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Recent research on the mechanical properties of hot-compacted polypropylene composites is reported. It is shown that incorporation of new high performance polypropylene tapes leads to significant improvements in tensile and flexural properties. The high temperature properties of these new composites are outstanding, showing higher stiffness and very low creep under permanent load. Comparisons are presented with both chopped glass-reinforced and continuous glass-reinforced polypropylene composites.

Static tests and ultrasonic measurements (2.25 MHz) have been carried out on a series of composite laminates of glass fibres in a polypropylene matrix. A range of angle ply laminates were prepared for this study, with laminate angles θ of ±0, 10, 20, 30 and 40^∘. The high frequency measurements were made using the ultrasonic immersion technique, which allows the determination of a complete set of the elastic constants of a material. The relationship between the ultrasonically determined elastic constants of the angle ply laminates was found to be in excellent agreement with theoretical predictions, as previously validated for carbon fibre/epoxy angle ply laminates. A comparison between the ultrasonic and statically measured values was made for two of the angle ply laminates (θ = 0 and 20^∘). It was found that the static values were lower than those measured at ultrasonic frequency, particularly those constants that were more matrix dominated (for example the transverse moduli of the laminates). Measurements on a pure polypropylene sample at both testing frequencies confirmed that the change in matrix properties with frequency was the cause of this difference. The change in properties with test frequency is likely to be much larger in this system than in other composite materials because the glass transition temperature of polypropylene is close to ambient temperature. Dynamic mechanical tests (1 Hz) were carried out on a sample of pure polypropylene to assess this effect. We also give an appropriate method of estimating the dependence of glass transition temperature on frequency. The results for polypropylene are compared with those for other commonly used polymer matrix materials: epoxy resin, nylon and polyetheretherketone (PEEK): DMTA measurements were also made on these samples. The effect of test frequency on matrix properties, for the glass/PP laminates, was further investigated by examining the relationship of the Poisson's ratios with laminate angle using a mixture of ultrasonic experiments and theoretical predictions. Previously we have shown that the degree of anisotropy between the reinforcing fibre and the matrix phase is paramount in determining whether the material will show a negative Poisson's ratio at a critical laminate angle. The ultrasonic measurements carried out in this study on the glass/PP laminates showed a minimum in one of the Poisson's ratio at a laminate angle of 32°, but the value did not become negative. However, theoretical predictions showed that for a static frequency measurement (1 Hz), where the matrix is softer and hence the anisotropy of each laminate ply is higher, the laminate will show a negative Poisson's ratio with a minimum at a laminate angle of around 28°.

A detailed study has been undertaken of the anisotropic mechanical behaviour of oriented hydroxyapatite reinforced polymer composite (HAPEXTM). The oriented material was produced by hydrostatic extrusion of isotropic HAPEXTM. It was found that the tensile and shear moduli and strengths of the oriented HAPEXTM are substantially increased compared with isotropic HAPEXTM. In particular, the properties along the extrusion direction exceed the lower range of the properties of cortical bone, showing that the oriented HAPEXTM is suitable for use in skeletal major load bearing applications.

The effect of the combination of pressure annealing and subsequent hydrostatic extrusion on some mechanical properties of composites of hydroxyapatite in a polyethylene matrix is examined. Both linear and branched polyethylenes have been used as the matrix and it is found that composites with the former can be processed to give the higher modulus. An important practical finding is that products with a Young's modulus well into the range shown by cortical bone can be produced. The critical step in the enhancement of the modulus is pressure annealing which alters the morphology of the linear polyethylene, encouraging the development of both crystallinity and crystallite size. The presence of butyl branches along the molecular backbone limits the extent to which these can be developed by pressure annealing and accounts for the failure of the process to improve the modulus of composites using the branched material. Comparison with similarly prepared samples of the pure polyethylenes shows that the development of orientation in the polyethylene is considerably restricted by the presence of hydroxyapatite particles, irrespective of whether pressure annealing is performed prior to extrusion. Consequently, the properties of these composites are less than might be expected from studies on the isolated polyethylenes.

Hydroxyapatite reinforced high density polyethylene composite (HAPEX™) has been developed for bone substitution. To improve its mechanical properties, HAPEX™ was hydrostatically extruded at different extrusion ratios after compression moulding. Substantial increases in the tensile and flexural properties of both unfilled polyethylene and HAPEX™ were achieved. It was evident that the higher the extrusion ratio, the stiffer and the stronger the extruded rods. The ductility of HAPEX™ was also significantly enhanced by hydrostatic extrusion. Hydrostatically extruded HAPEX™ possesses mechanical properties that are within the bounds for human cortical bone, which indicates its potential for load-bearing skeletal implant applications.

In this paper we describe the production, properties and morphology of hot compacted 2-dimensional woven high modulus polyethylene fibres. The aims of the work were to establish the optimum conditions for production of the compacted woven PE sheets using a combination of mechanical measurements at Leeds and morphological investigations at Reading. This joint approach had proved very successful in a previous study on the compaction of unidirectional arranged PE fibres, where the optimum compaction temperature was established as 138°C, where ∼10% of the original fibres were melted. Morphological studies clearly showed that the melted material had recrystallised, epitaxially, onto the original fibre backbones, forming a coherent network to bind fibres into a continuous structure. The current studies, using the woven PE material, showed that a higher temperature was needed to fill all the space between the woven polyethylene fibres, and so produce a coherent material. Peel tests, where two layers of cloth are compacted together and then pulled apart, were carried out over a range of compaction temperatures to measure the interlayer bond strength; this increased with increasing compaction temperature. Significantly, reasonable bond strengths were established at the optimum temperature established for the unidirectional samples (138°C measured on the mould or 136°C in the centre of the fibre assembly) which produces ∼10% melted and recrystallised material, although a higher interlayer strength was measured at higher temperatures where more of the melted phase was produced. Morphological investigations of woven samples with ∼10% melted material, showed that while the individual fibre bundles were well bonded, not all of the complicated junctions between the fibre bundles in the woven network were completely filled with melted and recrystallised material, and that a temperature 2°C higher than for 1D compactions was probably optimum. The optimum temperature was found to fall very close to the temperature at which complete melting of the fibre occurred.

The recovery behaviour after creep of oriented linear polyethylenes has been studied over the temperature range 20–60 °C. A range of samples was examined to identify the influence of draw ratio and molecular weight. It has been shown that in spite of significant differences in recovery strain level, the recovery kinetics are not affected over a wide range of the structural variations and experimental conditions. It is concluded that the time dependent recovery behaviour is consistent with a model where two thermally activated processes are acting in parallel. More exact values for the activation parameters for both processes of the model have been obtained by taking into account the time dependent distribution of the applied stress between these two processes.

The mechanical relaxation behaviour of high and linear low density polyethylenes has been examined using tensile dynamic mechanical measurements and compared to the deformation behaviour observed in tensile drawing. In this way the relationship between the mechanical relaxations and the tensile deformation has been investigated. It is shown that the brittle-ductile transition relates to the γ-relaxation while the yield behaviour is related to the interlamellar shear process and the c-shear process.

Dead loading creep and constant strain rate yield experiments have been used to study the tensile creep behaviour of three grades of isotropic polyethylene. This has provided further evidence for the existence of two yield points in isotropic polyethylene. Two different models have been used to attempt to describe this behaviour. Although the results can be described by to both the two process model of Wilding and Ward and the co-operative jump model of Fotheringham and Cherry, it appears that the two process model provides a more convincing quantitative fit to the data.

The morphology of woven polypropylene cloth compactions, prepared at a series of temperatures, has been examined by electron microscopy following permanganic etching. With increasing temperature, the interior structure of the fibres is seen to undergo progressively greater melting and recrystallization in the form of shish-kebab structures while the volume of melt surrounding the fibres increases. The regions of recrystallized extra-fibrillar melt show effects of flow-induced orientation in the form of row structures. The presence of boundaries persisting where recrystallization fronts have met is interpreted as an effect of segregation of low molecular weight material.

The deformation behaviour of four different grades of polyethylene has been examined as a function of the morphology produced by pressure annealing in the vicinity of the orthorhombic–hexagonal phase boundary. It is concluded that annealing within the hexagonal phase, to produce a “chain-extended” morphology, is appropriate for high molecular weight material as a precursor step in the production of high modulus by solid phase deformation. In low molecular weight material of the same morphology solid phase deformation does not produce improvements in mechanical behaviour. These results are interpreted in terms of the competition between increasing crystal size and the decrease in the coherence of the molecular network brought about by the pressure annealing. The critical parameter is the ratio of lamellar size to number average molecular chain length, rather than lamellar morphology. When this parameter is greater than 0.5 the molecules are incapable of completing two crystalline traverses and the network is affected detrimentally to the extent that it no longer holds the material together.

The compaction of gel-spun high molecular weight polyethylene (PE) fibre, SPECTRA 1000, has been investigated for a range of compaction temperatures between 142 °–155 °C. Differential scanning calorimetry (DSC), scanning electron microscopy (SEM) and broad-line nuclear magnetic resonance (NMR) techniques have been used to examine the structure of the compacted materials and to determine the compaction mechanisms. With increasing compaction temperature, the flexural properties of the compacted materials did not show any significant change up to 154 °C, but large changes were observed if the temperature was increased from 154 to 155 °C. DSC and SEM studies revealed that no evident surface melting and recrystallization occurred during hot compaction in the temperature range 144–154 °C, although the rigid crystalline fraction measured by NMR for all compacted materials is significantly lower than that for the original fibre. Significant transverse strength is also developed at the lower compaction temperatures, and this also only markedly increases on going from 154 to 155 °C. Structural investigations show how the fibres deform so as to interlock, and localized welding occurs, so as to bond each fibre to its neighbour. This is distinct from the melting and recrystallization at the surface of the fibres previously observed in melt spun fibres.

Investigation of the compaction of unidirectionally arranged high-tenacity polypropylene fibres is described. A combination of techniques, with the major emphasis being morphological studies, show that controllable “selective” surface melting is not achieved at a high enough proportion to give substantial fibre-to-fibre bonding, and hence good lateral strengths.

Polyethylene samples have been annealed under high pressures in order to transform the morphology and thereby make specimens, which on subsequent orientation by hydrostatic extrusion, produce high-modulus materials for comparatively low deformation ratios. The pre-extrusion morphology, as revealed by permanganic etching followed by transmission electron microscopy, is shown to be highly sensitive to the annealing conditions, especially in the vicinity of the orthorhombic/hexagonal phase boundary. The morphological changes have also been studied by gel permeation chromatography of nitrated samples and by differential scanning calorimetry. The moduli of samples, extruded to a standard extrusion ratio of 7, reflect differences in the morphology of the starting material. It is shown that the highest modulus is not obtained by annealing totally within the hexagonal phase but rather in the changeover region between it and the orthorhombic phase. Although the lamellar size can be dramatically increased by annealing within the hexagonal phase, this development occurs at the expense of reducing the integrity of the molecular network so that for high lamellar size the modulus falls and the samples become increasingly incoherent.

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.

The drawing behaviour of several syndiotactic polypropylenes is examined as a function of molecular weight and initial morphology. It is found that these materials can be drawn to a much lesser extent than comparable isotactic polypropylene. This limited drawability is attributed to the absence of an effective crystallinec-slip process, a view supported by dynamic mechanical measurements. Highest draw ratios are achieved in gel spun high molecular weight material where the degree of molecular entanglement is low enough to suppress the undesirable increase in drawing stress with extension.

An analytical method is presented which enables the elastic constants of the fibres in a unidirectional composite to be estimated from a knowledge of the elastic constants of the composite and the matrix resin. Results are presented for polyethylene fibre/epoxy resin and carbon fibre/epoxy resin composites, and it is shown that the predicted fibre constants are, in both cases, close to those obtained from other methods.

The compressive yield behaviour of poly(2,2′-m-phenylene-5,5′-bibenzimidazole) (PBI) has been studied over a wide range of temperatures. The tensile behaviour was also studied under superimposed hydrostatic pressure. In both cases wet and dry samples were examined and the results revealed the considerable effects of moisture on the mechanical properties of PBI. The results of all these tests show that PBI has a remarkably high shear yield stress at room temperature. This results in a very high compressive yield stress and a very high tensile yield stress observed under superimposed hydrostatic pressure when brittle failure from surface flaws is prevented. It is concluded on the basis of quantitative analysis that the yield mechanism in PBI at room temperature is initiation controlled, as in a metal or ceramic, rather than a velocity controlled, thermally activated, viscoelastic process which is generally considered applicable in polymers.

We report developments in the measurement of transverse elastic properties of a range of polymer fibres. The cylindrical fibre specimen is compressed across a diameter between a pair of parallel flat plates by a known load, while two quantities are measured; the width of the rectangular contact zone at one of the fibre-plate boundaries, and the degree of diametrical compression in the direction of the compressing force. Both measurements are used together with the relevant elastic solutions to give estimates of the transverse elastic modulus. For some fibres, plasticity is detected in the measurements of diametrical compression, and loading strategies are adopted to isolate this effect; the modulus values obtained from the two measurements are then in good agreement, provided the fibre is sufficiently large to permit accurate measurement of the contact zone. Finite-element modelling is used to show that the elastic-plastic behaviour is essentially understood and to estimate a value for yield stress. Results are presented for five fibres with diameters in the range 0.13–0.57 mm.

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.

The interlaminar shear strength of a unidirectional high-modulus polyethylene fibre-epoxy resin composite, measured using a short-beam bend test, increases when the fibre has been plasma treated but decreases with increasing fibre content or increasing filament diameter. Attempts have been made to explain these observations. From observation of the sample failure and the bending load/deflection curves, it was found that there are two possible failure modes dependent on fibre type, fibre volume fraction and fibre plasma treatment. These are (1) brittle failure of the resin and (2) failure at the fibre/resin surface. The present results appear to be consistent with the interlaminar shear strength data obtained from high-modulus polyethylene fibre samples made by the hot compaction process developed at Leeds.

The effect of filament diameter on the failure stress of polyethylene fibres has been studied using Weibull analysis. Both gel-spun and melt-spun fibres have been examined, so that differences might be observed for changes in draw ratio or modulus as well as molecular weight. It is concluded that the strength of high-modulus melt-spun fibres relates to the concentration of flaws and is significantly dependent on filament diameter. Conflicting results for gel-spun fibres are discussed in the light of the present investigation, and it is concluded that the mechanism of failure in these fibres is different from that of the melt-spun fibres.

The production of solid section highly oriented polyethylene by compaction of melt-spun polyethylene fibres is described. Differential scanning calorimetry, X-ray diffraction and electron microscopy have been used to determine the structure of the compacted polymer. The essential feature of the process is shown to be selective surface melting of the fibres to form a polyethylene/polyethylene composite of very high integrity, yet maintaining a very high proportion of the strength and stiffness of the fibres.

Series-parallel and parallel-series models are used to calculate upper and lower bounds for a unidirectional fibre composite. It is shown that this simple approach produces comparatively close bounds which agree well with those calculated from more sophisticated analytical methods. This approach may be used for initial estimates of composite properties, e.g. when characterizing a new material. In some cases this may be sufficient for the design of simple composite structures.

The microstructure of aggregates of parallel high-modulus polyethylene fibres, compacted at 138°C to give a composite with good mechanical properties, has been examined. Detail of transverse and longitudinal cross-sections has been revealed by permanganic etching and studied electron microscopically. The fibres pack together irregularly with coordination numbers typically between 4 and 7 and fibre diameters in the range 10–20 μm. Misalignment of fibres is generally close to zero but does occur incrementally, by 10° and more, between successive rows of fibres. A substantial proportion of fibres has deformed, often by shear, during treatment producing a broadening of interfibrillar contacts. A lamellar component of texture is present both between and within fibres. It is inferred to form by melting of those parts of the fibres where there is least constraint on the melt, followed by recrystallization on the fibres as nuclei. The lamellae therefore share the axial orientation of the fibres, while the crystallographic fibre/lamellar contact promotes good transverse properties.

The fracture behaviour of a range of polyethylenes with different molecular weights and side group content has been studied by means of a novel high-pressure tensile testing facility. The normally ductile specimens became brittle at high pressures, e.g. 700 MPa, so that the test results could be studied by means of linear elastic fracture mechanics. Oriented specimens were prepared by die-drawing to draw ratios of 5∶1 and 9.5∶1. The performance of the isotropic and oriented (longitudinal and transverse directions) materials were assessed using their KIC values. Results were carefully compared in order to study the effects of molecular weight and low concentrations of small side groups on the fracture behaviour of polyethylene. The structure of the oriented polymers was characterised using wide-angle X-ray diffraction.

The fracture initiation behaviour of injection-moulded plaques of a liquid-crystal polymer is described. Attention is restricted to cracks in the plane of the plaques propagating along the mould fill direction. The material is both anisotropic and inhomogeneous; the latter factor is accommodated by adopting a skin-core model of the plaque structure. The inhomogeneity leads to mixed mode (Modes I and II) fracture in some cases, and both single and mixedmode testing is used. The results can be represented in terms of either stress intensity or strain energy release rate by simple mixed-mode failure criteria.

An analysis of the mechanics of the die-drawing process for polymer orientation has been undertaken on the basis of the lower bound theory by Hoffman and Sachs for metals, with several important modifications. First it is accepted that the flow stress in polymers is strongly dependent on plastic strain, strain rate and temperature. Secondly, thermal effects during die drawing due to heat generated by plastic deformation are included in the analysis. Finally, the temperature distribution in the die is estimated, taking into account both heat condition and axial translation of the polymer during the drawing process. Good agreement was obtained between theoretically predicted drawing stresses and those determined experimentally for a range of drawing speeds.

The interlaminar shear strength of unidirectional ultra-high-modulus polyethylene composites was measured as a means of accessing the level of fibre/epoxy resin adhesion for a number of different reinforcing yarns, produced by melt and gel-spinning. The fibres were shown to possess poor adhesive properties due partly to inadequate wetting associated with the inert polyolefine surface and also because of a weak boundary layer, formed by the segregation of low molecular weight impurities to the surface during fibre formation. The interlaminar shear strength was significantly increased by pretreating the reinforcement with an oxygen plasma. This improved wetting by producing oxygen-containing groups on the fibre surface and removed the weak boundary layer by the formation of a cross-linked skin. For a fixed fibre volume fraction, the interlaminar shear strength was found to be inversely proportional to the filament diameter. The other mechanical properties were shown to be largely independent of fibre/resin adhesion, with plasma treatment having little or no effect.

Highly oriented polyethylene can be produced by a route which involves firstly annealing the material at a pressure of 450 MPa and a temperature 234 ° C so that ideally it enters the intermediate phase and chain extended material is produced. This is followed by hydrostatic extrusion of the material to align the molecular chains.

The stress relaxation behaviour of high-modulus oriented polyethylene fibre has been studied with regard to the response to successive small strain increments imposed on an initial relatively large strain deformation. For isotropic polymers, the results of such experiments have previously been interpreted in terms of a single thermally activated process modified by strain hardening. It has been found that, although this approach can describe satisfactorily some of the stress relaxation experiments on the oriented polyethylene fibres, it is unsatisfactory once the strain increments have exceeded a certain size, and that it is at variance with stress recovery experiments. It is shown that both the present stress relaxation and stress recovery experiments can be interpreted in terms of a model comprising two thermally activated processes acting in parallel. Furthermore, the parameters obtained for the stress relaxation data are consistent with those required to fit creep data obtained in a comparable stress range. The essential feature of the mechanical behaviour which was previously attributed to strain hardening can now be seen to arise from the transfer of stress between the two thermally activated processes in the two-process model.

An account is given of a series of hydrostatic extrusion experiments which has been carried out on linear polyethylene over a wide range of extrusion temperatures and environmental pressures. The results of pressure-to-pressure hot extrusions, which have been performed on melt crystallised polyethylene and also on pressure crystallised extended chain polyethylene, are discussed, with particular reference to the physical properties of the extruded products.

An analysis of the mechanics of the hydrostatic extrusion process for polymers is presented, in which the predicted extrusion pressure is considered to be influenced by the effects of strain, strain rate and pressure on the material flow stress, as well as by the billet—die friction. The extrusion behaviour of both crystalline and amorphous polymers is discussed with reference to experimental results for linear polyethylene, polyoxymethylene and polymethylmethacrylate. Particular attention is paid to the method of incorporating the flow behaviour of the polymer into the analysis. A modified form of the Eyring equation for an activated rate process is proposed, in which the effects of strain rate and pressure on the flow stress are assumed to be separable, but related to strain by the large strain dependence of the stress activation volume. Moreover, a direct equivalence between the pressure effect and the friction between the polymer and the die is proposed for hydrostatic extrusion, following previous work on the adhesive mechanism for friction in polymers. This results in a formally identical analysis for both crystalline and amorphous polymers, in which the strain rate sensitivity, pressure sensitivity and friction coefficients all increase markedly with material strain during the process.

An account is given of the creep behaviour, under tensile, compressive and torsional loading conditions, of oriented samples of a linear polyethylene (LPE) which have been produced by the process of hydrostatic extrusion. A comparison is made between the behaviour of oriented and isotropic LPE material and, with biomedical applications in mind, between the performance of the linear LPE polyethylene and the ultra-high molecular weight polyethylene grade commonly used in prostheses.

A theoretical treatment is given for the total diametral compression of a transversely isotropic elastic cylinder under compression between parallel rigid plates under conditions of plane strain. Experimental results are presented for the compression of isotropic and oriented cylinders of nylon 6.6 and linear polyethylene (Rigidex). These results confirm that the theoretical treatment is valid to a good degree of approximation. In the case of linear polythylene, where data are presented for ultra-highly oriented extrudates, the results are of some interest with regard to the elastic properties of these unusual materials.

Fracture measurements have been made using a high pressure technique on three batches of uPVC corresponding to distinct levels of gelation.Kc values correlate with methylene chloride attack in the established manner, but are low in comparison with room pressure values measured at comparable strain rates. This is linked with the presence of stress whitening on the fracture surface at room pressure, its absence at high pressure, and its diminution at room pressure when testing speed is increased, when lowKc values are also recorded. The results are successfully interpreted in terms of a multiple craze slow crack growth model, in which cracking in the tests at room pressure and slow speed is assumed to start at theKc measured at high pressure, and slow crack growth follows as the load on the specimen continues to increase until the specimen fails catastrophically. A reasonable choice of parameters then gives a good prediction of the observed collapse loads.

Yield-stress measurements on amorphous polyethylene terephthalate film are presented covering five decades of strain rate at temperatures from just below the glass transition (≈ 60° C) to −160° C. The data were obtained by combining measurements from a conventional Instron machine with data from a high-speed tensile tester capable of applying strain rates of up to 50 sec−1. Five different failure modes have been identified over this extensive range of test conditions. All data from tests which show a clear yield point can be described accurately by an extension of the Ree Eyring approach to include two processes. The process which is only important at low temperatures correlates well with theβ-process observed in dynamic mechanical experiments.

The plastic deformation behaviour of linear polyethylene and polyoxmethylene has been studied as a function of strain rate over a very wide range of plastic strains (deformation ratios up to ∼ 25). The results have been examined in terms of the concept of a true stress-strain-strain rate relationship. It has been shown that results for drawn fibres and extruded rods are compatible on this basis, and true stress-strain-strain rate surfaces can be defined. The significance of these results with regard to both tensile drawing and hydrostatic extrusion processes is considered. A preliminary discussion of the rate dependence is also presented.

The effect of plasma etching and chromic acid treatment on the surface adhesion of ultra-high modulus polyethylene fibres to an epoxy resin has been studied. The adhesion was determined from pull-out tests, and showed a significant improvement for both plasma and acid treatments. The mechanism of failure, however, appeared to be different in the two cases. Untreated and acid-treated monofilaments showed a fairly smooth surface and failure of the pull-out samples involved sliding along the monofilament/resin interface. Plasma treatment, on the other hand, produced a remarkable structure on the monofilament surface, into which resin penetrated to produce a mechanical keying between monofilament and resin. Failure in the pull-out test then involved rupture within the monofilament.

Notched impact fracture experiments have been conducted on specimens of polycarbonate in three different conditions, (a) as received, (b) annealed at 125° C, and (c) electron beam irradiated to reduce the molecular weight. By consideration of the behaviour of a wide range of notch geometries four different failure modes were identified which were present in different proportions for each material. (1) Razor notched specimens failed in a completely brittle manner, well described by linear elastic fracture mechanics. (2) Small notch tip radii specimens failed in an apparently brittle manner through the formation of a single craze. (3) Some intermediate notch tip radii specimens failed in a predominantly brittle manner with small shear lips indicative of plane stress yielding. A fracture mechanics approach was used here, the measured toughness correlating with the extent of plane stress yielding. (4) Fully ductile failure was observed for large notch tip radii for all materials. It has been established that the embrittlement of polycarbonate caused by annealing is due to an increase in the yield stress, whereas that caused by reducing the molecular weight is due to a reduction of the crazing stress. In both cases, more specimens of intermediate notch tip radius are caused to fail in the low energy brittle mode designated (2) above. By varying the yield stress and crazing stress independently we have thus been able to distinguish clearly how both influence the brittle-ductile transition.

The extensional compliance parallel to the draw direction, and the Poisson's ratio in the plane of the sheet, have been determined for one-way drawn polyethylene terephthalate sheet. The technique was to photograph an electron microscope grid printed on the surface of the sheets and thus measure changes in dimension under load. The results show good internal consistency and those for the extensional compliance agree well with other determinations.

Neck profiles, neck propagation velocities and draw loads were measured for a series of linear polyethylene samples drawn in a tensile testing machine at 21, 75 and 100° C. The results were used to determine the profiles of strain, strain rate and draw stress in the neck. Elementary considerations show that the strain rate field in the neck relates to the strain hardening and to the strain rate sensitivity. A point of particular interest is that strain hardening appears to relate primarily to the polymer molecular weight whereas the strain rate sensitivity is more sensitive to the initial thermal treatment. The relationship of the present work to studies of true stress-true strain-true strain rate surfaces is discussed.

The tearing behaviour of oriented linear polyethylene has been studied with particular emphasis on the effects of draw ratio, draw temperature and molecular weight distribution. In all cases the values of fracture surface energy were in the range 102 to 104 Jm−2. The differences between the low molecular weight grades and high molecular weight grades of linear polyethylene were large. At high molecular weight the energy for crack propagation parallel to the direction of orientation fell by a factor of approximately two as the draw ratio was increased from 10 to 20. At low and medium molecular weights, increasing the draw ratio above a value of 10 had no significant effect on the fracture surface energy. Observation of the fracture surfaces showed a simple relationship between the nature of the fracture surface and the value of the fracture surface energy. In general, the higher the value of fracture surface energy the less fibrillar is the fracture surface. Furthermore the fibrillar nature of the fracture surface is essentially determined by the void content of the material.

The fracture behaviour of notched specimens of polymethylmethacrylate has been examined for a wide range of geometries in Charpy impact tests, and in tensile and slow bend fracture tests. It was found that the failure of the very sharply notched specimens was consistent with linear elastic fracture mechanics and defined a constant fracture toughness KIC for a constant notch tip radius, whereas the blunt notched specimens failed at a constant critical stress at the root of the notch.

Two techniques have been used to measure the shear compliances of one-way drawn uniplanar-axially-oriented polyethylene terephthalate (PET) sheet, draw ratio 5∶1; a torsional method for all three compliancesS 44,S 55 andS 66, and a recently-developed simple-shear method forS 44 andS 66. The two techniques gave values for the compliances in common which agreed very well. An examination of end effects in torsion has also been made. The determination of the three shear compliances complete the determination of all nine independent compliances for this PET sheet. The overall mechanical anisotropy has been considered in the light of existing structural information, and comparison made with the elastic constants of isotropic PET and a highly oriented PET fibre on the basis of the single-phase aggregate model.

In this paper the influence of molecular weight and molecular weight distribution on the tensile strength (tenacity) of melt spun and drawn linear polyethylene are investigated with the aim of outlining the requirements for a high strength fibre. The tenacity was investigated over the molecular weight average ¯M w range 60×103 to 330×103 with polydispersities ¯M w/¯M n ranging from 1.1 to 13.3. It was found that both molecular weight and its distribution affected tensile strength. The drawing conditions were also found to be important, a high draw temperature and a high draw ratio being needed for a high strength, high modulus fibre. By using a polymer of high ¯M w and low polydispersity, and drawing at the optimum conditions, strengths of 1.65 GPa and moduli of 85 GPa have been achieved for test temperatures of −55° C.

Two of the Poisson's ratio compliances have been determined for oriented polyethylene terephthalate (PET) sheet. The method adopted was to determine the fringe shift under load when the specimens were placed in one arm of a Michelson interferometer operating in a vertical fringe mode. Data obtained for the specimens in air and in a water bath, with two polarization directions, can be used to provide two independent measurements of each compliance. Good agreement was shown between these measurements, and the values for the compliance constants are reasonable on physical grounds, as well as being consistent with previous work on PET fibres.