Fibre orientation structures and their effect on crack resistance of injection moulded transverse ribbed plate

Abstract

An extensive study of the fibre orientation structures developed in a transverse ribbed plate during injection moulding, and the use of these structures to investigate the effect of local fibre orientation state on crack initiation resistance, is reported. The fibre orientation results for the ribbed plate, measured using large area image analysis system developed at Leeds University, showed that after an initial settling down period, the central core region, where the fibres are aligned perpendicular to the flow direction, decreased in size monotonically, with an associated monotonic increase in the outer shell regions, where the fibres are aligned preferentially along the injection direction. Interestingly, the level of orientation in the two regions remained almost constant: only the proportions of the two regions were found to change with flow length. Across the plate, close to the gate, the central core region was found to have a lens-like shape, while at the other end of the plate the core was thinner and also consistent in thickness across the sample width. The transverse rib was found to cause little disturbance to the fibre orientation of the base plate. The different proportions of the shell and core regions at different locations over the ribbed plate provided an ideal case to test the proposition of Friedrich that the crack resistance of a short fibre reinforced material depends on the number of fibres that are perpendicular to the crack tip. The impact test results gathered in this way confirmed this hypothesis of Friedrich.     

Electrochemical rehabilitation methods for reinforced concrete structures: advantages and pitfalls

Abstract

There are still many unresolved issues concerning the efficiency of electrochemical realkalisation and electrochemical chloride removal as electrochemical rehabilitation methods for corroding reinforced concrete structures. The present paper seeks to answer a number of questions which, though seemingly elementary, continue to arouse controversy in scientific, technical and economic communities, despite the vast amount of work that has been devoted to research on corrosion in concrete embedded steel, such as whether corrosion can be stopped once it has started, whether corroded reinforced concrete structures can be repassivated, and whether it is sufficient to remove the sources of corrosion in order to stop rusting. A discussion is conducted on the relationship between the prerusting grade of rebars and the possibility of their repassivation; on whether electrochemical rehabilitation methods treatments are efficient, and if so, when and on whether a simple potential measurement can determine the passive or active state of a rebar. For this purpose an analysis is made, using electrochemical, gravimetric and metallographic techniques, of the response of clean and previously corroded steel electrodes in a Ca(OH)₂ saturated solution with and without a sodium nitrite corrosion inhibitor and in cement mortar. The effectiveness of electrochemical realkalisation and electrochemical chloride removal for repassivating prerusted steel in concrete is found to depend heavily on the prerusting grade.     

Effect of Air-Jet Texturing on Adhesion Behaviour of Technical Polyester Yarns to Rubber

Air-jet texturing of technical polyester yarns was performed in order to improve its adhesion to rubber. The air-jet texturing parameters were selected with great care to minimize the mechanical loss. H-adhesion tests were used to characterize the adhesion of the yarns to rubber. A significant increase in the adhesion of dimensionally stable polyethylene terephthalate yarn, textured with an overfeed level of 15% (DSPET15), was recorded, while a decrease in the adhesion of high tenacity polyethylene terephthalate (HTPET) yarn was observed for all overfeed levels. The effects of air-jet texturing on the adhesion of technical polyester yarns were discussed in terms of changes in the yarn geometry and changes on the single fiber surfaces. Changes in the yarn geometry were investigated by optical microscopy studies, while changes on the fiber surface were investigated by scanning electron microscopy (SEM), atomic force microscopy (AFM) and environmental scanning electron microscopy (ESEM) studies. It was observed that air-jet texturing alters both the yarn geometry and the single fiber surfaces, leading to a change in the adhesion to rubber.     

Functionalisation of polypropylene by solid phase graft polymerisation and its effect on the mechanical properties of silica nanocomposites

Abstract

To prepare macromolecular compatibiliser for grafted nano-SiO₂/polypropylene (PP) composites, solid phase graft copolymers of PP with styrene and ethyl acrylate were synthesised, respectively. It was found that both percentage grafting and grafting efficiency can be adjusted by changing initiator concentration, reaction temperature and reaction time. Due to partial chain scission and deterioration of the ordered structure of PP during the graft polymerisation, the grafted PP exhibits worse thermal stability and crystallisability than the unmodified PP. Mechanical tests of grafted nano-SiO₂/PP composites indicated that the addition of PP copolymer with the same species of grafting polymer as that on the nanoparticles further improves the ductility of the composites. Molecular rigidity of the grafting polymers, presence of the homopolymer produced during the graft polymerisation, and strain rate of the load applied have important influences on the toughening effect of the functionalised PP.     

Reinforced thermoplastic pipe using recycled PET reinforcement

Abstract

The present study describes the results of a pilot study to carry out the initial development and testing of a new class of high-pressure all thermoplastic pipe, using recycled PET as reinforcement. A prototype pipe design has been manufactured and initial qualification testing carried out demonstrating a new high added value product application for recycled materials.     

Mode I quasi–static and fatigue delamination characterisation of polymer composites for wind turbine blade applications

Abstract

Ambitious electricity generation targets from renewable sources set by many governments have lead to the rapid growth of the wind energy industry over the last two decades. The need for larger wind turbine blades for increasing energy generation has considerably increased the demand and use of high performance composites in wind turbine applications, mainly in blades. A common type of failure in composite materials is delamination of adjacent layers, which can occur either due to manufacturing inconsistencies or due to in service loads. Understanding and characterising delamination is very important in order to implement damage tolerant design methodologies. The present research work focuses on the assessment of the delamination behaviour of composites for wind turbine applications. Several composite systems were tested and their fracture toughness and fatigue delamination propagation behaviour under mode I (peeling) loading conditions were evaluated. Quasi–static tests were performed and delamination initiation values were evaluated. Fatigue delamination growth rate curves (da/dN versus G _Imax) were also produced. The carbon/epoxy and glass/epoxy material systems tested were compared in terms of resin type, fibre type and interfacial characteristics.     

Fracture mechanics testing of the bond between composite overlays and a concrete substrate

This paper presents a methodology for assessing the bond strength of composite overlays to concrete utilizing a fracture toughness test. The principles and practices of existing ASTM standards for determining the fracture toughness of adhesive bonds between double cantilever beam (DCB) metallic and composite specimens (D 3433-93 and D 5528-94a) have been extended to cover the case of an elastic composite layer bonded to a rigid concrete/masonry substrate. In the theoretical section, the dominant loading conditions, relevant ASTM standards, and the development of energy release rate concepts for analyzing a disbonding composite layer modeled as an elastic cantilever beam are presented. The experimental section covers specimen fabrication and preparation, experimental setup, test procedures, post-test evaluation of the specimens, and data processing. The discussion of test results focuses on explaining the variability in measured strain energy release rate, and identifies trends between the measured strain energy release rate and the fraction of the fracture surface retaining cement paste after disbonding. It was found that good-quality composite-to-concrete bond is associated with high fracture toughness of the adhesive and location of the crack path in the concrete substrate. Strict enforcement of surface preparation and adhesive handling procedures was found to play an important role in promoting good bond strength and high fracture toughness. The fracture toughness test developed in this paper can be used for screening various composite-repair systems, to assess the effect of different environmental attacks, and as a quality control tool.     

Microstructure and physical properties of Al/diamond composite fabricated by pressureless infiltration

Abstract

The Al/diamond composite was fabricated using a pressureless infiltration method. The microstructure and physical properties of the composite were investigated. The composite has a very low coefficient of thermal expansion (CTE) of 3·9 × 10^?6 K^?1. The thermal conductivity (TC) of the composite is 12% higher than that of the Al alloy matrix. The lower TC of the composite than the expected value was attributed to the existence of interfacial low conducting phases and the porocity of the composite.     

Novel approach to qualification of non-metallic pipe systems – as applied to reinforced thermoplastic pipe

Abstract

The present study discusses the possibilities offered by ramp pressure loading tests to generate data for determining the long term load-bearing characteristics of plastic pipe systems. In addition to ramp pressure loading, procedures are also considered where the pressure is held at a constant value for a period of time, after which the product is subjected to a ramped pressure burst test. The method discussed relies on the material in question obeying Miner's Law in its static fatigue behaviour. The pipe system to which the procedure has been applied is Reinforced Thermoplastic Pipe (RTP), in which the reinforcement is aramid fibre. However, the results are expected to apply to any polymeric system where there is evidence of conformity to Miner's Law. A set of ramp loading tests, and constant pressure plus burst tests are reported and compared with the results of conventional 'constant pressure' stress rupture tests and a method is proposed for converting these results into 'equivalent' constant pressure values. The results obtained on RTP using this new approach lie close to those generated using constant pressure.     

Reducing performance variations of compression moulded long glass fibre reinforced thermoplastics by targeted reinforcement

Abstract

Long glass fibre compression mouldings are prone to significant variations in mechanical properties. Stiffness and strength tend to vary across an individual moulding and in addition there are also variations when different mouldings are compared. Compression mouldings in the form of a top hat section were produced. This component was first modified to have ribs, then samples of both batches were reinforced with a thin thermoplastic composite plate with continuous fibres. In this way, four different batches were produced, which were tested under 3-point-bending. Further, this work investigated the changes in mechanical properties across the manufactured top hat section mouldings. Tensile test results strongly indicate that the scatter of mechanical properties increases towards the edges of the compression moulding. When the mouldings were tested under 3-point-bending, the experimental results were prone to a large scatter and it was not possible to predict the position of failure or the failure mode. However, this work successfully demonstrated that a targeted reinforcement of the moulding with continuous fibres can significantly reduce the performance variations, particular under loading with large strain. The findings of this work could lead to the development of new lightweight structural components, where there is a need for integrating functions into the moulding in order to reduce weight and costs for mass production.