Showing papers on "Flexural strength published in 1990"

Micromechanical fracture strength of silicon

Abstract: In order to test the statistical influence of some process and micromachining parameters on the fracture strength of silicon microelements, arrays of identical microsized cantilever beams were bulk micromachined in single‐crystalline silicon wafers. The beams were exposed to various surface treatments (diamond polishing with different grades, oxidization, stripping of oxide) in different combinations. The influence on fracture strength was investigated by bending the beams to fracture in a micromanipulator mounted in situ in a scanning electron microscope while registering force‐versus‐deflection curves. Average fracture strengths, standard deviations, Weibull moduli, crack‐initiating flaw sizes, and in some cases elastic moduli were evaluated. Diamond polishing was found to decrease the fracture strength drastically, but polishing followed by oxidization not only restored the original strength, but actually increased it, due to crack healing. Polishing, oxidization, and subsequent stripping of oxide resu...

Premature Failure of Externally Plated Reinforced Concrete Beams

Abstract: Reinforced concrete beams can be strengthened and stiffened by gluing mild steel plates to the tension face of the beam. However, experimental tests show that these externally bonded plates have a tendency to peel away after the formation of shear diagonal cracks or when the curvature in the beam is increased. Fifty‐seven plated reinforced concrete beams are tested to study peeling induced by increasing curvature; in these tests, the geometry and material properties of the beams are varied, and the reinforced concrete beams are subjected to pre‐cracking and pre‐cambering, as might occur in an existing structure. A method is derived for determining the moment at which peeling starts (serviceability limit) and the moment that causes complete separation of the plate (ultimate limit). This method, which depends on the flexural rigidity of the plated beam, the thickness of the plate, and the tensile strength of the concrete, can be used to adjust the size and extent of plating so that flexural peeling does not...

The effect of cell size on the mechanical behavior of cellular materials

Abstract: The Young's modulus, strength and fracture toughness, of a brittle reticulated vitreous carbon foam, was measured as a function of cell size at a constant density and compared to a theoretical model Image analysis was used to characterize the macrostructure of the samples and provided a basis for evaluating the mechanical behavior It was determined that both the compressive and bend strength scale inversely with cell size The change in compressive strength is due to a change in the strut strength with cell size The bend strength behavior may be due to a reduction in the critical flaw size, as well as the increasing strut strength at smaller cell sizes The fracture toughness and elastic modulus were found to be independent of cell size Comparison of these results with previous work on open cell alumina clearly indicates a very different behavior and is attributed to a change in the microstructure of the solid phase with cell size in the alumina materials

Behavior of surface and corner cracks subjected to tensile and bending loads in Ti-6Al-4V alloy

Abstract: The behavior of part-through flaws with regard to failure under monotonic loading and their growth under fatigue loading was studied experimentally and analytically. Comparisons are made of experimental values of toughness obtained using surface and corner cracked specimens with those obtained using standard test specimens, and also experimental growth cycles were compared with numerical predictions using the NASA/FLAGRO computer program. Tests were conducted on various types of surface and corner cracks under tensile and bending loads. Room temperature lab air provided the test environment. The material used in this study was the Ti-6Al-4V alloy in the solution treated and aged (STA) and stress relieved condition. Detailed tabulation of the fracture toughness data and results of life prediction using the NASA/FLAGRO program are presented. Fatigue crack growth rates for the part-through cracked specimens are compared with a base curve fitted from the data obtained using standard specimens. The fatigue loading used in the crack growth testing was constant-amplitude sinusoidal type.

In‐Plane Resistance of Reinforced Masonry Shear Walls

Abstract: Flexural and shear strengths of reinforced masonry shear walls are examined, based on experimental results obtained from 22 6-ft. by 6-ft. masonry wall specimens. These results are summarized and current design formulae examined. It is found that the simple flexure theory based on the plane-section assumption can be applied to square wall panels with good accuracy. Moreover, it appears to be consistently conservative. The 1988 Uniform Building Code specifications for the shear strength tend to be very conservative for the square wall panels studied and less conservative for walls with lower aspect ratio. Furthermore, the code specifications tend to overestimate the shear strength contributed by the horizontal reinforcement and neglect the influence of axial stress. Hence, a new shear formula that takes into account the influence of axial stress and flexural reinforcement is proposed. The formula appears to have good correlation with experimental results obtained in this study as well as those of others.

Mixed‐Mode Fracture of Concrete Subjected to Impact Loading

Abstract: An analytical and experimental investigation of mode I (tensile) and mixed mode (combined tensile and shear) fracture of concrete subjected to impact loading was conducted. The rate of loading ranged from a slow strain rate of 10\u-\u6/s to an impact strain rate of 0.5/s. Mixed-mode tests were conducted using beams with notches at different locations along the span of the beam. A new clip gage was developed to measure crack-opening displacement at slow and impact rates of loading. A nonlinear fracture-mechanics model was developed to predict the rate sensitivity of mode I and mixed-mode fracture of concrete. This approach was based on the observation that the prepeak nonlinearity may be attributed to prepeak stable crack growth and that this prepeak crack growth decreased with an increase in the rate of loading. The mixed-mode-fracture-mechanics study used finite element analysis with singular quarter-point elements at the crack tip. Mixed-mode experimental and analytical results indicate that impact loading could result in brittle diagonal tension-shear failure of concrete structures as opposed to ductile flexural failure at a slow rate of loading.

Bending strength of lumber

Abstract: This is the description of a model that explains the relationship between the three strength properties, tension, compression and bending. The model which included nonlinear compresion behavior and size dependent brittle tension behavior of wood, has been calibrated and verified with reference to test results. Although it could be used for a single piece of lumber, the model is more useful to predict the bending strength of lumber throughout the distribution of strength, given the distribution of tension and compression strengths for a given size, species, and grade.

Mechanical properties of heat treated restorative resins for use in the inlay/onlay technique

Abstract: Inlay/onlay materials based on composite resins have recently been introduced. It was the aim of the present study to investigate the effect of post-curing temperature on selected mechanical properties. Four different composite resins were produced. The composite resins had identical filler content, but varied with respect to the content of the monomers BISGMA, TEGDMA, UEDMA, and HEMA. After initial curing by light, the materials were post-cured at 37, 100, 125, 150, 175, or 200 degrees C for 1 h. We then measured diametral tensile strength, flexural strength, and modulus of elasticity. These properties were found to vary with the composition of the material. Composites containing UEDMA or UEDMA/HEMA were stronger and stiffer than BISGMA/TEGDMA formulations. The optimum post-curing temperature was 150 degrees C, but the increase in strength and stiffness was moderate, only about 9%.

Mode II interlaminar fracture of the center notch flexural specimen under impact loading

Abstract: The Mode II interlaminar fracture of unidirectional graphite/epoxy (AS4/2220-3) and graphite/polyetheretherketone (APC-2) composite laminates subjected to impact loading is investigated using the Center Notch Flexural (CNF) test specimen. Experimentally, instrumented impact testing of the CNF specimen enables the impact force history and the absorbed energy during delamination propagation to be estimated. Scanning electron microscopic examination is conducted to assess the influence of impact loading on fracture morphology in the crack initiation and propagation regions of the frac ture surface. Data reduction for evaluating the Mode II interlaminar initiation toughness is based upon beam theory that includes kinetic energy effects and dynamic finite element analysis of the specimen in conjunction with virtual crack closure techniques. The initia tion toughness under impact loading was approximately 20 and 28% lower than the static values for AS4/2220-3 and APC-2, respectively. Data reduction for evaluati...

Thermal stresses, differential subsidence, and flexure at oceanic fracture zones

Abstract: Geosat geoid undulations over four Pacific fracture zones have been analyzed. After correcting for the isostatic thermal edge effect, the amplitudes of the residuals are shown to be proportional to the age offset. The shape of the residuals seems to broaden with increasing age. Both geoid anomalies and available ship bathymetry data suggest that slip must sometimes occur on the main fracture zone or secondary faults. Existing models for flexure at fracture zones cannot explain the observed anomalies. A combination model accounting for slip and including flexure from thermal stresses and differential subsidence is presented. This model accounts for lateral variations in flexural rigidity from brittle and ductile yielding due to both thermal and flexural stresses and explains both the amplitudes and the shape of the anomalies along each fracture zone. The best fitting models have mechanical plate thicknesses that are described by the depth to the 600-700 C isotherms.

Characterization of edge effects in cellular materials

Abstract: Cellular solids constitute a unique class of materials possessing a high stiffness to weight ratio. Due to the high level of porosity in these materials (70 to 99.7%) one must utilize special testing considerations in order to obtain accurate results. One of these items has been referred to as edge effects and stems from the large scale macrostructure of cellular materials. This results in a dramatic decrease in the mechanical properties when testing small specimens having a large cell size. A reticulated vitreous carbon was used to characterize these effects for both elastic modulus and flexural strength measurements. It was observed that a critical specimen to cell size ratio is required to overcome these effects and achieve accurate results. A simple model is presented to help in predicting these edge effects.

Repaired reinforced concrete beams

Abstract: The repair of cracks is a necessity, and a comparison between repair methods for reinforced concrete beams subjected to different levels of cracking was made experimentally. Four methods of repair were studied: epoxy injection; ferrocement; steel-plate bonding; and combined method of epoxy injection and ferrocement. Levels of damage studied range from beam cracking at service load to complete failure of the beams. Experimental data on strength and ductility characteristics of repaired beams were obtained and comparisons were made. Epoxy injection is shown to restore strength and ductility for all levels of damage studied while ferocement increases the strength and partially restores ductility, depending on the level of damage. The steel-plate bonding repair technique leads to an increse in strength, but concomitantly with considerable reduction in ductility of the repaired beams, regardless of the level of damage. The combined method of repair leads to both increase in strength and ductility. The increase in ductility will depend on the level of damage.

Strengthening of Glass by Sol-Gel Coatings

Abstract: The affects of alkoxide-derived coatings on the bending strength of silica glass were investigated. For densified coatings, increases in strength of 120% to 130% were found. A model of the strengthening process, based on the filling-in and partial healing of surface flaws, is proposed. The model accounts for the temperature dependence of strengthening, the inensitivity of strengthening to the size of oligomers in the coating solution, and the insensitivity of strengthening to the thickness of the coating. The presence of a significant residual tensile stress in sol-gel-strengthened material is also indicated.

Strength and deformation characteristics of reinforced concrete walls under load reversals

Abstract: The effects of loding history and repair methods on the structural characteristics of reinforced concrete walls was investigated. Large scale slender wall models were tested to failure, then unloaded, repaired, and retested to destruction under various regimes of cyclic horizontal loading. It was found that, while repairing only the damaged regions of the compressive zone was sufficient to fully restore wall strength, the additional use of epoxy resins to heal major flexural and inclined web cracks led only to a marginal improvement of the structural characteristics, the latter being distinctly inferior to those of the original walls. Such results are in compliance with the concept of the compressive force path and demonstrate that, in contrast to widely held views, the compressive zone is the main contributor to shear resistance.

Some engineering properties of slag concrete as influenced by mix proportioning and curing

Abstract: This paper presents a simple method to obtain a 50 MPa 28-day strength concrete having 50 and 65 % by weight cement replacements with slag having a relatively low specific surface. The method produces slag concrete with strengths comparable to ordinary portland cement concrete from 3 days onward. The compressive and flexural strengths and the elastic modulus of these two concretes as affected by curing conditions are then presented. Prolonged dry curing is shown to adversely affect tensile strength and elastic modulus, and to create internal microcracking, as identified by pulse velocity mearurements. High swelling strains at high slag replacement levels show the need for longer wet curing for such concretes. The results emphasize that even 7-day wet curing was inadequate for high levels of slag replacement, and that continued exposure to a drying environment can have adverse effects on the long-term durability of inadequately cured slag concrete.

Joining of ceramics demonstrated by the example of SiC/Ti

Abstract: The reaction behaviour of titanium sputtered on silicon carbide was investigated in a series of tests. It was ascertained that, in the temperature range between 1250 and 1500° C, mainly the ternary phase Ti3SiC2 is formed (>90%). In joining experiments of silicon carbide with 1–3 μm thick titanium layers at 1450° C at compaction pressures between 5 and 30 MPa the formation of the ternary phase leads to a high joining strength. The achieved bending strength (σM ≈ 286 MPa; m ≈ 10) is comparable to that of the starting materials (σM ≈ 303 MPa; m ≈ 15).

Tension-stiffening and moment-curvature relations of reinforced concrete elements

Abstract: The different parameters that influence the tension stiffening of reinforced concrete are critically examined. A simple equation based on previous experimental results is derived to define the softening characteristics of the concrete, and can be used to compute deflections. Reinforcement parameters, such as bar diameter, reinforcement spacing, and cover affect the numerical coefficients in the equation describing the falling branch of the concrete component. Incorporating these material nonlinearities in the tension and compression zones of a 2-zone model, moment-curvature relations are developed. The analytical resutls are verified by comparison with test data. The effect of the resultant tensile force location and magnitude on the overall respnse of the structure is demonstrated in flexural members with low percentages of reinforcement.

Creep behaviour of post-weld heat-treated 2.25Cr-1Mo ferritic steel base, weld metal and weldments

Abstract: The evaluation of the creep rupture behaviour of 2.25Cr-1Mo ferritic steel at 773 and 823 K over a stress range of 130–300 MPa has been carried out. The material conditions examined include base material, weld metal and weldments (comprising base, weld and heat-affected zones). Specimens for creep testing were taken from single-V-weld pads, fabricated by manual metal arc welding using basic coated 2.25Cr-1Mo electrodes, and were given a post-weld heat treatment (973 K for 1 h). Microstructure and hardness in the as-welded, post-weld heat-treated and creep-tested conditions were evaluated. The heat-affected zone consisted of coarse-grain bainite, fine-grain bainite and intercritical structure regions. Generally, the weld metal exhibited significantly higher creep rupture strength than the base material, while the composites showed inferior creep strength. Ductility was found to decrease with increasing rupture time for all the material conditions. The rupture elongation exhibited by the base material was significantly higher than that shown by weld metal and composite specimens. Composite specimens crept at a faster rate while weld metal deformed at a slower rate than base metal. The applied stress (σ) dependence of the secondary creep rate (es was found to follow a power-law relationship of the form e = Aσ n . At all test conditions, transgranular fracture was observed. Failure in the composite specimens occurred in the intercritical structure of the heat-affected zone. Interrupted creep tests at 823 K on composite specimens have revealed progressive localization of creep deformation in the intercritical structure prior to fracture. An attempt has been made to establish equations that can predict the weld joint creep properties on the basis of the base and weld metal properties.

Effect of span-depth ratio on the ultimate steel stress in unbonded prestressed concrete members

Abstract: The effect of member span-depth ratio on the predicted steel stress f sub ps in unbonded prestressed concrete members at their nominal moment resistance is implemented in an approximate manner of ACI 318-83. The recognition of member span-depth ratio and its implementation as an independent design parameter was made based on comparison of limited experimental results. However, there is not yet a clear phenomenological explanation that helps in understanding the mechanism of this parameter and its level of influence on the predicted f sub ps at ultimate flexural strength.

Flexural plate-wave gravimetric chemical sensor

Abstract: We describe the use of flexural plate waves traveling in thin composite plates of zinc oxide, aluminum and silicon nitride for gravimetric chemical sensing. Theoretical and experimental results show that, compared to bulk- and surfacewave gravimetric sensors, flexural plate-wave sensors are more sensitive to added mass while operating at significantly lower frequencies. Experiments with sorptive poly(dimethylsiloxane) films exposed to vapors of toluene, 1,1,1-trichloroethane and carbon tetrachloride demonstrated the high sensitivity of the plate sensor and detection in the parts-per-million range.

The fatigue properties of wood in flexure

Abstract: A detailed review of the wood fatigue literature is presented and the need for experimental work under load control at a range of moisture contents and Rratios is emphasized. A laminated hardwood, Khaya ivorensis, a softwood, Sitka spruce, and compressed beech laminates were fatigue tested under load control in four point flexure. Tests were conducted in repeated and reversed loading over a range of five Rratios at three moisture contents, and the accumulation of fatigue damage was followed by microtoming fatigued wood and observing the formation of cell wall kinks by polarized light optical microscopy. Fatigue life is largely species independent when normalized by static strength. Increased moisture content reduces the static strength and fatigue life and reversed loading results in the lowest fatigue life. A constant life diagram for sliced Khaya laminate has been constructed which summarizes the effect of Rratio on fatigue life. Optical microscopy demonstrates that fatigue damage is progressive commencing on the compression face of flexural samples as fine scale cell wall kinks and developing into macroscopic creases.

mechanics of solids and structures

Abstract: Stress and strain transformation plane elasticity theory structures with symmetry bending of beams and plates theories of torsion moment distribution flexural shear flow energy methods instability of columns and plates finite elements yield and strength criteria plasticity and collapse creep and visco-elasticity high and low cycle fatigue fracture mechanics. Appendices: properties of areas matrix algebra stress concentrations.

Chemical toughening of epoxies. II: Mechanical, thermal, and microscopic studies of epoxies toughened with hydroxyl-terminated poly(butadiene-co-acrylonitrile)

Abstract: Diglycidyl ether–bisphenol-A-based epoxies toughened with various levels (0–12%) of chemically reacted liquid rubber, hydroxyl-terminated poly(butadiene-co-acrylonitrile) (HTBN) were studied for some of the mechanical and thermal properties. Although the ultimate tensile strength showed a continuous decrease with increasing rubber content, the toughness as measured by the area under the stress-vs.-strain curve and flexural strength reach a maximum around an optimum rubber concentration of 3% before decreasing. Tensile modulus was found to increase for concentrations below 6%. The glass transition temperature Tg as measured by DTA showed no variation for the toughened formulations. The TGA showed no variations in the pattern of decomposition. The weight losses for the toughened epoxies at elevated temperatures compare well with that of the neat epoxy. Scanning electron microscopy revealed the presence of a dual phase morphology with the spherical rubber particles precipitating out in the cured resin with diameter varying between 0.33 and 6.3 μm. In contrast, a physically blended rubber–epoxy showed much less effect towards toughening with the precipitated rubber particles of much bigger diameter (0.6–21.3 μm).