Caroline Baillie

Review: Current international research into cellulosic fibres and composites

The following paper summarises a number of international research projects being undertaken to understand the mechanical properties of natural cellulose fibres and composite materials. In particular the use of novel techniques, such as Raman spectroscopy, synchrotron x-ray and half-fringe photoelastic methods of measuring the physical and micromechanical properties of cellulose fibres is reported. Current single fibre testing procedures are also reviewed with emphasis on the end-use in papermaking. The techniques involved in chemically modifying fibres to improve interfacial adhesion in composites are also reviewed, and the use of novel fibre sources such as bacterial and animal cellulose. It is found that there is overlap in current international research into this area, and that there are complementary approaches and therefore further combining of these may make further progress possible. In particular a need to measure locally the adhesion properties and deformation processes of fibres in composites, with different chemical treatments, ought to be a focus of future research.

Effects of environmental conditions on mechanical and physical properties of flax fibers

The environmental degradation behaviour of flax fibers and their mechanical properties were investigated. Upgraded Duralin flax fibers, which have been treated by a novel treatment process for improved moisture and rot sensitivity, were studied. Results showed that upgraded Duralin flax fibers absorbed less moisture than untreated Green flax fibers, whereas the mechanical properties of the upgraded fibers were retained with moisture absorption, if not improved. In addition electrochemical studies were conducted on these fibers. These data agreed well with conventional moisture absorption data. Zeta (ζ)-potential measurements at different pH-levels showed differences for Duralin fibers, which can be attributed to differences in morphological features.

On the mechanical properties, deformation and fracture of a natural fibre/recycled polymer composite

Abstract A composite laminate based on natural flax fibre and recycled high density polyethylene was manufactured by a hand lay-up and compression moulding technique. The mechanical properties of the composite were assessed under tensile and impact loading. Changes in the stress–strain characteristics, of yield stress, tensile strength, and tensile (Youngs) modulus, of ductility and toughness, all as a function of fibre content were determined experimentally. A significant enhancement of toughness of the composite can be qualitatively explained in terms of the principal deformation and failure mechanisms identified by optical microscopy and scanning electron microscopy. These mechanisms were dominated by delamination cracking, by crack bridging processes, and by extensive plastic flow of polymer-rich layers and matrix deformation around fibres. Improvements in strength and stiffness combined with high toughness can be achieved by varying the fibre volume fraction and controlling the bonding between layers of the composite.

A study of transcrystallinity and its effect on the interface in flax fibre reinforced composite materials

Abstract Cellulose fibres have long been used in the plastics industry as cost-cutting materials. Nowadays they are recognised as a potential replacement for glass fibres for use as reinforcing agents in composite materials. They have a number of certain advantages over glass fibres, such as low cost, high strength-to-weight ratio, biodegradability and ease of processing. In this study crystallisation from the melt of two different isotactic polypropylene matrices (iPP) in the presence of flax ( Linum usitatissinum ) fibres of four different types (green flax, dew retted flax, Duralin ® treated flax and stearic acid sized flax) was examined. The effect of processing parameters such as crystallisation temperature and cooling rate was investigated using hot stage optical microscopy. Differential scanning calorimetry (DSC) was used to investigate the inner morphology of the transcrystalline (TC) layer. Scanning electron microscopy (SEM) and X-ray diffraction were used in an attempt to identify the origin of the TC layer in connection with the structural characteristics of the fibres. The effect of transcrystallinity upon the mechanical properties of the interface was assessed using the single fibre fragmentation test. It was found that the interfacial adhesion is improved by the presence of a TC layer.

Engineering and characterisation of the interface in flax fibre/polypropylene composite materials. Part II. The effect of surface treatments on the interface

Abstract Natural fibres have attracted much attention recently for use as reinforcing agents in composite materials. However, even though natural fibres possess many advantages over glass fibres, such as lower density, lower cost and recycleability, they are not totally free of problems. Natural fibres are comprised mostly of cellulose, a highly hydrophilic macromolecule with strong polarity and, as a result, problems of compatibility with very apolar matrices (e.g. polyolefins) almost certainly arise. Surface treatments, although having a negative impact on economics, may improve the compatibility and strengthen the interface in natural fibre composite materials. In Part I of the present study two such surface treatments, acetylation and stearation, have been developed and applied to flax fibres. In this second part, the effect of these treatments upon the interface of flax fibre/polypropylene composites is assessed by means of fragmentation tests. It has been found that both treatments led to improvement of the stress transfer efficiency at the interface, and both applied treatments were optimised, accordingly.

Studies of microstructural and mechanical properties of nylon/glass composite : Part I The effect of thermal processing on crystallinity, transcrystallinity and crystal phases

Experimental investigations were conducted to study the effect of thermal processing on microstructures of GF/PA6 composites. Different degree of crystallinity and phases of the thermoplastic matrix, and transcrystallinity of the composite interfaces were achieved at different cooling rates during the thermal processing. XRD and DSC results indicated that when the cooling rate was varied from fast to slow, the crystallinity and the ratio of the α/γ phases of nylon6 were increased in the PA6 matrix. The microscopy observations showed that columnar spherulites grew along the glass fibre in the slow cooled thin film samples associated with larger diameter of spherulite structures in the matrix. The columnar spherulite structures around the fibres may be transcrystalline layers and they disappeared with increasing cooling rate during thermal processing. It was also found that samples with large amount of voids and poor interfacial bond was found in the low holding pressure samples during the thermal processing.

Building knowledge about the interface

Abstract In order to explore the current views of the interface community and to develop an understanding of the present state of play, research specialisation (from education research) which studies knowledge development, has been applied to research variation in the communitys ways of experiencing the ‘interface’ as a phenomenon in composite materials. Ten leaders of the field were interviewed and the transcripts of those interviews collated and analysed. Four dimensions of variation in the ways researchers view the interface were uncovered. It is found that there is not even general agreement as to whether the interface is an object, which can be measured, or an illusive concept. The resulting ‘outcome space’ or ‘collective consciousness’ may lead to a broader understanding of the issues involved.

The ‘Power Test’: its impact on student learning in a materials science course for engineering students

Abstract This paper explores the impact on students’ learning of a change in assessment in a materials science course for engineers. The theory behind the move is discussed with reference to previous work on developing deeper approaches to learning in students at university. The new concept adopted was a ‘power test‘ — an open book format final exam, with extended time and opportunities for colleague interaction. Student response was evaluated by interviews and structured group discussions using a nominal group technique and outcomes were assessed by classification of student exam answers using the SOLO taxonomy of Biggs & Collis (1982). Results were compared with those of students taking the same course in the previous year but who were assessed by standard closed book exam. Problems associated with the introduction of such a novel course assessment to an otherwise traditional degree are discussed.

Studies of microstructural and mechanical properties of Nylon/Glass composite Part II The effect of microstructures on mechanical and interfacial properties

This is a part of a series of studies on the influence of thermal processing on microstructures and mechanical properties of thermoplastic composites. In this paper, the effect of cooling rate during thermal moulding processes on the mechanical properties of bulk unidirectional commingled yarn GF/PA6 composites (Iosipescu shear strength, transverse flexural tensile strength and elastic modulus) has been investigated. Cooling rate from fast to slow, −60°C/min, −3°C/min and −1°C/min, were achieved at 1.5 MPa pressure. Scanning electron microscopy (SEM) was used to analyse the damaging mechanisms of the fracture surfaces of the tested samples. The different dynamic responses of the samples were observed by polarised optical microscopy (POM) during the mechanical tests. The results indicated that when the cooling rate was varied from fast to slow, the interfacial tensile and shear strength were improved associated with enhanced elastic modulus. These results may be attributed to the slow cooling achieved a high transcrystallinity between the glass fibres and PA6 matrix, and high crystallinity of α phase in the PA6 matrix.

Effects of crystallinity, transcrystallinity and crystal phases of GF/PA on friction and wear mechanisms

Relationships between manufacturing processes, microstructures, mechanical and tribological properties of GF/PA6 composite were revealed in this paper by experimental investigations and theoretical analysis. The study indicated that the wear resistance was significantly influenced by the thermal history of the composite. Decreasing cooling rate during the components thermal moulding would result in enhancing approximate 28% of wear resistance and reducing almost 14% of friction in the GF/PA6 composites. A slow cooling gives the PA6 a high crystallinity and high ratio of α/γ phases which resulted in higher thermal stabilities, higher density and harder matrix in the composites than that of fast cooling. It also improves the interfacial properties and the composite hardness. Due to the thermal decomposition, abrasion and adhesion dominated the wear mechanisms, these improvements led to a higher wear resistance and higher pv service limit in the slow cooled GF/PA6 samples than that of fast cooled samples. Interfacial debonding dominated the friction mechanisms in the unlubricated pin-on-disk tests. A poor interfacial bond led to more glass fragments falling off from the GF/PA6 pins which scratched the sliding surfaces, damaged the oxidative film and resulted in severe abrasion and adhesion in the fast cooled samples, hence leading to a high friction coefficient.