Showing papers on "Compressive strength published in 1987"

Crack propagation in ceramics under cyclic loads

Abstract: Stable crack growth is observed in notched plates of polycrystalline alumina subject to fully compressive far-field cyclic loads at room temperature in a moist air environment andin vacuo. The fatigue cracks propagate at a progressively decreasing velocity along the plane of the notch and in a direction macroscopically normal to the compression axis. The principal failure events leading to this effect are analysed in terms of notch-tip damage under the far-field compressive stress, microcracking, frictional sliding and opening of microcracks, and crack closure. An important contribution to such Mode I crack growth arises from the residualtensile stresses induced locally at the notch-tip when the deformation within the notch-tip process zone leaves permanent strains upon unloading from the maximum nominal compressive stress. It is shown that the phenomenon of crack growth under cyclic compressive stresses exhibits a macroscopically similar behaviour in a wide range of materials spanning the very ductile metals to extremely brittle solids, although the micromechanics of this effect are very different among the various classes of materials. The mechanisms of fatigue in ceramics are compared and contrasted with the more familiar examples of crack propagation under far-field cyclic compression in metallic systems and the implications for fracture in ceramic-metal composites and transformation toughened ceramic composites are highlighted. Strategies for some important applications of this phenomenon are recommended for the study of fracture mechanisms and for the measurement of fracture toughness in brittle solids.

Effects of Cure Pressure on Resin Flow, Voids, and Mechanical Properties

Abstract: The effects of cure pressure on resin flow, compaction, void content, and mechanical properties of fiber reinforced composites were investigated. Tests were performed on lami nates made of Fiberite T300/976 and T300/934 unidirectional prepreg tape. Data are pre sented on compaction, resin flow, and void content which lend support to the validity of the Springer-Loos model. Data are also given showing the effects of cure pressure on void content and on the compressive strength and short-beam shear strength and modulus.

Tensile recoil measurement of compressive strength for polymeric high performance fibres

Abstract: Recoil forces acting on the broken ends of a fibre after tensile failure are known to cause substantial damage to polymeric high performance fibres. This damage is the result of compressive stresses developed during snap-back, or recoil, whose magnitude exceeds the compressive strength of the fibre. An analysis describing the axial stress history experienced by a fibre following a tensile failure has been performed and the results have led to the development of a simple, single filament, recoil technique for measuring fibre compressive strength. A number of polymeric high performance fibres were examined using the technique and compressive strengths measured are in excellent agreement with values obtained from composite tests.

Vacuum mixing of acrylic bone cement

Abstract: A partial-vacuum (500–550 mmHg), slow-speed (2 Hz) system for optimal blending of the liquid and powder components of Simplex-P acrylic bone cement was developed to eliminate five different sources of porosity observed with x-ray during the course of cement preparation and specimen fabrication. The vacuum mixing system produces set speciments of less than 1% porosity that have significant improvements over specimens prepared with conventional mixing in the mechanical properties of tensile and compressive strength and uniaxial tensile fatigue life. Hence, a much stronger cement can be available in surgery without any change in original chemical composition.

Compressive strength, ash weight, and volume of vertebral trabecular bone in experimental fluorosis in pigs.

Abstract: The aim of the investigation was to measure the effect of fluoride on vertebral trabecular bone compressive strength and to correlate this with fluoride-induced changes in bone density. This correlation would express changes in the quality of bone during fluoride treatment. Pigs were used in the experiment because their trabecular bone structure and remodeling sequences are very similar to the human. Eight animals receiving a supplement of 2 mg F−/kg b.w. per day from age 8–14 months were compared with 8 control animals. Morphologic measurements in the animals receiving fluoride supplement showed a significant increase of 17% in bone density and a smaller, insignificant increase of 3% in ash weight analyses. Meanwhile, the mechanical parameters for the fluorotic animals were unchanged (maximum compressive strength, maximum stiffness, and energy-absorption capacity) or decreased (normalized compressive strength=maximum compressive load corrected for ash density). It is concluded that the increased bone mass during the initial stages of fluoride treatment does not necessarily indicate an improved bone quality. The discrepancy between bone mass and strength could be either a permanent or a temporary phenomenon and requires further investigation.

The tensile strength of the lithosphere and the localization of extension

Abstract: Summary Continents appear to rift in preference to oceans. Furthermore, some areas of continents appear to be more susceptible to rifting than others. Experimental rock mechanics data are used to estimate lithospheric strength for lithospheres of different structure, thereby to investigate the possible causes of rift localization. Using optimum creep parameters for silicic, mafic and ultramafic rocks, we find that lithospheric strength is inversely related to both crustal thickness and heat flow. By virtue of its thinner crust, oceanic lithosphere is inherently stronger than continental lithosphere. We find that oceanic lithosphere older than about 10 Ma should be able to withstand the lithospheric forces exerted on it by gravity sliding and plate interactions. Rifting or ridge-jumps are therefore only likely to occur in very young oceanic lithosphere. Low heat flow continental shields should also be able to withstand likely lithospheric forces without significant deformation. As the heat flow increases, however, the lithosphere is weakened dramatically. A smaller amount of weakening is associated with crustal thickening. Thus, unless rifting is localized by a strongly heterogeneous stress field, it will be by anomalously weak lithosphere. This lithosphere is likely to have anomalously high heat flow and/or crustal thickness prior to extension.

Rapid hardening cements for repair of concrete

Abstract: This paper presents the results of an investigation on very rapidly hardening cements. Rapid hardening cements are those that can develop several thousand psi compressive strength within a few hours. Since these cements were developed just recently, very little laboratory and even less practical experience have been obtained with them. Therefore, this paper presents the results of a laboratory investigation on four such cements. These cements are: 1. magnesium phosphate cement for cold and regular weather use (MPC); 2. magnesium phosphate cement for hot weather use (MPH); 3. aluminum phosphate cement (MAP); and, 4. regulated set cement (RS). A combination of both mechanical and physicochemical tests were performed. Several chemical admixtures were also tried with these cements. The tests revealed that the magnesium phosphate base cements exhibit the most rapid strength development. The presented investigation concentrates on the strength development at early ages. Other important properties of these mixtures (durability, etc.) will be presented in another paper.

Relationship between strength and volumetric composition of moist-cured cellular concrete*

Abstract: Synopsis The effect of the volumetric composition of cellular concrete, particularly water and air voids, on its compressive strength has been demonstrated to follow Feret's general formula. The increase in strength at all ages with a corresponding increase in water/cement ratio (opposite to that of mortar mixes) as obtained in the experiment has been shown to be consistent with Feret's formula. The inclusion of the degree of hydration in the modified form of Power's gel/space ratio further improves the correlation with strength when this is taken as the parameter.

Shrinkage and Creep of High-, Medium-, and Low-Strength Concretes, Including Overloads

Abstract: The influence of drying and of sustained compressive stresses, at and in excess of the normal working stress levels, on the shrinkage and creep properties of high-, medium-, and low-strength concretes were experimentally studied and compared. The 28-day compressive strength of the materials studied ranged from 3000 to 10,000 psi (21 to 69 MPa). The long-term shrinkage was found to be greater for low-strength concrete and smaller for medium- and high-strength concretes. Creep strain and normalized creep were smaller for high-strength than for concretes of medium and low strengths. The creep-stress proportionality limit (as a percent of fc) was higher for the high-strength concrete than for the others by about 20 percent. Data on creep recovery for the three materials are also presented.

Constitutive behavior of a microcracking brittle solid in cyclic compression

Abstract: A constitutive formulation is presented to determine the “driving force” for Mode I fatigue crack growth in notched plates of brittle solids stressed in uniaxial cyclic compression. For the particular case of a microcracking medium, it is demonstrated that residual tensile stresses are induced ahead of the notch during unloading from the maximum far-field compressive stress. We propose that it is this region of residual tensile stresses at the notch-tip which promotes fatigue crack growth in ceramics along the notch plane in a direction normal to the compression axis. The predictions of the analysis are compared with new experimental results on compression fatigue in brittle solids. Specifically, it is shown that the numerical estimates of the near-tip tensile zone size for microcracking ceramics compare favorably with the experimentally measured distance of stable Mode I fatigue crack growth after the first compression cycle. Experimental information on the threshold stress for microcracking, transition stress for the inducement of residual tension during unloading, and the effect of mean stress on fracture, as well as direct observations of microcracks and crack growth in compression fatigue, corroborate the assumptions and implications of the analysis.

Temperature-dependence of Compressive Properties of Human Dentin

Abstract: The effect of temperature on compressive stress/strain behavior of human dentin obtained from recently extracted permanent lower molar teeth has been determined over the range 0-80°C. Dentin specimens were loaded uni-axially in a direction perpendicular to the tubule orientation. A statistically significant, linear regression relationship was found between modulus (E) and temperature (T): The observed temperature coefficient of the modulus is in close agreement with that observed for cortical bone. Proportional limit, compressive strength, and resilience were also found to undergo a linear decrease with increasing temperature. Mechanical failure of specimens generally occurred along lines determined by maximum shear stresses, approximately 45° to the axial load direction.

Enhancement of Strength through Sinter Forging

Abstract: Packing defects in powder compacts can develop into microcracks during the sintering process. For example, an agglomerate which sinters differently than its surroundings can open up large, cracklike defects which are difficult to remove in free sintering. Traditionally, hot-pressing or hot isostatic pressing has been used to prevent the formation of such defects. Here we show that processing defects can be virtually eliminated by sinter forging. In sinter forging the powder preform is compacted under a uniaxial compressive stress without any lateral constraint. In this way both densification and shear strain are imposed on the specimen. We have found that the shear strain can eliminate large pores by changing their shape. Eventually the flattening of the pore under compressive strain causes it to disintegrate into small pores which are then removed by sintering, leading to an increase in the strength of the ceramic. In alumina, a sigmoidal curve for the change in strength with strain is obtained where the strength begins to increase at 20% shear strain and reaches a maximum value at 60% strain. The shear strain required for the improvement in strength is compared with theoretical estimates.

Failure Stress Criteria for Composite Resin

Abstract: In previous work (Peters and Poort, 1983), the stress distribution in axisymmetric models of restored teeth was analyzed by finite element analysis (FEA). To compare the tri-axial stress state at different sites, they calculated the Von Mises equivalent stress and used it as an indication for weak sites. However, the use of Von Mises' theory for material failure requires that the compressive and tensile strengths be equal, whereas for composite resin the compressive strength values are, on the average, eight times larger than the tensile strength values. The objective of this study was to investigate the applicability of a modified Von Mises and the Drucker-Prager criterion to describe mechanical failure of composite resin. In these criteria, the difference between compressive and tensile strength is accounted for. The stress criteria applied to an uni-axial tensile stress state are compared with those applied to a tri-axial tensile stress state. The uni-axial state is obtained in a Rectangular Bar (RB) specimen and the tri-axial state in a Single-edge Notched Bend (SENB) specimen with a chevron notch at midspan. Both types of specimens, made of light-cured composite, were fractured in a three-point bend test. The size of the specimens was limited to 16 mm x 2 mm x 2 mm (span, 12 mm). Load-deflection curves were recorded and used for linear elastic FEA. The results showed that the Drucker-Prager criterion is a more suitable criterion for describing failure of composite resins due to multi-axial stress states than are the Von Mises criterion and the modified Von Mises criterion.

Use of Reclaimed Wastewater for Concrete Mixing

Abstract: This feasibility study of using reclaimed wastewater for concrete mixing was carried out in the laboratory. The compressive strength of concrete cubes cast with varying percentages of reclaimed wastewater in the total mixing water was studied. In general, slight increases in strength at early ages were observed from cubes cast with increased percentages of reclaimed wastewater compared with those cast with potable water. At three months or beyond, compressive strengths were similar. No adverse effect on compressive strength was observed when concrete was cured in reclaimed wastewater. In fact, concrete cubes cured in reclaimed wastewater showed increases in strength at earlier ages compared to those cured in potable water. The long-term compressive strengths are similar.

Truss-core corrugation for compressive loads

Abstract: A corrugated panel structure for supporting compressive loads includes curved cap strips 22 separated by truss-core web segments 30. The truss-core web segments 30 are formed from first and second flat panels 32 and 34 with a corrugated filler 36 therebetween. The corrugated filler 36 extends in the direction of the compressive load, thereby providing load carrying capability for the compressive load. As a result, all components of the panel structure have a compressive load carrying capability resulting in a high strength-to-weight ratio when the compressive load is limiting.

Influence of shape and size on residual stress in ceramic/metal joining

Abstract: Effects of shape and size on the residual stress on the surfaces of silicon nitride/Invar alloy joints have been examined by means of the strain-gauge method. The highest residual stress perpendicular to the interface appeared near the corners in the rectangular bond face joint. It was tensile in the silicon nitride and compressive in the Invar alloy. The highest tensile stress in the rectangular bond face joint was larger than that in the circle bond face joint. The larger the diameter of the cylindrical joint the larger was the tensile stress induced. The residual stress parallel to the interface was compressive in silicon nitride while that in the Invar alloy was tensile.

Strength and deformational behaviour of jointed rocks

Abstract: The most rational approach to design of structures on or in a rock mass is based on the strength and deformational response of the rock mass. Realizing the importance, the present investigation was undertaken to study the strength and deformational response of jointed rock mass. The objective was achieved by simulating joints in rock cores in the laboratory. Three materials, namely, plaster of Paris, Jamrani sandstone and Agra sandstone were selected. These materials provided a wide range of compressive strength (0-Ci=11.32 to 110 MN/1112 ). The nature and mineralogical composition of the materials were estimated using X-ray diffration and scanning electron micrographs. The physical and engineering properties of the materials were determined using ISRM and Indian Standard (IS) test procedures. Specimens of 38 mm diameter and 76 mm height were prepared. Anisotropy was induced into the intact specimens by developing joints at various orientations (13 =0, 30, 40, 50, 60, 70, 80 and 90°) by adopting special techniques. Triaxial tests on plaster of Paris were conducted at O3=0, 0.3, 0.5, 1.0, 1.5, 2.0, 2.5, 5.0 and 7.0 MPa. Jamrani sandstone was tested under confining pressures of 0, 2.5, 5.0 and 10.0 MPa, whereas Agra sandstone was tested at (5=0, 2.5, 5.0, 7.5 and 10.0 MPa. Joint frequency or number of joints per meter, Jn, joint inclination, p, and joint roughness have been identi-