Volume 18, Issue 1-2

No abstract available

No abstract available

In this study, wear behaviour of TiB 2 particle reinforced aluminium (Al) composites produced by the mechanical alloying method was investigated. TiB 2 ceramic particles of four different volume fractions were mechanically alloyed with Al and A356 alloy matrix materials. The mechanically alloyed particles were cold presses and then sintered at 550°C. After the sintering process, the composites were characterised through hardness measurements, scanning electron microscope (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD). Wear tests were also carried out on a pin-on-disc type wear apparatus under 10, 25 and 50 N loads. Wear loss of the composites were found to decrease with increasing hardness which increased with increasing TiB 2 particle volume fraction. However, increasing the applied load and sliding distance increased the wear loss. Wear surface examinations showed that various wear mechanisms were effective in wear of the composites.

Cracks are vitally detrimental to the load-bearing capacity of materials and further to the durability and service-life of various structures. Crack-repairing technology via embedded capsules with repair agent is becoming a promising approach to sustain the performance of structural materials. However, the appropriate dosage of capsulated repair agent for autonomic healing is not theoretically solved in the literature. In this study, taking cementitious materials as an example, the surface cracks in materials caused by various mechanisms are firstly simplified as linear cracks and zonal cracks in two-dimensional plane. Then, from the viewpoint of geometrical probability, the theoretical solutions on the exact dosage of capsules required are developed for different types of crack models via the knowledge of integral geometry and the concepts of probability distribution. Finally, reliability of these theoretical solutions is verified via computer modeling technology.

In this study, non-linear static analysis of a cantilever Timoshenko beam composed of functionally graded material (FGM) under a non-follower transversal uniformly distributed load is studied with large displacements and large rotations. Material properties of the beam change in the thickness direction according to a power-law function. In this study, the finite element of the beam is constructed by using the total Lagrangian Timoshenko beam element approximation. The non-linear problem is solved by using the incremental displacement-based finite element method in conjunction with the Newton-Raphson iteration method. To use the solution procedures of the Newton-Raphson method, there is a need to linearize equilibrium equations, which can be achieved through the linearization of the principle of virtual work. In this study, the effects of large deflections, large rotations and various material distributions on displacements and normal stress and shear stress distributions through the thickness of the beam are investigated in detail. The convergence study is performed for various numbers of finite elements. In addition, some of the particular results of the present study which are obtained for the homogeneous material case are compared with the results of the SAP2000 packet program. Numerical results show that geometrical non-linearity and material distribution play very important roles in the static responses of FGM beams.

The automated fiber placement (AFP) of thermoplastic composites has a complex thermal history, which seriously leads to residual stresses in the products. An ANSYS simulation model is proposed to predict thermal history and induced thermal stresses in thermoplastic composite rings. It attempts to simulate closely the AFP process by adding materials incrementally. The processing speed was modeled by incrementally moving the hot air heater along the outer surface of the composite cylinder. The analysis is time-dependent, including two steps of transient heat transfer and induced thermal stress analyses. The heat transfer characteristics and the accumulated thermal effect during processing are investigated. Thermally induced stresses are examined.

Owing to the huge interface region existing in a polymer nanocomposite, the effects of thermal processes on properties of nanocomposites are much more complicated than in a pure polymer. It is therefore important to determine the effects of thermal processes on nanocomposites with different interfacial interactions between the nanofillers and the polymer matrix. It is also important to explore the performance changes for nanocomposites under elevated temperatures over pure polymers. In this investigation, we examined the correlation of thermal treatment with dielectric properties of carbon nanofiber (CNF) reinforced high-density polyethylene nanocomposites. The thermal treatment of specimens was conducted for up to 120 h at 87°C and 127°C. Then, alternating current (AC) conductivity and dielectric properties were tested after definite intervals of time. Their changing rates over treatment time were analyzed. The results revealed the approximate linear relationships of AC conductivity and dielectric constant vs. heating time. Modified CNF reinforced nanocomposites had less influence by the heating treatments exhibiting better thermal resistance. The change rates of AC conductivity σ and dielectric properties have higher sensitivity to the treatment at a higher temperature. This study provides potential for further research on application of electrical and dielectric signals to detect the effects of heating process on lifetime of polymeric materials.

This paper defines two parameters to describe the effects of fatigue load adjustment on the reliability degradation of composites. The transition period, n 2 a , indicates the free failure time period from the instant of high-low adjustment until the residual strength distribution meets the low-level stress. The reliability drop, ∣Δ R ∣, is shown at the instant of low-high adjustment. Based on the strength-life equal rank assumption, a typical Yang model of the residual strength is considered to derive these two items by retrieving the residual strength distributions to the corresponding initials. The predicted parameters are verified to be in good agreement with the Monte Carlo simulation of fatigue failure. It is found that n 2 a increases dramatically as the reliability R a at which the stress level changes, in the range of 1> R a >0.95. The relationship between ∣Δ R ∣ and R a exhibits a parabolic-like curve with a peak near the mean cycles to failure of the process at low-level stress.

The strength and post-peak performance of reinforced concrete corbels, strengthened with epoxy bonded glass fiber reinforced polymer (GFRP) overlays, were experimentally investigated. The test variables were the corbel shear span to depth ratio, corbel main reinforcement ratio, and the number and orientation of the GFRP fibers. In total, 24 normal strength concrete, one-third scale, corbel specimens, without hoop reinforcement, were tested to failure under quasi-static gravity loading. Test results revealed that GFRP overlays can easily be used for the enhancement of corbel load bearing capacity, depending on the fiber orientation. The main reinforcement ratio and the number of GFRP plies were found to be the two main variables affecting the level of strength gain in the corbel specimens.

Flax is a type of natural fiber widely used as reinforcing materials for polymer composites. The commercially available flax fibers in Canada consist of a significant amount of shive and other impurities, which could act as stress concentration regions to negatively affect the mechanical property of composites. In this study, the shive was manually removed from the commercial flax fibers by screening and combing to obtain different shive contents from 0 to 30 wt%. By contrast, the obtained flax fibers were further treated with alkaline solution. The fibers obtained from mechanical and alkali treatment were compared on their thermal and mechanical properties. As expected, it was found that the thermal stability and mechanical properties of the flax reinforced polypropylene composites increased significantly with the removal of the shive content. However, the alkali treatment on flax fiber did not further improve the composites properties. The possible reason was that the proper mechanical treatment (screening and combing) prior to alkaline treatment effectively loosened the fiber bundles for better single fiber separation in matrix and significantly removed the impurities, thus the effect of alkaline treatment did not become obvious.

This study investigates the effects of water immersion on tensile properties of jute fiber reinforced polypropylene (PP) composites. The long fiber type jute/PP pellet and neat PP pellet were used as materials and the jute/PP specimens with different fiber content were prepared by injection molding with dry-blending of jute/PP and neat PP pellets. All the specimens were aged in hot distilled water at 80°C, and after the fixed periods of immersion, the weight changes and the tensile properties were measured. The weight gain by water absorption was significantly affected by the fiber content. The specimens with the jute fiber content of 30 wt% more easily absorbed water and it reached more than 10%. In these specimens, significant material loss by immersion also occurred. The tensile strength after immersion decreased remarkably in the specimens with the jute fiber content of 30 wt% and more, and all the jute/PP composites showed lower strength than neat PP after immersion of 1000 h.

In 1981, many detached houses collapsed owing to the earthquake which took place at the coast of Miyagi Prefecture in Japan, because ferroconcrete was not used as the base of most of the collapsed houses. Therefore, the earthquake-proof design criterion was revised and it was obliged to use ferroconcrete as the base of detached houses ever since. However, there are still more than 10 million detached houses using aged concrete as the base in Japan. In this paper, a reinforcement method for the aged concrete is demonstrated by using the high performance carbon fiber reinforced plastic (CFRP) strips. Three kinds of concrete beams reinforced with the CFRP strips with different vertical insert positions and different numbers of the CFRP strips were tested by a four point bending test method. In order to clarify the effects of CFRP strips, a concrete beam without reinforcement was also tested. The results of the experiment showed that the maximum deflections of the concrete beams reinforced with the CFRP strip were more than 100 times larger than those of the concrete beams without reinforcement. Although the concrete beams without the CFRP strips collapsed instantly, the concrete beams reinforced with the CFRP strips gradually failed, which were similar to the plastic behavior of metals. This result means that the residents can afford to escape safely from the houses in the event of possible earthquakes. The experimental results were compared with the analytical ones obtained from the composite beam theory and the failure criterion. Both results showed good agreement and the effectiveness of base concrete reinforced with CFRP strips was demonstrated.

As a step towards ensuring the sustainability of concrete as a construction material, an investigation was carried out on the mechanical and durability properties of concrete made without natural sand, termed “sandless” concrete, and with cement partially replaced by fly ash. Four groups of concrete mixes, with water-cement ( w/c ) ratios of 0.40, 0.45, 0.50 and 0.55, respectively, were studied. For each w/c ratio, there were six mixes with 0, 10%, 20%, 30%, 40% and 50% of cement content replaced by fly ash by mass, and one normal concrete mix containing natural sand. The difference in mechanical properties between normal and “sandless” concrete was not significant. “Sandless” concrete mixes with cement replaced by fly ash by <30% showed comparable compressive, splitting tensile and flexural strengths as those without fly ash. However, the elastic modulus was reduced with the incorporation of fly ash. In addition, use of fly ash led to reduced drying shrinkage of “sandless” concrete, and significantly improved the resistance to chloride ion penetration. The resistance to sulfate attack, on the other hand, seemed to decrease with higher fly ash content. From the study, it appears that “sandless” concrete with cement replaced by fly ash up to 30% could be considered for structural applications.

Advanced polymer matrix composite (PMC) materials have been more frequently employed for aerospace applications due to their light weight and high strength. Fiber-reinforced PMC materials are also being considered as potential candidates for elevated temperature applications such as supersonic vehicle airframes and propulsion system components. A new generation of high glass-transition temperature polymers has enabled this development to materialize. Clearly, there is a requirement to better understand the mechanical behaviour of this class of composite materials. In this study, polyimide-coated fiber optic sensors are employed to continuously monitor strain in a woven carbon fiber bismaleimide (BMI) matrix laminate subjected to tensile static and fatigue loading at elevated temperatures. A unique experimental test protocol is utilized to investigate the capability of the optical sensors to monitor strain and track stiffness degradation of the composite material. An advanced interrogation system and an optical spectrum analyzer are utilized to track the variation in the optical fiber wavelength and the wavelength spectrum for correlation with strain gage measurements. Isothermal tensile static and fatigue tests at room temperature, 105°C, 160°C and 205°C suggest that these optical sensors are capable of continuously monitoring strain and tracking the stiffness loss of a highly compliant PMC specimen during cyclic loading. The results illustrate that employing optical sensors for elevated temperature applications has significant advantages when compared to conventional strain gages.