Showing papers on "Flexural strength published in 1975"

Reinforced Concrete Structures

Abstract: The Design Approach. Stress--Strain Relationships for Concrete and Steel. Basic Assumptions of Theory for Flexural Strength. Strength of Members with Flexure. Strength of Members with Flexure and Axial Load. Ultimate Deformation and Ductility of Members with Flexure. Strength and Deformation of Members with Torsion. Bond and Anchorage. Service Load Behavior. Strength and Ductility of Frames. Shear Walls of Multistory Buildings. The Art of Detailing.

Uniaxial Failure of Composite Laminates Containing Stress Concentrations

Abstract: Two previously developed failure criteria for predicting the uniaxial tensile strength of a laminated composite containing through-the-thickness material discontinuities (notches) are subjected to further experimental scrutinization. In particular, the two-parameter (unnotched tensile strength of the laminate and a characteristic length) models, which are capable of predicting observed discontinuity size effects without resorting to the concepts of linear elastic fracture mechanics, are based on limited experimental verification. In the present paper, and experimental program is presented which examines the effect of changes in the material system, the laminate fiber orientations, and the notch shape and size (stress gradient), on the model predictions. This is accomplished by obtaining experimental data on two material systems, glass/epoxy and graphite/ epoxy, in conjunction with two orientations of fiber-dominated laminates containing through-the-thickness circular holes and sharp tipped cracks of several sizes. In addition to the test results, two observations based on the models are presented. First, the statistical failure distribution for a composite containing a circular hole is predicted using the models and shown to agree well with experimental observations. Second, an Irwin type correction factor applied to the stress intensity factor is shown to result in nearly constant values of the critical stress intensity factor for all values of crack length. The correction factor is shown to be related to the characteristic length of the present models.

Intermittent bonding for high toughness/ high strength composites

Abstract: High strength and high toughness are usually mutually exclusive in brittle filament/brittle matrix composites. The high tensile strength characteristic of strong interfacial filament/matrix bonding can, however, be combined with the high fracture toughness of weak interfacial bonding, when the filaments are arranged to have alternate sections of high and low shear stress (and low and high toughness). Such weak and strong areas can be achieved by appropriate intermittent coating of the fibres. The strong regions ensure that the filament strength is picked up; weak areas randomly in the path of running cracks serve to blunt them by the Cook/Gordon mechanism which, in turn, produces long pull-out lengths with an associated large contribution to toughness. Boron-epoxy composites of volume fraction 0.20 to 0.25 have been made in this way which have fracture toughnesses of over 200 kd m -2, whilst retaining rule of mixtures tensile strengths (~ 650 MN m-2). At the volume fractions used, this apparently represents Kic values greater than 100 MN m -3/2, An analysis is presented for toughness and strength which demonstrates, in broad terms, the effects of varying the coating parameters of concern. Results show that the "toughness" of interfaces is an important parameter, differences in which may not be shown up in terms of interfacial "strength". The choice of coating material is crucial in getting the desired effect. Some observations are made upon methods of measuring the components of toughness in composites.

Shear transfer in reinforced concrete with moment or tension acting across the shear plane

Abstract: A comprehensive study of the shear transfer strength of reinforced concrete, subject to both single direction and cyclically reversing loading (the latter simulating earthquake conditions), is currently in progress at the University of Washington. This paper reports that part of the study concerned with the effect of normal force and moment in the shear plane on single direction shear transfer strength. Tests are reported of corbel type push-off specimens and of push-off specimens with tension acting across the shear plane. It was found that: (1) moments in the shear plane less than or equal to the flexural ultimate moment of the shear plane do not reduce the shear transfer strength; and (2) tension across the shear plane results in a reduction in shear transfer strength equal to that which would result from a reduction in the reinforcement parameter by an amount equal to the tension stress. A future paper will extend the results of this investigation to lightweight concrete and provide a firm basis for corbel design. /Author/

The fracture stress and its dependence on slow crack growth

Abstract: An analysis is presented which enables the fracture strength, (under constant stress-rate conditions) to be predicted from fracture mechanics data obtained during slow crack growth — by identifying and evaluating several key parameters. The predicted strength characteristics are illustrated using fracture mechanics data obtained for a soda lime silicate glass. Finally, the predicted strengths are compared with strengths measured in flexure on abraded soda lime silicate glass specimens. A good correlation is obtained, indicating an equivalence between micro- and macro-crack propagation conditions for this material.

Partial-interaction design of composite beams

Abstract: Situations are described in which it is advantageous to use composite steel-concrete beams with fewer shear connectors than the number required for full interaction. From a study of the results of tests and computations, simple rules are derived for estimating the ultimate flexural strength of such beams and for checking deflections in service, both by calculation and by the use of limiting span-depth ratios. /Author/TRRL/

Seismic shear loading at flexural capacity in cantilever wall structures

Abstract: An investigation is described into the effect of various combinations of the normal modes of vibration of cantilever shear wall structures on the maximum shears at flexural capacity. It is shown that the base shear can be much higher than would be derived by assuming a normal code lateral load distribution of sufficient magnitude to cause flexural yielding. The results of elastic normal mode response spectrum analyses of a 10-storey building considering several structural variables are presented in terms of envelope values of the ratio of maximum base shear at flexural capacity to that assuming a code lateral load distribution. The same effect is investigated with a series of step-by-step numerical integration dynamic analyses of cantilever wall structures responding inelastically to a range of earthquakes. On the basis of the results suggestions are made for the shear design of cantilever walls.

Characterization of bone cements.

Abstract: Properties of acrylic bone cements during and after curing were determined for three brands of bone cement. Curing time and consistency were chosen for the characterization of the handling and working behavior of these materials. The performance of bone cements after curing may be related amongst other things to the following properties: water resorption, solubility/disintegration, flexural modulus of elasticity, yield stress, proportional limit, flexural strength and impact strength. Methods to determine these handling and material properties are described. The influence of radiopacifying and antibiotic additives on these properties is evaluated as well as the influence of porosity on flexural strength and impact strength. The results indicate that considerable differences in the handling properties occur. The material properties of the three brands tested do not show marked differences. Radiopacifying and antibiotic additives appear to have a negative effect on material properties; the effect of porosity as it develops during curing under simulated clinical conditions is more pronounced.

Crack Growth Resistance of Random Fiber Composites

Abstract: Crack growth resistance of a random glass fiber composite has been studied by applying the concepts of Linear Elastic Fracture Mechanics. The crack growth resistance curves (R-curves) for epoxy and polyester composites have been developed to study the total fracture behavior of these composites. It is shown that the R-curve approach is suitable for these type of materials. An attempt has been made to use the information from the R-curves to predict the fracture strength of plate specimens with a hole at the center and subjected to tensile loading.

Communication cable utilizing optical transmission elements

Abstract: An optical cable is constructed using a core of high tensile strength steel wires around which is disposed a radial system of chambers in the form of a helix retaining the optical conductors so that they are movable radially thereby protecting the optical conductors from tensile, compression and flexural stresses.

On the extensional and flexural vibrations of rotating bars

Abstract: The non-linear equations of motion of a slender bar rotating at constant angular velocity about a transverse axis are formulated. Under the assumption that a small perturbed motion occurs about an initially stressed equilibrium configuration, linearized equations of motion for the longitudinal and flexural deformations of a rotating bar carrying a tip mass are derived. Numerical computations for the natural frequencies of the lowest three modes of free vibration reveal that the values of the extensional frequencies increase monotonically, contrary to previously published results, as the angular velocity of rotation increases.

Development of a structural design procedure for flexible airport pavements

Abstract: A design procedure is presented for three types of flexible pavement: conventional, bituminous concrete, and chemically stabilized. These represent nearly all flexible pavements being constructed at this time. Designs are based on analytically determined strain values and experimental and laboratory determined material fatigue strengths. Thus, the procedure can handle in a rational manner the possible variations in the properties of different pavement materials. An adaptation of the cumulative damage concept permits the consideration of cyclic variation in bituminous materials due to variations in temperatures and the variation in subgrade strength resulting from freeze-thaw cycles. (Author)

Properties of glass fibre cement — the effect of fibre length and content

Abstract: The properties of glass fibre reinforced cement composites (grc) containing alkali-resistant fibres of lengths 10 to 40 mm and volume fractions 2 to 8% have been studied. At 28 days the optimum properties of the composite were achieved with 6 vol % fibre addition. These were 4 to 5 times the bending strength, 3 to 4 times the tensile strength and 15 to 20 times the impact strength of the unreinforced cement paste. Further increase in the fibre content increases the porosity of the composite resulting in the lowering of bending and tensile strengths. The stress and strain of the composite at matrix cracking increased with increasing fibre contents. No significant improvements in the modulus of the composite were observed over the range of fibre additions investigated. The trends in the properties of grc as affected by the variations in volume fraction and length of the fibre, and environmental conditions of curing of the composites, are qualitatively related to the degree of cement hydration, changes in porosity of the composites and fibre/matrix interfacial effects. The properties of grc change with time, (strengths tend to decrease) and long term studies are in progress.

Influence of Microstructure on Crack Propagation in ZnSe

Abstract: The rate of crack propagation as a function of the stress intensity factor, K1, for chemically vapor-deposited (CVD) ZnSe was significantly larger in H2O than in air. Lower flexural strengths were measured in H2O (29 MN/m2) than in air (40 MN/m2). Fracture surface analysis of these flexural specimens showed that failure usually resulted from flaws contained within 1 or 2 large grains. These flaws were shown to propagate at a stress-intensity factor of 0.33 MN/m3/2 compared to the measured value of 0.70 MN/m3/2 for polycrystalline ZnSe. Fracture mechanics parameters obtained in the present study were used to show that failure time predictions using values of K1C for propagation through a single grain are 25 times shorter, and proof-to-service stress ratios are 1.1 times greater, than those obtained for polycrystalline ZnSe.

Manufacture of sulfur concrete

Abstract: Sulfur concrete which exhibits improved compressive strength and flexural strength, and improved corrosion characteristics, is manufactured by mixing sulfur, dicyclopentadiene and aggregate at from 120° to 160° C. The period for which the sulfur and dicyclopentadiene interact should not exceed certain limits.

Lamination residual stresses in fiber composites

Abstract: An experimental investigation was conducted to determine the magnitude of lamination residual stresses in angle-ply composites and to evaluate their effects on composite structural integrity. The materials investigated were boron/epoxy, boron/polyimide, graphite/low modulus epoxy, graphite/high modulus epoxy, graphite/polyimide and s-glass/epoxy. These materials were fully characterized. Static properties of laminates were also determined. Experimental techniques using embedded strain gages were developed and used to measure residual strains during curing. The extent of relaxation of lamination residual stresses was investigated. It was concluded that the degree of such relaxation is low. The behavior of angle-ply laminates subjected to thermal cycling, tensile load cycling, and combined thermal cycling with tensile load was investigated. In most cases these cycling programs did not have any measurable influence on residual strength and stiffness of the laminates. In the tensile load cycling tests, the graphite/polyimide shows the highest endurance with 10 million cycle runouts at loads up to 90 percent of the static strength.

Strength and Elastic Modulus of a Randomly-Distributed Short Fiber Composite:

Abstract: Experimental results for tests of strength and elastic modulus of a randomly-distributed short fiber composite are reported in this paper. Three different tests were employed: tension, 3-point flexure, and 4-point flexure. A statistical approach based on Weibull distribution was applied to account for the difference between tensile and flexural test results. Mate rial variation, as exhibited in the experimental scatter for elastic modulus, and its effect on strength were shown not to be negligible. Test results also showed wider experimental scatter than observed with continuous fiber composites. Consequently considerable penalty had to be paid by reducing the design strength in order to assure the same level of confidence and reliability.

Strength and Related Properties of Brick Masonry

Abstract: Sixteen important controllable variables in the strength of conventional brick masonry are related in 10 equations for strength in compression, flexure, and shear. Axial compressive strength of standard brick masonry prisms is a function of the compressive strengths of brick and mortar and quality of workmanship. The compressive strength of brick is a function of unit size and shape, raw material, manufacturing process and degree of burning. Compressive strength of mortar is a function of cement, lime, water and air content, curing condition and batch life. Flexural strength of brick masonry is a function of brick-mortar bond, joint thickness, and mortar cement content. Bond strength of brick to mortar is a function of mortar flow, air content, and exposure time, and brick suction and surface condition. Shear strength of brick masonry is a function of brick-mortar bond strength, compressive stress and coefficient of internal friction.

Effect of Boron on Long Period Creep Rupture Strength of 12%Cr Heat Resisting Steel

Abstract: Synopsis: The effect of boron on creep rupture properties of steels has been investigated in order to develop new 12% Cr heat resisting steel with improved strength at elevated temperatures. 1) The creep rupture strength of a 12% Cr—Mo—V—Nb steel is improved by boron addition. Especially, the creep rupture life of the steel containing 0.037% boron is five to six times as long as that of the steel free from boron in long term creep rupture tests at 650•Ž and above. 2) Boron forms boro—carbide M23(C,B)6, substituting some amounts of carbon in carbide, type M23C6. It is observed under electron microscope that M23(C,B)6 is finer than M23C6 at elevated temperature. This is one of the reason for that the 12% Cr—Mo—V—Nb-B steel has superior creep rupture strength to the 12% Cr-Mo-V-Nb steel.

Behavior of Cold-Rolled Stainless Steel Members

Abstract: Results obtained from a research project dealing with the structural behavior of cold-formed members of cold-rolled stainless steel are presented. The distinctive mechanical properties include anisotropy, nonlinear and unsymmetrical stress-strain relationships in tension and compression, and the pronounced effect of cold working. The post-buckling behavior of thin compression elements stiffened along one or both unloaded edges by thin webs has been found to agree with Koiter’s theory, with von Karman’s relationship on effective width, as modified by Winter for carbon steel, as the lower bound. Using an iterative numerical procedure, the flexural strength and deflection of thin-walled cold-rolled stainless steel beams can be predicted accurately. A brief analysis on column behavior is also presented. Design methods taking into account the material characteristics and the post-buckling strength of the thin compression elements of these structural members are recommended.

Effect of heattreatment variations on strength and ductility of Cr-Mo-V steels during creep at 550°C

Abstract: The performance of Cr-Mo-V steels, as widely used for steam pipe work, turbine rotors and casings, and associated bolts in steam power generating plant, may be limited by a susceptibility to failure at low ductility, especially when heat treated to high strength levels. To investigate the ductility in creep conditions of such steels, an extensive study of the interrelation of heat treatment, microstructure, creep strength, rupture strength, rupture ductility, and fracture characteristics has been made in commercially produced 0·5Cr-0·5Mo-0·25V and 1Cr-1 Mo-0·25V steels tested at 550°C for times up to >20000h. The results show that the rupture strength and ductility can be varied widely by changing the austenitizing temperature and the rate of cooling therefrom, and that a heat treatment at a high temperature (1300°C), given to simulate the microstructure in the vicinity of a weld, can produce very low creep-rupture ductility (<1%). Low ductility in these steels is associated with an intergranular ...

A Fracture‐Mechanics Study of ZnSe for Laser Window Applications

Abstract: Fracture-mechanics studies of chemically vapor-deposited ZnSe show that this material has a low fracture toughness and is susceptible to water-(moisture-) dependent slow crack growth. Investigations of biaxial flexural strength indicate that fracture (in this configuration) is controlled primarily by surface flaws and that macrofracture mechanics adequately describe the propagation of typical preexisting flaws in this ZnSe. Crack growth can be detected consistently by the acoustic emission technique; fracture in air is preceded by precursor events which occur up to 2 s prior to catastrophic propagation. Failure diagrams generated from the slow-crack-growth parameters show that the lifetime requirements of the laser window can be achieved by overload proof-testing to a proof ratio of 1.35 when the proper proof-test precautions are implemented.

Determination of Fracture Strength in Orthotropic Graphite-Epoxy Laminates

Abstract: A fracture test program performed on orthotropic graphite-epoxy laminates is reported. The test data are used to evaluate the effects of specimen size and specimen configuration on the measured value of laminate fracture strength. It is shown that apparent fracture strength is independent of specimen configuration and specimen thickness, but varies significantly with respect to crack length. As crack length increases, however, the apparent fracture strength is found to asymptotically approach a limiting value, taken to be the valid laminate fracture toughness. The test data are also used to evaluate two analytical models for predicting laminate fracture toughness. The first model links basic ply properties and the fracture toughness of angle-ply laminates, while the second model relates the fracture toughness of an arbitrary orthotropic laminate to the fracture properties of its angle-ply components. Both models show good agreement with the test data. It is concluded that linear elastic fracture mechanics does provide a meaningful characterization of crack growth in orthotropic composite laminates, if some specified conditions are met. Study of cases which do not meet these conditions is recommended for future work.