Showing papers on "Compressive strength published in 1986"

The effect of resin formulation on the degree of conversion and mechanical properties of dental restorative resins

Abstract: The goal of this study was to determine the effects of resin formulation variables, such as diluent concentration, catalyst type and concentration and cure mode, on the degree of conversion of carbon double bonds and mechanical properties of dental restorative resins. Diametral tensile strength, compressive strength, hardness, flexural modulus and strength, and dynamic mechanical properties were tested, and the results were correlated to the degree of conversion results obtained by infrared analysis. The results showed a significant correlation between increased mechanical properties and higher degrees of conversion. Enhanced conversions were achieved by incorporating higher diluent and lower inhibitor concentrations into the resins. Ambient temperature properties were similarly enhanced by lower inhibitor concentrations, but were not enhanced by higher diluent concentration. Dynamic mechanical properties testing at oral and elevated temperatures elucidated possible differences in resin microstructure and network quality. The storage moduli decreased over the dental temperature range and was lower at all temperatures for resins with lower conversions. The glass transition temperature was also lower in resins with poorer conversions, suggesting that these resins may be more unstable at oral temperatures than more highly converted resins. Dynamic mechanical properties were most closely correlated to degree of conversion in these polymeric systems.

Tensile Tests and Failure Analysis of Concrete

Abstract: The tensile strength of concrete is receiving an increasing amount of attention since the loading capacity and durability of structures are being studied more thoroughly, and since nonlinear fracture mechanics and numerical methods require a complete stress deformation relation. The theoretical background of the application of the tensile strength of concrete in fracture analysis is given. Test results are presented that were obtained from deformation‐controlled uniaxial tensile tests under static and cyclic loading of lightweight and normalweight concrete. The results have been used to establish material models for finite element calculations. Numerical examples show the usefulness of these models for static and cyclic loading conditions.

Experimental studies of scale effects on the shear behaviour of rock joints

Abstract: The effect of scale on the shear behaviour of joints is studied by performing direct shear tests on different sized replicas cast from various natural joint surfaces. The result show significant scale effects on both the shear strength and deformation characteristics. Scale effects are more pronounced in the case of rough, undulating joint types, whereas they are virtually absent for planar joints. The key factor is the involvement of different asperity sizes in controlling the peak behaviour of different lengths of joints. It is shown that as a results both the joint roughness coefficient (JRC) and the joint compression strength (JCS) reduce with increasing scale. The behaviour of multiple jointed masses with different joint spacing is also considered. It is found that despite unchanged roughness, jointed masses consisting of many small blocks have higher peak shear strength than jointed masses with larger joint spacing. These scale effects are related to the changing stiffness of a rock mass as the block size or joint spacing increases or decreases. Economic methods for obtaining scale-free estimates of shear strength are described.

Tension and flexural strength of silicon carbide fibre-reinforced glass ceramics

Abstract: The use of silicon carbide-type fibres to reinforce lithium aluminosilicate glass ceramics results in composites with exceptional levels of strength and toughness. It is demonstrated that composite strength and stress-strain behaviour depend onin situ fibre strength, matrix composition, test technique and atmosphere of test. Both linear and non-linear tensile stress-strain curves are obtained with ultimate strengths at 22° C approaching 700 MPa and failure strains of 1%. Flexure tests performed at up to 1000° C in air are compared with data obtained in argon to demonstrate a significant dependence of strength and failure mode on test atmosphere. Finally, glass ceramic matrix composite performance is compared with a silicon carbide fibre-reinforced epoxy system to demonstrate the importance of matrix failure strain on strength and stress-strain behaviour.

Shear Strength of Fibrous Concrete Beams Without Stirrups

Abstract: Test data are presented on the shear strength of a series of longitudinally reinforced fibrous concrete beams in which the shear span‐to‐effective depth ratio, volume fraction of fibers, percentage of reinforcement, and strength of the concrete were varied. Test results indicate that the fibers have significant influence on the mode of failure and ultimate shear strength of a longitudinally reinforced concrete beam. Conventional reinforced concrete analyses with some modification, as suggested in this paper, to account for the effect of fibers give good predictions of the ultimate strength and the mode of failure for rectangular beams with no shear reinforcement.

Shear Capacity of Prestressed Concrete Beams Using High-Strength Concrete

Abstract: Results of an experimental investigation of the shear strength of prestressed concrete beams using concrete with compressive strength ranging to approximately 12,000 psi (83 MPa) are summarized. A total of 34 beams was tested, half designed for flexure-shear cracking and half for web-shear cracking. In each set, nine beams were without web reinforcement and eight had web reinforcement in the form of vertical stirrups. In addition to concrete strength, variables included longitudinal prestressed and nonprestressed steel ratios, shear span-to-depth ratio, amount of prestress force, and amount of web reinforcement. Test results are compared with strengths predicted using the equations of the 1983 ACI Building Code (ACI 318-83). The ACI Code approach is basically sound in considering two cases, webshear cracking and flexure-shear cracking; both modes were observed and behavior in each was distinctly different. Present code equations for cracking load gave conservative results for all concrete strengths. For beams with stirrups, code equations for total shear strength were also conservative for all concrete strengths, but the degree of safety depended on several parameters, including concrete strength, that are not adequately considered by present equations.

Influence of width/height ratio on post-failure behaviour of coal

Abstract: The post-failure slope immediately after strength failure decreases on the negative side with the increase inw/h ratio. Atw/h equal to around 10 or 11 this slope becomes almost zero and then again the slope increases but on the positive side with further increase inw/h ratio.

Compression failure mechanisms in unidirectional composites

Abstract: The present paper examines compression failure mechanisms in unidirectional composites. 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 material 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 macroscopic-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, for stiff resins, failure was characterized by the formation of a kink band. For unidirectional laminates, compressive strength, and compressive modulus to a lesser 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.

Mechanical Stress Dependence of Radiation Effects in MOS Structures

Abstract: The effects of mechanical stress on radiation damage in polycide-gate MOS capacitors have been investigated as a function of gate-oxide thickness and silicide-gate electrode material (TiSi2, MoSi2 and WSi2). It was found that compressive stress on the SiO2/Si interfacial region reduces both positive charge build-up and interface-trap generation. The positive charge build-up exhibits a smaller stress effect, as compared with the interface-trap generation. The magnitude of stress effect depends only on the compressive strength, and not on the silicide material, if the annealing conditions are the same. In addition, as the gate-oxide thickness decreases, the stress effect on positive charge build-up increases, while the interface-trap generation remains nearly constant. These results can be explained on the basis of the bond reformation process, i. e., in a region where compressive stress exists, the broken bonds are reformed with high probability. Futhermore, radiation response of MOS transistors with different gate-oxide stress values have been evaluated, and compared with those observed in MOS capacitors. Similar stress effects were also obtained.

Microstructural development in cured soil-lime composites

Abstract: The bond which develops during curing between soil particles in the presence of lime and moisture is a result of the growth and the development of a newly formed cementitious phase (or phases). For the particular soil studied, appreciable reaction occurs between soil particles and lime only at elevated temperature, in a moist environment The growth and development of the new phase is accompanied by an increase in compressive strength of the soil-lime composites. Scanning electron microscopy (SEM) studies show the phase to consist of an interlocking network of fine platelets and fibres, and although no direct determination of composition was possible, evidence from X-ray analysis and thermal analysis shows that the new phase is poorly crystalline and probably consists of a hydrate of calcium silicate or calcium aluminate.

Compressive Strength of Concrete Masonry Prisms

Abstract: Experimental tests on prisms and constituent materials (mortar, grout, and hollow-core concrete blocks) are used to calibrate linearly elastic finite element models for hollow and grouted concrete ma­ sonry prisms. These finite element models are then used to develop simplified relationships which closely predict the compressive strengths and failure modes of prisms.

Combined Bending and Axial Loading in Lumber

Abstract: This paper describes the development of a model for predicting the strength of lumber in bending, and in combined bending and axial loading, on the basis of axial tension and compression behavior of similar members. Both instability and material strength failures are included. The model includes the effects of variability in lumber strength, both within a member and between members. Size effects which predict decreasing strength with increasing member size are incorporated. An extensive experimental program on a large quantity of structural lumber has been used for calibration and verification. Output from the model will contribute to the development of more efficient design methods for wood trusses and other structures where lumber is subjected to combined bending and axial loading.

Stress corrosion cracking of Lac du Bonnet granite in tension and compression

Abstract: Rocks subjected to long-term loading have been known to suffer microcracking. The rate of cracking is sensitive to the type of the applied stress (tensile or compressive), and the magnitude of the stress relative to the instantaneous strength. In addition, crack growth is influenced by the environment (pressure and temperature) including the presence or absence of moisture. For tensile loading, the sensitivity of granite to time-dependent cracking is demonstrated through a fracture mechanics test known as double torsion. The crack velocity versus stress intensity function is established for two environments, room temperature and humidity and room temperature and 100 percent humidity. For compressive loading, time dependent cracking is evaluated from creep tests conducted in uniaxial compression in the same two environments. The rate of cracking is defined by finding the functional relationship between the rate of crack growth, expressed as the rate of crack volume strain, and uniaxial compressive stress. A variety of mathematical functions has been fitted to the obtained data. The traditionally-used power and exponential relationships give good correlation for both crack velocity and crack volume strain rate. The crack volume strain rate versus stress function can be integrated to obtain a lifetime estimate for Lac du Bonnet granite. After 1 000 years of loading in uniaxial compression at room temperature and 100 percent humidity, the strength of this granite could reduce from 225 MPa to 90–100 MPa.

Strain of concrete during first cooling from 600°C under load

Abstract: Synopsis This is the third of three papers presenting the results of an investigation into the transient thermal strain behaviour of concrete during the first heat cycle to 600°C under load. The strains measured during the first cooling of concrete from 600°C are presented here, together with the results of residual compressive strength and elastic modulus tests. These are analysed with reference to the complete thermal cycle and constituent materials property data already given in the two previous papers. Strains during first cooling were primarily influenced by the thermal strain of the coarse aggregate and were unaffected by initial moisture content (as-cast, air-dry or pre-dried at 105°C), heating rate (0·2 or 1°C/min) or concrete age (1 year or 9 years). The applied load level did not affect the cooling strains of concretes with the more thermally stable aggregates of basalt or sintered pulverized-fuel ash, or the siliceous gravel concrete despite the extensive cracking it suffered during heating. Th...

The effect of resin toughness and modulus on compressive failure modes of quasi-isotropic graphite/epoxy laminates

Abstract: Compressive failure mechanisms in quasi-isotropic graphite/epoxy laminates were characterized for both unnotched and notched specimens and also following damage by impact. Two types of fibers (Thornel 300 and 700) and four resin systems (Narmco 5208, American Cyanamid BP907, and Union Carbide 4901/MDA and 4901/mPDA) were studied. For all material combinations, failure of unnotched specimens was initiated by kinking of fibers in the 0-degree plies. A major difference was observed, however, in the mode of failure propagation after the 0-degree ply failure. The strength of quasi-isotropic laminates in general increased with increasing resin tensile modulus. The laminates made with Thornel 700 fibers exhibited slightly lower compressive strengths than did the laminates made with Thornel 300 fibers. The notch sensitivity as measured by the hole strength was lowest for the BP907 resin and highest for the 5208 resin. For the materials studied, however, the type of fiber had no effect on the notch sensitivity.

Mechanical characterization of commercially made carbon‐fiber‐reinforced polymethylmethacrylate

Abstract: Acrylic bone cement is significantly weaker and of lower modulus of elasticity than compact bone. It is also weaker in tension than in compression. This limits its use in orthopedics to areas where tensile stresses were minimum. Many authors have shown that addition of small percentages of fiber reinforcement by hand mixing improved the mechanical properties significantly but with variable results. In this investigation we have examined the mechanical properties of machine-mixed, commercially available carbon-fiber-reinforced bone cement. Appropriate samples of normal low-viscosity cement and carbon-fiber-reinforced cement were prepared and tested mechanically. Carbon fiber increased the tensile strength and modulus by 30% and 35.8% respectively. The compression strength and modulus, however, increased by only 10.7%. Similarly, bending and shear strengths improved by 29.5% and 18.5%, respectively. Diametral compression strength, which is an indirect measure of tensile strength, however, showed only 6.2% improvement. The maximum temperature rise during polymerization was also reduced significantly by the fiber reinforcement.