SAGE Publications

This research investigated the effect of fiber volume content on the electromechanical behavior of strain-hardening steel-fiber-reinforced cementitious composites under direct tension. There is strong correlation between the change of electrical resistivity and the tensile response of strain-hardening steel-fiber-reinforced cementitious composites: the electrical resistivity of strain-hardening steel-fiber-reinforced cementitious composites clearly decreased during strain hardening as the tensile strain of them increased. The electrical conductivity, tensile resistance, and damage-sensing capacity of strain-hardening steel-fiber-reinforced cementitious composites were generally increased as the volume content of twisted steel fibers added in a mortar matrix increased from 0.0 to 2.0%. The strain-hardening steel-fiber-reinforced cementitious composites with fiber content more than 1% by volume produced high damage-sensing capacity with high nominal gauge factor: absolute value over 150. Besides, the addition of carbon black or ground granulated blast furnace slag in mortar matrices significantly reduced the electrical resistivity but slightly enhanced the damage-sensing capability of strain-hardening steel-fiber-reinforced cementitious composites.

Hybrid composite laminates are currently receiving researchers’ attention due to their specific advantages in designing laminates with improved specific strength and stiffness. One of the main disadvantages of polymeric laminated composites is their high sensitivity to notches, which cannot be avoided in design. This paper presents a comparison between two common hybridization techniques, namely sandwich and intra-ply hybridization. The study adopts experimental observations to investigate the influence of hybridization method on the flexural properties of notched carbon–aramid hybrid laminates. After four-point bending tests, the results show that the damage nature in both laminates is different. A catastrophic damage is observed for intra-ply hybrid laminates, while sandwich laminates show progressive damage. In terms of the strength, sandwich specimens show 1.3 times higher specific strength, compared to intra-ply specimens. Moreover, the bottom layers of the laminate manufactured in the sandwich fashion show minimal damage due to the high capability of the aramid/epoxy core to absorb the energy in deformation and concentrate the damage at the top layers (the compression side).

This research investigated the effect of fiber volume content on the electromechanical behavior of strain-hardening steel-fiber-reinforced cementitious composites under direct tension. There is strong correlation between the change of electrical resistivity and the tensile response of strain-hardening steel-fiber-reinforced cementitious composites: the electrical resistivity of strain-hardening steel-fiber-reinforced cementitious composites clearly decreased during strain hardening as the tensile strain of them increased. The electrical conductivity, tensile resistance, and damage-sensing capacity of strain-hardening steel-fiber-reinforced cementitious composites were generally increased as the volume content of twisted steel fibers added in a mortar matrix increased from 0.0 to 2.0%. The strain-hardening steel-fiber-reinforced cementitious composites with fiber content more than 1% by volume produced high damage-sensing capacity with high nominal gauge factor: absolute value over 150. Besides, the addition of carbon black or ground granulated blast furnace slag in mortar matrices significantly reduced the electrical resistivity but slightly enhanced the damage-sensing capability of strain-hardening steel-fiber-reinforced cementitious composites.

Batches of multi-walled carbon nanotubes having an average length of 2091 nm were aggressively tip-ultrasonicated to produce shortened carbon nanotubes having average lengths of 1689 nm, 1332 nm, 992 nm and 503 nm. Raman spectroscopic analysis confirmed that the shortened carbon nanotubes retained their crystallinity after the shortening process. Carbon nanotubes were then dispersed in the epoxy matrix using high-shear mixing technique (calendering). The mechanical properties were measured for the cured epoxy–0.1 wt% carbon nanotube nanocomposites having carbon nanotubes of different lengths. It was found that the nanocomposites containing long carbon nanotubes (2091 nm and 1689 nm) possess higher tensile strength, elastic modulus, fracture strain and fracture toughness as compared to nanocomposites containing short carbon nanotubes (1332 nm, 992 nm and 503 nm).

The AC-impedance and dielectric properties of hybrid polymer composites made up of epoxy (diglycidyl ether of bisphenol-A) matrix filled with various zinc oxide concentrations (0, 4.9, 9.9, 14.9 and 19.9 wt%) and reinforced with conductive carbon black nanoparticles (0.1 wt%) have been investigated as a function of fillers concentrations, applied frequency in the range from 20 kHz to1 MHz and temperature in the range from 30℃ to 110℃. The observed data were analyzed using the dielectric permittivity and electric modulus formalisms (i.e. the inverse quantity of complex permittivity). The dielectric constant of the composites increases with increasing temperature and filler concentrations, and this case can be explained by hopping and tunneling processes. The AC-conductivity is apparently enhanced with increasing frequency, temperature, zinc oxide and carbon black fillers. The observed increase in the AC conductivity was explained based on the concept of conductive paths and connections between the zinc oxide–particles and the conductive carbon black–nanoparticles. The activation energy has been estimated from fitting the AC conductivity–temperature data and it was found that it is decreased by the addition of the zinc oxide content in the epoxy matrix reinforced with carbon black, which means that the composites have better electrical conduction. The scanning electron microscopy images revealed that the dispersed zinc oxide-particles and carbon black-nanoparticles were randomly distributed within the epoxy matrix with some paths and surface contacts between the fillings. It was found that the addition of carbon black nanoparticles to epoxy/zinc oxide composites enhances the electrical conduction due to the electronic and impurity contributions.

Experimental results were obtamed for the natural frequencies and the mode shapes of graphite-epoxy and boron-epoxy composite materials having fiber orientations of 0°, 15°, 30° and 90° with respect to cantilever beam axes Certain elastic constants were experimentally determined and used in a programmed numerical solution in which rotary inertia, trans verse shear, and coupled bending-torsion effects were included. The experi mental results for the 15° and 30° beams gave a clear indication of the interaction between bending and twisting and good agreement was obtained with the numerical results.

The self-consistent approach originally proposed by Hill has been adopted to derive the effective elastic stiffness constants of unidirectional short-fiber composites. The short-fibers are modeled as ellipsoidal inclu sions uniformly distributed in the matrix and the transverse isotropy of the composite has been taken into account. The method of analysis is valid for multi-component systems and hence, applicable to hybrid composites. Comparisons of this analysis with existing theories are made for binary composites.

Pyrolysis is a well known method to recover carbon fibers from composite waste. In order to bring these recycled carbon fibers back into new composites, and to provide a `closed loop' for this material, their properties have to be investigated and proved suitable for new products. In a former study a strong influence of the pyrolysis process on the surface of the recovered fibers was found. This in turn influenced other properties like fiber strength, electrical properties and fiber—matrixadhesion. These results offer the possibility to control individual properties of recycled carbon fibers and their composites, but on the other hand they indicate a necessity for process optimization in order to provide a high quality of the recycled fibers. In this study different process parameters during pyrolysis were investigated and optimized in order to provide the reclaimed carbon fibers with properties close to new fibers. The optimization was done by labscale pyrolysis experiments performed in a thermogravimetric analyzer (TGA), with variation of pyrolysis temperature, isothermal dwell time and oven atmosphere, respectively. The recovered fibers were analyzed by scanning electron microscopy and Raman spectroscopy. Finally a pyrolysis test on a semi-industrial-scale was successfully performed to demonstrate the technical viability of the optimized process parameters.

The development and application of benchmark examples for the assessment of quasi-static delamination propagation capabilities is demonstrated. The methodology proposed, based on fracture mechanics, is used to develop benchmarks for the double cantilever beam, end-notched flexure, single leg bending as well as mixed-mode bending specimens for 20%, 50% and 80% mode II. The methodology proposed for the development of quasi-static benchmark results is discussed in detail, and will be presented using the double cantilever beam specimen as an example. The practical application of the benchmarks generated is illustrated by using the double cantilever beam and mixed-mode bending (50% mode II) benchmarks to assess the automated procedure implemented in Abaqus/Standard®. This assessment proved to be valuable by highlighting the issues (e.g. overshooting, saw-tooth behavior, non-convergence) associated with choosing the input parameters unique to the Virtual Crack Closure Technique implementation in Abaqus/Standard®. Additional studies should include the assessment of the propagation capabilities in more complex specimens and on a structural level.

This study aims to enhance carbon nanotube (CNT)-reinforced magnesium (Mg) matrix composites fabricated by spark plasma sintering (SPS). Ultrasonic treatment with organic solvent (DMAc) and dispersant (K₂CO₃) were used for the processes to make each CNT apart. Nano-order ceramic powders of zirconia (ZrO₂) were found to be effective to keep uniform dispersion of CNT in Mg matrix. Low melting point metals such as Sn were added to fill voids and cracks in the composites. Post-SPS rolling was also employed aiming to improve the microstructures. The fabricated CNT-reinforced Mg composites were investigated on mechanical properties such as hardness and three-point bending strength to be related to the microstructures.