Showing papers on "Flexural strength published in 2008"

Analysis of crack formation and crack growth in concrete by means of fracture mechanics and finite elements

Abstract: A method is presented in which fracture mechanics is introduced into finite element analysis by means of a model where stresses are assumed to act across a crack as long as it is narrowly opened. This assumption may be regarded as a way of expressing the energy adsorption GC in the energy balance approach, but it is also in agreement with results of tension tests. As a demonstration the method has been applied to the bending of an unreinforced beam, which has led to an explanation of the difference between bending strength and tensile strength, and of the variation in bending strength with beam depth.

Tough, bio-inspired hybrid materials.

Abstract: The notion of mimicking natural structures in the synthesis of new structural materials has generated enormous interest but has yielded few practical advances. Natural composites achieve strength and toughness through complex hierarchical designs extremely difficult to replicate synthetically. Here we emulate Nature's toughening mechanisms through the combination of two ordinary compounds, aluminum oxide and polymethylmethacrylate, into ice-templated structures whose toughness can be over 300 times (in energy terms) that of their constituents. The final product is a bulk hybrid ceramic material whose high yield strength and fracture toughness ({approx}200 MPa and {approx}30 MPa{radical}m) provide specific properties comparable to aluminum alloys. These model materials can be used to identify the key microstructural features that should guide the synthesis of bio-inspired ceramic-based composites with unique strength and toughness.

Measurements of near-ultimate strength for multiwalled carbon nanotubes and irradiation-induced crosslinking improvements.

Abstract: The excellent mechanical properties of carbon nanotubes are being exploited in a growing number of applications from ballistic armour to nanoelectronics. However, measurements of these properties have not achieved the values predicted by theory due to a combination of artifacts introduced during sample preparation and inadequate measurements. Here we report multiwalled carbon nanotubes with a mean fracture strength >100 GPa, which exceeds earlier observations by a factor of approximately three. These results are in excellent agreement with quantum-mechanical estimates for nanotubes containing only an occasional vacancy defect, and are ∼80% of the values expected for defect-free tubes. This performance is made possible by omitting chemical treatments from the sample preparation process, thus avoiding the formation of defects. High-resolution imaging was used to directly determine the number of fractured shells and the chirality of the outer shell. Electron irradiation at 200 keV for 10, 100 and 1,800 s led to improvements in the maximum sustainable loads by factors of 2.4, 7.9 and 11.6 compared with non-irradiated samples of similar diameter. This effect is attributed to crosslinking between the shells. Computer simulations also illustrate the effects of various irradiation-induced crosslinking defects on load sharing between the shells. The mechanical properties of carbon nanotubes rarely match the values predicted by theory owing to a combination of artefacts introduced during sample preparation and inadequate measurements. However, by avoiding chemical treatments and using high-resolution imaging, it is possible to obtain values of the mean fracture strength that exceed previous values by approximately a factor of three.

Dimensional stability and mechanical behaviour of wood–plastic composites based on recycled and virgin high-density polyethylene (HDPE)

Abstract: This paper investigated the stability, mechanical properties, and the microstructure of wood–plastic composites, which were made using either recycled or virgin high-density polyethylene (HDPE) with wood flour ( Pinus radiata ) as filler. The post-consumer HDPE was collected from plastics recycling plant and sawdust was obtained from a local sawmill. Composite panels were made from recycled HDPE through hot-press moulding exhibited excellent dimensional stability as compared to that made from virgin HDPE. The tensile and flexural properties of the composites based on recycled HDPE were equivalent to those based on virgin HDPE. Adding maleated polypropylene (MAPP) by 3–5 wt% in the composite formulation significantly improved both the stability and mechanical properties. Microstructure analysis of the fractured surfaces of MAPP modified composites confirmed improved interfacial bonding. Dimensional stability and strength properties of the composites can be improved by increasing the polymer content or by addition of coupling agent. This project has shown that the composites treated with coupling agents will be desirable as building materials due to their improved stability and strength properties.

Effects of surfactant treatment on mechanical and electrical properties of CNT/epoxy nanocomposites

Abstract: Surfactant has been successfully applied to enhance the dispersion of carbon nanotubes (CNTs) in polymer and the properties of nanocomposite. CNTs were treated with a nonionic surfactant Triton X-100, and its effects on dispersion state, surface chemistry, structure and morphology of CNTs, as well as on the thermomechanical, mechanical and electrical properties of CNT/epoxy nanocomposites were evaluated. The mechanical properties such as impact fracture toughness, flexural strength and modulus, the thermomechanical properties, as well as the electrical conductivity of the nanocomposite all showed significant improvements after the treatment. The above observations are attributed to the “bridging” effects between the CNT and epoxy, which are introduced by the hydrophobic and hydrophilic segments of the nonionic surfactant. The enhanced interfacial interactions gave rise to improved dispersion and wetting of CNTs in polymer matrix, enhancing the mechanical and fracture properties of the nanocomposite. Unlike chemical functionalization techniques, however, the surfactant treatment exhibited little adverse effect on electrical conducting behavior of the nanocomposite.

Mechanical properties of polypropylene hybrid fiber-reinforced concrete

Abstract: This paper investigates the mechanical properties of polypropylene hybrid fiber-reinforced concrete. There are two forms of polypropylene fibers including coarse monofilament, and staple fibers. The content of the former is at 3 kg/m3, 6 kg/m3, and 9 kg/m3, and the content of the latter is at 0.6 kg/m3. The experimental results show that the compressive strength, splitting tensile strength, and flexural properties of the polypropylene hybrid fiber-reinforced concrete are better than the properties of single fiber-reinforced concrete. These two forms of fibers work complementarily. The staple fibers have good fineness and dispersion so they can restrain the cracks in primary stage. The monofilament fibers have high elastic modulus and stiffness. When the monofilament fiber content is high enough, it is similar to the function of steel fiber. Therefore, they can take more stress during destruction. In addition, hybrid fibers disperse throughout concrete, and they are bond with mixture well, so the polypropylene hybrid fiber-reinforced concrete can effectively decrease drying shrinkage strain.

Comparative flexural behavior of four fiber reinforced cementitious composites

Abstract: This research investigates the flexural behavior of fiber reinforced cementitious composites (FRCC) with four different types of fibers and two volume fraction contents (0.4% and 1.2%) within a nominally identical mortar matrix (56 MPa compressive strength). The four fibers are high strength steel twisted (T-), high strength steel hooked (H-), high molecular weight polyethylene spectra (SP-), and PVA-fibers. The tests were carried out according to ASTM standards. The T-fiber specimens showed best performance in almost all aspects of behavior including load carrying capacity, energy absorption capacity and multiple cracking behavior, while the PVA-fiber specimens exhibited comparatively the worst performance in all aspects of response. The only category in which SP-fiber specimens outperformed T-fiber specimens was deflection capacity, where SP-specimens exhibited the highest deflection at maximum load. By comparing the test results to data from an additional test program involving the use of a higher strength mortar (84 MPa) with both H- and T-fibers, it is shown that, again, T-fibers perform significantly better than H-fibers in a higher strength matrix. The test results from both experimental programs were used to critique the new ASTM standard [C 1609/C 1609M-05], and a few suggestions were made for improving the applicability of the standard to deflection-hardening FRCCs.

Structure-property correlations in Al 7050 and Al 7055 high-strength aluminum alloys

Abstract: The 7XXX series age-hardenable high-strength aluminum alloys find useful applications in the field of aerospace engineering. Constant efforts are being made to tailor the mechanical and corrosion properties of these alloys as per requirements for a particular application. These properties are a function of factors like microstructure, chemical composition and processing parameters. An effort has been made to collate the information available from different studies conducted on alloys Al 7050 and Al 7055. Databases were created to consolidate the information about microstructure, mechanical properties and corrosion behavior for the two alloys. Existing models were utilized to predict strength and fracture toughness for these alloys and these models were validated using experimental values and a qualitative evaluation was made for the corrosion behavior of these alloys. Available data were utilized to prepare maps that are intended to serve as guides to design aluminum alloys with desired combination of properties.

Fabrication and characterization of carbon/epoxy composites mixed with multi-walled carbon nanotubes

Abstract: In this study, a high-intensity ultrasonic liquid processor was used to obtain a homogeneous mixture of epoxy resin and multi-walled carbon nanotubes (CNTs). The CNTs were infused into epon 862 epoxy resin through sonic cavitation and then mixed with W curing agent using a high-speed mechanical agitator. The trapped air and reaction volatiles were removed from the mixture using a high vacuum. Flexural tests and fracture toughness tests were performed on unfilled and CNT-filled epoxy to identify the effect of adding CNTs on the mechanical properties of epoxy. The highest improvement in strength and fracture toughness was obtained with 0.3 wt% CNT loading. The nanophased matrix filled with 0.3 wt% CNT was then used with weave carbon fabric in a vacuum-assisted resin transfer molding (VARTM) set up to fabricate composite panels. Flexural tests, thermogravimetric analysis (TGA), and dynamic mechanical analysis (DMA) were performed to evaluate the effectiveness of adding CNTs on the mechanical and thermal properties of the composite. The glass transition temperature, decomposition temperature, and flexural strengths were improved by infusing CNTs. Based on the experimental result, a linear damage model has been combined with the Weibull distribution function to establish a constitutive equation for neat and nanophased carbon/epoxy. Simulated result show that that infusing CNTs increases Weiubll scale parameter, but decrease Weibull shape parameter.

How to improve mechanical properties of polylactic acid with bamboo fibers

Abstract: Bamboo fibers (BF) were mixed in polylactic acid (PLA) to improve its mechanical properties: impact strength and heat resistance. Three different types of BF were extracted from raw bamboo by either sodium hydroxide (NaOH) treatment or steam explosion in conjunction with mechanical processing. They were designated as “short fiber bundle,” “alkali-treated filament” and “steam-exploded filament,” respectively. Composite samples were fabricated by injection molding using PLA/BF pellets prepared by a twin-screw extruding machine. Among them, the highest bending strength was obtained when steam-exploded filaments were put into PLA matrix. Impact strength of PLA was not greatly improved by addition of short fiber bundles as well as both filaments. In order to improve the impact strength of PLA/BF composites, PLA composite samples were alternatively fabricated by hot pressing using medium length bamboo fiber bundles (MFB) to avoid the decrease in fiber length at fabrication. Impact strength of PLA/MFB composite significantly increased, in which long fiber bundles were pulled out from the matrix. The addition of BF improves thermal properties and heat resistance of PLA/BF composites due to the constraint of deformation of PLA in conjunction with crystallinity promoted by anneal (at 110 °C for 5 h).

Fabrication and characterization of TiO2–epoxy nanocomposite

Abstract: A systematic study has been conducted to investigate the matrix properties by introducing nanosize TiO2 (5–40 nm, 0.5–2% by weight) fillers into an epoxy resin. Ultrasonic mixing process, via sonic cavitations, was employed to disperse the particles into the resin system. The thermal, mechanical, morphology and the viscoelastic properties of the nanocomposite and the neat resin were measured with TGA, DMA, TEM and Instron. The nano-particles are dispersed evenly throughout the entire volume of the resin. The nanofiller infusion improves the thermal, mechanical and viscoelastic properties of the epoxy resin. The nanocomposite shows increase in storage modulus, glass transition temperature, tensile modulus, flexural modulus and short beam shear strength from neat epoxy resin. The mechanical performance and thermal stability of the epoxy nanocomposites are depending on with the dispersion state of the TiO2 in the epoxy matrix and are correlated with loading (0.0015–0.006% by volume). In addition, the nanocomposite shows enhanced flexural strength. Several reasons to explain these effects in terms of reinforcing mechanisms were discussed.

Fiber-Reinforced Composites

Abstract: Part A. Fiber-Reinforced Concrete 22.1 Historical Development 22-2 22.2 General Characteristics 22-2 22.3 Mixture Proportioning 22-4 22.4 Mechanics of Fiber Reinforcement 22-5 First Cracking Load • Critical Fiber Length: Length Factor • Critical Fiber Spacing: Space Factor • Fiber Orientation: Fiber Efficiency Factor • Static Flexural Strength Prediction: Beams with Fibers Only 22.5 Mechanical Properties of Fibrous Concrete Structural Elements 22-8 Controlling Factors • Strength in Compression • Strength in Direct Tension • Flexural Strength • Shear Strength • Environmental Effects • Dynamic Loading Performance 22.6 Steel-Fiber-Reinforced Cement Composites 22-14 General Characteristics • Slurry-Infiltrated Fiber Concrete • DSP and CRC Cement Composites • Carbon-Fiber-Reinforced Cement-Based Composites • Super-Strength Reactive-Powder Concretes 22.7 Prestressed Concrete Prism Elements as the Main Composite Reinforcement in Concrete Beams 22-17 Part B. Fiber-Reinforced Plastic (FRP) Composites 22.8 Historical Development 22-18 22.9 Beams and Two-Way Slabs Reinforced with GFRP Bars 22-19 22.10 Carbon Fibers and Composite Reinforcement 22-20 Carbon Fibers • Hybrid GFRP and CFRP Reinforcement for Bridges and Other Structural Systems • Use as Internal Prestressing Reinforcement • Use as External Reinforcement 22.11 Fire Resistance 22-16 22.12 Summary 22-25 Acknowledgments 22-25 References 22-25

On the modulus of elasticity and strain capacity of Self-Compacting Concrete incorporating rubber aggregates

Abstract: Cement-based materials suffer from low tensile strength and poor strain capacity. They are brittle and highly sensitive to cracking, notably to shrinkage cracking, which is particularly detrimental for large surface areas. This paper focuses on the properties of a Self-Compacting Concrete (SCC) incorporating rubber aggregates, obtained by grinding end-of-life tyres, as a partial replacement for natural aggregates. Results show that the new cementitious material goes against some governing principles of mechanical behaviour of ordinary cement-based concrete. In particular, the modulus of elasticity of rubberized SCC is reduced and its variation with rubber aggregate content does not obey the conventional empirical relationship of modulus of elasticity with compressive strength. The strain capacity of SCC was quantified through flexural bending tests, which demonstrated that strain capacity increased when rubber aggregates were incorporated in concrete. This response is interpreted as a result of the ability of rubber aggregates to reduce the stress singularity at the first crack tips running into the rubber/cement–matrix interface, a mechanism slowing the cracking kinetics and delaying macrocrack localization. In such conditions, rubberized SCC is expected to be suitable when resistance to the cracking due to imposed deformation is a priority. This type of composite with low modulus of elasticity is also suitable for Controlled Modulus Columns (CMC) foundations, the ultimate solution for improving very soft soils subjected to settlement or stability problems caused by insufficient bearing capacity. Incidentally, the use of rubber aggregates in SCC provides an opportunity to recycle non-reusable end-of-life tyres.

Flexural Phonons in Free-Standing Graphene

Abstract: Rotation and reflection symmetries impose that out-of-plane (flexural) phonons of freestanding graphene membranes have a quadratic dispersion at long wavelength and can be excited by charge carriers in pairs only. As a result, we find that flexural phonons dominate the phonon contribution to the resistivity $\ensuremath{\rho}$ below a crossover temperature ${T}_{x}$ where we obtain an anomalous temperature dependence $\ensuremath{\rho}\ensuremath{\propto}{T}^{5/2}\mathrm{ln} T$. The logarithmic factor arises from renormalizations of the flexural-phonon dispersion due to coupling between bending and stretching degrees of freedom of the membrane.

Investigations on the thermal effect caused by laser cutting with respect to static strength of CFRP

Abstract: To increase production volume and efficiency in the area of CFRP (carbon fiber-reinforced plastics) component production, fast, flexible and cost-efficient technologies are needed. One process that is necessary during CFRP component production is trimming and cutting. Although laser cutting in principle meets these requirements, it is often not used for component trimming and contour cutting, due to insufficient knowledge about the influences of thermal machining on the material behavior. It is a common argument that lasers, as a thermally acting tool, may damage the CFRP, thus reducing its strength properties. This, however, has never been proven or disproven. Therefore, this paper presents investigations on the influence of laser cutting on the static strength of a CFRP laminate. The material is cut using three different high-power laser sources: a pulsed Nd:YAG laser, a disk laser and a CO2 laser. Appropriate cutting parameters have been found, and the results in cut quality and heat-affected zone are discussed. With these parameter sets, specimens for tensile strength and bending tests have been prepared. These specimens have been tested under static tensile and bending conditions, and the results have been compared to conventional milling as well as abrasive water-jet cut samples. Though a clear dependency of the static strength values on the heat-affected zone was detected, all strengths were found to be far above the material values given by the producer of the laminate.

On flow properties, fibre distribution, fibre orientation and flexural behaviour of FRC

Abstract: For improving the mechanical properties of fibre reinforced concrete one can either increase the fibre content, use hybrid fibre systems, or one can attempt to align fibres in the direction of stress. In this paper, it is attempted to use the flow-properties of the fresh (self-compacting) concrete to change the fibre distribution and orientation. Using a single mixture of fibre reinforced concrete, containing 3% of 30 mm long straight steel fibres, the fibre distribution and orientation was determined in three different parts of a ‘U-shaped specimen’ where the concrete could flow in three different directions. The fibre distribution and orientation was determined from a CT-scan. Flexural tests show that the mechanical behaviour depends on the fibre distribution and orientation, which can be affected by changing the viscosity of the fresh mixture.

Engineering porosity in polymer-derived ceramics

Abstract: By employing carefully controlled processing methods, a large amount of porosity (>70 vol%) was introduced in ceramic materials derived from preceramic polymers (silicone resins) after pyrolysis at 1000–1200 °C in inert atmosphere. The resulting components have a bulk density ranging from ∼250 to 950 kg/m3. Three main fabrication methods have here been employed: (1) direct foaming of a solution of a thermosetting silicone resin in a suitable solvent (with or without the addition of polyurethane precursors), acting also as a blowing agent; (2) the use of sacrificial fillers that decompose during pyrolysis, consisting in polymeric microbeads; (3) the mixing of preceramic polymers possessing different characteristics, in particular ceramic yield, depending on their molecular structure. In addition to that, several methods for developing micro- or meso-pores within the resulting SiOC macro-porous ceramics were explored, with the aim of fabricating components with hierarchical porosity. These include a controlled thermal treatment, the addition of fillers with a high specific surface area (SSA), the deposition of zeolites or meso-porous silica coatings, the infiltration with aerogels, selective etching of the SiOC material and the in situ formation of C-based nanostructures. Depending on the fabrication procedure adopted, cells with an average size ranging from the micrometer to the millimeter were obtained. All these processes are simple, economical and versatile, and large bodies with various shapes (tubes, plates, blocks) can be produced, possessing a wide range of morphologies and properties. Compression strength, flexural strength and Young's modulus vary with the morphology and density of the porous components. It is also possible to add to the preceramic polymers some filler powders, for instance possessing electrical conductivity or magnetic properties, leading to the production of functional cellular ceramics.