Showing papers on "Flexural strength published in 1991"

External Reinforcement of Concrete Beams Using Fiber Reinforced Plastics

Abstract: A series of 16 under-reinforced beams was tested to study the effectiveness of external strengthening using fiber reinforced plastic (FRP) plates. Plates of glass, carbon and aramid fibers were bonded to the tension side of the beams using a 2-part epoxy. An interative analytical method was developed to predict the stiffness and maximum strength in bending of the plated beam. Increases in stiffness (over the working load range) from 17 to 99 percent and increases in strength (ultimate) from 40 to 97% were achieved for the beams with FRP plates. Predicted and acutal load-deflection curves showed fairly good agreement, although generally the theoretical curves were stiffer. Experimental failure did not occur in the maximum moment region on many of the beams, despite attempts at end anchorages to postpone local shear failure. The ulltimate loads of the beams that did fail in the maximum region were within about 5% of predicted values.

Strength and ductile-phase toughening in the two-phase Nb/Nb 5 Si 3 alloys

Abstract: The effect of heat treatment on the mechanical properties of Nb-Nb5-Si3 two-phase alloys having compositions Nb-10 and 16 pct Si (compositions quoted in atomic percent) has been investigated. This includes an evaluation of the strength, ductility, and toughness of as-cast and hot-extruded product forms. The two phases are thermochemically stable up to ∼1670 °C, exhibit little coarsening up to 1500 °C, and are amenable to microstructural variations, which include changes in morphology and size. The measured mechanical properties and fractographic analysis indicate that in the extruded condition, the terminal Nb phase can provide significant toughening of the intermetallic Nb5Si3 matrix by plastic-stretching, interface-debonding, and crack-bridging mechanisms. It has been further shown that in these alloys, a high level of strength is retained up to 1400 °C.

RC Beams Strengthened with FRP Plates. II: Analysis and Parametric Study

Abstract: Analytical models based on the compatibility of deformations and equilibrium of forces are presented to predict the stresses and deformations in concrete beams strengthened with fiber composite plates epoxy-bonded to the tension face of the beams. The models are given for beams having rectangular and T cross sections. A parametric study is conducted to investigate the effects of design variables such as plate area, plate stiffness and strength, concrete compressive strength, and steel reinforcement ratio. The moment versus curvature diagrams for various combinations of these variables are plotted and compared. The results indicate that bonding composite plate to a concrete beam can increase the stiffness, yield moment, and flexural strength of the beam. The method is particularly effective for beams with a relatively low steel reinforcement ratio.

Fiber-type effects on the performance of steel fiber reinforced concrete

Abstract: The paper reports the results of an experimental study on the relative effectiveness of different types of steel fiber in concrete. A constant fiber volume fraction of 2 % was used throughout this investigation. The fresh fibrous mixes were characterized by their slump, inverted slump-cone time, and subjective workability, and the hardened materials by their compressive and flexural load-deformation relationships. The overall workability of fresh fibrous mixes was found to be largely independent of the fiber type, with crimped fibers producing only slightly higher slumps. Hooked fibers were found to be more effective than straight and crimped ones in enhancing the flexural and compressive behavior of concrete. Under flexural loads, crimped fibers were slightly less effective than straight ones in improving the strength and energy absorption of concrete.

Mechanical Strength of Clay Minerals as Influenced by an Adsorbed Polysaccharide

Abstract: The water-stabilizing action of polysaccharides on soil aggregates has been ascribed to the binding together of soil particles by polymer bridges. This study was conducted to investigate the ability of polysaccharides to increase the strength of interparticle bonds by measuring the mechanical strength of clay-polysaccharide complexes. Kaolinite, montmorillonite, and the polysaccharide scleroglucan were used. Cores were prepared from dilute suspensions of the complexes, air dried, and manufactured either into small briquets or into spherical aggregates to measure respectively the modulus of rupture and the tensile strength. The polysaccharide increased the mechanical strength of both clay minerals. The modulus of rupture of montmorillonite increased from 265 to 580 × 10⁵ Pa at the maximum adsorption of scleroglucan, which was 140 g kg⁻¹ clay. In the case of kaolinite, modulus of rupture measurements indicated that particle-orientation effects were small and could be neglected. The tensile strength of kaolinite increased from 1 to 20.6 × 10⁵ Pa at the maximum adsorption of scleroglucan (29.9 g kg⁻¹ clay). In this case, the mechanical properties were not due to changes in the clay microorganization, since scleroglucan affected neither the fabric nor the porosity of the kaolinite. Increases in strength were thus intercepted as the formation of polymer bridges between the clay particles and were related to the progressive coverage of the surface area of the clay by the polysaccharide. We concluded that the aggregating action of polysaccharides is predominantly due to an increased strength of interparticle bonds.

Chain silicate glass-ceramics

Abstract: Glass-ceramics based on chain silicates: enstatite (MgSiO 3 ), potassium fluorrichterite (KNaCaMg 5 Si 8 O 22 F 2 ), and fluorcanasite (K 2 Na 4 Ca 5 Si 12 O 30 F 4 ), display a combination of high flexural strength ( > 200 MPa) and high fracture toughness (> 3 MPa rmm 1 2 ). The relationships between composition, microstrcuture, and properties are discussed. Recent and potential applications are described.

Wood Flour/Polypropylene Composites: Influence of Maleated Polypropylene and Process and Composition Variables on Mechanical Properties

Abstract: The mechanical properties of wood flour/polypropylene composites may be improved by using coupling agents to enhance the bonding between filler and matrix. We used a two-level, full-factorial experiment to examine the effectiveness of a commercially available additive, Epolene E-43, on strength (tensile, flexural, and cantilever-beam), modulus, impact energy, density, and melt viscosity of composites. The effect of Epolene E-43 was studied alone and in combination with three other variables: the weight ratio of wood flour to total polymer in the composites (45/55 or 55/45), size of wood flour particles (nominal 20 or 40 mesh), and extruder residence time (one or three extrusions). Of all the variables, Epolene E-43 had the greatest effect on strength. This effect was somewhat enhanced by using the 40-mesh wood flour and three extrusions; for this combination of variables, Epolene E-43 caused a 30 percent increase in strength. The combination of Epolene E-43 with a 55/45 wood flour/polymer ratio a...

Review of effects of loading rate on reinforced concrete

Abstract: A survey of the behavior of reinforced concrete subjected to dynamic loading is presented. Provisions for seismic design in current design codes have been developed on the basis of results obtained from static tests. Realistic methods of design should take into consideration the strain‐rate‐dependent properties of reinforced concrete in order to accurately predict the behavior of a reinforced concrete structure subjected to dynamic loads. The response of reinforced concrete materials and elements to different strain rates is reviewed and discussed. As rate of loading increases, concrete compressive strength, steel yield strength, and flexural capacity of reinforced concrete member also increase. The increase in flexural capacity of individual members as a result of high strain rates, if significant, may shift the failure mode of a structure from a preferred ductile manner to a less desirable brittle mode.

Variables influencing the repair strength of dental composites.

Abstract: Differences in the flexural strength of composites (P-30) repaired with four different adhesive techniques were determined and compared to the flexural strength of unrepaired specimens. Besides investigating the effects of differences in repair methods, both the effect of composite age at repair time and the effect of water storage on flexural strength were evaluated. No significant differences were found between specimens which had been stored for different times (1 or 60 days) before they were repaired. All specimens, including the unrepaired ones, became weaker with time in water. After both 90 and 360 days at 37 degrees C in water, the strength of the repaired specimens ranged between 25 and 50% of the strength of the unrepaired specimens. The weakest group was the one which had been repaired after acid etching, water rinsing, and air drying only.

A curved beam test specimen for determining the interlaminar tensile strength of a laminated composite

Abstract: A curved beam type of test specimen is evaluated for use in determining the through-the-thickness strength of laminated composites. Two variations of a curved beam specimen configuration (semicircular and elliptical) were tested to failure using static and fatigue loads. The static failure load for the semicircular specimens was found to be highly sensitive to flaw content, with the specimens falling into two distinct groups. This result supports the use of proof testing for structural validation. Static design allowables are derived based on the Weibull distribution. Fatigue data indicates no measured increase in specimen compliance prior to final fracture. All static and fatigue failures at room temperature dry conditions occurred catastrophically. The elliptical specimens demonstrated unusually high failure strengths indicating the presence of phenomena requiring further study. Results are also included for specimens exposed to a wet environment showing a matrix strength degradation due to moisture content. Further testing is underway to evaluate a fatigue methodology for matrix dominated failures based on residual static strength (wearout).

Glass Fibre Reinforced Cement

Abstract: Glass fibre reinforced cement is introduced as a new composite material, based on a matrix of cement or cement and fine filler reinforced with a relatively small addition of strands of alkali-resistant glass. The glass content is relatively low, being restricted by increasing difficulty of mixing and compacting as the glass content is increased. For any method of mixing and placing and varying with the mix proportions there is an optimum value of glass content in relation to bending strength; impact strength rises steadily with glass content. When premixing an additive is required, commonly polyethylene oxide or methylcellulose. Premixed formulations can be placed by tamping and vibration, pumping or extrusion. An effective method is to mix and place simultaneously by spraying the matrix as a slurry into a stream of chopped glass fibre from a spray chopper. The combined sprays are directed on to a suction mould where excess water is removed. Properties of composites prepared by the various methods are compared. The impact strength and fire resistance of glass fibre reinforced cement are particularly good. Examples of practical applications are given.

Coir-Glass Intermingled Fibre Hybrid Composites

Abstract: The aim of the investigation is to evaluate the enhancement in the proper ties of coir-polyester composites by incorporating glass as intimate mix with coir. Addi tion of relatively small volume fraction (0.05) of glass enhances the tensile strength by about 100%, flexural strength by more than 50 % and impact strength by more than 100 % . Coir-glass intermingled hybrid composites (CGP) absorb moisture 4 to 5 times less than that of coir-polyester composite (CRP) on immersing in boiling water for 2 hrs and the former thereby shows considerably better resistance to weathering. It has also been ob served that CGP core-GRP shell laminates possess better flexural and interlaminar shear strength compared to that of CRP core-GRP shell laminates containing equivalent volume fractions of the fibres as in the former.

Deformation and fracture behaviour of wheat starch plasticized with glucose and water

Abstract: The mechanical properties of wheat starch and the effects of plasticizers upon them are studied in flexure at 293 K. For compositions low in water and glucose the material is glassy, with a flexural modulus between 0.7 and 5.0 GPa. The addition of water and glucose to wheat starch plasticizes the material through its glass transition into a rubbery state. The flexural moduli of the rubbery samples are in the range 50 to 200 MPa, which is indicative of a partially crystalline polymer. For starch-water mixtures the glass transition occur in the water content range 18 to 20%. The addition of glucose progressively shifts the glass transition to lower water contents. At strains below 0.04 brittle failure is only observed in the glassy samples. The surface morphology of the fractured samples shows features typical of pure synthetic glassy polymers.

Structures and properties of injection moldings of crystallization nucleator‐added polypropylenes. I. Structure–property relationships

Abstract: Flexural test specimens were injection-molded from polypropylenes added with 0.5 wt % of calcium carbonate, talc, p-tert- dibutyl-benzoic acid monohydroxy aluminum, or p-di-methyl-benzylidene sorbitol under cylinder temperatures of 200–;320°C. Properties such as flexural modulus, flexural strength, heat distortion temperature, Izod impact strength, hardness, and mold shrinkage and higher-order structures such as crystalline texture, crystallinity, a*-axis-oriented component fraction, and degree of crystalline orientation were measured and structure–property relationships were studied. By the addition of crystallization nucleators, the flexural modulus, flexural strength, heat distortion temperature, hardness, and mold shrinkage were increased and Izod impact strength was decreased. The degrees of crystalline orientation such as the orientation fraction OF and c-axis orientation function fc were increased by the addition of nucleators. The degree of the increase was higher as the crystallization temperature was higher. Close relationships were observed between some properties and the degrees of crystalline orientation.

Microstructure and mechanical properties of hot-pressed SiC-TiC composites

Abstract: Hot-pressed SiC-TiC composite ceramics with 0–100 wt% TiC have been investigated to determine the effect of composition (amount of TiC) on the elastic modulus, hardness, flexural strength and fracture toughness,KIC. The composites exhibited superior mechanical properties compared to monolithic SiC and TiC, especially in fracture toughness,KIC, value for 30–50 wt% TiC composite. The maximum values ofKIC and room-temperature flexural strength were 6 MPa m1/2 for a 50 wt % TiC and 750 MPa for a 30 wt% TiC composite, respectively. The observed toughening could be attributed to the deflection of cracks due to dispersion of the different particles. Although no third phases were detected by both TEM and XRD studies, an EDAX study and resistivity measurements indicated some possibility of solid solutions being present. The composites containing more than 30 wt% TiC, exhibited resistivity lower than 10−3Ω cm which is favourable for electro-discharge machining of ceramics.

Creep and creep fracture in hot-pressed alumina

Abstract: The creep and creep fracture behavior of two hot-pressed aluminas are presented, for both flexural and tensile testing. Steady-state power-law creep is observed with a stress exponent of about 2 for each material. Three distinct fracture regimes are found. At high stress in flexure, fracture occurs by slow crack growth with a high stress dependence of the failure time. At intermediate stresses, in both flexure and tension, creep fracture occurs by multiple microcracking after modest strains. Failure times exhibit a modest stress dependence (stress exponent of 2.5 in tension and 3 in flexure), with a constant failure strain equal to 0.09. The failure times are considerably longer in flexure than in tension, because of the constraint imposed on crack growth by the bending geometry. We conclude that flexure cannot be used for creep lifetime assessment, even in simple, single-phase materials such as Al2O3. At low stresses, in tension, failure also exhibits a modest stress dependence but with a much higher failure strain. The material shows the onset of super-plastic behavior.

Fracture toughness of brittle honeycombs

Abstract: Existing models for the fracture toughness of brittle cellular materials are based on the assumptions that the crack is large relative to the cell size and that the modulus of rupture of the cell wall material is constant: both may, in practice, be invalid. Here, we account for the effect of short cracks by using a finite element analysis and for that of the variability in the cell wall modulus of rupture by describing it by a Weibull distribution. The results of the analysis indicate that if the crack half length/cell size is less than 7 the fracture toughness is reduced and that the way in which the fracture toughness varies with cell size depends on the Weibull modulus of the cell wall material.

Effects of interfacial coating and temperature on the fracture behaviours of unidirectional kevlar and carbon fibre reinforced epoxy resin composites

Abstract: The enhancement of transverse fracture toughness of unidirectional Kevlar and carbon fibre reinforced epoxy resin composites (KFRP and CFRP) has been studied using polymer coatings on the fibres. The results obtained show a substantial improvement in the impact fracture toughness of both KFRP and CFRP with polyvinyl alcohol (PVAL) coating without any loss of flexural strength; but there is only a moderate increase in impact toughness with other types of coating (i.e. carboxyl-terminated butadiene acrylonitrile (CTBN) copolymer and polyvinyl acetate (PVA)) with some reduction in flexural strength. The dependence of impact fracture toughness of these composites (with and without PVAL coating) on temperature was analysed on the basis of existing theories of toughening mechanisms from measurements of fibre-matrix interfacial properties, debond and fibre pull-out lengths and microscopic observations. The beneficial effect of fibre coating with PVAL on transverse fracture toughness is shown to sacrifice little damage tolerance of the composites against delamination fracture.

Fracture toughness of brittle foams

Abstract: Existing models for the fracture toughness of brittle cellular materials have two limitations: the crack is assumed to be large relative to the cell size and the modulus of rupture of the cell wall material is assumed to be constant. In the previous companion paper, we reanalyzed the fracture toughness using a finite element analysis for short cracks and assuming that the modulus of rupture of the cell wall material followed a Weibull distribution. The finite element analysis gave a reduction factor for the effective fracture toughness for short cracks which was independent of the cell geometry. The Weibull analysis showed that the effect of cell size on the fracture toughness of brittle honeycombs is dependent upon the Weibull modulus of the cell wall material. We now apply similar ideas to the analysis of the fracture toughness of brittle foams.

Flexural Behavior of Reinforced High-Strength Lightweight Concrete Beams

Abstract: Flexural tests were conducted on 6 singly reinforced beams. The variables were strength of concrete and the ratio of tensile steel content r as a ratio of the balanced steel content. No compression or lateral reinforcement was used in this study. Test results are presented in terms of load-deformation behavior, ductility indexes, and cracking patterns. It is concluded that to achieve a ductility index of 3, rho/rho sub b should not exceed 0.40 for beams with concrete strengths of 8000 psi or 0.20 for beams with concrete strength of 11,000 psi. The flexural design provisions of the ACI Building Code are found to be adequate to predict the strength of reinforced high-strength concrete beams.

Splitting tensile strength and compressive strength relationships at early ages

Abstract: The paper reports on investigation of the relationship between concrete compressive strength and its splitting tensile strength, especially at early ages, as well as the examination of the applicability of some of the existing relations between these properties to concrete at early ages. Analyses of test results show that the compressive strength and the spliltting tensile strength are related, and an increase in one, in general is similarly reflected in an increase in the other. The commonly accepted 0.5 power relationship between the compressive strength and the spliltting tensile strength was found to be inaccurate at all ages. In fact the tensile strength was found to be proportional to the 0.79 power of the cylinder compressive strength. An alternate relationship between the tensile strength and the compressive strength is proposed. Tables of results and figures supporting these observations and conclusions are included.

Fracture Resistance of a Transparent Magnesium Aluminate Spinel

Abstract: The fracture resistance of a fully dense, transparent, polycrystalline magnesium aluminate spinel was measured from room temperature to 1400°C using the chevron-notched beam and the straight-notched beam macroflaw techniques, as well as the indentation-induced, controlled-microflaw test method, all in three-point bending. Flexural strengths were also measured for the same range of temperatures to compare with the fracture toughness measurements. From the load vs load-line displacement (P-u) curves of the chevronnotched test specimens, the crack growth resistance curves (R-curves) and the total work-of-fracture were determined. It was observed that polycrystalline MgAl2O4 exhibits rising R-curve behavior which increases with increasing test temperature. The R-curve increases are attributed to the geometric constraints due to grain bridging and grain wedging phenomena as well as secondary grain boundary microcracking processes, all of which occurred in the wake region behind the advancing crack. The work-of-fracture and the R-curves increased rapidly above 800°C coincident with the onset of increased secondary grain boundary microcracking.