Showing papers on "Ultimate tensile strength published in 1994"

Mechanical strength of repairs of the rotator cuff

Abstract: We have studied the mechanical properties of several current techniques of tendon-to-bone suture employed in rotator-cuff repair. Non-absorbable braided polyester and absorbable polyglactin and polyglycolic acid sutures best combined ultimate tensile strength and stiffness. Polyglyconate and polydioxanone sutures failed only at high loads, but elongated considerably under moderate loads. We then compared the mechanical properties of nine different techniques of tendon grasping, using 159 normal infraspinatus tendons from sheep. The most commonly used simple stitch was mechanically poor: repairs with two or four such stitches failed at 184 N and 208 N respectively. A new modification of the Mason-Allen suture technique improved the ultimate tensile strength to 359 N for two stitches. Finally, we studied the mechanical properties of several methods of anchorage to bone using typically osteoporotic specimens. Single and even double transosseous sutures and suture anchor fixation both failed at low tensile loads (about 140 N). The use of a 2 mm thick, plate-like augmentation device improved the failure strength to 329 N. The mechanical properties of many current repair techniques are poor and can be greatly improved by using good materials, an improved tendon-grasping suture, and augmentation at the bone attachment.

Tensile-compressive yield asymmetries in high strength wrought magnesium alloys

Abstract: Owing to magnesium's very low density (of 17 g/cm[sup 3]) there is potential for producing Mg alloys with higher specific strengths than those of existing aluminium alloys for applications where weight savings are at a premium Magnesium alloys can also have greater creep resistance and superior elevated temperature specific tensile properties than those of Al However, the hexagonal crystal structure of magnesium severely limits its available slip systems and preferred orientations can develop during working processes, slip occurring predominantly on the basal planes which leads to their alignment parallel to the direction of metal flow As a geometric consequence of the c/a ratio of magnesium being less than ideal, twinning can theoretically only occur in compression, parallel to the basal planes, which would be expected to result in there being large asymmetries between the tensile and compressive yield stresses in textured wrought products The compressive yield stresses in wrought magnesium alloys have been previously found in forgings to the 06--07 times those of the tensile Such an asymmetric yield behavior, resulting in a poor compressive yield stress, would prohibit many potential applications for wrought Mg alloys The aim of this paper is to investigate how significant the problem of yieldmore » asymmetry can be in two typical high strength wrought Mg alloys, the Mg-Y-Nd mischmetal based alloy, WE54, and the Mg-Zn-Cu based alloy, ZC71« less

Mechanical properties of kaolinite/fiber soil composite

Abstract: The mechanical properties of a kaolinite/fiber soil composite were evaluated by a series of laboratory unconfined‐compression, splitting‐tension, three‐point‐bending, and hydraulic‐conductivity tests. The inclusion of randomly distributed fibers significantly increased the peak compressive strength, ductility, splitting tensile strength, and flexural toughness of kaolinite clay. The increase in strength and toughness was a function of fiber length and content, and the water content of the composite. Increasing fiber content increased the compressive and tensile strength, and the toughness index of kaolinite clay, with the effect being more pronounced at lower water contents. The contribution of fibers to peak compressive and tensile strengths were reduced, and ductility increased, with increasing fiber length. The fiber inclusion increased the hydraulic conductivity of the composite and the increase was more pronounced at higher fiber contents. Despite the increase, the hydraulic conductivity of the compo...

Fracture Behavior of Polymers

Abstract: Fracture analysis is complicated for polymers since, besides of temperature and time dependence, there are involved effects from plastification, chain orientation and adiabatic temperature rise. It is the aim of this section to describe several fracture processes which are specific of polymers. Fracture behavior is determined experimentally by the mode and time profile of loading. Stress- and strain controlled loading yields different behavior as well as loading in tension, compression or shear (torsion). Compressive strength is higher than tensile strength; shear strength is the lowest one.

Mechanical and thermal properties of dragline silk from the spider Nephila clavipes

Abstract: Dragline silk from the spider, Nephila clavipes, was characterized by thermal analysis (TGA, DSC, DMA), computational modeling, scanning electron microscopy and by quasi-static as well as high rates of strain. Thermal stability to about 230°C was observed by TGA, two transitions by DMA, −75°C, representative of localized motion in the amorphous domain, and a main chain motion associated with partial melt at 210°C. Tensile tests indicated average initial modulus, ultimate tensile strength and ultimate tensile strain of 22 GPa, 1.1 GPa and 9%, respectively. The corresponding properties of the best fibers tested were 60 GPa, 2.9 GPa and 11%, respectively. High strain rates (>50,000%/sec) indicated similar mechanical properties to the average values indicated above. Microscopy showed compressive and tensile strains to failure of 34%. Computational modeling yielded a crystal modulus of 200 GPa.

Failure mechanisms of 3D woven composites in tension, compression, and bending

Abstract: Observations of failure mechanisms in monotonic loading are reported for graphite/epoxy composites containing three-dimensional (3D) interlock weave reinforcement. The key phenomena are delamination and kink band formation in compression, tow rupture and pullout in tension, and combinations of these in bending. The materials exhibit great potential for damage tolerance and notch insensitivity. This is partly due to the presence of geometrical flaws that are broadly distributed in strength and space; and partly to the coarseness of the reinforcing tows, which leads to extensive debonding and reduced stress intensification around sites of failure. Rules of mixture corrected for the effects of tow irregularity suffice to estimate elastic moduli. Rough estimates of the stress at which the first failure events occur in compression or tension can be made from existing micromechanical models. Ultimate tensile failure might be modeled by regarding failed tows that are being pulled out of the composite as a cohesive zone. The characteristic length estimated for this zone, which is a direct measure of damage tolerance and notch insensitivity, has very large values of order of magnitude 0.1–0.5 m.

Testing system for determining the mechanical behaviour of early age concrete under restrained and free uniaxial shrinkage

Abstract: A modified uniaxial restrained shrinkage test was developed, characterized by complete automation and high accuracy of measurements. The system developed was such that tensile stresses remained constant throughout the cross-section, excluding any premature failure of specimen. This testing arrangement enables separation of creep strain from shrinkage by means of simultaneous testing of twin specimens, one under restrained shrinkage, and the other under free shrinkage. A variety of mechanical characteristics of the concrete (individual components of strain, shrinkage stresses, moduli of elasticity, creep coefficient and tensile strength) may be determined in one test. Results using this testing arrangement are presented for concrete cured in sealed conditions for 1 day and then exposed to drying at 30° C/40% RH.

Mechanical properties of LDPE/granular starch composites

Abstract: The mechanical properties of composites of granular starch and low density polyethylene (PE) have been studied as functions of starch volume fraction ϕ, granule size, and presence of compatibilizer. Property–volume fraction relationships were interpreted using various theories of composite properties. The dependence of elongation (ϵ ∼ ϕ1/3) and tensile strength (σ ∼ ϕ2/3) agree with theoretical predictions, although the proportionality constants are less negative than theoretical values. The addition of compatibilzer (ethylene-co-acrylic acid copolymer, EAA) did not significantly affect the elongation or tensile strength, but significantly increased the composite tensile modulus. The cornstarch/PE moduli could be described by the Kerner or Halpin-Tsai equations. Analysis of the composite moduli data using the Halpin-Tsai equation allowed the estimation of the modulus of granular starch. The value obtained, 15 GPa, is considerably greater than most unfilled synthetic polymers of commercial importance, but significantly lower than the modulus of cellulose. It is also greater than a previously reported value of 2.7 GPa. © 1994 John Wiley & Sons, Inc.

Size effects on tensile fracture properties: a unified explanation based on disorder and fractality of concrete microstructure

Abstract: Tests were carried out using three independent jacks orthogonally disposed, making it possible to apply a purely tensile force, so that the secondary flexural stresses, if kept under control, constitute a degree of error comparable with the values allowed for normal testing apparatus. The method enables a stress versus strain curve to be plotted with the descending (softening) branch up to the point where the cross-section of the tensile specimen breaks away. The principal purpose is to avoid any spurious effect that might provide a fallacious explanation of the recurring size effects on apparent tensile strength and fictitious fracture energy. Once the secondary effects have been excluded, only the disorder and fractality of the concrete microstructure remain to explain such fundamental trends. In the case of tensile strength, the dimensional decrement represents self-similar weakening of the material ligament, due to pores, voids, defects, cracks, aggregates, inclusions, etc. Analogously, in the case of fracture energy, the dimensional increment represents self-similar tortuosity of the fracture surface, as well as self-similar overlapping and distribution of microcracks in the direction orthogonal to that of the forming macrocrack.

Flexural/tensile-strength ratio in engineered cementitious composites

Abstract: In this paper the flexural behavior of a strain-hardening engineered cementitious composite (ECC) is studied and compared with that observed in a regular fiber/reinforced cementitious composite (FRC). Unlike concrete or regular FRC, ECC materials are characterized by their ability to sustain higher levels of loading after first cracking while undergoing additional straining. This strain-hardening behavior gives ECCs a significant advantage under flexural loading. In quasi-brittle materials such as regular FRC, the ratio of flexural strength (the modulus of rupture) to tensile strength is known to vary between 1 and 3, depending on the details of the reinforcement and the geometry of the specimen. In this paper, the strain-hardening behavior observed in an ECC led to a high flexural-strength-to-tensile-strength ratio. In a third-point bending test, the flexural strength of an ECC was measured to be five times its tensile (first-cracking) strength. This result was also predicted by a simple theoretical model. The model can be used for the purpose of optimizing the flexural strength of ECCs.

(Co)polymers of L‐lactide, 2. Mechanical properties

Abstract: The effect of copolymerization of L-lactide with several lactones and cyclic carbonates and the processing variables in compression moulding of poly(L-lactide) homopolymers on the mechanical properties was investigated. Sharp maxima in the impact strength versus comonomer composition and heat of fusion were obtained, reflecting the important role of the presence of a crystalline fraction in the material. Crystalline domains increase the interconnectivity of a physical polylactide network and decrease the molecular weight between these physical crosslinks. The brittle strength concomitantly increases, and as the yield stress remains more or less constant, an increase in impact strength is observed. Very high crystalline as-polymerized poly(L-lactide) prepared with less active catalysts on the other hand has an extremely low entanglement density and therefore a low brittle tensile strength.

Recycled Newspaper Fibers as Reinforcing Fillers in Thermoplastics: Part I-Analysis of Tensile and Impact Properties in Polypropylene:

Abstract: Recycled newspaper fibers (ONP) are potentially outstanding nonabrasive reinforcing fibers with high specific properties. In this study, a high energy thermokinetic mixer was used to mix these fibers in a polypropylene (PP) matrix, and the blends were then injection molded in order to observe the tensile and impact strengths of the compos ites. A 40% (weight) of ONP in PP resulted in a tensile strength of 34.1 MPa and an un notched Izod impact strength of 112 J/m. Small quantities of maleic anhydride-grafted polypropylene (MAPP) and acrylic acid-grafted polypropylene (AAPP) were used to im prove the interaction between the hydrophobic PP and the highly polar fibers. The im provement in properties by using MAPP depended on the amount of maleic anhydride in the graft copolymer and the molecular weight of the copolymer. Tensile strengths as high as 57 MPa and an unnotched Izod impact strength of 212 J/m were achieved with the addi tion of one of the MAPP, while smaller improvements were found with AAPP Reinf...

Measurement and prediction of the strength of rubberized concrete

Abstract: Illegally discarded piles of automobile tires are sources of potential hazards. Current disposal methods are wasteful and costly as they require either consumption of landfill space or continuous costly maintenance. A solution to the problem of scrap-tire disposal is the potential use of tire chips and crumb rubber as mineral aggregate substitutes in Portland cement concrete mixes. In this study, some of the engineering properties of rubberized concrete were examined and a neural network was developed to predict its compressive and tensile strengths. Rubberized concrete was found to possess good esthetics, acceptable workability, and a smaller unit weight than normal concrete. However, rubberized concrete did not perform as well as normal concrete under repeated freeze-thaw cycles. It exhibited lower compressive and tensile strength than that of normal concrete. Unlike normal concrete, rubberized concrete had the ability to absorb a large amount of plastic energy under compressive and tensile loads. It did not demonstrate the typical brittle failure, but rather a ductile, plastic failure mode. Test results were analyzed so that a model can be developed to predict the strength of rubberized concrete. Two neural network models were developed to predict the reduction in the compressive and tensile strength as a result of replacing mineral aggregate with rubber aggregate. A maximum difference of 9·2% between test results and model prediction was detected during the testing of the neural networks.

The large strain compression, tension, and simple shear of polycarbonate

Abstract: Polymeric materials subjected to large strains undergo an evolution in molecular orientation. The developing orientation and corresponding strengthening are highly dependent on the state of strain. In this paper, we examine and compare the very different stress-strain results of polycarbonate produced from four types of mechanical testing: uniaxial compression, plane strain compression, uniaxial tension, and simple shear. These tests produce different states of orientation within the material and, in the case of simple shear, the principle axes of orientation rotate during the deformation. The ability of the recent constitutive model of Anuda and Boyce (1992) to predict the observed behavior is evaluated. The model has been incorporated into a finite element code in order to properly simulate the material behavior during the inhomogeneous deformations of tension (cold drawing) and simple shear. The material properties of the model are obtained from the uniaxial compression test and the model is then found to be truly predictive of the other states of deformation demonstrating its fully three dimensional capability. The disadvantages of the tensile and simple shear tests for obtaining the data needed to accurately quantify the large strain material behavior of polymers are shown and discussed.

Stereocomplex formation between enantiomeric poly(lactic acid). VIII. Complex fibers spun from mixed solution of poly(D‐lactic acid) and poly(L‐lactic acid)

Abstract: To obtain poly(lactic acid) (PLA) complex fibers, spinning was performed by wet and dry methods from 5–10 g/dL chloroform solutions of poly(D-lactic acid) (PDLA) and poly(L-lactic), both with a viscosity-average molecular weight of 3 × 105. The dope was extruded from a monohole nozzle into coagulation baths from ethanol and chloroform for wet spinning and into a drying column kept at 60°C for dry spinning. Scanning electron microscopic observation of the as-spun fibers showed that the surface of the wet-spun fiber had large basins with diameters of 50–100 μm and many pores with diameters from sub μm to 10 μm, whereas the surface of dry-spun fiber had a microporous structure with the pore diameter of 1–3 μm. The tensile strength of the wet-spun complex fiber was very low and could not be drawn at high temperatures, in contrast to the dry-spun fiber. The tensile strength of dry-spun complex fiber increased upon hot drawing and showed the tensile strength of 94 kg/mm2 by drawing at 160°C to the draw ratio of 13. Differential scanning calorimetry revealed that the complex fibers contained both the stereocomplex crystallites (racemic crystallites) and the crystallites of the single polymers, PDLA and PLLA, regardless of the spinning methods. The ratio of the racemic crystallites to the single-polymer crystallites increased with the draw ratio of the complex fiber. © 1994 John Wiley & Sons, Inc.