Rehan Umer

Synergistic toughening of epoxy with carbon nanotubes and graphene oxide for improved long-term performance

Epoxy based nanocomposites using graphene oxide (GO) sheets dispersed multi-walled carbon nanotubes (CNTs) as combination fillers were prepared using an in situ polymerization technique. A remarkable synergetic effect was observed between CNTs and GO sheets which improved the mechanical properties of the epoxy. It was confirmed by optical and field-emission scanning electron microscopy (FESEM) images that the dispersion of CNTs in epoxy matrix can be significantly improved by adding GO sheets. The overall mechanical properties of CNT–GO/epoxy composites were greatly enhanced with only adding 0.04 wt% (percent by weight) CNTs and 0.2 wt% GO sheets. Moreover, the fatigue and creep rupture lives of pure epoxy was also significantly increased by the addition of GO dispersed CNTs. Approximately a 950% improvement in fatigue life, and 400% improvement in creep rupture life were observed at the applied stress levels tested.

Experimental and numerical characterizations of flexural behavior of VARTM-infused composite sandwich structures

Experimental and numerical characterizations of sandwich materials are required to fully exploit the benefits offered by these materials and efficiently design them. In this study, the flexural behavior of resin-infused composite sandwich structures was investigated. Panels with two different polyvinylchloride foam densities and thicknesses were studied. The S-2 glass fabric face sheets were resin-infused and bonded to the core in a single-step process using epoxy resin. Sandwich beams machined from the panels were subjected to three- and four-point bending tests. Finite element-based simulations predicting the flexural response of the sandwich panels were performed and compared with experiments. Excellent agreement in finite element-predicted failure loads and experiments were observed.

The mechanical properties of 3D woven composites

In this work, three types of 3D woven fabric (orthogonal, angle interlock, and layer-to-layer) were used to study the effect of weaving architecture on processing and mechanical properties. In order to characterize the fabrics for liquid composite molding processes, the compaction and permeability characteristics of the reinforcements were measured as function of fiber volume fraction. High compaction pressures were required to achieve a target fiber volume fraction of 0.65, due to presence of through-thickness binder yarns that restricts fiber nesting. In-plane permeability experiments were completed and flow front patterns were obtained to understand the anisotropy in the laminates. The resin transfer molding process was then used to manufacture panels that were then tested under quasi-static flexure and low-velocity impact conditions. It was found that the flexural strength and modulus were higher along the weft direction, where high in-plane permeability of the reinforcement was observed, due to fiber alignment. Impact tests on composite plates based on the three types of fabric indicated that the orthogonal system offered a slightly higher perforation resistance and lower levels of damage at any given energy.

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.

Singularity-Free Workspace Aimed Optimal Design of a 2T2R Parallel Mechanism for Automated Fiber Placement

A bstract: This paper introduces a new concept of applying a parallel mechanism in automated fiber placement for aerospace part manufacturing. By investigating the system requirements, a 4-DOF parallel mechanism consisting of two RPS(revolute-prismatic-spherical joints) and two UPS(universal-prismatic-spherical joints) limbs with two rotational and two translational motions is proposed. Both inverse and forward kinematics models are obtained and solved analytically. Based on the overall Jacobian matrix in screw theory, singularity loci are presented and the singularity-free workspace is correspondingly illustrated. To maximize the singularity-free workspace, locations of the two UPS limbs with the platform and base sizes are used in the optimization which gives a new design of a 4-DOF parallel mechanism. A dimensionless Jacobian matrix is also defined and its condition number is used for optimizing the kinematics performance in the optimization process. A numerical example is presented with physical constraint considerations of a test bed design for automated fiber placement.

The energy-absorbing characteristics of polymer foams reinforced with bamboo tubes

This paper investigates for the first time, the energy-absorbing characteristics of a range of lightweight bamboo-reinforced foam structures. Initial attention focuses on characterizing the energy-absorbing characteristics of the individual bamboo tubes and assessing the influence of tube geometry on energy absorption. Here, it has been shown that the specific energy absorption of the bamboo tubes decreases with increasing tube length as a result of axial splitting associated with barreling of the samples under compressive loading. The influence of the tube inner diameter/thickness (D/t) ratio was also investigated using a number of tube sizes, where it was shown that there is a small increase in specific energy absorption with decreasing D/t. The tubes were then embedded in crosslinked PVC foams in order to investigate the influence of varying degree of external support applied to the reinforcement on the failure modes in the tubes as well as the measured specific energy absorption values. Finally, alternative techniques for enhancing the energy-absorbing capacity of the tubes were investigated. Here, tubes of different length were wrapped circumferentially in epoxy-impregnated kenaf fibers to enhance their resistance to axial spitting. It was shown that reinforcing the tubes in this manner can significantly enhance the ability of the tubes to absorb energy under conditions of axial quasi-static crushing.

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.

Modelling Liquid Composite Moulding Processes Employing Wood Fibre Mat Reinforcements

Liquid Composite Moulding (LCM) processes are commonly used techniques for the manufacture of advanced composite structures. This study explores the potential of wood fibres as reinforcement for LCM preforms, considering mats produced using dry and wet methods. The compaction response of these mats has been investigated with and without the presence of a test fluid. Permeability of these mats was also measured as a function of fibre volume fraction. Reinforcement permeability and compaction response data were used to model two different LCM processes. The simulation results have been compared with experiments.

Electrospinning: Current Status and Future Trends

With the emerging nanotechnology, nanoscaled materials have drawn much attention to wide research communities for many years. Nanoscaled fibrous materials offer a multitude of fascinating features such as high surface area-to-volume ratio and tuneable porosity, making them attractive for widespread applications. Among different methods for nanofibre fabrication, electrospinning is a simple and versatile process for generating ultrathin fibres from a variety of polymeric materials or composites. This chapter gives a holistic review on current approaches and developments in electrospinning and its future trends in manufacturing advanced materials.

The energy-absorbing properties of internally reinforced composite-metal cylinders with various diameter-to-thickness ratios

In order to enhance the energy-absorbing behaviour of both new and existing metallic structures, metal cylinders are reinforced by inserting an internal composite cylinder. Initially, the energy-absorbing characteristics of the individual composite and metal rings with a large variety of the diameter-to-thickness ratios is assessed separately through a series of compression crushing tests, prior to testing the metal-composite hybrids. It was found that the values of specific energy absorption for the composite rings varied from 48.1 to 93.3 kJ/kg and those for the aluminium systems from approximately 20.9 to 70.1 kJ/kg. Here, the smaller aluminium rings failed in a localised buckling mode, whereas some of the larger rings failed in a low-energy fracture mode. In contrast, the small diameter composite samples failed in a combination of splaying and fragmentation modes, reducing the rings to small fragments and fine debris. The larger diameter rings failed in a delamination/splitting mode involving lower levels of energy absorption. However, the specific energy absorption values for the aluminium-composite structures were higher than the individual aluminium rings. The percentage increase in specific energy absorption ranged from 20 to 70% as the outer ring diameter was increased from approximately 19 to 66 mm. This evidence suggests that composite cylinders and tubing can be inserted into new or existing metallic components in order to greatly enhance their energy-absorbing behaviour.