Showing papers on "Flexural strength published in 1992"

Experimental and numerical analysis of micromechanisms of fracture of cement-based composites

Abstract: In this paper experimental evidence of fracture toughening of concrete and mortar through a mechanism called crack face bridging is presented. The classical explanation for softening of concrete, viz. the formation of a zone of discontinuous microcracking ahead of a continuous macrocrack seems only partially true. Instead, crack face bridging in the wake of the macrocrack tip seems a physically sounder explanation. The crack face bridges are flexural ligaments between ovelapping crack tips. The failure of the flexural ligaments occurs in a stable and controlled manner because the two overlapping crack tips shield each other. The cohesive stress over the macrocrack is directly related to the size of the crack face bridges, which depends on the heterogeneity of the material. The typical failure mechanism can be simulated using a simple numerical lattice model. First the grain structure of the material is generated either by manual methods or by adopting a random generator. Secondly a tringular lattice of brittle breaking beam elements is projected on the grain structure. Aggregate, matrix and bond properties are assigned to the lattice elements at the respective locations, and a simple algorithm allows for crack growth simulation. The main conclusion is that the crack patterns and the associated load-deformation response are largely governed by the properties of the constituents. The bond between aggregates and matrix is the weakest link in the system, and variation of this parameter leads to profoundly different crack patterns.

Mechanical behaviour of bamboo and bamboo composite

Abstract: The tensile, flexural and impact strengths of bamboo and bamboo fibre-reinforced plastic (BFRP) composite have been evaluated. The high strengths of bamboo, in the fibre direction, have been explained by its anatomical properties and ultra structure. Bamboo fibres and bamboo orthogonal strip mats (bamboo mat) have been used to reinforce epoxy resin. BFRP composites with unidirectional, bidirectional and multidirectional strengths have been made. In bamboo mat composites, the fibre volume fraction,Vf, achieved was as high as 65%. The tensile, flexural and impact strengths of bamboo along the fibres are 200.5 MN m−2, 230.09 MN m−2 and 63.54 kJ m−2, respectively, whereas those of bamboo fibre composites and bamboo mat composites are 175.27 M N m−2, 151.83 MN m−2 and 45.6 kJ m−2, and 110.5 MN m−2, 93.6 M N m−2 and 34.03 kJ m−2, respectively. These composites possess a close to linear elastic behaviour. Scanning electron microscopy studies of the fractured BFRP composite specimens reveal a perfect bonding between bamboo fibres and the epoxy. Furthermore, high strength, low density, low production cost and ease of manufacturing make BFRP composite a commercially viable material for structural applications.

Effects of particle size on mechanical and impact properties of epoxy resin filled with spherical silica

Abstract: Effects of particle size on the mechanical and impact properties of cured epoxy resins are studied. This resin was filled with spherical silica particles prepared by hydrolysis of silicon tetrachloride. Particles were sorted into five kinds of different mean sizes in the range from 6–42 μm. A static flexural and tensile tests and an instrumented Charpy type impact test were carried out. Flexural strength, tensile strength, and impact-absorbed energy increased with a decrease in the particle size. Fractured surfaces were observed using a scanning electron microscope to clarify the initiation point of fracture.

Heat-pressed ceramics: technology and strength.

Abstract: The flexural strength of a new heat-pressed ceramic material (IPS-Empress) was measured before and after pressing and/or simulated firing treatments (eg, veneering, surface coloring, glazing). Heat pressing the material significantly improved its flexure strength whereas heat treating the material alone did not. Additional firings (heat treatments) after heat pressing further increased material strength. The final strength values ranged between 160 and 180 MPa and were comparable to some other all-ceramic systems. No clinical implications were drawn from these data.

Evaluation of hydroxylapatite/poly(L-lactide) composites: mechanical behavior

Abstract: By mixing hydroxylapatite (HA) into L(-)-dilactide monomer, prior to polymerization to poly(L-lactide) (PLLA), hydroxylapatite filled poly(L-lactide) composites were obtained. This study reports about the mechanical properties of these composites compared with unfilled PLLA. It was concluded that a 30 wt% HA/PLLA composite has better compressive and tensile strengths, higher stiffness and Vickers hardness number than unfilled PLLA (Mv: 125-150,000). Gas sterilization (ethylene oxide) affects molecular weight and flexural strength significantly. Implantation studies revealed loss of 50% of initial flexural strength within 3 weeks, and a faster decline of flexural strength was observed in phosphate buffered saline than in the subcutis of goats. From a mechanical point of view storage at -20 degrees C proved to be a safe method. In its current state HA/PLLA composites can not be used as implant materials that have to resist major forces. However, such composites might be useful in non-loadbearing applications in orthopedic or maxillofacial surgery.

Strength and Creep Behavior of Rare‐Earth Disilicate–Silicon Nitride Ceramics

Abstract: The flexural strength and creep behavior of RE2Si2O7–Si3N4 materials were examined. The retention in room-temperature strengths displayed by these ceramics at 1300°C was 80–91%, with no evidence of inelastic deformation preceding failure. The steady-state creep rates, at 1400°C in flexural mode, displayed by the most refractory materials are among the lowest reported for sintered Si3N4. The creep behavior was found to be strongly dependent on residual amorphous phase viscosity as well as on the oxidation behavior of these materials. All of the rare-earth oxide sintered materials, with the exception of Sm2Si2O7–Si3N4, had lower creep strains than the Y2Si2O7–Si3N4 material.

Flexural Analysis of Reinforced Concrete Beams Containing Steel Fibers

Abstract: The flexural behavior of reinforced concrete beams containing steel fibers is investigated in the present study. An experimental program was set up and nine reinforced concrete beams have been tested, including two series of singly reinforced concrete beams and one series of doubly reinforced concrete beams. The fiber contents of reinforced concrete beams for each series were 0%, 1%, and 2% by volume. The load was applied in four-point flexural loading condition. The deflections and the steel and concrete strains were automatically measured at the various locations. The crack widths and crack spacings at each loading step were observed during the loading process. It was found from these measurements that the crack widths increase almost linearly with the increase of steel stress and the crack widths at the same loading stages are greatly reduced as the content of steel fibers increases. The present study also indicates that the ductility and the ultimate resistance are remarkably enhanced due to the addition of steel fibers. The design implication of fiber-reinforced concrete beams is also discussed. A method for incorporating fiber effects in the flexural analysis of singly reinforced concrete beams is redrived, slightly modified and used to analyze the singly reinforced concrete beams. A new method to analyze doubly reinforced concrete beams containing steel fibers is derived and proposed for future use. It was found from this study that the neglect of fiber contribution may considerably underestimate the flexural capacity of fiber-reinforced concrete beams. The present study allows more realistic analysis and design of reinforced concrete beams containing steel fibers.

Absorbable self-reinforced polylactide (SR-PLA) composite rods for fracture fixation: strength and strength retention in the bone and subcutaneous tissue of rabbits

Abstract: The strength and strength retention of self-reinforced (SR) absorbable polylactic acid composite rods were evaluated after intramedullary and subcutaneous implantation in rabbits. Rods made of poly-l-lactic acid (SR-PLLA) and of poly-dl-lactic acid + poly-l-lactic acid composite (SR-PDLLA/PLLA) were used. The molecular mass (Mv) of PLLA was 260.000 and that of PDLLA 100.000. The bending and shear strengths were measured after a follow-up of 1–48 weeks. The initial bending strength of the SR-PLLA rods was 250–271 MPa and the shear strength was 94 98 MPa. After intramedullary and subcutaneous implantation of 12 weeks the bending strength of the SR-PLLA implants was 100 MPa. At 36 weeks the bending strength had decreased to the level of the strength of cancellous bone (10–20 MPa). There were no changes in the shear strength during 12 weeks hydrolysis. The initial bending strength of SR-PDLLA/PLLA implants was 209 MPa and during the follow-up the implants lost their bending and shear strength faster than the SR-PLLA implants. The present investigation gave us the impetus to continue the studies with the fixation of experimental cortical bone osteotomies with SR-PLLA intramedullary rods.

Flexural toughness of steel fiber reinforced concrete

Abstract: A relatively new procedure was used to measure the deflections in this suudy of the flexural behavior of steel fiber reinforced concrete (SRC). The variables investigated were fiber type, length and volume fraction; and matrix composition. The results indicate that fiber content in the range of 50 to 100 lb/cu yd provides excellent ductility for normal strength concrete. The fiber content has to be increased to about 150 lb/ cu yd for high strength concrete. Hooked-end fiber geometry provides better results than corrugated and deformed-end geometry. Fiber length in the range of 1.18 to 2.36 in. does not have a significant effect on toughness for hooked-end fibers. Ductility behavior and toughness values are also discussed.

Evaluation of the Strength and Creep–Fatigue Behavior of Hot Isostatically Pressed Silicon Nitride

Abstract: The strength of a commericially available hot isostatically pressed silicon nitride was measured as a function of temperature. To evaluate long-term mechanical reliability of this material, the tensile creep and fatigue behavior was measured at 1150°, 1260°, and 1370°C. The stress and temperature sensitivities of the secondary (or minimum) creep strain rate were used to estimate the stress exponent and activation energy associated with the dominant creep mechanism. The fatigue characteristics were evaluated by allowing individual creep tests to continue until specimen failure. The applicability of the four-point load geometry to the study of strength and creep behavior was also determined by conducting a limited number of flexural creep tests. The tensile fatigue data revealed two distinct failure mechanisms. At 1150°C, failure was controlled by a slow crack growth mechanism. At 1260° and 1370°C, the accumulation of creep damage in the form of grain boundary cavities and cracks dominated the fatigue behavior. In this temperature regime, the fatigue life was controlled by the secondary (or minimum) creep strain rate in accordance with the Monkman–Grant relation.

Mechanical properties of ultra-fine grained zirconia ceramics

Abstract: The mechanical properties of tetragonal zirconia (TZP) materials doped with Y, Ce or Ti were studied as a function of temperature and grain size. Fine grained Y-TZP (grain size < 0.3 mgrm) shows values for fracture toughness and strength at room temperature, which are comparable with the coarse grained transformation toughened materials, despite lacking transformation toughening. The morphology of the fracture surface points to crack deflection as the most important toughening mechanism. At 800 °C fracture toughness and strength are higher than in coarse grained Y-TZP materials. Doping Y-TZP with Ce or Ti results in a similar trend in mechanical properties, for fine grained material, as for the Y-TZP materials.

Fracture Toughness of Advanced Ceramics at Room Temperature.

Abstract: Results of round-robin fracture toughness tests on advanced ceramics are reported. A gas-pressure silicon nitride and a zirconia-toughened alumina were tested using three test methods: indentation fracture, indentation strength, and single-edge precracked beam. The latter two methods have produced consistent results. The interpretation of fracture toughness test results for the zirconia alumina composite is shown to be complicated by R-curve and environmentally assisted crack growth phenomena.

The Relationship between Tensile and Flexural Strength of Unidirectional Composites

Abstract: Weibull theory predicts a higher strength in bending than in tension. How ever it assumes that failure initiates from a critical defect, whereas many unidirectional composites fail gradually in bending. A model has been developed of the composite as a bundle, each element of which consists of a small bundle of fibres. The resin is assumed to stabilise the bundle against buckling under compression, whilst allowing the individual elements to split and behave independently in tension. This model also predicts higher strength in bending than in tension, with a ratio of similar order of magnitude to that pre dicted by Weibull theory. The new model provides an explanation for the different strengths in bending and tension which is consistent with the progressive, noncatastrophic failure process which is often observed in bending tests of unidirectional composites.

Effect of particle size on mechanical properties of epoxy resin filled with angular‐shaped silica

Abstract: Effect of particle size on the mechanical properties of cured epoxy resins has been studied. Resin was filled with angular-shaped silica particles prepared by crushing fused natural raw quartz. These particles were sorted into six groups having different mean sizes ranging from 2–47 μm. Flexural and compressive moduli of the cured epoxy resin slightly decreased with decrease in the particle size of the silica, whereas tensile modulus slightly increased. Flexural and tensile strengths increased with decrease in particle size. Fractured surfaces were observed using scanning electron microscopy to clarify the initiation point of fracture.

Mechanical properties of concrete materials reinforced with polypropylene or polyethylene fibers

Abstract: An experimental study was conducted to compare the effectiveness of fibrillated polypropylene and high-modulus polyethylene fibers, both used at relatively low volume fractions, in enhancing the mechanical properties of concrete materials. Replicated flexure, impact, and compression tests were conducted, and the results were analyzed statistically. It was concluded that lower volume fractions of high-modulus polyethylene fibers can produce flexural and impact strengths comparable with those obtained at 0.1% volume fraction of fibrillated polypropylene fibers.

Effect of Firing Temperature on Bending Strength of Porcelains for Tableware

Abstract: Porcelains for tableware in the quartz–feldspar (sericite)–kaolin system were investigated to study the effect of firing temperature on the bending strength. The maximum bending strength of a body was attained at about 75°C below the temperature of complete sintering; therefore, its apparent porosity and the total porosity were almost 0% and 8.0%, respectively. Control of the finely dispersed, round, and isolated pores in the body was found to be necessary to attain strengthening of the body despite its relatively large porosity, and at the same time it was important that a large amount of fine quartz particles remain undissolved. Furthermore, a dense layer, formed close to the surface, was observed to strengthen the body more than that expected from its relative density when the body was fired above 1200°C.

Evaluation of the long-term properties of concrete

Abstract: Variations in concrete compressive strength, flexural strength, and modulus of elasticity with time are discussed. Concrete specimens made from five types of portland cement, portland blast-furnace slag cement, portland-pozzolan cement and air-entraining cement were tested at ages between 1 day and 34 years. Relationships between the compressive strength at various ages and the 28-day compressive strength are developed for specimens stored in moist and dry environments. Flexural strength and modulus of elasticity are related to the compressive strength.

Effects of freezing on mechanical properties of rat skin.

Abstract: Two test specimens of skin were cut from the lateral aspect of each hind limb of 9 rats. Specimens were contiguous, thereby providing matched pairs. One specimen was immediately placed in liquid nitrogen for 5 minutes, then stored at -70 C and tested within 3 to 4 weeks. Within 5 minutes of harvest, the second specimen was used for immediate material testing. Basic engineering material tests were used to measure strength, loading response, and elastic and viscous properties. Each matched pair of tissues was used for the same procedure. Quasistatic uniaxial tensile tests were used to apply deformations to the test specimens, and resulting loads were recorded. Stress and strain were calculated from the recorded data, providing information on yield strength, ultimate strength, fracture strength, and loading response. Each matched pair of specimens represented 1 repetition; 6 repetitions were made of each observation. Statistical analysis indicated that tissue freezing significantly (P less than 0.05) increased fracture strength, but did not affect strength, ultimate strength, or loading response. Dynamic vibration response tests were used to find mechanical mobility of the specimens, thereby providing information on elastic and viscous behaviors, which were quantified by calculation of spring and damping coefficients, respectively. As before, 6 repetitions were used. Statistical analysis indicated that tissue freezing did not affect these coefficients.

Significance of quenching stress in the cohesion and adhesion of thermally sprayed coatings

Abstract: In thermal spraying, molten particles strike a solid surface, where they are flattened and quenched within a very short time. Considerable in-plane tensile stress on the order of 100 MPa can develop within each splat during quenching after solidification because thermal contraction of the particle is constrained by the underlying solid. Ni-20Cr alloy and alumina powders have been plasma sprayed in air onto steel substrates that were maintained at about 473 K. The influence of spraying conditions such as spray distance on the magnitude of the quenching stress have been studied by measuring the curvature of the substrate during spraying. Mechanical properties such as Young’s modulus and bend strength of the deposited coatings have also been measured. A strong correlation was found between the quenching stress and the strength of Ni-20Cr coatings, which suggests that the strength of interlamellar bonding limits the quenching stress at such temperature.

Progressive Collapse Analysis of a Ship's Hull under Longitudinal Bending (2nd Report)

Abstract: This paper describes the further improvement of the simplified method of progressive collapse analysis of a ship's hull under longitudinal bending proposed by the authors. Here, the method of analysis of flexural behaviour of stiffener elements was extended to deal with coupled flexural-torsional behaviour of angle bar stiffeners welded to continuous plating.A series of elastoplastic large deflection analysis was performed and the rationality of the proposed method was examined.With the improved computer code 'HULLST', two alternative analyses were performed on existing bulk carrier including and neglecting tripping of stiffener elements. It was found that the ultimate strength of stiffener elements decreases when stiffener tripping takes place, which results in reduction of ultimate bending moment of the cross-section. The load carrying capacity of stiffener elements in the post-ultimate strength range was more decreased and so that of the cross-section. The allowable bending moment according to the Lloyd's Register of Shipping was calculated and the reserve strength above it until the initial local collapse was 22.4% of the allowable bending moment. Interaction diagram for longitudinal hull strength under bi-axial bending is also presented.Progressive collapse analysis was performed also on a large scale frigate model tested by Dow, and the applicability of the present method was demonstrated.

Plasma surface modification of advanced organic fibres

Abstract: The interlaminar shear strength, interlaminar fracture energy, flexural strength and modulus of extended-chain polyethylene/epoxy composites are improved substantially when the fibres are pretreated in an ammonia plasma to introduce amine groups on to the fibre surface. These property changes are examined in terms of the microscopic properties of the fibre/matrix interface. Fracture surface micrographs show clean interfacial tensile and shear fracture in composites made from untreated fibres, indicative of a weak interfacial bond. In contrast, fracture surfaces of composites made from ammonia plasma-treated fibres exhibit fibre fibrillation and internal shear failure as well as matrix cracking, suggesting stronger fibre/matrix bonding, in accord with the observed increase in interlaminar fracture energy and shear strength. Failure of flexural test specimens occurs exclusively in compression, and the enhanced flexural strength and modulus of composites containing plasma-treated fibres result mainly from reduced compressive fibre buckling and debonding due to stronger interfacial bonding. Fibre treatment by ammonia plasma also causes an appreciable loss in the transverse ballistic impact properties of the composite, in accord with a higher fibre/matrix interfacial bond strength.