S. Rao

Manufacturing and Recycling of Sisal-Polypropylene Composites

The current study focuses on manufacturing, recycling and mechanical testing of thin (1.5 mm) sisal-PP extruded composite sheets. An 3-factor two-level experimental design based on the Taguchi method was applied in the manufacturing of the composite sheets to maximise their mechanical properties. The sisal-PP composite sheets obtained by setting the factors predicted by the Taguchi analysis were mechanically recycled by pelletising the sheets and feeding through the extruder. The effects of recycling on crystallinity, fibre length, mechanical properties and stress-relaxation were evaluated. By selecting a fibre mass of 30%, polymer MFI of 1.3 g/10 min and 1% of lubricant, ~12%, ~20% and >100% increases in tensile strength, impact strength and tensile modulus respectively were observed. The fibre lengths dropped from 7 mm before extrusion to 6mm and under after extrusion, and under 5 mm after recycling. A marginal change (± 5%) in modulus values was observed in both directions after recycling, the ultimate tensile strength of the recycled sisal-PP specimens dropped down from 41.4 ± 2.5 to 36.4 ± 0.75 MPa along the machine direction, and increased from 19.2 ± 0.5 MPa to 21.4 ± 0.1 MPa transverse to the machine direction mainly due to the decrease in the reinforcing fibre lengths. The recycled composites exhibited greater relaxation compared to the sisal-PP composites; this is because, at higher temperatures, the polymer matrix is in a softened state and the bonding between the fibre and matrix is expected to be weaker and the short fibres behave like polymer rich areas and fail to share the imposed load, thus exhibiting greater relaxation at elevated temperatures.

Fire Performance of Flax Laminates and their Hybrids

In the current age of growing environmental awareness, natural fibre composites have gained wide acceptance in various facets of engineering. However, in industries, such as aerospace and mining, their acceptance is primarily dependent on them meeting the stringent fire test requirements. In this paper, symmetric laminates consisting of only glass, glass/flax hybrid and only flax as reinforcements in thermoset matrices were tested for their time to ignition, heat release rate and smoke constituents as per standard ASTM E 1354 in a cone calorimeter. Four fire retardant versions of resin systems, were used in this study. The laminates were manufactured using wet hand-layup technique that was vacuum bagged and cured between hot platens of a hydraulic press. A constant fibre volume fraction of 0.5 for all the laminates was obtained by maintaining a constant laminate thickness of 4mm. The results from the cone calorimeter tests were compared to examine the influence of natural fibres on the fire properties of the laminates. It was observed that the degree of fire retardance in the polyester based composites decreased with the increase in the flax fibre content; however, in the modified urethane composites, flax fibre composites performed better by exhibiting higher ignition time compared to the hybrid and glass fibre composites. Another important observation was that the carbon monoxide emissions during testing decreased with the increase in flax content in the composites, no matter what resin system was used. These preliminary tests suggest that, by judiciously incorporating natural fibres in a synthetic system, a hybrid system could be designed to sustain loads in environments with high fire risks.

Investigation of peel resistance during the fibre placement process

In this study, the influence of compaction load, layup speed and temperature on the adhesive properties of automated fibre placement grade towpreg has been investigated on the ply-tool interface where higher peel forces are required to permit the deposition of subsequent plies. The automated layup process was simulated on a CNC milling machine, using a roller assembly and the adhesion properties of the towpreg were determined using a floating roller peel test. The processing window for the towpreg was determined using a dynamic mechanical analyser and a two-level, full factorial design of experiments was developed for the three factors, to understand their effects on the peeling force, both individually and synergistically. The design of experiments analysis indicates a strong temperature effect, with the towpregs requiring a higher layup temperature to accommodate higher layup speeds. A strong load-temperature interaction was detected, with a negative temperature effect at lower loads and a strong positive temperature effect at higher loads. The predicted factor settings to achieve a peeling force of 246 N/m are, 1 kN compaction load, 65℃ layup temperature, and a layup speed of 120 mm/min. Experimental tests, carried out at the predicted factor settings, agree well with the analysis, yielding a peel force of 256 N/m with a standard deviation of 25 N/m.

Electrospun nanofibre cores containing graphene oxide for sandwich films: manufacturing and analysis

The motivation for the need of small-scale devices has made thin films technologically important in the recent years. They have found applications in broad fields, such as microelectronic integrated circuits, magnetic information storage systems, optical coatings and wear resistant coatings. However, the mechanical performance of these materials tends to depend on fabrication and post-processing parameters. With the intent of improving the mechanical properties of the films, a relatively novel concept of sandwich composite films has been tried in this research. Poly-methyl methacrylate (PMMA) and Graphene Oxide (GO) have been used to manufacture the sandwich films, where PMMA films have served as the facings and electrospun PMMA/GO nanofibre mat forms the sandwich core. Dimethylformamide (DMF) and Tetrahydrofuran (THF) solvents are used in suitable proportions to dissolve PMMA, and then GO is added to this solution to obtain a uniform suspension of PMMA/GO for electrospinning. The mechanical and functional properties depend on the fibre quality and their distributions in the mat, which in turn depends on the concentration of the solution. Therefore, design of experiments based on mixture analysis was used to identify the solution concentration for obtaining uniform fibre diameters and their distribution throughout the electrospun core. The analysis suggested 23% PMMA and 2% GO concentrations in the solution would give uniform fibre diameters and dispersion throughout the mat.

Compressive Behavior of Fibre Reinforced Honeycomb Cores

Honeycomb core sandwich panels have found extensive applications particularly in the aerospace and naval industries. In view of the recent interest in alternative, yet strong and lightweight materials, honeycomb cores are manufactured from sisal fibre reinforced polypropylene (PP) composites and the out-of-plane compressive behaviour of these cores is investigated. The cell wall material is modeled as a linear elastic, orthotropic plate/lamina and also as a linear elastic, quasi-isotropic material. The failure criteria for the reinforced honeycombs are theoretically developed. Failure maps that can be used for the optimal design of such honeycombs are constructed for a wide range of honeycomb densities. The results indicate a significant improvement in the load carrying capacity of the honeycomb cores after fibre reinforcement.

The Low Velocity Impact Response of Nano Modified Composites Manufactured Using Automated Dry Fibre Placement

The low velocity impact response of composite materials manufactured from dry fibre tows using the automated fibre placement technique has been investigated. Following fibre placement, the dry preforms were infused with an epoxy resin. The influence of incorporating graphene oxide (GO) nanoparticles into the matrix was investigated, and the impact response of these samples was compared to that of its plain resin counterpart. Flexural and low velocity impact tests were undertaken in this study to understand the influence of GO nano-filler loading on mechanical behaviour of the unreinforced epoxy resin. The introduction of GO into the resin showed nearly 50% increase in ductility compared with that of the neat polymer at a mere 0.1 wt% filler loading. However, GO had a negligible effect on the impact response of these novel composites. There was no observable difference between the load-displacement traces or the resulting damage in the plain and unmodified composites. It is possible that the polymers ability to undergo larger non-linear deformation at lower rates of strain is suppressed when it is subjected to impact rates of loading.

Welding Heat Transfer Analysis Using Element Free Galerkin Method

Mesh-less methods belong to a new class of numerical methods in computational mechanics and offer several advantages over the conventional mesh-based methods. They enable modelling of processes involving high deformation, severe discontinuities (e.g. fracture) and multiple physical processes. These types of situations are usually encountered in arc welding, rendering its modelling suitable via mesh-less methods. In this paper, a mesh-less Element Free Galerkin (EFG) method has been developed to model the heat transfer during welding. The results predicted by the EFG method are found to be in close agreement with those obtained by the finite element method and those observed in welding experiments. This demonstrates the effectiveness and utilities of the EFG method for modelling and understanding the heat transfer processes in arc welding.

An Evaluation of the Compression Response of High-Performance Prepregs for Afp Applications

In this study, the compaction response of unidirectional out-of-an autoclave (OOA) prepreg tape has been evaluated experimentally at 25°, 45° and 65 °C. Instantaneous and incremental displacement compaction experiments have been carried out in a universal testing machine until a compressive strain of 0.42 ±0.02 was achieved. The fiber-bed stress was determined by transposing the results of displacement-controlled experiments onto data from corresponding load-controlled experiments. The material was then modelled as a viscoelastic material using a third order exponential decay function, based on a generalized Maxwell model. This procedure enabled the extraction of the relaxation time constants and the peak stress. The results highlight a linear relationship between the operating temperature and the load/stress relaxation response of the prepreg/tape, suggesting that if the fiber placement procedure is performed at 65 °C, the effect of roller stiffness on tape/fiber laying is negligible. The maximum contribution of fiber stress to the overall relaxation is approximately 6% at 45 °C and 65 °C, indicating the formability to be matrix dependent. A minimum spring-back is observed in specimens that were tested under incremental displacement conditions. The difference in thickness of the as-laid laminate and the cured laminate at 45 °C and 65 °C were similar. The linear stress relaxation model has been shown to be capable of predicting the maximum stress accurately using the viscosity values of the dashpots and the moduli of the springs.

The crushing characteristics of reinforced Nomex honeycomb

A Nomex honeycomb core has been reinforced with small diameter composite rods and tubes in order to enhance its compression properties and energy-absorbing characteristics. The influence of the areal density of the rods and tubes on the strength and energy-absorbing properties of the reinforced honeycomb was investigated by introducing increasing numbers of rods/tubes in square samples with bonded composite skins. An initial series of crushing tests on arrangements of small tubes and rods resulted in a stable model of failure yielding specific energy absorption values of approximately 45 kJ/kg for the tube and rod-based structures. A subsequent observation of the failed tubes highlighted the similar failure processes to those observed previously following the tests on much larger composite cylinders. Mechanical tests on the Nomex cores have shown that the compression strength and energy-absorbing characteristics of the reinforced honeycombs increase rapidly with increasing composite reinforcement. At low and intermediate values of core density, the rod and tube-reinforced cores exhibited similar properties, in terms of their compression strengths and specific energy absorption, an effect that is likely to be due to the dominance of the heavier Nomex core in these samples. At higher densities, the rod-reinforced systems tended to out-perform their tube-reinforced counterparts. Tests at impact rates of strain have shown that the compression strength and energy-absorbing capabilities of the reinforced cores are higher under the dynamic conditions, with the rod-reinforced cores offering values of specific energy absorption as high as 78 kJ/kg.

Manufacturing and characterization of multifunctional polymer-reduced graphene oxide nanocomposites

In this chapter, multifunctional nanocomposite films using reduced graphene oxide and poly(methyl methacrylate) were produced using electrospinning and compression molding techniques. Electrical conductivity and gas-barrier properties of the composite films were investigated experimentally using the four-probe method and oxygen permeability testing as per ASTM Standard D3985-02, respectively. Based on the experimental results, the films are highly suitable for applications in food and beverage packaging and chemical-processing plants. The analytical model outcomes for EMI-shielding effectiveness were positive, and the theoretical cases suggest increased shielding efficiency values with increase in the number of layers in the film.