Showing papers on "Flexural strength published in 1978"

Nonlinear Flexural-Flexural-Torsional Dynamics of Inextensional Beams. II. Forced Motions

Abstract: The nonplanar, nonlinear, resonant forced oscillations of a fixed-free beam are analyzed by a perturbation technique with the objective of determining quantitative and qualitative information about the response. The analysis is based on the differential equations of motion developed in Part I of this paper which retain not only the nonlinear inertia but also nonlinear curvature effects. It is shown that the latter play a significant role in the nonlinear flexural response of the beam.

Glass-ceramic structures and sintered multilayer substrates thereof with circuit patterns of gold, silver or copper

Abstract: Sintered glass-ceramic substrates containing multi-level, interconnected thick-film circuit patterns of highly conductive metals such as gold, silver or copper are provided which can be fired in air (for gold and silver) or in neutral atmospheres (for copper) at temperatures below the melting points of these metals. This has been made possible by the discovery that finely divided powders of certain glasses described herein sinter to essentially zero porosity at temperatures below 1000° C. while simultaneously maturing to glass-ceramics of low dielectric constant, high flexural strength and low thermal expansivity.

Static and Dynamic Fatigue of a Polymer-Coated Fused Silica Optical Fiber

Abstract: The dependence of fracture strength on tensile strain rate (dynamic fatigue) was used to establish the characteristic slow-crack-growth parameters in ethylene vinyl acetate-coated optical fibers. Strength was measured with five strain rates at each of four relative humidities (from 2 to 97% rh) on specimens 5 cm long. The inert fracture strength was measured at a low temperature in a dry atmosphere. The results of these tests were incorporated into a fracture mechanics model, including slow crack growth, for static fatigue. An increase in relative humidity promotes dynamic fatigue and leads to a predicted acceleration of static fatigue. Measurements of the static fatigue time-to-failure at three applied stress levels (on specimens 61 cm long) in 97% rh agree with the predicted behavior based on dynamic fatigue. The applicability of fracture mechanics crack-growth laws to fused silica fibers is discussed in terms of the Charles and Hillig stress-corrosion model.

Applications of the Average Stress Failure Criterion: Part II — Compression

Abstract: Results are presented for the compressive fracture strength of several Gr/Ep (AS/3501-5) laminates containing countersunk holes with both loaded and unloaded fasteners in place. Finite element stress analyses are presented which approximate the countersink geometry and the elastic contact problem. The average stress failure criterion is successfully used to predict the observed strengths in all cases considered.

Evaluation of a Fundamental Approach for the Statistical Analysis of Fracture

Abstract: Generalized relations between the strength distribution function and derivatives of the fracture probability are developed for five widely used testing methods for measuring the strengths of ceramic materials. The derived relations for three of these methods are used to analyze room-temperature fracture strength data for hot-pressed silicon nitride. Fracture of this material is shown to be controlled by internal flaws at strength levels up to and including the range where the tensile and flexural distributions overlap, but it is primarily surface-flaw-controlled toward the upper end of the flexural distribution.

Carbon fiber-reinforced carbon as a potential implant material.

Abstract: A carbon fiber-reinforced carbon is being evaluated as a promising implant material. In a unidirectional composite, high strengths (1200 MN/m2 longitudinal flexural strength) and high modulus (140 GN/m2 flexural modulus) may be obtained with an interlaminar shear strength of 18 MN/m2. Alternatively, layers of fibers may be laid in two directions to give more isotopic properties. The compatibility of the material with bone has been studied by implanting specimens in holes drilled in rat femora. For a period of up to 8 weeks, a thin layer of fibrous tissue bridged the gap between bone and implant; but this tissue mineralizes and by 10 weeks, bone can be observed adjacent to the implant, giving firm fixation. Potential applications include endosseous dental implants where a greater strength in the neck than that provided by unreinforced carbon would be advantageous.

The Flexural Strength of Aramid Fiber Composites

Abstract: An analysis is presented for the strength in three-point flexural loading of a rectangular unidirectional composite beam reinforced with Keviar®1 49 aramid fibers. The material behavior is assumed to be linearly elastic in tension and elastic-perfectly plastic in compression. The statistical effects of volume and stress distribution on tensile strength are included in the model. The predicted strength agrees reasonably well with experimental data.

Stress-strain characteristics of high-strength concrete

Abstract: The stress-strain relationship and flexural stress distribution for ultimate strength design has been well established from previous work. Generally, normal-weight concretes with strengths ranging from 1,000 psi to 7,500 psi (6.9 MPa to 51.7 MPa) have been investigated. In the present study, flexural characteristics of high-strength concretes were obtained from a series of specimens tested at the Portland Cement Association laboratories. The test series included concrete strengths ranging from 6,500 psi to 14,850 psi (44.8 MPa to 102.4 MPa) for normal-weight concretes and from 3,560 psi to 12,490 psi (24.5 MPa to 86.1 MPa) for lightweight concretes. Concretes containing three different normal-weight aggregates and two different light- weight aggregates were included in the study. Stress-strain curves, flexural constants, and moduli of elasticity are reported for the complete range of concrete strengths. Results of this investigation have been combined with those of other investigators. The data are compared with the latest ACI Building Code revisions pertaining to flexural constants for strength design. /Author/

On the influence of a flexural biasing state on the velocity of piezoelectric surface waves

Abstract: A system of linear electroelastic equations for small fields superposed on a bias is applied in the determination of the velocity of acoustic surface waves in piezoelectric substrates subject to flexural biasing stresses. The influence of the biasing stresses appears in the boundary conditions as well as the differential equations. Direct calculations performed for both quartz and lithium niobate when the spatial variation of the flexural biasing state is omitted indicate that the biasing stresses in the boundary conditions have an important influence of the surface wave velocity. In addition, perturbation calculations are performed which include the influence of the spatial variation of all flexural biasing terms and it is shown that, for substrate thickness-to-wavelength ratios well within the practical range, the spatial variation in the biasing state has an appreciable effect on the velocity of acoustic surface waves.

Ultimate Steel Stresses in Unbonded Prestressed Concrete

Abstract: Current design practice in the United States for unbonded post-tensioned flat plate structures is partially based on the assumption that the stress in the post-tensioned tendons at the time of flexural failure can be predicted using the equation given by the American Concrete Institute (ACI) Code for stresses in beams However, data from recent tests of a number of thin flat plate structures and of beams having comparable span-depth ratios show that this is generally not valid if the span-thickness ratio is about 45, or more The purpose of this paper is to present the test data and to demonstrate that an additional factor, the span-depth ratio, l/d, needs to be introduced into the prediction equations for ultimate steel stress in unbonded post-tensioned members

Effect of an External Stress on Moisture Diffusion and Degradation in a Graphite-Reinforced Epoxy Laminate

Abstract: The effect of external stress on moisture diffusion and mechanical properties degradation of a graphite/epoxy composite, Scotchply SP-313, has been investigated. Moisture gain as a function of time is determined for the composite subjected to four tensile stress levels in the range of 0 to 65 percent of its ultimate tensile strength (UTS). Experimental techniques to determine moisture gain in a composite subjected to external stress have been developed and discussed. In addition, the residual (after drying) and the current (wet) short-beam shear strength and the flexural strength as a function of moisture content and external stress level have been determined for the composite. The formation of environmental damage by the combined stress and humid environment conditions as well as the mode of failure in the short-beam test was investigated by scanning electron microscope (SEM) studies. Results indicated that the moisture absorption process into the graphite/epoxy composite subjected to external stress, at the levels used in this study, can be predicted effectively by Fick's second law of diffusion. Increasing the external stress level produces higher effective diffusivities, and higher initial absorption rates. A significant increase was observed in laminates subjected to more than 45 percent of their UTS and containing more than 1 percent moisture. This was attributed to early formation of cracks within the 90-deg plies caused by moisture degradation of the matrix or interface strength. The current and residual short-beam shear strength was reduced with increase in moisture content. By enhancing the moisture penetration into the composite, the external stress accelerates the mechanical degradation process. The flexural strength, which is a fiber-dominated property, showed a very low sensitivity to application of external stress for the levels used in the present study.

Carbon-carbon composite material and method for its production

Abstract: The present invention provides a carbon-carbon composite material consisting of a matrix carbon and a fibrous reinforcing carbon, said matrix carbon and fibrous carbon consisting substantially of amorphous carbon and forming an interface without an intervening third material, wherein said composite material has a fracture surface showing a uniform vitreous light reflection, but when it is used as an anode and electrolytically etched in sulfuric acid-acidified water, a difference in the degree of etching arises between the matrix carbon and the fibrous carbon, and a method for the preparation thereof. The composite material has a high flexural strength and a very low gas permeability and is useful as a molding material in the fields of high temperature chemistry, the atomic energy industry and medicine.

Cracking and toughening of concrete and polymer-concrete dispersed with short steel wires

Abstract: A fibre-strengthened brittle solid can crack and fracture in a number of ways and simple models can be used to describe quantitatively the fracture processes. This paper discusses some of these models and compares experimental measurements of cracking stress and toughness for two brittle fibrous composites with the theoretical predictions. The two brittle matrices are concrete and concrete impregnated with polymethylmethacrylate reinforced by discontinuous (short) high strength steel wires. It involved extracting a single steel wire from each brittle matrix to evaluate the debonding stress and pull-out stress as a function of fibre embedded length. These key material parameters and the energetics of cracking determined in three-point flexural experiments, together with the cracking and toughening equations are then used to characterize the fracture behaviour of fibrestrengthened concrete and polymer-concrete composites.

Bending Deformation Studies of Orthodontic Wires

Abstract: Bending deformation has been measured for orthodontic wires ranging in diameter from .010 to .051 inch. Results establish a revised expression for Young's modulus and show that either the stiffness tester or the torque meter will yield essentially the same measured values of bending properties. The dependence of test span length in determining Young's modulus and flexural yield strength is discussed.

Process for strengthening a shaped article of a polyester

Abstract: Heat treatment of shaped articles, larger than films or fibers, formed from anisotropic-melt-forming polymers provides an increase of at least 25% in energy to break in flexure. In at least one direction of measurement the heat treated articles are characterized by a flexural break strength of at least 10,000 psi, maximum strain to break in flexure of at least 2% and a flexural modulus of at least 500,000 psi. In many instances the heat treated articles can be further characterized by a notched Izod impact strength of at least 2 foot-pounds per inch. Since the heat-treated shaped articles have high levels of strength, stiffness and toughness, they are useful as replacements for some metals, e.g., brass.

Effect of long-term water exposure on properties of carbon and graphite fiber reinforced epoxies

Abstract: The effects of long term exposure to dry and humid environments on carbon-epoxy and graphite-epoxy composites have been studied. Filament wound Naval Ordnance Laboratory rings were fabricated in 1965 and were placed in dry, distilled water and sea water for 11 years. Moisture desorption tests were conducted in order to determine the water content of specimens exposed to the water environments, The effect of the history on moisture absorption characteristics was investigated by re-exposure of partially and completely dried specimens to two different environments: distilled water at 60°C and 98% relative humidity at 60°C. The weight gain was measured and diffusion coefficients were calculated. Horizontal shear tests and flexural tests were performed on “wet” specimens (current properties) and on partially and completely dried specimens (residual properties). The shear strength of the carbon-epoxy composites is degraded by the exposure while that of the graphite-epoxy composite is slightly increased. The composite flexural strengths are not degraded by the exposure. These results are discussed in terms of failure modes.

Strength of a dense alumina ceramic after aging in vitro.

Abstract: Rods of dense alumina ceramic were aged in Ringer's solution for time periods up to 12 weeks, with half of the rods being aged while mechanically stressed. Pyrex-type glass rods were similarly tested for comparison purposes. No reduction in flexural strength was observed in dense alumina rods after aging, providing no permeation of the solution into the ceramic occurred. However, a marked reduction in flexural strength was experienced by alumina rods, nominally dense and nonporous, into which some permeation of the saline solution was observed after aging; the largest reduction in strength occurred in such rods aged while mechanically stressed. The results indicate the importance and advantage of using densely sintered alumina ceramics as implants for optimum structural performance over an extended time period.

Mechanical Fatigue Characteristics of High Voltage Generator Insulation

Abstract: This paper presents the deformation and fracture behavior of stator windings subjected to static and cyclic bending loadings, and the reliability of newly developed insulation systems. The flexural deformation characteristics are revealed on the curved end portion of the stator windings for a 1300 MVA water cooled generator. The fatigue characteristics are examined on the test bars with three different kinds of the developed insulation systems. Comparing those results with calculated thermal strain, it has been confirmed that the insulation systems have sufficient mechanical strength to withstand thermal cycling in service.

Axial Hysteresis Behavior with End Restraints

Abstract: Parametric study of axially loaded steel members with symmetrical rotational end springs (connections) was performed. The main parameters are the connection flexural strength, the effective slenderness ratio, and cross-sectional properties. The rotational end spring is a simulation of the end restraint due to flexural rigidity of the connections of an axially loaded (bracing) member. The member with balanced strength connections, i.e., where plastic hinges form simultaneously at midspan and at the ends, is more efficient with respect to compressive load and energy dissipation capacities than members of the same length and same cross-sectional properties with nonbalanced strength connections. The hysteresis behavior of a balanced strength member can be adequately represented by that of a pin-connected member of the same cross section and same effective slenderness ratio.

Force–displacement evaluation of macromolecular materials in flexural impact tests. II. Influence of rubber content, degree of grafting, and temperature on the impact behavior of ABS resins

Abstract: The impact behavior of ABS (acrylonitrile–butadiene–styrene) resins was studied by measuring the force–deflection curve generated during flexural impact tests. Each curve is characterized by a limited number of significant parameters. Their dependence on the following constitutive or ambient variables was investigated: content of butadiene rubber, degree of grafting of acrylonitrile and styrene onto polybutadiene, and temperature. The results are analyzed phenomenologically and are tentatively interpreted in the light of possible micromechanical mechanisms.

Flexural Effects in Disc Brake Pads

Abstract: Details of the automobile disc brake are outlined, leading to the flexural requirements and construction of friction pad assemblies. A theoretical treatment of these is given, introducing the conce...

Effect of particle size on the mechanical properties of poly(vinylchloride)-copper particulate composite

Abstract: The effect of particle size ratios on tensile strength, tensile modulus, impact strength, Vickers hardness and transverse rupture strength of particulate PVC-Cu composite has been studied. The strength values reach a maximum at a particular metal loading where the formation of a segregated network is believed to occur. This critical metal loading shifts towards lower values with increasing polymer-to-metal size ratios. The mean free path calculated from the proposed model is found to be same in all metal loadings, corresponding to strength maxima irrespective of the particle size ratios. A correlation between strength properties and the mean free path has been suggested.

Composite Beam Criteria in LRFD

Abstract: Methods are proposed for the computation of flexural strength of composite beams and for design of shear connectors based on limit states of plastic strength. Both fully and partially composite beams with formed steel deck or flat soffit slabs are considered. LRFD resistance factors are derived using a first-order second-moment probability analysis. Linear plastic strength equations for partially composite beams and a direct design procedure for the steel section and shear connectors are presented. Also treated are effective slab width, beams under construction, and approximate formulas for effective moment of inertia and section modulus of partially composite beams.