Deformation micromechanics in high-modulus fibres and composites

It is demonstrated that Raman spectroscopy can be used to study the deformation micromechanics of aramid fibres and of the fibres in a model single-fibre composite with an epoxy resin matrix. It is shown that the peak position of the 1610cm⁻¹ aramid Raman band shifts to lower frequency under the action of stress or strain as a result of the macroscopic deformation leading to direct stretching of the aramid molecules. The strain-induced band shifts can be used to follow the deformation of the aramid fibres in a composite matrix. This allows the distribution of strain to be mapped along a fibre, and it is shown that the behaviour is consistent with that predicted by the classical shear-lag analysis. It is also demonstrated that the interfacial shear stress can be calculated from the distribution of strain along the fibre. Finally, the technique is extended to measure the strain in fibres in a single-fibre composite which are aligned at an angle to the tensile axis. In this case it is shown that the strain in the centre of the fibres is identical to that predicted by classical elasticity theory.     

Measuring the elastic properties of high-modulus fibres

The measurement of the elastic constants of several highly oriented thermoplastic polymer fibres is described. The method makes use of the hot-compaction process, developed and patented in this laboratory, which enables a solid section of highly oriented polymer to be produced from an aggregate of highly oriented fibres. As only a small fraction of the original fibre is melted and recrystallized during the process, the compacted materials offer a unique opportunity for measuring fibre properties in the bulk. An ultrasonic immersion technique is used to measure the elastic properties of the compacted materials, from which the properties of the polymer fibres are inferred. The experimentally determined fibre elastic properties have been compared with other oriented polymer materials to assess any similarities in elastic anisotropy between different methods for producing fibre orientation, and compared with theoretical upper limits for the fibre elastic properties based on theoretical estimates for the polymer crystal unit cell appropriately averaged for hexagonal symmetry using the aggregate model.     

On the internal morphologies of high-modulus polyethylene and polypropylene fibres

High modulus polyethylene fibres, both melt and gel-spun, contain the same longitudinal density deficient regions, revealed by permanganic etching, as were found in specimens of the same fibres after treatment by the Leeds high temperature compaction process. For these materials a new model of fibre structure is proposed, which develops as a consequence of nucleation on an extended network of entangled molecules permeating the fibres. Subsequent growth into spaces between the network will encounter stresses due to the contraction on crystallization leading to distributed density deficient regions of high free volume. Each of the four commercial PE fibres examined differs from the others in its characteristic outline and the details of its internal substructure. The structure of commercial polypropylene fibres is also compared.     

Differential melting in compacted high-modulus melt-spun polyethylene fibres

The compaction of high-modulus melt-spun polyethylene fibres has been investigated for compaction temperatures above the optimum. After such treatment the specimens are liable to be non-uniform because of differential melting. Individual compacted fibres are observed to melt not only from the outside inwards, but also in certain internal regions, depending upon the availability of local free volume. The regions of different stability have been identified and inferences drawn concerning the structure of the initial fibres. It is suggested in particular that the longitudinal regions of deficit density (which exhibit cratering in transverse sections and melt before their surroundings) are a result of initial crystallization occurring within a rigid framework inside the fibre, possibly nucleated on a strained molecular network. The presence of banded recrystallization around residual fibres demonstrates that this phenomenon develops via interaction of neighbouring lamellae as they grow.     

Phase transformation of inviscid melt spun (IMS) alumina-zirconia eutectic fibres

Al₂O₃-ZrO₂ (AZ) eutectic fibres containing 41.05 wt% ZrO₂ and 2.03 wt% Y₂O₃ were produced via inviscid melt spinning (IMS). Scanning electron microscopy (SEM) was used to examine the morphology of the as-spun AZ fibre. Differential thermal analysis (DTA) and X-ray diffraction (XRD) were used to investigate the phase transformations in this fibre. The XRD pattern of the as-spun AZ fibre showed tetragonal ZrO₂, -Al₂O₃, and non-equilibrium -Al₂O₃ phase formation. The DTA curve of the as-spun AZ fibre showed only one endothermic peak representing the phase transformation of -Al₂O₃ to -Al₂O₃. This phase transformation was confirmed by analysing the XRD pattern of heat-treated AZ fibre.     

Flexural fatigue and surface abrasion of Kevlar-29 and other high-modulus fibres

This investigation deals with some flexural fatigue and abrasion studies of Kevlar-29, glass and carbon fibres. The test methods included in the study are fatigue by pure flexing, buckling and rotation over a wire, and abrasion by rubbing against a rotating rod. Kevlar-29 fibres were found to perform well in these tests because they could survive the relatively high bending strains by yielding in axial compression. Carbon and glass fibres, although unable to survive at these high strains, did perform well when very low bending strains and tensions were used. Kevlar-29 fibres were found to be less abrasion-resistant than glass fibres, probably because of their low radial strength. The fracture morphologies of Kevlar-29 fibres in nearly all these tests showed axial splitting, confirming indications of low strength in the fibre perpendicular to its axis.     

The mechanism of reinforcement of polyurethane foam by high-modulus chopped fibres

The morphology and fracture behaviour of polyurethane foams reinforced by short chopped fibres have been investigated. The presence of the fibres is shown to give rise to localized change in the foam morphology and the extent of this depends upon the fibre bundle size which is affected by the surface treatment. The changes in morphology are correlated with changes in the tensile properties of the foams at ambient and cryogenic temperatures. The systems are shown to be matrix limited with failure occurring remote from the interface which assumes a poassive role during tensile fracture. A critical fibre length for reinforcement of polyurethane foam, which depends on matrix shear strength and foam density, is defined.     

A study on the achievement of high-modulus polyethylene fibres by drawing

Several methods of preparing spherulitic sheets of high-density polyethylene from which high draw ratios (∼ 30x) and high moduli (∼ 800 kbar) may be obtained are described. It is shown that, independent of the method of preparation of the initial sheet, and provided certain conditions are met, the modulus is a unique function of draw ratio. The maximum draw ratio (and hence modulus) achievable from a particular sheet is shown to depend on its morphology and its molecular weight distribution; in particular, the presence of some segregated low molecular weight material appears to be essential. When viewed in the polarizing microscope a “black” region often is observed bounding the spherulites, particularly in those sheets which give high draw ratios. This region is correlated with segregated low molecular weight material. In addition, the recovery properties of high modulus fibres are reported, both after isothermal strain and on annealing when near complete recovery is observed.     

Compaction of high-modulus melt-spun polyethylene fibres at temperatures above and below the optimum

In the process of hot compaction developed at the University of Leeds, high-modulus fibres are compacted to form coherent thick-section products with stiffnesses unobtainable by current processing techniques. Using high-modulus polyethylene fibres (trade name TENFOR) produced by the melt-spinning/hot-drawing route as the starting material, it was discovered that under optimum conditions of pressure and temperature it is possible controllably to melt a small proportion of each fibre. On cooling, this molten material recrystallizes to bind the structure together and fill all the interstitial voids in the sample, leading to a substantial retention of the original fibre properties. For a hexagonal close-packed array of cylinders, only 10% of melted material is needed for this purpose. If the compaction temperature is too low, there is insufficient melt to fill the interstices, the fibres deform into polygonal shapes, and insufficient transverse strength is developed. Above the optimum temperature, the proportion of melt increases, causing the stiffness of the composite to be reduced. The recrystallization of the melt is nucleated on the oriented fibres, giving similarly oriented cylindrulitic growth. Where the regions of melt are large enough, and cooling sufficiently rapid, development away from the nucleus is accompanied by a cooperative rotation in chain orientation, analogous to banding in spherulites.     

Solid phase change controlling the tensile and creep behaviour of gel-spun high-modulus polyethylene fibres

Tensile and creep tests have been conducted on monofilaments of gel-spun high-modulus polyethylene fibres. Fibre strength has been determined over a temperature range –175–100 °C. The creep studies have revealed changes in behaviour which depend on the applied stress and the temperature. The results of these studies are explained by a change in crystallographic structure from orthorhombic to hexagonal which can take place under certain conditions of temperature and applied stress. It has therefore been possible to determine the applied stress necessary to obtain the change of phase as a function of temperature.     

Structure and properties of high-modulus alloys of the Ti-B system

We study the influence of alloying with Al, Zr, and Si on the structure and mechanical properties (including the modulus of elasticity) of cast and deformed alloys of the Ti-B system. It is shown that, by optimizing the compositions of titanium alloys subjected to combined silicoboride hardening and additionally alloyed with aluminum, it is possible to get the modulus of elasticity E as high as 160 MPa with a strength σ of 1500 MPa and a level of plasticity δ of 2–5% at room temperature. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 42, No. 3, pp. 27–32, May–June, 2006.     

The compressive deformation of cross-linkable PPXTA fibres

A study has been conducted on the compressive deformation behaviour of thermally cross-linkable poly(p-1,2-dihydrocyclobuta phenylene terephthalamide) (PPXTA) fibres. The morphology of the failure zones was examined by scanning electron microscopy and dark-field transmission electron microscopy. On increasing the heat-treatment temperature from 260–400°C, and therefore with increasing cross-link density, fewer kinks per unit length were displayed after compressive deformation. The kink specific energy was estimated to increase by a factor of 30, as determined by quantitative measurements of kink density at a given strain and of the critical strain to kink formation. Thus, the intermolecular cross-links still allowed deformation to proceed by kinking, but significantly raised the energy of kink formation. Finally, rupture zones were predominantly observed in axially compressed PPXTA fibres heat-treated at 440°C. Compressive failure of the fibres changed from kink-dominated failure to brittle rupture with increased heat-treatment temperature, evidently as the result of cross-linking or of chain degradation. A dislocation model of the kink boundary developed by Vladimirov et al. was analysed and critically compared with our data. The analysis of this theory with our experimental results suggested that the dramatic change in compressive behaviour with cross-linking was due to a transition from fine intermolecular shear to blocky interfibrillar shear. This revised version was published online in November 2006 with corrections to the Cover Date.     

The lateral deformation of cross-linkable PPXTA fibres

The lateral deformation properties of oriented polymer fibres were examined by transverse compressive and torsional experiments. A modified interfacial test system machine was used to study the transverse compressive deformation behaviour of thermally cross-linkable poly(p-1,2-dihydrocyclobutaphenylene terephthalamide) (PPXTA) fibres and of a number of commercially available polymers (Nomex, nylon, Kevlar, Dacron) and ceramic (Nicalon and FP) fibres. The torsional (shear) modulus G of PPXTA and Kevlar poly(p-phenylene terephthalamide) (PPTA) fibres was measured by pendulum experiments. During both fibre torsion and transverse compression, the deformation involves materials slip on (h k 0) planes, in the [0 0 1] direction for the torsion and the [h k 0] directions for transverse compression. The intermolecular crosslinks in PPXTA did not significantly modify the elastic transverse modulus Et and caused only slight (13%) increase in shear modulus G. However, the plastic transverse properties of cross-linked PPXTA were significantly different than those of uncross-linked PPXTA. The stress at the proportional limit σp, determined from the transverse load–displacement curves, was substantially higher for the cross-linked fibres than for the uncross-linked fibres. In addition, the cross-linked PPXTA fibres exhibited a large strain recoverable response reminiscent of elastomers, whereas the PPTA and uncross-linked PPXTA fibres exhibited a large strain irreversible response. This revised version was published online in November 2006 with corrections to the Cover Date.     

Comparative study of zirconia-mullite and alumina-zirconia composites

Several zirconia-mullite and alumina-zirconia composites were prepared and their physical, chemical, mechanical and thermal properties determined. The phase assemblage, size of ZrO₂ crystals and microstructure of the composites were ascertained. This paper embodies a comparison of the alumina-zirconia and zirconia-mullite composites with respect to their properties and also the constitution-property relationships between each group of composites. Alumina-zirconia composites (AZ) were found to be superior to zirconia-mullite composites (prepared through ZMS or ZMC route) in almost all respects. Thermal-shock resistance and hydration resistance of the clay-based zirconia-mullite (ZMC) composites and alumina-zirconia (AZ) composites were almost at par.     

Structure and properties of coir fibres

In this paper, early research on the structure and properties of coir fibres has been critically reviewed. Gaps in the scientific information on the structure and properties of coir fibre have been identified. Attempts made to fill some of these gaps include the evaluation of mechanical properties (as functions of the retting process, fibre diameter and gauge lengths of fibre, as well as of the strain rates) and fracture mechanisms using optical and scanning electron microscopy. The deformation mechanism of coir fibre resulting in certain observed properties has been discussed with the existing knowledge of the structure of plant fibres as a basis. It is concluded that more refined models need to be developed for explaining the observed mechanical properties of coir fibres. Some of the suggestions for further work include relating properties of fibres to factors like the chemical composition of the fibre and the size and number of cells, size of lumen, variation in micro-fibril angle within each cell and between different cells of the same fibre, and understanding the deformation of the whole fibre in terms of deformation of individual micro-components. Further work is required on the effects of mechanical, thermal and thermomechanical, chemical treatments to modify the structure and mechanical properties of these fibres in such a way as to make them more suitable as reinforcements in polymer, clay and cement matrices.     

Molecular deformation processes in gel-spun polyethylene fibres

The structure/property relationships in the PE fibres have been interpreted quantitatively using a microfibrillar model and the low-strain mechanical properties have been analysed using the Takayanagi models. Information obtained from Raman spectroscopy in the previous paper has been analysed to determine the molecular deformation behaviour of the gel-spun polyethylene (PE) fibres. It is demonstrated that there is a bimodal distribution of stress in the crystalline regions due to the two-phase microstructure of the fibres and it has been shown that the molecular deformation behaviour can be interpreted quantitatively using a parallel-series model. It is found that the Young's modulus of the crystalline regions increases with the degree of chain extension and for the highest-modulus fibres may be close to the theoretical modulus of polyethylene. The fibre modulus is reduced by the presence of low-modulus non-crystalline material in parallel with the crystals.     

Crystallization of gel-derived alumina and alumina-zirconia ceramics

The crystallization behaviour and phase relations in gel-derived alumina and alumina-zirconia ceramics has been investigated. Zirconia was found to form a limited metastable solid solution with the alumina matrix. When present in solid solution, zirconia phase appeared to enhance rapid growth of corundum grains. There was no apparent grain refinement in the alumina-zirconia composites. The implications of microporous gel structures on the modification of microstructures in these gel-derived ceramics are discussed.