flexor had the lowest bond strength to cured PMMA and ranged from 9.6 to 11.3 kg/cm2. Super-Soft, Novus, and Molloplast-B demonstrated better bond strengths and ranged from 16.7 to 17.6 kg/cm2. The bond strength of Novus could be improved by using the recommended bonding agent and bonded Novus at 26.1 kg/cm2 had the highest bond strength of all materials tested.

Tensile properties of polypropylene/kaolin composites

Abstract: Tensile properties of isotactic polypropylene filled with particulate kaolin fillers were evaluated in the composition range 0–60 wt % kaolin. Tensile modulus increased with filler concentration while breaking elongation and tensile strength decreased. The modulus increase was attributed to the restriction on the molecular mobility of the polymer imposed by kaolin particles. The decrease in elongation was also an effect of this restriction coupled with interference to stress transfer by the filler particles. Generation of discontinuity in the composite structure through formation of stress concentration points accounted for the tensile strength decrease. Morphology studies by SEM also indicated the introduction of stress concentration points by the presence of bare and nonadherent kaolin particles and their agglomerates with sharp edges in these composites.

Correlation of deformation mechanisms with the tensile and compressive behavior of NiAl and NiAl(Zr) intermetallic alloys

Abstract: To identify the mechanisms controlling strength and ductility in powder-extruded NiAl and NiAl + 0.05 at. pct Zr, tensile and compressive testing was performed from 300 to 1300 K for several grain sizes. Grain size refinement significantly increased yield stress in both alloys and, in some cases, slightly lowered the ductile-to-brittle transition temperature (DBTT), although no room-temperature tensile ductility was observed even in the finest grain size specimens. The small Zr addition increased the DBTT and changed the low-temperature fracture mode from intergranular in NiAl to a combination of intergranular and transgranular in the Zr-doped alloy. Scanning electron microscopy (SEM) of compression specimens deformed at room temperature revealed the presence of grain-boundary cracks in both alloys. These cracks were due to the incompatibility of strain in the poly crystalline material, owing to the lack of five independent slip systems. The tendency to form grain-boundary cracks, in addition to the low fracture stress of these alloys, contributed to the lack of tensile ductility at low temperatures. The operative slip system, both below and above the DBTT, was {110} 〈001〉 for both alloys. The change from brittle to ductile behavior with increasing temperatures was associated with the onset of diffusional processes.