CONFERENCE REPORTS Aeronautical Research Laboratories, Melbourne, Australia, 12 July 19 72
Carbnn fibres and fibre reinforced composites During the morning session, factors related to the production of carbon fibres from a PAN precursor),2 and the relationship between the structure of the carbon fibres and the mechanical and physical properties were presented.3 6 Factors responsible for the decrease in strength at high moduli in some forms of the PAN-based fibres were a point of considerable scientific dispute, although most of the models used for describing the structure of the fibres were based on Ruland's original two-dimensional model of undulating ribbons of turbostratic graphite. in particular Dr B. J. Wicks 4 proposed that the loss in strength was caused by the formation of platelets of threedimensional graphite - probably catalysed by metal impurities in the fibres. These platelets were thought to act either as Griffith-type cracks or as stress concentrators caused by internal dislocation pile-up at the platelet boundary. Experimental evidence for this mechanism was the relationship found between the average platelet size and strength. Dr M. Stewart 5 considered that the fibre strength was connected with the stress concentration caused by the unbending o f the curved turbostratic ribbons. As the ribbons become thickened by high temperature heat treatment, so the stress concentration increases and the strength falls. Finally Dr C. W. Le Maistre 6 provided both experimental and theoretical evidence to show that the decrease in strength, at elevated temperature heat treatment, was connected with longitudinal thermal cracking of the anisotropic fibre. Further evidence presented in favour of this model was the fact that fibres with a 'dog-bone' type of structure do not show a decrease in strength with heat treatment. In the first afternoon session, factors influencing the selection of suitable matrices for carbon fibres were discussed.
Dr A. A. Baker 7 suggested that, on the basis ot chemical compatability and oxidation resistance, aluminium was the only metal with much potential as a matrix for structural applications; even with aluminium, aqueous corrosion was a problem. Resin matrices were discussed by Dr J. Williams 8 together with some proposed work on interface analysis. Applications of carbon fibre composites were discussed in the final afternoon session and papers by Dr Baxter 9 on 'Pressure Vessels' and Mr M. Campbell i0 on the 'Reinforcement of Girder Structures' were presented.
A, A. Baker
*Aeronautical Research Laboratories, Melbourne. Australia
Ennis, B. (Defence Standards Lab), 'Chemistry and structure of the PAN precursor' Kilpin, D. (Weapons Research Establishment), 'Chemistry of pyrolysis of PAN' Coyte, R. (Aeronautical Research Lab), 'Structure of carbon fibres' Wicks, B. (Aeronautical Research Lab), 'l:actors inllucncing the mechanical properties of carbon fibres' Stewart, M. (University of New South Wales), 'Fracture mechanisms in carbon fibres' t,e Maistre, C. (Weapons Research Establishment l, "Relationship between structure and physical properties of carbon fibres' Baker, A. A. (Aeronautical Research Lab), 'Chemical compatibility' Williams, J. l l)efence Standards Lab), 'Resin materials" Baxter, J. (Weapons Research Fstablishmentk Pressure VeSSels'
Campbell, M. (Monash University I, 'Reinforcement of girder slructures"
Brighton, 10 12 October 1972
Eighth international reinforced plastics conference Organized by the Rcinlbrced Plastics Group of the British Plastics Federation The continuing success of these conferences was assured by the attendance of more than 400 delegates from about 20 different countries and by the high quality of the 27 papers presented, in his welcoming address the chairman, S. R. Badley (BP Chemicals International kid), stressed the urgent need for the industry to get over to the consumer how much poorer his way o f life would be in the absence of reinforced plastics. In addition to this public relations task the different members o f the reinforced plastics industry from material producers to fabricators should cooperate fully to solve the disposal problenr. If this was not done then all plastic products whether intended for long term or short term use would come under public censure and their use be possibly subject to restraints. Prosperity for the industry depended on collective endeavour,
COMPOSITES . JANUARY 1973
and a free exchange of information and advice across tile various sectors of the industry was essential. Tile theme of tile conference was 'Reinforced Plastics are the materials o f today', and to support this claim the proceedings were divided into 5 sessions - (1) Achievements; (2) Research exploited; (3) Research for exploitation; (4) International reinforced plastics; and (5) Tile way ahead - industrial growth, ideas and innovation as they affect reinforced plastics. The opening paper in the achievenrents session was given by J. Mountifield (Fibreglass Ltd) on the use o f GRP in naval applications and in particular on the construction of HMS Wilton by Vosper Thorneycroft ktd for the Ministry of Defence. Regrettably a full account was not possible
due to a prior publication requirement of the Ministry but sufficient details were given to emphasise the scale and success of the exercise. Both the hull and the superstructure are in GRP and at 153 ft long HMS Wilton is the largest GRP ship in the world. From the initial feasibility studies to the actual launching the project required a careful integration of materials, design and production technology. The properties and durability of the materials had to be tested under operating conditions from tropical to arctic service, and ease of repair was a major requirement. To determine structural design, shock resistance tests simulating underwater explosions were performed on test sections to prove that a top hat/single skin structure was superior to a double skin construction. The overall design had to basically follow the original design for a conventional metal ship so that available components such as engine mountings could be used. A design factor of five was also imposed so that although a very safe structure was obtained, it was not the most efficient possible. The important point was that HMS Wilton would provide data from the seagoing experience of a large GRP structure. Designing for impact resistance was also the main point of the paper by 13. E. Cleathero (British Rail) on the development of reinforced plastics for rolling stock and buildings. The problem was to ensure the safety of train crew when loose objects struck the front of a train travelling at high speed. Tests consisting of firing a 4 lb steel shot at speeds of up to 175 mph were performed on various steel panels and sandwich structures of GRP/polyurethane foam. The3 showed that within the weight limits imposed on the component only the sandwich structures were capable of arresting the shot without penetration of the inner surface. The most effective construction was obtained by injecting polyurethane foam into a cavity between two skins each composed of an outer surface of chopped strand mat/ unidirectional rove cloth/polyester and an inner surface of a continuous filament mat which had been pressed against the wet laminate but was not impregnated with polyester. This mat became thoroughly impregnated by the foam and became distributed throughout the foam. This was the feature which was fundamentally responsible for the good energy absorption characteristics. Low density foams were found to be better than high density foams as the low support given to the glass caused fibre pull out rather than fracture. The use of reinforced plastics in building applications was introduced by P. C. Oliver and E. C. Roach (Mickleover Ltd). Their paper dealt with the Airport Terminal at Dubai, Trucial States where the use of a GRP roof structure meant a considerable saving in weight and allowed a subsequent economy in the depth of the foundations. The problems were described of designing and preparing structural components which could be manufactured in England, transported 4 000 miles and erected easily by unskilled labour. The roof structure itself could simply be described as an assembly of 56 inverted umbrellas, each umbrella being about 30 ft square and 19 ft deep and consisting of GRP/phenolic foam sandwich panels in a framework of steel ribs. The largest GRP building in the world - an impulse generator tower built by Ferranti Ltd for the Central Electricity Research Laboratories at Leatherhead -- was described by J. E. Swainson and D. J. Wade of Permali Ltd. The tower is 16.5 ft (5 m) in diameter and 60 ft
(18 m) high. It supports, protects and electrically insulates the 70 tons of generator components and can withstand gales of up to 100 mph. The choice of construction techniques had lain between a filament wound or hand lay-up cylindrical structure and a modular panel design. The latter was preferred on economic and ease of erection considerations. Each module panel was 6 x 4 ft (1 "8 x 1"2 m) and was prepared by a hot press moulding of 245 lb (111 kg) of sheet moulding compound and continuous filament polyester prepreg. A complex lay-up pattern was needed to provide ribs and flanges having the correct fibre orientation. The steel mould used was 98 in by 79 in (2"5 x 2-0 m) and weighed 12 tons. Cure conditions were 45 minutes at 120°C and 660 tons pressure. The panels were attached to a vertical framework of high strength, high modulus (4.5 x 106 lbf/in 2, 31 GN/m 2) bidirectional epoxy/continuous-glass laminate. A design factor of six was used to give a tower of excellent stability and rigidity. A paper by H. Keesen (Bayer) on experience with sheet moulding compounds and dough moulding compounds in the German electrical industry highlighted their widespread use in electrical applications compared with practice in other countries. Several factors were possibly responsible for this difference. Firstly, German companies were very adventurous and were prepared to take financial risks in new developments. Authorities were probably more prepared to accept novel materials and/or construction techniques. Also, German engineering companies were able to supply such a wide range of high quality hydraulic presses and other machinery that processing was never a difficulty. Considering the materials themselves, the combination of good mechanical and electrical properties, the resistance to corrosion, and the ease of fabrication of intricate profiles by pultrusion techniques had established many successful industrial applications. For example, the excellent track resistance was important in switch and relay construction in high voltage applications. The higll resistance to corrosion allowed 15 year guarantees to be freely given for switch and cable terminal bases. Insulation and corrosion were also the problem areas mentioned by G. Harrison (BP Chemicals International Ltd) in which there was an increasing use of GRP. There were many such applications in terminal installations and distribution engineering for oil and petrol products. In addition to the well-established usage for pipes, mooring buoys, etc there were now such diverse applications as cladding for North Sea platform legs and sieve tray/bubble caps for distillation columns. More important, there was now a growing acceptance of GRP storage tanks, but this was an area where Europe still lagged a very long way behind America. The 'Research exploited' session opened with a paper by W. Brandt Goldsworthy (Glastrusions lnc, USA) on the rapidly expanding use of pultruded profiles in architecture and construction. Major growth areas exist in mobile homes, exhibition halls, portable nrilitary shelters and low cost housing developments for under-developed areas. When enough service data and construction experience had been gained from these applications, then use could well be extended to the conventional building industry where the conservatism of the builder and the user had to be overcome.
The ability to produce a building material in a continuous profile gave the architect remarkable freedom and flexibility in design. Furthermore the concept of modular prefabricated systems where the user could change the size and shape of his buildings to suit his requirements was an exciting prospect for the bold designer. An existing project by a US airline was a 'people pipe' for directing passengers between airport terminal buildings and the new generation of wide bodied jet aircraft. Such a facility could obviously be easily adapted to suit changing demands of air traffic. The basic component for this 'people pipe' was an 8 ft (2-4 m) square, 40 ft (12.2 m) long modular unit which could be joined end to end or at other angles using appropriate joint fittings. Material evaluation showed the most economical construction used pultruded profiles and sandwich panels having a low density core, This gave a 50-65% reduction ~n costs compared to conventional materials despite a doubling up of design loads by airport authorities due to the novelty of the material and construction techniques. To satisfy these static flow loadings of 100 lbf/ft 2 (4.8 kN/m 2), snow loadings of 50 lbf/ft 2 and wind loadings up to 150 mph a core thickness of 6 in ( 152 ram) and a heavy skin were required. These would be reduced as soon as service experience had allowed a moderation in the requirements. The only paper dealing with fabricating techniques was given by P. R. Chant (Shell Plastics Laboratories, Delft) in the session 'Research for exploitation'. He posed the general question of whether the stagnation of the reinforced plastics industry is due to an overall economic depression or whether the market ceiling is being approached with the materials and processes currently available. If the latter were even partly true, he could provide a possible answer in a new process called 'Reservoir moulding technique'. This was less labour intensive than conventional processes and gave products having a higher stiffness to weight ralio than was commonly obtained. Essentially the process comprised: • impregnation of a soft foam sheet, typically a polyurethane, with a resin system: • assembly of this impregnated sheet between two layers of dry fibrous reinforcement in a mould; • closing the mould which involves three stages the foam presses the dry reintbrcement against the mould surfaces most of the air in the foam is squeezed out to pass through the still dry reinforcelnent, and resin is squeezed from the foam and impregnates the reinforcement. Since only one layer an easily handled, easily impregnated foam - has to be impregnated with the resin there is a definite reduction in the labour involved and automation was a further possibility. A range of thicknesses can be obtained by using varying degrees of compression. The amount and type of reinforcement can also be altered to provide an extremely broad range of mouldings. Processing conditions are easy.
Operating temperatures are froln ambient to 180"C and cycle times of less than one minute are feasible. Times could even be shortened further by B-staging the resin. Moulding is always to stops and the only pressure needed is that to compress a soft foam. This is generally about 1 kgf/cm 2 (98 kN/m 2) and the process is obviously suitable for exploitation without a press. This could significantly reduce the costs of the operator. The critical feature of the process is the viscosity of the resin. The room temperature viscosity must be sufficiently low for the initial impregnation process, but tot) low a viscosity at the moulding temperature would result in poor retention in the foam and cause dripping problems or premature impregnation of the reinforcement. The latter would destroy one of the features of the process namely that the reinforcelnent is put into its final shape before it is impregnated by the resin and is consequently easier to ['orul. The mouldings are cbaracterised by a low specilic gravity, a high stiffness to weight ratio, an optimised use of the reinforcement and a wide and easily variable range of properties. Possible applications include skis, lightweight suitcases, decorative building panels and small boats. New furane-resin/catalyst systems were the subject of a paper by A. T. Radcliffe (Quaker Oats Ltd, UK). Exploitation of furane resins for GRP had been severely restricted due to the strong acidity of the usual catalysl required and the highly exothermic cure. The new systems had overcome these limitations and in addition had appeared to optimise the already good chemical resistance of furane laminates. Further beneficial characteristics were excellent fire resistance, low smoke emission and good high temperature properties. Laminates made from three layers of chopped strand mat and two layers of tissue mat with a total glass content of 25% had a room temperature flexural strength of 154 MN/m 2 and there was 48-0'J retention at 150°C, 38-1% at 230°C and 27-2'/~ at 260°C. Processing may be by normal techniques including filament winding. The only restriction was that metal fittings in contact with the catalyst system should be of stainless steel The gel time may vary from 4 rain to a few days, and a range of cure cycles is possible depending on the type and concentration of catalyst. Typical conditions are 16 hours at 22°C or 2 hours at 82°C. Applications are primarily those where a high degree of chemical resistance and/or temperature resistance is required, eg storage and effluent tanks, pipes and ducts in chemical plant, fume stacks and scrubbers in exhaust systems, A contribution of great significance to the designer and user was given by R. J. Howe and M. J. Owen of Nottingham University on 'Cumulative damage in chopped strand mat/ polyester resin laminates'. The development of damage during both simple tensile loading and tensile fatigue conditions was studied microscopically. The first sign of damage was always debonding between fibres and matrix where the fibres were perpendicularly oriented to the line of debonding. No debonding occurred below 10% of the ultimate stress but debonding increased rapidly thereafter especially above the 30% level and reached saturation conditions at about 70% of the ultimate stress. Microphotographs of the debonds show that, in the case of the fatigue loading, the fibre -matrix
separation becomes much larger than for the simple tensile testing condition and a considerable amount of debris is present in the debond. For both loading conditions the debonding was followed by resin cracking. This did not develop extensively before failure under tensile test conditions and no meaningful measurements were possible. Under fatigue conditions however there was extensive cracking prior to failure and a direct correlation existed between the extent of cracking and the reduction in strength of the laminates. While simple debonding was not therefore a primary contributor to the loss in strength it was extremely important in that it provided active sites for the propagation of transverse cracks. These cracks then caused debonding of fibres aligned with the direction of loading and the extent of resin cracking was further increased. Howe and Owen were able to establish a cumulative damage law relating the residual strength of the landnate to their fatigue loading treatment. Two papers dealt With the use of carbon fibre as reinforcement. The first by J. McAinsh (ICI Ltd) described the preparation of unidirectional specimens of carbon fibre (modulus 184 + 20 GN/m2; strength 1.73 + 0-64 GN/m 2) and a variety of thermoplastics. The more important of these were three different grades of polyethersulphones (ICI Ltd) which could give composites having an interlaminar shear strength of 97.3 MN/m 2, a flexural modulus of 100"9 GN/m 2 and a flexural strength of 1 '25 GN/m 2. Boiling water immersion tests showed that there was good retention o f these properties and in particular the interlaminar shear strength showed negligible change after 90 days. Preparation of these polysulphone/carbon-fibre laminates was perhaps more critical than for epoxy laminates and a correct balance between time, temperature and pressure was necessary to achieve optimum properties. Further drawbacks to exploitation were the brittle, non-tacky nature of the prepregs and the high moulding temperature (300°C) required. The second paper on carbon fibre was by T. Hayashi (Chuo University, Japan) and concerned the preparation of hybrid composites with improved mechanical properties. Theoretical values were derived for composites containing carbon fibre, boron fibre and glass fibre. Tensile tests results were obtained for an epoxy laminate whose construction was 3-ply GRP/2-ply CFRP/3-ply GRP. The surprising feature was that while the CFRP part always broke first as expected, there was a definite retardation of its fracture. Its primary fracture strength and corresponding strain were approximately 40% greater than had been theoretically estimated. This was ascribed by Dr Hayashi to the influence of the greater ductility of the adjacent GRP parts. The international session of the conference started with reports on the consumption and uses of GRP in 1971. In all countries there had been a marked reduction in the rate of growth compared with previous years, but most speakers attributed this to the worldwide general econonnc recession rather than the approach o f a market ceiling. Major growth in the marine and building industries was still confidently expected. An increasing use of GRP products for decorative and aesthetic applications was also obvious, and the GRP furniture in President Pompidou's state dining room was an indication o f this social acceptance.
The conference closed with a session entitled 'The way ahead' and fittingly the final word was with Brian Parkyn (Scott Bader Company Ltd) who has been involved in reinforced plastics from their earliest days. Mr Parkyn saw the way ahead for the reinforced plastics industry was not through moulding materials which by their nature and processing requirements would always be subject to size and shape constraints and would always be merely a part of the general plastics industry. The future clearly lay in major structures, and HMS Wilton was hopefully an important turning point for the GRP industry. To compete successfully with metallic or other conventional building materials however it was imperative that GRP structures could be made in one piece. A contact moulding process was needed which would give an accurate control of the consistency of the product and which would also be less labour intensive than present manufacturing techniques. New catalysts and resins were needed, and Mr Parkyn envisaged the provision of a precatalysed preimpregnated material which could be consolidated and cured at the same time by the use of heated rollers. In the tradition of its predecessors the conference offered ample time for both formal and informal discussion. Since the prosperity of the industry depends not so much on the consolidation of existing fields but more and more on successful innovation, it is increasingly necessary for the marketing expert and the materials technologist to meet on common ground for an interchange of ideas and opinions. The importance of these conferences to the British plastics industry cannot be overestimated.
*Materials Department, Royal Aircraft Establishn]ent, Farnborough, UK
PAPERS PRESENTED I 2 3 4 5 6 7 8 9 10 11
Mountificld, J. (Fibreglass Ltd), 'The Ministry of Dcfencc (Navy t: (; RP applications' ('leathern, B. E. (British Railways Board, Rail~ay Technical ('entre), 'Developments of reinforced plastics by British Rail in rolling stock and buildings' ()liver,P. C. and Roach, I'_'.C. (Mickleover Ltd), 'The use of reinforced plastics in civil engineering works with particular reference to the Dubai International Airport Terminal" Keescn,H. (l:arbenfabriken Bayer), 'Experience ~ith glass reinforced lIP resins (SMC, DMC) in the German electrical industry' llarrison, G. (BP Chemicals International Ltd), 'Plastics applications developments in a large oil company" Swainson,J. 1". and Wade, D. J. (Permali ktd), 'The design and use of reinforced plastics for outdoor impulse generators' Guest, R. R. and Reid, I. W. (Prodorite Ltd), 'The use of reinforced plastics in the chemical industry and allied fields' Ermolli,E. R. (}:acuity of Architecture, University of Naples), 'Composite floors in glass reinforced plastics' Goldswozthy, W. Brandt (Glastrusions lnc), 'The rapidly expanding use of reinforced plastics composites/pultruded profiles in architecurc and construction Cameron, A. B. and Taylor, K. J. (Turner Brothers Asbestos Co Ltd), 'The properties of reinforced thermoplastics, including glass reinforced and asbestos reinforced compounds' Merten,J. and Protoschill, K. (Farbenfabriken Bayer), 'Glass filled polycarbonatcs: engineering materials for solving many-sided problems'
COMPOSITES • JANUARY
12 13 14 15 16 17 18
Burns,R., Hankin, A. G. and Jolmson, A. E. (l'ilkington Brothers Ltd), 'Development and exploitation of new concepts in glass fibre reinforcements' Raymond, J. A. (Scott Bader Co Ltd), 'The practical significance of resin content and properties in laminate bchaviour" Turner, R. M. (ICI Ltd), 'The effect of resin ductility on composite properties' Chant, P. R. (Koninklijke/Shell Plastics Laboratorium l)clft), 'Reservoir moulding technique: a new process for glass reinforced resins' Lens,T. J. (Quaker Oats Company mbtl) and Radcliffe, A. T. (Quaker Oats Ltd), '(;lass fibre reinforccd furane resin laminates; a unique combination of properties' Thomas, W. 1.. (t!niversity College, Cardiff), 'The explmtation of high strength glass fibre research results in commercial fibre reinforced polymers' Shimamura, S., Shimizu, M. and Collision Test Group (Mechanical Engineering Laboratory, Tokyo), 'Impact resistance of FRP car bodies'
19 20 21 22
McAinsh,J. (ICI Ltd), 'The reinforcement ot polysulphones and other thermoplastics with continuous carbon fibre" Banks,W. M. (University of StrathclydeL 'An introduction to design analysis for reinforced plastics plates subjected to in-plane loading' Itowe, R. J. and Owen, M. J. (University of Nottingham), "Cumulative damage in chopped strand mat/polyester resin laminates' Itayashi, T. (Chuo University), 'On the improvement of mechanical properties of composites by hybrid composition"
Papers 23 to 27 were not pre-prmted or titled. They were delivered by I~romley, C. M. (National Economic Developlnent Office) I.ocke, II. B. (National Research l)evelopment Corp) Reilly, Sir Paul (Design Council) Bacon, C. E. (Owens Corning l:iberglas ('orp) Parkyn, Brian (Scott Bader Co Ltd'~
BOOK REVIEW Engineers' handbook of adhesives D. F. Aitken (Editor) The Machinery Publishing Co Ltd, Brighton, UK (1972), 176 pp, £3"50 The book is written primarily as a guide for the mechanical engineer in the use of adhesives though others, particularly students, will find it an interesting and informative book on one end use of polymers. A description is given of the different types of adhesive and their properties together with information on joint design, surface preparation, application techniques and equipment. Illustration is provided by examples taken from the aerospace and automotive industries. The closing chapter covers health and fire hazards and the associated regulations. Sensibly, the two main groups of adhesives, the structural and the flexible types, are dealt with separately although the sections on surface treatment could usefully have been combined to avoid repetition and to provide one thorough treatment of the subject. The authors have, in the main, avoided the use of tables and this makes for easier reading. However, ill the case of
the structural adhesives, the description of types and properties would have been clearer with less on chemical derivation and more graphical or tabulated data on environmental performance and the interaction between time, temperature and the ability of the different adhesive types to accept stress concentrations. For a book of this size, which aims to introduce the reader to the subject, the range of adhesives covered is good though naturally not exhaustive. I would, however, have expected to see a little more space devoted to the anaerobic and cyanacrylate adhesives in view of their increasing use as rapid assembly materials; also, the silicone adhesives as the higher temperature range flexible systems. On the credit side, though not mentioned in the title or chapter headings, the sections covering flexible adhesives include sealants as assembly and caulking materials. There is alsa a useful bibliography for further reading and an index. At £3-50 it represents fair value t\)r money.
.L /lamer* *Senior Materials Engineer, British Aircraft ('orporation, ('onuncrcial Aircraft Division, Webridge, UK
The properties of fibre composites Proceedings of a one day conference held at the National Physical Laboratory (NPL), Teddington, UK in November 1971.
Order your copy of .The Properties of Fibre Composites now from
IPC Science and Technology Press Ltd, IPC House, 32, High Street, Guildford, Surrey, England. 88 pages, international A4 size, 1SBN 0 902852 08.6 Price £5.00 (~ 14.00) per copy.
COMPOSITES . JANUARY 1973