Showing papers on "Flexural strength published in 1968"

The effect of carbon fibre reinforcement on the friction and wear of polymers

Abstract: Thermosetting and thermoplastic polymers have been reinforced with carbon fibres of high strength and elastic modulus, and their friction and wear properties examined. During dry sliding against steel, randomly oriented chopped fibres reduce both the coefficients of friction and the wear rates of polymers to levels which are approximately independent of the matrix material. Fibre orientation is also important, and minimum wear is obtained when the fibres are normal to the sliding surface. Carbon fibre reinforced polymers are less abrasive than those containing glass, exhibit lower friction and have greater moduli of elasticity; the flexural strengths are also often greater, and particularly so with polytetrafluoroethylene (PTFE). It is concluded that the carbon fibres reduce the friction and wear of polymers by preferentially supporting part of the applied load, and in addition by smoothing the surface of the steel counterface.

Maximum Crack Width in Reinforced Concrete Flexural Members

Abstract: THE MAXIMUM CRACK WIDTH MEASUREMENTS FROM A NUMBER OF CRACK WIDTH INVESTIGATIONS WERE EXAMINED USING STATISTICAL METHODS. PREVIOUSLY PROPOSED EQUATIONS FOR MAXIMUM CRACK WIDTH WERE COMPARED AND NEW EQUATIONS WERE PROPOSED AS A RESULT OF THE ANALYSES. INTEREST IN CRACK WIDTHS IN FLEXURAL MEMBERS HAS BEEN STIMULATED BY THE TREND TO USE HIGH STRENGTH DEFORMED REINFORCING BARS. VARIOUS INVESTIGATIONS HAVE PRODUCED MUCH INFORMATION ABOUT THE WIDTH AND SPACING OF CRACKS. HOWEVER, DUE TO THE RELATIVELY LARGE SCATTER IN THE WIDTH OF THE LARGEST CRACKS, AND TO THE LARGE NUMBER OF INTERRELATED VARIABLES PRESENT IN REINFORCED CONCRETE BEAMS, AGREEMENT WAS LACKING AMONG THE INVESTIGATORS AS TO THE MOST PERTINENT VARIABLES INFLUENCING THE SIZE OF THE CRACKS. BY USING DATA FROM SIX DIFFERENT INVESTIGATIONS, THE SAMPLE WAS LARGE AND DIVERSE ENOUGH TO PERMIT RELIABLE STATISTICAL ANALYSES. THE MAXIMUM CRACK WIDTH MEASURED ON A BEAM AT A CERTAIN STEEL STRESS LEVEL REPRESENTED A STATISTICAL SAMPLE. IN THE COMPUTER INPUT, EACH SAMPLE WAS ACCOMPANIED BY ALL BEAM DETAILS THAT COULD INFLUENCE THE CRACK WIDTH. THE MAXIMUM CRACK WIDTH ON THE TENSION FACE OF THE BEAM AND THE MAXIMUM CRACK WIDTH ON THE SIDE OF THE BEAM AT THE LEVEL OF THE TENSILE REINFORCEMENT WERE ANALYZED SEPARATELY. THE FOLLOWING MAJOR CONCLUSIONS RESULTED FROM THE ANALYSIS OF THE MAXIMUM CRACK WIDTH: (1) THE STEEL STRESS IS THE MOST IMPORTANT VARIABLE, (2) THE DIAMETER OF THE REINFORCING BAR IS NOT A MAJOR VARIABLE, (3) THE THICKNESS OF THE CONCRETE COVER IS IMPORTANT, BUT IT IS NOT THE ONLY FACTOR REFLECTING GEOMETRY, (4) THE AREA OF THE CONCRETE SURROUNDING EACH REINFORCING BAR, A, ALSO INFLUENCES THE CRACK WIDTH, (5) THE SIZE OF THE SIDE CRACK WIDTH IS REDUCED BY THE PROXIMITY OF THE COMPRESSION ZONE (NEUTRAL AXIS) IN FLEXURAL MEMBERS, (6) THE BOTTOM CRACK WIDTH IS INFLUENCED BY THE STRAIN GRADIENT FROM THE LEVEL OF THE STEEL TO THE TENSION FACE OF THE BEAM. /ACIJP/

The critical tensile stress criterion for cleavage

Abstract: An elastic-plastic analysis was used to accurately measure the microscopic cleavage strength σ inff sup* of notched bars of high nitrogen steel in bending. It was found that σ inff sup* increases as the root radius of the notch ϱ decreases. For ϱ > 0.010″, the variation of σ inff sup* with ϱ, and the difference between σ ingff sup* and the cleavage fracture strength of a plane tensile specimen, σf, may result from a statistical effect, due to differences in the volume of highly stressed material in the plastic zone. For ϱ < .010″, the primary reason for the apparent increase in σ inff sup* with decreasing ϱ, is the steep stress gradient at the notch tip, which forces the critical plastic zone size to extend further to insure that unstable microcraek propagation can occur. Both the statistical and stress gradient effects have been quantitatively evaluated and found to be in good agreement with the experimental data.

Exploratory Shear Tests EmphasizingPercentage of Longitudinal Steel

Abstract: THE SHEAR STRENGTH OF REINFORCED CONCRETE BEAMS AS GIVEN BY ACI 318-63 IS KNOWN TO BE UNCONSERVATIVE WHEN THE RATIO OF THE LONGITUDINAL REINFORCEMENT IS SMALL. THE AIM OF THE PRESENT INVESTIGATION WAS TO ASSESS THE EXTENT OF THIS INADEQUACY. TEN RECTANGULAR BEAMS WITHOUT WEB REINFORCEMENT AND HAVING BETWEEN 0.0173 AND 0.0025 WERE TESTED; RESULTS OF 27 OTHER BEAMS WITH LESS THAN 0.012 FROM VARIOUS OTHER INVESTIGATORS WERE ALSO ANALYZED. ALL THE BEAMS CONSIDERED HAD A/D RATIOS GREATER THAN 2.75. THREE RECTANGULAR BEAMS WITH WEB REINFORCEMENT ARE ALSO REPORTED. /AUTHOR/

Chemical Strengthening of Polycrystalline Alumina

Abstract: Low-expansion solid solution surface layers formed by reaction of Cr2O3 and Al2O3 were used to chemically strengthen alumina bodies. The surface layers were formed by packing the samples in Cr2O3 powder and heating to high temperatures. Fluorides or chlorides were added to the packing material, leading to substantial additional improvement. The flexural strengths of the control samples were increased by refiring in the presence of the products of decomposition of the fluorides. At high concentrations of fluorides in the packing material, the outside dimensions and the weights of the samples decreased during the treatments.

The Effect of Carbon Fibre Reinforcement on the Friction and Wear of Polymers

Abstract: Thermosetting and thermoplastic polymers have been reinforced with carbon fibres of high strength and elastic modulus, and their friction and wear properties examined. During dry sliding against steel, randomly oriented chopped fibres reduce both the coefficients of friction and the wear rates of polymers to levels which are approximately independent of the matrix material. Fibre orientation is also important, and minimum wear is obtained when the fibres are normal to the sliding surface. Carbon fibre reinforced polymers are less abrasive than those containing glass, exhibit lower friction and have greater moduli of elasticity; the flexural strengths are also often greater, and particularly so with polytetrafluoroethylene (PTFE). It is concluded that the carbon fibres reduce the friction and wear of polymers by preferentially supporting part of the applied load, and in addition by smoothing the surface of the steel counterface.

Seismic response of single-story interaction systems

Abstract: The influence of the flexibility of the foundation medium on the seismic response of a class of typical single-story structures is examined. The foundation medium is represented by an isotropic elastic half-space, while the ground acceleration is simulated by a transient time dependent function which exhibits characteristics resembling those of strong motion earthquakes. The solution of the equations of motion shows that the flexural displacement of an interaction system can be greater or less than the response of the same structure on a rigid foundation. On the basis of a parametric study, simple relationships are given which permit the use of spectral velocity curves for estimating the maximum flexural response of single-story structures on flexible foundations. The proposed technique yields, for both pseudo and actual strong motion earthquakes, results which are in good agreement with the maximum flexural response obtained from the solutions of the equations of motion.

Ultimate strength of reinforced concrete beams in combined torsion and shear

Abstract: CLASSICAL THEORY SHOWS THAT BOTH TORSION AND FLEXURAL SHEAR HAVE THE SAME POINT OF MAXIMUM STRESS FOR RECTANGULAR SECTIONS. HOWEVER, THE STRESS DISTRIBUTION THROUGHOUT THE REST OF THE CROSS SECTION VARIES SIGNIFICANTLY. A SERIES OF RECTANGULAR REINFORCED CONCRETE BEAMS WITH NORMAL PERCENTAGES OF BOTH LONGITUDINAL AND TRANSVERSE STEEL WAS TESTED AND THE INTERACTION OF THEIR TORSIONAL AND FLEXURAL SHEAR CAPACITIES WAS DEVELOPED. THE EFFECTS OF BENDING AND AMOUNT OF REINFORCEMENT ARE DISCUSSED. VARIOUS INTERACTION FORMULAS ARE COMPARED. /AUTHOR/

Discrete Model Analysis of Elastic-Plastic Plates

Abstract: By invoking the usual assumptions of the classical theory of plates and shells, a discrete flexural model of a plate is deduced from a discrete model of three-space solids. All the flexural relations and equations, including boundary conditions, pertaining to a discrete set of field quantities can then be formulated directly through the model; these relations can also be shown consistently to be central difference analogs of the corresponding classical differential equations. On this basis, the treatment of nonlinear-inelastic material properties is quite transparent; material properties are treated in their most basic form and general properties are handled in the same manner as that of linearly elastic material. All these lead to a simple set of recursive equations which constitute the basis for an algorithmic approach to the flexural analysis of nonlinear-inelastic problems of plate structures. The solution for several square plates of elastic-perfectly plastic material are illustrated with different boundary conditions. In all cases, the numerically predicted load-carrying capacities are shown to be consistently within the bounds of limit analysis.

Strength of glass.

Abstract: : The report discusses the development of improved methods for testing mechanical strength of glass and other brittle materials and results of an attempt to evaluate the influence of various types of defects on the strength of glass and to assist in developing specifications for strengthened glass for use in deep-diving vehicles. (Author)

Dynamic Response of Finite-Length Cylindrical Shells to Nearly Uniform Radical Impulse

Abstract: The response of a cylindrical shell to a uniform radial impulse is a simple harmonic motion in which the cross section remains circular. In the presence of small nonuniformities in the impulse, this motion may be unstable. The instability is characterized by the parametric excitation of flexural modes which exchange energy with the basic motion in a cyclic manner. The stability criterion derived here for finite length cylinders shows this instability may occur over a wide range of shell geometries. The flexural modes that exhibit significant growth are identified. With this, the finite time response of the shell is obtained by numerical integration of the nonlinear equations. The nonlinear response is dominated by a few flexural modes that initially have high growth rates. The amplification of these modes results in displacements and stresses considerably in excess of those associated with the unperturbed response. Finally the imperfection sensitivity of the nonlinear motion is examined.

Effects of molecular weight and strain rate on the flexural properties of polycarbonate

Abstract: The effects of changes in molecular weight (7000 to 22,000) and strain rate (0.0001 to 4 min.−1) on the flexural properties of polycarbonate have been examined in detail with the use of speciments of different molecular weight prepared by high-energy electron irradiation. The results have been plotted as surfaces which show the dependence of both stress and strain on molecular weight and strain rate, and these surfaces have been described in terms of brittle, transitional, and ductile regions. The relationships between stress or strain and molecular weight in the brittle region have been shown to be hyperbolic. A single failure locus has been found to include all the corresponding stress and strain data obtained at the various molecular weights and strain rates. In the low strength region this locus exhibits a proportionality between stress and strain, while at high strength values, strain becomes a logarithmic function of stress. Stress–molecular weight data obtained at the various rates have been superimposed to form a single composite curve, and the corresponding crossplots of stress–log rate have been treated similarly. It is concluded from these superpositions that an equivalence exists between changes in both molecular weight and strain rate such that a tenfold change in strain rate corresponds approximately to a change of 1000 in molecular weight. Strain-strain rate data obtained at the various molecular weights have also been superimposed in a similar manner. Modulus is shown to increase slowly with decrease in molecular weight and appears to be relatively insensitive to changes in strain rate.

Thermoplastic polyester moulding compositions containing polyacetals

Abstract: THERMOPLASTIC MOULDING COMPOSITIONS COMPRISING A LINEAR SATURATED POLYESTER AND POLYACETALS, FROM WHICH SHAPED ARTICLES CAN BE MADE HAVING A GOOD SURFACE HARDNESS AND A HIGH IMPACT STRENGTH AND BENDING STRENGTH.

Effect of Rate of Loading on Strength and Young's Modulus of Elasticity of Rock.

Abstract: : The effect of loading rates on the strength and tangent modulus of elasticity of a basalt, a granite, and a tuff was investigated using static-loading, rapid-loading, and shock-loading techniques. During static loading, specimens of each rock were loaded at a rate of 50 psi/sec. The specimens tested using static and rapid techniques were loaded in uniaxial compression, while those specimens tested using the shock-loading method were loaded in one-dimensional strain. Static testing was conducted in a 440,000-lb-capacity universal hydraulic testing machine. (Author)

Prevention and control of cracking by use of short random fibers

Abstract: THE CORPS OF ENGINEERS AND CARNEGIE INSTITUTE OF TECHNOLOGY HAVE SEPARATELY INVESTIGATED THE RESISTANCE OF FIBROUS REINFORCED CONCRETE TO IMPULSIVE LOADS. THE TESTS BY THE CORPS WERE CENTERED UPON THE RESISTANCE OF RECTANGULAR SLABS TO EXPLOSIVE LOADS ON ONE SIDE. THE FINDINGS ARE: (1) NYLON AND STEEL FIBERS SIGNIFICANTLY REDUCE THE SPALL FRAGMENT VELOCITY OF CONCRETE SLABS AND ARE EFFECTIVE IN RESISTING BOTH TENSILE AND COMPRESSIVE SPALLING OR FRAGMENTING GENERATED BY IMPACT LOADING, (2) FIBROUS REINFORCED CONCRETE POSSESSES GOOD ENERGY-ABSORPTION CHARACTERISTICS, (3) SYNTHETIC FIBERS USED AS RANDOM REINFORCING DO NOT INCREASE THE FLEXURAL CRACKING STRESS OF CONCRETE, HOWEVER, NYLON MONOFILAMENTS WILL INCREASE THE ULTIMATE FLEXURAL STRENGTH AND (4) NO APPRECIABLE INCREASE IN ULTIMATE FLEXURAL STRENGTH IS ACHIEVED UNTIL THE WIRE SPACING IS LESS THAN 0.2 IN. FOR 17 MIL WIRE AND 0.1 IN. FOR 4 MIL WIRE. THE RESULTS OF THESE TESTS SHOW THAT BEAMS MADE WITH FIBER REINFORCED CONCRETE SHOW A SUBSTANTIAL INCREASE IN THE ENERGY ABSORBING ABILITY OF THE BEAM. /AUTHOR/

Finite indentation of elastic-perfectly plastic membranes by a spherical indenter.

Abstract: The beryllium wire epoxy composites displayed a yieldingtype tensile stress-strain curve with a proportional limit 30 to 40% of ultimate strength and yield strengths 80 to 96% of ultimate. In the 0-90° bidirectional composites tested in tension, there was 100% conversion of the strength and modulus of the beryllium wire. In compression, the beryllium wire plastic composites failed by plastic deformation, and in the long span flexure, they failed by excessive flexural deformation followed by filament fracture. Short beam interlaminar shear specimens failed by excessive flexural deformation rather than delamination. Inter laminar shear strength of the beryllium wire epoxy composites was not degraded by a two hour water boil. The ballistic impact resistance of beryllium wire epoxy composites was about 5 times that of glass fiber reinforced epoxy composites having similar lay up and thickness. V. Conclusions Based on preliminary processing and mechanical property investigations of plastic composites incorporating intermediate and high modulus graphite fibers, silicon carbide filaments and beryllium wire reinforcements, all three types of fibers warrant further investigation in composites for aerospace structural use. From the standpoint of further improvement, especially in modulus of elasticity, the graphite fiber composites have outstanding potential because of anticipated future availability of higher modulus graphite fibers.

Effect of porosity and particle size on the mechanical strength of sintered iron

Abstract: The flexural and tensile strengths and the fatigue limits of sintered iron materials have been measured as a function of porosity and pore size. The resulting experimental values were lower than the theoretical. This discrepancy is attributed to a pore-size effect, analogous to the results of earlier work in which the influence of pore size on elastic modulus was studied (G. Artusio, V. Gallina, G. Mannone, and E. Sgambetterra, Powder Met., 1966, 9, (17), 89).

Componets of Creep in Mature Concrete

Abstract: TESTS ARE REPORTED ON THE CREEP AND RECOVERY OF CONCRETE UNDER UNIAXIAL COMPRESSION AND TENSION, APPLIED AT ADVANCED AGES OF OVER A YEAR. THE CHARACTERISTICS OF THE CREEP COMPONENTS (IRRECOVERABLE OR FLOW, RECOVERABLE OR DELAYED ELASTIC) FOR YOUNG AND MATURE CONCRETE ARE COMPARED, AND FOUND TO BE IN REASONABLE AGREEMENT. THE RATE OF FLOW METHOD (BASED ON THE TWO COMPONENTS) FOR CALCULATING STRAIN UNDER VARIABLE STRESS IS DISCUSSED. IN PARTICULAR, IT IS COMPARED FAVORABLY WITH SUPERPOSITION. /AUTHOR/

Prevention and Control of Cracking by Use of Short Random Fibers

Abstract: THE CORPS OF ENGINEERS AND CARNEGIE INSTITUTE OF TECHNOLOGY HAVE SEPARATELY INVESTIGATED THE RESISTANCE OF FIBROUS REINFORCED CONCRETE TO IMPULSIVE LOADS. THE TESTS BY THE CORPS WERE CENTERED UPON THE RESISTANCE OF RECTANGULAR SLABS TO EXPLOSIVE LOADS ON ONE SIDE. THE FINDINGS ARE: (1) NYLON AND STEEL FIBERS SIGNIFICANTLY REDUCE THE SPALL FRAGMENT VELOCITY OF CONCRETE SLABS AND ARE EFFECTIVE IN RESISTING BOTH TENSILE AND COMPRESSIVE SPALLING OR FRAGMENTING GENERATED BY IMPACT LOADING, (2) FIBROUS REINFORCED CONCRETE POSSESSES GOOD ENERGY-ABSORPTION CHARACTERISTICS, (3) SYNTHETIC FIBERS USED AS RANDOM REINFORCING DO NOT INCREASE THE FLEXURAL CRACKING STRESS OF CONCRETE, HOWEVER, NYLON MONOFILAMENTS WILL INCREASE THE ULTIMATE FLEXURAL STRENGTH AND (4) NO APPRECIABLE INCREASE IN ULTIMATE FLEXURAL STRENGTH IS ACHIEVED UNTIL THE WIRE SPACING IS LESS THAN 0.2 IN. FOR 17 MIL WIRE AND 0.1 IN. FOR 4 MIL WIRE. THE RESULTS OF THESE TESTS SHOW THAT BEAMS MADE WITH FIBER REINFORCED CONCRETE SHOW A SUBSTANTIAL INCREASE IN THE ENERGY ABSORBING ABILITY OF THE BEAM. /AUTHOR/

An evaluation of a chromium-base alloy for high-temperature service

Abstract: An extensive evaluation of a 2% tantalum, 0.5% silicon, 0.1% titanium, chromium-base alloy involving tensile, bend, impact, mechanical fatigue, thermal fatigue, stress/rupture and oxidation tests, has been completed. Some improvement in the ductility was obtained by prior vacuum annealing. At temperatures over 550 °C the material appeared insensitive to notches. Stress/rupture properties were significantly better than the best wrought nickel alloys. The thermal fatigue endurance was good but crack propagation rates were very high. The alloy possessed useful mechanical fatigue resistance at 900 ° and 1000 °C. Its oxidation resistance at 1100 °C was adequate, but it was prone to nitridation embrittlement. The rupture strength of the alloy melted and processed under different conditions in Australia, was superior to that of the United Kingdom material.

Bending Strength of Aluminum Formed Sheet Members

Abstract: An experimental and analytical investigation produced a method of calculating the bending strength of aluminum formed sheet members such as are used for roofing and siding. These members fail by crippling of the flange or web elements at stresses that may be well in excess of the local buckling strength. Specimens (numbering 46) were tested under either uniform bending moment or air pressure loading. Formulas are presented for local buckling and crippling of flange and web elements. Crippling failures occur simultaneously with local buckling in sections with relatively thick plate elements. In other sections, weighted average crippling stresses for the compression elements are used to predict ultimate bending strengths. Agreement with test results is achieved.

The effect of cement type, aggregate type and mix water content on the properties of lean concrete mixes

Abstract: DETAILS ARE PRESENTED OF PRELIMINARY TESTS CARRIED OUT TO ASSESS THE EFFECT OF INTRODUCING CHANGES IN THE COMPOSITION OF LEAN CONCRETE MIXES SUITABLE FOR USE IN ROAD BASE CONSTRUCTION. IT IS SHOWN THAT SUBSTANTIAL CHANGES IN THE STRENGTH AND ELASTIC PROPERTIES, ESPECIALLY AT EARLY AGES, CAN BE PRODUCED BY USING CEMENTS OTHER THAN ORDINARY PORTLAND CEMENT AND THEIR PRACTICAL IMPLICATIONS ARE DISCUSSED. THE EFFECT OF USING CRUSHED GRANITE AGGREGATE INSTEAD OF IRREGULAR GRAVEL AND NATURAL SAND IS EXAMINED USING CONSTANT MIX PROPORTIONS. THE USE OF GRANITE AS THE COARSE AGGREGATE, AS THE COARSE AGGREGATE AND PART OF THE FINE AGGREGATE, AND FINALLY AS THE WHOLE OF THE AGGREGATE PRODUCES SUCCESSIVE CHANGES IN THE PROPERTIES MEASURED. THE WORKABILITY IS REDUCED, THE COMPRESSIVE AND FLEXURAL STRENGTHS ARE INCREASED BUT THE ELASTIC PROPERTIES ARE NOT GREATLY AFFECTED. AN INCREASE IN MIX WATER CONTENT TO A VALUE WHICH WOULD PERMIT COMPACTION BY SURFACE VIBRATION RATHER THAN BY ROLLING IS SHOWN, FOR A GIVEN CEMENT CONTENT, TO CAUSE A SUBSTANTIAL REDUCTION IN STRENGTH AND AN APPRECIABLE REDUCTION IN THE ELASTIC MODULUS. RELATIONSHIPS ARE SUGGESTED BETWEEN THE MAIN PROPERTIES EXAMINED AND AN INDICATION IS GIVEN OF THE EFFECT OF MIXING ACTION AND OF COMPACTION ON THE REPRODUCIBILITY OF RESULTS. /RRL/A/