Showing papers on "Flexural strength published in 1974"

The mechanical behaviour of some molluscan hard tissues

Abstract: Pieces of shell from 19 species of molluscs exhibiting various microstructures were tested for tensile strength, modulus of elasticity in bending and modulus of rupture. In tensile strength most shells with cross-foliated, foliated, homogeneous and crossed-lamellar structures did not exceed 60 MNm 2 but prismatic and nacreous structures often exceeded this value. Nacreous structure was generally superior to all others in modulus of rupture tests; that of Turbo being about equal to bone. Values of modulus of elasticity were more uniform between structures. There is a general relation between mechanical properties, microstructure and the life style of each animal. Nacreous structure, which is very strong but not widely used, apparently evolved earlier than the less strong but widely used crossed-lamellar structure.

Waste Glass as Coarse Aggregate for Concrete

Abstract: The paper reports on the performance of 34 different concrete mixes containing glass crushed to ¾-in. (19-mm) maximum size as coarse aggregate and six reference mixes made with gravel of the same size. Two cements of alkali equivalent 0.58 and 1.13, classifiable as low and high alkali (ASTM C 150-72), in amounts ranging from 400–900 lb/yd3 (237–534 kg/m3 were used in combination with glass both with the fines removed and in the as-crushed condition. Partial cement replacement with fly ash and mixing of glass with gravel aggregate were included in an attempt to find a suitable method of overcoming the expected adverse effects of the reaction between glass and cement alkalis. On the basis of compressive strength, flexural strength, expansion, and visible surface deterioration recorded up to an age of one year, the results show that in many cases the direct combination of glass with portland cement yields concrete which exhibits marked strength regression and excessive expansion due to alkali-aggregate reaction. The conditions under which performance is satisfactory appear to relate to limiting maximum values of cement content and alkali equivalent. Replacement of 25 to 30 percent by weight of the cement, whether low or high alkali, appears to be an effective and widely applicable method of ensuring good long-term concrete performance, although the minimum required in any given case may be related to cement composition.

Potential application of graphite fiber and methyl methacrylate resin composites as internal fixation plates.

Abstract: The use of conventional rigid metal plates (modulus of elasticity 30 × 106 psi) for internal fixation of difficult long bone fractures may paradoxically weaken the fractured bone through a process of osteoporosis secondary to protection of the bone from stress. The hypothesis being studied which has stimulated the present work is that a new composite plate made of less rigid material but with similar geometry to stainless steel plates may promote fracture union without the complication of osteopenia. This new composite material has a modulus of elasticity similar to that of bone (approximately 3 × 106 psi) which is more than an order of magnitude lower than that of stainless steel, yet the fracture strength of this new composite material (35,000 psi) is approximately one-half that of stainless steel. The ability to fix diaphyseal fractures by composite plates was studied by implanting these plates in the left (experimental) radii of six dogs with transverse mid-shaft osteotomies. The right (control) radii had similar transverse osteotomies, but were plated by conventional metal plates equal in size to the new composite plates. Four months postsurgery, all experimental and control fractures had healed. Biomechanical tests of the excised radii using a torsional loading apparatus showed there were no significant differences in strength, fracture energy, angular deformation, or maximum shearing stress when comparing the experimental to the control radii. The results are sufficiently promising to warrant additional long-term experiments investigating this concept of internal fixation.

Process for making a reinforced board from lignocellulosic particles

Abstract: A reinforced board manufacturing process whereby a compressed uniform mixture of comminuted lignocellulosic particles, binder and other additives has a plurality of elongate reinforcing filaments having substantially greater tensile strength and modulus of elasticity than the mixture, embedded therein and bound to the particles by the binder. For some board applications, the reinforcing filaments comprise a plurality of short filaments, exemplarily glass fibers or steel wires, of about one-quarter to one and one-half inches in length distributed uniformly throughout the particle and binder mixture in a random orientation so as to extend generally in all directions. For board applications having greater flexural strength and stiffness requirements, the reinforcing filaments comprise a series of long filaments specifically oriented straightly in a predetermined direction corresponding to the direction of expected tensile stress and in a parallel arrangement spaced transversely from one another. The distribution of such long filaments is normally not uniform throughout the board, but rather concentrated in certain areas where greatest flexural strength and stiffness is needed. A combination of short and long reinforcing filaments embedded in the board is preferred for many applications.

No-fines concrete - its properties and applications

Abstract: No-fines concrete consists solely of normal portland cement, water and coarse aggregate. It has been used in Europe and the United Kingdom since the 1930s for the building of single story and multistory dwellings, but had found little acceptance in North America. In recent years, however, due to increased awareness of the need for conservation of nonrenewable mineral resources, increased consideration is being given to the use of no-fines concrete in Canada and the United States. The compressive strength of no-fines concrete is considerably lower than that of conventional portland cement concrete and varies between 200 to 2000 psi (1.37 to 13.73 MN/square meters ). Young's modulus of elasticity is usually between 1.0 x 10,000,000 to 1.5 x 10,000,000 psi (0.7 x 100,000 to 1.2 x 100,000 MN/square meters) depending on the strength level of the concrete. The ratio of modulus of rupture to compressive strength expressed as a percentage varies between 10.8 and 42.0 percent. The shrinkage of no-fines concrete made with crushed limestone or river gravel, is of the order of 200 x 0.000001. This is about half for that of conventional concrete. Investigations at CANMET have indicated that no-fines concrete prisms with no air-entraining agent had poor resistance to freeze-thaw cycling; the corresponding prisms incorporating an air-entraining agent were able to withstand up to 274 freeze-thaw cycles compared with 56 for prisms without an air-entraining agent. The principal advantages claimed for no-fines concrete are economy in materials, somewhat higher thermal insulating values, lower shrinkage, and lower unit weight. The major disadvantages are its low compressive, flexural, and bond strength, and higher permeability. The principal applications of no-fines concrete are for load-bearing cast-in-place external walls of single story and multistory housing, small retaining walls and as a dampproofing subbase material for concrete floors cast on grade. This type of concrete is also eminently suitable for construction in northern Canada because of its somewhat higher thermal insulating property and low cement content.

Stress analysis of the compact specimen including the effects of pin loading

Abstract: An improved method of boundary collocation was applied to the two-dimensional stress analysis of the compact specimen. The effects of the pin-loaded holes on stress-intensity factors and crack-opening displacements were investigated for various crack-length-to-specimen-width ratios, hole locations, and internal loadings. The stress-intensity factors for the 'standard' compact specimen under plane-stress or plane-strain conditions were found to be within 1 percent of the stress-intensity factors reported in the ASTM Test for Plane-Strain Fracture Toughness of Metallic Materials (E 399-72) over a range of crack-length-to-specimen-width ratios of 0.4 to 0.7. However, for crack-length-to-specimen-width ratios less than 0.4, the pin-load holes (which were not previously accounted for) had a significant effect on stress intensity and crack-opening displacements.

Slow Crack Growth in Cement Paste

Abstract: The dependence of crack velocity on stress intensity was measured in hardened cement paste. Fracture toughness, modulus of rupture, and dynamic elastic modulus were also measured to permit calculation of the time-to-failure (life expectancy) of stressed cement paste. The double torsion method was used for slow crack growth studies and fracture toughness measurements. Fracture toughness was also measured by a notched-beam method; the agreement between the two methods was excellent. The crack-velocity-stress-intensity curve for cement paste resembles that of glass in water. The slope of the log V-log KI curve is ∼35.

Imparting strength and toughness to brittle composites

Abstract: Appropriate intermittent coatings of fibers can produce areas of low and high toughness in brittle composites. Experiments using silicon vacuum grease (SVG) and polyurethane varnish (PUV) coatings that achieve weakly and strongly bonded interfaces are described. Tensile strength and edge-crack fracture toughness for both SVG and PUV coatings were plotted against the percentage coating (C). Both coating materials maintain tensile strengths in the order of the rule of mixture strength values up to a large C. It is suggested that both materials produce similar coated interfacial shear strengths while producing different effects on toughness.

Fracture strength of soda-lime glass after etching

Abstract: The strength of soda-lime glass at liquid nitrogen temperature after various amounts of etching was measured. A median crack length of 6 μm was calculated from the results and a model of the etching process. It was found that the rate of etching at the crack tip was much lower than on the external surface. Measured distributions of strength for samples etched different depths were also in reasonable agreement with calculated distributions. The etching process itself was found to cause some weakening of the glass.

Short‐ and long‐term strength characteristics of particulate‐filled cast epoxy resin

Abstract: Particulate-filled thermosetting composites are widely used, yet little systematic work has been done on their long-term strength characteristics. In this study short-term tensile, flexural, and impact tests as well as tensile creep-rupture tests were made for unfilled and filled epoxy to clarify the effects of filler size, filler content, and temperature. Fillers used were silica, alumina particles, and glass beads. Test temperatures were varied from 25 to 110°C. As a result of short-term testing, it was found that the Petch relation held between strength and filler size if brittle fracture occurred, while a strength and filler size if brittle fracture occurred, while a strengthening effect existed when ductile fracture occurred. On creeprupture testing, a strengthening is observed with filler size and content for silica and glass beads. The Arrhenius plot of rupture time for various filler sizes and contents converges to a characteristic point corresponding to the glass transition temperature of the material. Using this relation, a modified Larson-Miller master rupture curve is proposed which can predict the long-term strength of particulate-filled thermosetting composites as functions of rupture time, temperature, filler size, and content.

Influence of fibre-aggregate interaction on some properties of steel fibre reinforced concrete

Abstract: In the practical applications of fibre reinforced concrete, particularly for load-bearing purposes, it is desirable to incorporate coarse aggregates for economic and other reasons. Tests are reported in this paper in which the fibre-aggregate interaction is related to the properties of steel fibre concrete both in the fresh and hardened states. It is shown that compactibility, flexural and compressive strengths are reduced progressively by the presence of coarse aggregates. From the test results, a mix design for fibre concrete containing coarse aggregates to produce twice the flexural strength of the unreinforced matrix without bundling of fibres is presented. The data presented on flexural strength show good agreement with theory.

Effects of Moisture on the Properties of High-Performance Structural Resins and Composites

Abstract: Graphite, boron, S-glass, and DuPont's PRD-49-III fiber reinforced composites, as well as castings of current epoxy resin systems, were evaluated to determine the effects of moisture and high humidity on their physical properties and their room- and elevated-temperature mechanical properties. All of the neat resin castings were found to absorb moisture and swell. Associated with moisture absorption is a loss in elevated-temperature tensile strength. All of the composite systems showed weight gains and thickness increases when subjected to a high-humidity environment. However, the effect of absorbed moisture on the elevated-temperature mechanical properties of composites is determined principally by fiber orientation and test method employed. Unidirectional composites may show a significant reduction of 350°F (177°C) flexural strength due to absorbed moisture, whereas a multidirectional lay-up may show only a minor loss of 350°F tensile strength after equivalent moisture absorption. Fiber-controlled composite properties are relatively unaffected by absorbed moisture whereas matrix-controlled properties are adversely affected.

Molded plastic article and method

Abstract: A rigid plastic article includes an outer skin surface of a polyolefin plastic material such as cross-linked high density polyethylene tightly secured to a foam core the latter preferably of a high density or medium density polyethylene. Such a laminated plastic article has substantial rigidity and strength, particularly flexural strength, the latter being greater for the laminate than the sum of the individual flexural strengths of the skin and foam separately. Due to the thermal and sound insulation, rigidity, flotation, impact resistance, relative lightweight and weatherability characteristics, the laminate offers particular advantages for use in complex shapes such as automotive doors and tops, camper tops, boats and recreational vehicles and containers such as water tanks, carboys and the like. Such products may be manufactured by a rotational molding procedure in which each of the plastic materials is sequentially introduced into a rotating heated mold. The first plastic material coats the mold surface and the second is sequentially released so that both plastics are fused together in a single cycle with the result that a strong natural bond is formed at the interface of the laminate thus contributing substantially to the overall strength of the resultant product. A third plastic material may be used to form a second surface skin. Typical materials are high, medium, low density polyethylene, cross-linked polyethylene, polycarbonates, and acrylic modified vinyl chloride plastics. The foam core is preferably of high or medium density linear polyethylene having a closed cell structure and a density of 12 to 20 pounds per cubic foot.

Polyphenylquinoxalines: Synthesis, characterization, and mechanical properties

Abstract: The synthesis of polyphenylquinoxalines (PPQ) is discussed in detail. Solutions of PPQ at 20% solids content from stoichiometric quantities of pure reactants are generally metastable, increasing in viscosity and, in some cases, gelling. This can be alleviated without any appreciable detrimental effect upon the polymer by upsetting the stoichiometry of the reactants. Relatively high molecular weight polymer (ηinh = 1.84 dl/g; H2SO4, 0.5%, 25°C) can be obtained when the stoichiometry is upset by 2%. Several new PPQ are reported which contain multiple phenyl ether groups located in the para position of the substituted phenyl ring. Introduction of a phenoxy group decreased Tg by 70°C. The properties of random and block copolymers of phenylquinoxaline and phenyl-as-triazine units are reported. Depending upon the processing conditions, PPQ Ti tensile shear specimens gave RT strength of 4900 psi and 316°C strength after 500 hr at 316°C of 1500 psi. Unidirectional high-modulus graphite composites provided RT flexural strength of 123,000 psi, flexural modulus of 15.6 × 106 psi, and inter-laminar shear strength of 7000 psi.

Development of Engineering Data on the Mechanical and Physical Properties of Advanced Composites Materials

Abstract: : Data were generated on the effect of various environments on the physical, thermal, and mechanical properties of Thornel 300/Narmco 5208 composites. The laminates were prepared from three-inch wide prepreg tape in accordance with current aerospace specifications. The environments included steady state humidity conditioning for two exposure periods, cyclic humidity conditioning which conductivity, density, steady state thermal exposure, thermo- humidity cycling, intralaminar shear, flex tests, Thornel 300 Graphite/Narmco 5208, graphite/epoxy composites, laminate data, laminate fabrication, moisture weight gain. The mechanical properties investigated included tension, compression, in-plane shear, interlaminar shear and flexural static properties, fatigue, creep and stress-rupture resistances.

Prediction of particleboard mechanical properties at various moisture contents

Abstract: For this study, urea-bonded particleboards with three resin levels and three specific gravity values were made in the laboratory. Specimens for modulus of rupture, modulus of elasticity, and internal bond strength tests were conditioned at 14 relative humidity values over the range 13–97 per cent. Results for the three mechanical properties were fitted to cubic equations in moisture content with a high degree of accuracy (correlation coefficients of over 0.99 in most cases).

Analysis of three-point-bend test for materials with unequal tension and compression properties

Abstract: An analysis capability is described for the three-point-bend test applicable to materials of linear but unequal tensile and compressive stress-strain relations. The capability consists of numerous equations of simple form and their graphical representation. Procedures are described to examine the local stress concentrations and failure modes initiation. Examples are given to illustrate the usefulness and ease of application of the capability. Comparisons are made with materials which have equal tensile and compressive properties. The results indicate possible underestimates for flexural modulus or strength ranging from 25 to 50 percent greater than values predicted when accounting for unequal properties. The capability can also be used to reduce test data from three-point-bending tests, extract material properties useful in design from these test data, select test specimen dimensions, and size structural members.

Fracture Topography of Ceramics

Abstract: Fracture topography is a permanent record of the fracture process, providing valuable information on crack propagation and failure mechanisms. This paper describes mostly features observed in flexural failure at room temperature. Previous papers1–3 are complemented by emphasizing features and techniques for finding and characterizing fracture origins to aid in use of such important, too often neglected, steps in strength studies.

Flexural Vibrations and Timoshenko's Beam Theory

Abstract: This paper is a study of flexural elastic vibrations of Timoshenko beams with due allowance for the effects of rotary inertia and shear. Two independent formulations are developed, one based on the concepts proposed by Timoshenko and the other on the extended Rayleigh-Ritz energy method. The results obtained from the two formulations are matched in order to determine the correcting coefficients in the simplified formulae which are proposed for the frequency of the first two flexural branches. The proposed formulae are shown to achieve greater accuracy in describing the flexural motions. New accurate solutions are offered for several cross-sectional geometries which enable the accuracy of the available methods to be assessed. Correction coefficients are evaluated for several sections and are compared with previous work.