Showing papers on "Flexural strength published in 1988"

Plate separation and anchorage of reinforced concrete beams strengthened by epoxy-bonded steel plates

Abstract: This paper deals with the problem of Anchorage at the ends of steel plates glued to the tensile faces of reinforced concrete beams. A simple theoretical study of the force systems at the plate/glue and the glued concrete interfaces is presented. This suggests that high stress concentrations and peeling forces are present at the ends of the plates when the composite beam is loaded in flexure. Tests carried out to investigate the effectiveness of different Anchorage arrangements are described in detail. The results from these tests confirm that, at the ends of the plates, interface stress concentrations exist, which have limiting peak values in the region of root 2 x tensile splitting strength of the concrete. Theoretical interface bond stresses, based on simple elastic behaviour, are found to have no consistent relationship to the measured peak values. However, if the maximum (unreduced) plate thickness is always used in these calculations, a simple method is proposed for obtaining a reasonable assessment of the peak stress. The efficiency of the different Anchorage details is discussed, and it is shown that the use of additional glued anchor plates gives the best results. These plates overcome the problem of Anchorage failure and enable the full theoretical flexural strength to be achieved, together with ductile behaviour.(a)

Fracture testing of silicon microelements in situ in a scanning electron microscope

Abstract: Fracture testing of silicon cantilever beams (thicknesses 10–20 μm) was performed in situ in a scanning electron microscope by means of an equipment specially designed for this purpose. Beams of various sizes and orientations (〈011〉 and 〈001〉) were manufactured in Si (100) wafers by two different micromachining procedures. The beams were tested by simple bending to fracture, and a number of fundamental fracture parameters were determined from an analytical model of elastic fracture. To verify its validity, the model was utilized to evaluate an experimental E modulus, which was found to agree well with previous results. Fracture limits, fracture strains, and initiating flaw sizes were determined. The maximum fracture limit was very high; about 10 GPa. The strengths of different beams scattered from this value down to practically zero strength, with an average close to 4 GPa. The corresponding fracture strains and initiating flaw sizes were 6% and 3 nm, respectively (maximum strength), and 2% and 17 nm (ave...

Recent Development in Thermoplastic Composites: A Review of Matrix Systems and Processing Methods

Abstract: Manufacturers of composite materials have been developing high strength fiber rein forced composites with thermoplastic matrix systems exhibiting enhanced performance in fracture toughness and damage tolerance with the potential of reduced manufacturing cost for both primary and secondary structure components. A review will be made of principal thermoplastic matrix systems and processing methods used to fabricate the fiber rein forced composites, which have been developed during recent years. Included is a com parison of the neat matrix resins' chemical structure, thermal/tensile properties, fracture toughness, melt viscosity, solvent resistance, morphology and melt processability. The matrix-dominated mechanical properties of thermoplastic composites are given, including flexural, short beam shear and compression strengths, interlaminar fracture toughness and/or compression-after-impact performance. Innovative process development in thermoplastic composites including thermoformable sheets, filament windi...

Survey of quartz bulk resonator sensor technologies

Abstract: The term 'bulk resonator' is used to include a variety of vibrational modes. The survey is broken down by type of resonant mode, namely thickness shear, single-ended flexural, double-ended flexural, and torsional. Where appropriate, the discussion of each type of resonant mode includes items related to the frequency-control applications of the particular mode to emphasize the cross fertilization occurring between frequency control and sensor work. >

Composite abrasive compact having high thermal stability and transverse rupture strength

Abstract: A composite compact adapted for high-temperature uses, such as a cutter on a rotary drill bit, which includes a relatively thick table of diamond or boron nitride particles with a strong, chemically inert binder matrix and a thin metal layer bonded directly to the table in a HP/HT press. The table is characterized by having high thermal stability at temperatures up to 1200° C. The thickness of the thin metal layer, which does not exceed one-half that of the table, is selected such that at temperatures up to 1200° C. the differential forces due to thermal expansion do not exceed the fracture strength of the table.

Rising Fracture Toughness from the Bending Strength of Indented Alumina Beams

Abstract: The analytical function of crack extension to a fractional power is used to represent the fracture resistance of a vitreous-bonded 96% alumina ceramic. A varying flaw size, controlled by Vickers indentation loading between 3 and 300 N, was placed on the prospective tensile surfaces of four-point bend specimens, previously polished and annealed. The lengths of surface cracks were measured by optical microscopy. Straight lines were fitted to the logarithmic functions of observed bending strength versus indentation load in two series of experiments: (I) including the residual stress due to indentation and (II) having the residual stress annealed out at an elevated temperature. Within the precision of measurement these lines have the same slope, being about 32% less than the -1/3 slope which a fracture toughness independent of crack extension would indicate. Considering the criteria for crack extension and specimen failure, the fracture mechanics equations were solved for the conditions of the two series of experiments. Approximately the same values of fracture toughness, rising as a function of indentation flaw size, were obtained from both series of experiments.

Influence of Filler Type and Water Exposure on Flexural Strength of Experimental Composite Resins

Abstract: The objective of this investigation was to determine the influence of water exposure on the flexural strength of three experimental composite materials. The composites consisted of an experimental resin system which contained silane-treated filler particles of quartz, barium glass, and porous silica. The amorphous silica particles were spheres approximately 5 micron in diameter. In addition to the different composites, pure resin samples were investigated as a control group. The results of this investigation did not support the hypothesis that use of porous amorphous silica filler particles reduced the hydrolytic degradation effect of composites containing such silica particles. Instead, the results indicated that water may have a more detrimental effect on the strength of the matrix than on the filler-matrix interface. However, the relatively high frequency of fracture lines in the porous silica particles after storage in water indicate that water had a weakening effect on this type of filler.

Engineering Properties of Concrete Affected by Alkali-Silica Reaction

Abstract: A detailed study of the effects of alkali-silica reaction (ASR) on the engineering properties of concrete such as compressive and tensile strength, elastic modulus, and pulse velocity is presented. Two types of reactive aggregate - a naturally occurring Beltane opal and synthetic fused silica - were used. The tests wee carried out at 20 C and 96 percent relative humidity (RH). The results showed that losses in engineering properties do not all occur at the same rate or in proportion to the expansion undergone by the ASR-affected concrete. The two major properties affected by ASR were flexural strength and dynamic modulus of elasticity. Compressive strength was not a good indicator of ASR, but the flexural strength proved to be a reliable and sensitve test for mointoring ASR. Nondestructive tests like dynamic modulus and pulse velocity were also able to identify deterioration of concrete by ASR. The data indicate that critical expansion limits due to ASR would vary depending on the type and use of a concrete structure.

Mechanical strength of silica aerogels

Abstract: Pure silica aerogels are obtained by hypercritical evacuation of the solvent. The strength is measured by the three-point flexural test on monolithic parallelepipedic samples and by a diametral compression test on cylindrical samples. The stress-strain curve shows a perfect elastic behaviour and the “conchoidal” fracture morphology indicates that the material is as brittle as a conventional glass. The mechanical properties are followed as a function of the bulk density. Aerogels with the highest porosity ( P > 95%) reveal a maximum flexural strength lower than 10 −2 MPa. A model is proposed to account for the obtained mechanical properties.

Physical and mechanical properties of YBa2Cu3O7-δ superconductors

Abstract: The physical and mechanical properties of YBa2Cu3O7−° superconductors are examined. These properties are related to powder preparation method, powder characteristics, sintering behaviour and sintered microstructure. The sintering atmosphere and sintering schedules affect the final microstructure very strongly and determine, in conjunction with starting powder characteristics, the sintered density. The mechanical properties such as Young's modulus, bend strength and critical stress intensity factor (fracture toughness) are measured and related to microstructure as determined by electron microscopy. Control of microstructure by careful powder selection and sintering schedule is seen as key to optimizing the physical and mechanical properties of the material. Finally attention is drawn to fabrication techniques and how these must be optimized in order to realize the mechanical properties which are necessary if these are to be useful as engineering materials. Comparisons between fabrication techniques show that uniaxial powder pressing suffers from limitations in terms of specimen complexity and densification whereas the favoured route, termed viscous processing, gives a more homogeneous microstructure, higher strength and allows near theoretical density to be achieved.

Flexural analysis of laminated composites using refined higher-order C ° plate bending elements

Abstract: A finite element formulation for flexure of a symmetrically laminated plate based on a higher-order displacement model and a three-dimensional state of stress and strain is presented here. The present higher-order theory incorporates linear variation of transverse normal strains and parabolic variation of transverse shear strains through the plate thickness, and as a result it does not require shear correction coefficients. A nine-noded Lagrangian parabolic isoparametric plate bending element is described. The applications of the element to bending of laminated plates with various loading, boundary conditions, and lamination types are discussed. The numerical evaluations also include the convergence study of the element used. The present solutions for deflections and stresses are compared with those obtained using the three-dimensional elasticity theory, closed-form solutions with another high-order shear deformation theory, and the Mindlin's theory. In addition, numerical results for a number of new problems, not available in the literature, are presented for future reference.

Mechanical properties of polycrystalline diamonds

Abstract: The room temperature mechanical properties of polycrystalline diamonds, i.e. tensile strength, transverse rupture strength, compressive strength, impact strength, fracture toughness, and elastic constants, have been determined. The applied test techniques are described and the results compared with those obtained by other authors. The fracture mode under the present experimental conditions was primarily transgranular. A grain size dependence, where strength increases with decreasing grain size, has been found. Fracture toughness was found to go through a maximum for grain sizes between 10 to 30 μm. The modulus of elasticity increases with increasing grain size. An influence of cobalt content on strength and modulus of elasticity has been found, while no significant influence on toughness could be determined. Increasing the cobalt content increases strength, but has the inverse effect on the modulus of elasticity. The results of strength, toughness, and elastic constants measurements are discussed ...

The Acousto-Ultrasonic Approach

Abstract: The nature and underlying rationale of the acousto-ultrasonic approach is reviewed, needed advanced signal analysis and evaluation methods suggested, and application potentials discussed. Acousto-ultrasonics is an NDE technique combining aspects of acoustic emission methodology with ultrasonic simulation of stress waves. This approach uses analysis of simulated stress waves for detecting and mapping variations of mechanical properties. Unlike most NDE, acousto-ultrasonics is less concerned with flaw detection than with the assessment of the collective effects of various flaws and material anomalies. Acousto-ultrasonics has been applied chiefly to laminated and filament-wound fiber reinforced composites. It has been used to assess the significant strength and toughness reducing effects that can be wrought by combinations of essentially minor flaws and diffuse flaw populations. Acousto-ultrasonics assesses integrated defect states and the resultant variations in properties such as tensile, shear, and flexural strengths and fracture resistance. Matrix cure state, porosity, fiber orientation, fiber volume fraction, fiber-matrix bonding, and interlaminar bond quality are underlying factors.

Effect of residual stresses on the fracture of ground ceramics

Abstract: Grinding induces residual stresses and cracks at and near the surfaces of ceramic workpieces. The residual stresses in several ground ceramics are measured using the curvature method. Four-point bend tests are conducted to measure the fracture strength of the ground specimens. A fracture mechanics analysis includes the measured residual stress to calculate the grinding-induced surface crack size. It is shown that the residual stresses sometimes have a significant effect on the strength- controlling flaw size.

Morphology of polycrystalline alumina brackets and its relationship to fracture toughness and strength.

Abstract: Inherent defects seen in the morphology of polycrystalline ceramic brackets severely limit their fracture strength. Only by improving the surface characteristics can those mechanical properties which are necessary for sufficient strength and efficient orthodontic tooth movement be more fully realized.

The oxidative stability of carbon fibre reinforced glass-matrix composites

Abstract: The environmental stability of carbon fibre reinforced glass-matrix composites is assessed. Loss of composite strength due to oxidative exposure at elevated temperatures under no load, static load and cyclic fatigue as well as due to thermal cycling are all examined. It is determined that strength loss is gradual and predictable based on the oxidation of carbon fibres. The glass matrix was not found to prevent this degradation but simply to limit it to a gradual process progressing from the composite surfaces inward.

Mechanical and Microstructural Characterization of Mullite and Mullite‐SiC‐Whisker and ZrO2‐Toughened‐Mullite—SiC‐Whisker Composites

Abstract: High-purity mullite-SiC-whisker composites and mullite-ZrO2-SiC-whisker composites were fabricated in situ by hot-pressing using a matrix prepared by the alkoxide process. Varying degrees of ZrO2 stabilization were achieved by varying amounts of Y2O3 or MgO addition. Microstructural characterization was accomplished using SEM and energy dispersive analysis. Room-temperature flexural strength and fracture toughness were determined as a function of SiC-whisker content (0% to 30%) and ZrO2-stabilizer content. The flexural strength of mullite varied with composition and was increased ∼50% by the addition of ∼30% ZrO2 phase. The flexural strength of mullite and mullite + 30% ZrO2 was increased ∼50% for 30% SiC-whisker additions. The fracture toughness of mullite + 30% ZrO2 was nearly twice that of mullite. For a 30% SiC-whisker addition, the fracture toughness of mullite was doubled, and the fracture toughness of mullite + 30% ZrO2 was increased 25% to 50%.

Acousto-ultrasonics : theory and application

Abstract: Keynote.- The Acousto-Ultrasonic Approach.- Theory - Wave Propagation for Acousto-Ultrasound.- Interaction of Ultrasonic Waves with Layered Media.- Diffuse Waves for Materials NDE.- Low Frequency Flexural Wave Propagation in Laminated Composite Plates.- Preliminary Evaluation of Non-Contact Acousto-Ultrasonic Displacement Fields in Polymeric Matrix Composite.- Utilization of Oblique Incidence in Acousto-Ultrasonics.- Data Interpretation.- Ultrasonic Velocity Studies of Composite and Heterogeneous Materials.- Acousto-Ultrasonic Wave Propagation in Composite Laminates.- Ray Propagation Path Analysis of Acousto-Ultrasonic Signals in Composites.- Nondestructive Evaluation of Composite Material Using Ultrasound.- Experimental and Theoretical Analysis of Backscattering Mechanisms in Fiber-Reinforced Composites.- Statistical Evaluation of Quality in Composites Using the Stress Wave Factor Technique.- Application of Acousto-Ultrasonics to Quality Control and Damage Assessment of Composites.- Predicting Damage Development in Composite Materials Based on Acousto-Ultrasonic Evaluation.- Sources/Detectors.- Laser Generated Ultrasound.- Electromagnetic Transducers for Generation and Detection of Ultrasonic Waves.- Applications and Advantages of Dry Coupling Ultrasonic Transducers for Materials Characterization and Inspection.- Fiber Waveguide Sensors of Stress Waves in Solids.- Calibration and Method Implementation.- Considerations for Developing Calibration Standards for Acousto-Ultrasonic Inspection.- Test Conditions in Stress Wave Factor Measurements for Fiber Reinforced Composites and Laminates.- Measurement of the Energy Content in Acousto-Ultrasonic Signals.- Applications - General.- Acousto-Ultrasonics: Applications to Wire Rope, Wood Fiber Hardboard, and Adhesion.- An Acousto-Ultrasonic Method for Evaluating Wood Products.- Acousto-Ultrasonic Characterization of Physical Properties of Human Bones.- Applications - Composite Materials.- Multi-Parameter, Multi-Frequency Acousto-Ultrasonic for Detecting Impact Damage in Composites.- Trans-Ply Crack Density Detection by Acousto-Ultrasonics.- Application of Acousto-Ultrasonics for Predicting Hygrothermal Degradation of Unidirectional Glass-Fiber Composites.- Applications - Bonding.- Bond Quality Evaluation of Bimetallic Strips: Acousto-Ultrasonic Approach.- Acousto-Ultrasonic Evaluation of the Strength of Composite Material Adhesive Joints.

Tensile performance of carbon-fibre-reinforced glass

Abstract: The tensile behaviour of unidirectional and [±θ]s angle-ply HMU carbon-fibre-reinforced borosilicate glass was determined as a function of the angle between the fibre and the applied load. Both the longitudinal and transverse strain of the composite are reported and discussed relative to the microstructural features responsible for the observed composite behaviour. Stress-strain behaviour for static and cyclic loading conditions is presented. The experimentally determined values of the composite elastic modulus and strength are also compared with those predicted from classical laminate theory. The composite tensile strength is accurately predicted by the Tsai-Hill failure criterion. The elastic modulus measurements indicate that the shear modulus of the uniaxially reinforced composites is higher than that of the multiaxially reinforced composites. This observation is attributed to more extensive matrix microcracking being present in the multiaxially reinforced composite as a result of fabrication. The failure modes present in the composite are also documented.

Temperature dependence of strengthening by whisker reinforcement: SiC whisker-reinforced alumina in air

Abstract: The high fracture toughness of alumina reinforced with 20 vol% SiC whiskers is maintained at temperatures up to {approx} 1,100{degree}C in air. The fracture strength values ({sigma}{sub f}) decrease slowly with increasing temperature to 1,100{degree}C; however, when subjected to an applied stress (= 2/3 {sigma}{sub f}) for periods of up to 1,000 h in air at 800{degree}, 1,000{degree}, or 1,100{degree}C, the subsequent fracture strength values (retained strengths) at these temperatures increase with increasing exposure times. On the other hand, above 1,100{degree}C, the fracture strength values decrease while the apparent toughness value increases. At 1,200{degree}C, samples first subjected to applied stresses of {le} 310 MPa for up to {approx} 240 h exhibit a loss in retained strengths with increasing exposure time. The temperature dependence of the mechanical behavior is shown to be related to oxidation reactions in these composites with creep phenomena contributing to the loss in strength and increased toughness above 1,100{degree}C.

Impulse excitation technique for dynamic flexural measurements at moderate temperature

Abstract: The impulse excitation technique (IET), which is presently a precise and reliable technique for measuring dynamic moduli at room temperature, has been adapted to measure dynamic flexural modulus at temperatures in the range of 25° to 300 °C. This modified technique involves a sensitive microphone and electronics to record and analyze the sound waves emitted from a specimen vibrating in the fundamental flexural mode. The fundamental resonant frequency and geometry of the specimen are used to obtain the modulus. The location of the microphone relative to the specimen is critical and is a major factor once the specimen is placed within the heated environment. Problems were identified and solved, and test data for aluminum are presented to support the modification of the IET for use at elevated temperatures.

Structural ceramic materials having refractory interface layers

Abstract: Ceramic composite articles having high flexural and tensile strength are produced by introducing multiple layers of a ceramic matrix material onto a ceramic reinforcement fabric. Crack propagation in the resulting structure is inhibited by applying refractory interface layers intermediate successive ceramic matrix layers. Such ceramic composite articles are particularly useful for high temperature applications requiring a high degree of strength, such as rocket motor insulation and turbine blades, combustion chambers, and after burners for jet motors.

Influence of Contraction Mismatch and Cooling Rate on Flexural Failure of PFM Systems

Abstract: The interactive influence of cooling rate and the sign and magnitude of thermal contraction difference between metals and ceramic veneers on bond strength have not been extensively analyzed, although numerous bond-test studies have been reported during the past two decades. A previous analytical study of residual incompatibility stress in bond-test specimens indicated that bond strength values may be of relatively little value if the residual stress state of the metal-ceramic specimens is not considered. The objective of this study was to determine the influence of cooling rate and contraction mismatch on the flexural failure resistance of metal opaque-porcelain strips. Specimens were subjected to four-point loading in an Instron testing machine until crack initiation occurred at the metal-ceramic interface. The residual stress states in the ceramic region were estimated from finite element stress analyses of the bond-test specimens by use of dilatometry data obtained at the cooling rate of 3 degrees C/min. The total stress induced from the residual stress and the applied flexural load was also determined for these specimens. Statistical analyses of the experimental data revealed that the slowly cooled specimens exhibited a significantly lower (p < 0.05) flexural strength compared with rapidly cooled specimens. Regardless of the cooling technique, metal-ceramic specimens with a negative thermal contraction difference (alpha m - alpha p < 0) failed at significantly lower (p < 0.05) flexural loads than did specimens with a positive thermal contraction difference.

Mechanical behaviour of polymer concrete systems

Abstract: The mechanical behaviour of epoxy and polyester polymer concrete systems was studied under different loading conditions at various temperatures, resin content, and glass fibre content. While polymer content varied between 10 and 20% of the total weight of polymer concrete, the fibre content was limited to 4% by weight. The temperature was varied between 22 and 110°C, depending on the glass transition temperature of the resin. Compared to vibration, the compaction method of preparation reduces the void content and enhances the strength and modulus of polymer concrete. The compressive and flexural strength and stiffness of the polymer concrete systems increase up to a certain limit of polymer content at which they exhibit maximum strength and stiffness. They subsequently decrease or remain almost constant with further increase in polymer content. The strength and stiffness of polymer concrete are very much dependent on the temperature. The stiffness model, based on inclusion theory, yields satisfactory results for the three-phase polymer concrete. Using this model, the compression and flexural modulus of polymer concrete can be predicted from the properties of the constituents and their composition. Incremental strength and stiffness models developed in this study are effective in predicting the increase in strength and stiffness of glass-fibre-reinforced polymer concrete.

Damage‐Enhanced Creep in a Siliconized Silicon Carbide: Mechanics of Deformation

Abstract: Continuum mechanics methods were employed to analyze creep deformation of a grade of siliconized silicon carbide at elevated temperatures. Three loading modes (tension, compression, and bending) are considered in this analysis. In tension, deformation is accompanied by cavitation at stresses in excess of a temperature-dependent threshold level, resulting in bilinear power-law creep. In compression, greater applied stresses are required to achieve the same rate of strain, and although bilinear creep behavior is also observed, a single power-law creep equation was assumed to simplify the mathematical analysis of the flexure problem. Asymmetrical creep in siliconized silicon carbide leads to a number of unique features in flexural creep. At steady state, a threshold bending moment exists below which no damage occurs. The neutral axis shifts from the geometric center toward the compressive side of the specimen by an amount that depends on the level of applied stress. Cavitation zone shapes, which are predicted to develop in a four-point bend specimen as a function of load, are found to be in qualitative agreement with those obtained experimentally. For transient creep under bending, the time-dependent neutral axes for stress and strain do not coincide, although they do converge toward a single axis at steady state. Quantitative predictions are given for relaxation of tensile stresses at the outer fiber, reverse loading in the midplane region, and the growth of the damage zone toward the compressive side of the flexural specimen. This load redistribution leads to a prolonged transient stage as compared to its counterpart in uniaxial creep.

Flexural strength and internal defects of some dental porcelains.

Abstract: The flexural strength, number (n), size, and form of pores in seven different dental porcelains were examined. The flexural strength was approximately 100 MPa for the core materials for metal bonding, 116 MPa for traditional alumina porcelains (NBK 1000, Vitadur N), 150 MPa for two new high-alumina porcelains (Vita Hi Ceram, Cerestore), and 240 MPa for a castable glass (Dicor). The mean pore diameter was approximately 10μm in all porcelains except Dicor, but with a large and varying range. Dicor showed a mean pore size of 1 μm. The pore density varied from 36.5/mm2 in one of the porcelains for metal bonding to 4367/mm2 in Cerestore. The mean cross-sectional area occupied by pores was below 10% for most porcelains except Vitadur N and Cerestore, showing 17.5% and 32.5%, respectively. No correlation was found between flexural strength and frequency or cross-sectional area of pores of the various products.