Winter 2005: Society for Adhesion and Adhesives One Day Symposium on 'Bonding of Composite Materials'
One Day Symposium on Bonding of Composite Materials, held, on 8th December 2005, at the Society of Chemical Industry, 15, Belgrave Square, London
This meeting covered a range of topics from several industries. These included aircraft, marine and racing car applications. It is amazing how far composites have expanded in these industries since the last meeting with a similar title. For aircraft, in particular, composites have progressed from usage as fairings and other lightly loaded parts to the stage of all-composite fuselage and wing structures.
The first paper entitled: ‘Addressing the Challenges of Bonding the Next Generation of Aeroplanes: Control of Interfacial Morphology’, was given by Alex Baitak of Cytec Engineered Materials.
He studied the morphology of multi-phase materials at, and in the vicinity of, interfaces. His team found that control of the substrate surface energy was important. They first studied aluminium and found that when a primer is applied on the surface, the morphology at the interface is comparable to that of the bulk. However, without the primer, there is preferential wetting of the substrate by one of the components of the multi-phase adhesive. The analysis of the resin composition as a function of the distance from the interface was interesting. It showed an oscillating nature of the resin composition, which seemed to be independent of the nature of the interface. This was presented as figures. The composition oscillates around a value which corresponds to the overall bulk concentration of the modifier component. This was an interesting paper; Alex concluded that:
a) The bonding of composite structures needs new surface preparation techniques.
b) New adhesives may need to be developed to accommodate a variety of surfaces.
c) New protocols for better characterisation of adhesives are needed.
d) Long term durability of adhesive bonds to composites needs further study.
e) Faster and more reliable inspection methods would be helpful.
f) Repair techniques need further study.
The second paper: ‘Fracture in Bonded Composite Joints’ was given by Bamber Blackman of Imperial College.
This paper discussed work on test methods. Mode 1 fracture toughness is now studied using BS 7991, which is an improvement on ASTM-D-3433. A new Mode II test was also mentioned that is currently being assessed by an International set of “round robin” tests. Tests on joints containing pre-bond moisture showed significant loss of strength. Previous work on these lines showed similar results.
Work on the effects of rate of loading was also mentioned as composites in primary structures may need to cope with impact loading.
Conclusion: The increasing use of composites will require better test methods and more information to allow prediction of performance.
The third paper: ‘Structural Paste Bonding in Composite Structure’ was given by Benjamin Hawtin of Airbus.
This was interesting as paste adhesives have not been much used for manufacture of composite parts. It seems that paste adhesives have been found useful for assembling stringers and other parts to skin panels because it is easy to cope with profile tolerance variations by allowing a variable glue line thickness to be used. Their ability to cure at a lower temperature, and hence require cheaper and less complex tooling, has proved a cost advantage. One problem has been that even greater care is required with surface preparation because the cure temperature is lower and contaminants are less easily absorbed.
Tests for fracture energy G1c were found to be the most important. He found that grit blasting the surface was essential to remove all peel ply footprint. All other processes such as open time, cure temperature and cure time needed to be controlled and the adhesive needed to be applied to both surfaces. Bonded brackets for electrical cables were shown to be effective and to require less design input than drilled holes.
The fourth paper: ‘Bonding of Marine Composites’ was given by Peter Davies of IFREMA, the French Ocean Research Establishment at Brest.
This talk discussed pleasure boats, military boats and racing yachts. He first explained that conditions are different for marine applications:
a) Large and variable bond line thickness (up to 25mm in some cases)
b) Low temperature cure
c) Industrial bonding conditions
d) Durability is required but in a marine environment.
a) Continuous lamination is best
b) Delay then over-lamination is next
c) Peel ply is worst; as with all bonding, the quality of surface preparation is important
Adhesive bonding is becoming more common for superstructure parts.
These are used for the development of techniques and materials. Work is under way to study the adhesive properties, fabrication conditions and analysis for several applications.
The fifth paper: ‘Dielectric Studies of Adhesive Bonded Composite Joints’ was given by Prof. Dick Pethrick of the University of Strathclyde.
This paper reported work to evaluate the potential use of dielectric methods for the assessment of ageing in adhesively bonded structures. Typical behaviour was presented to show the type of signals that can be obtained with Al-epoxy-Al systems where a good conductor is used. Data was also presented show that the same type of signals can be obtained in the case of carbon fibre composite. In this case the changes in the dielectric signal can be explained purely on the basis of the type of distribution of moisture in the bond line.
The penetration of water in polymeric materials can therefore cause chemical and physical ageing. Chemical ageing is a non-reversible process that alters the molecular arrangement of the polymer. This can be caused by hydrolysis or plasticization of the material. In contrast to chemical ageing, physical ageing is reversible: by heating the material to above its glass transition temperature the original thermodynamic equilibrium is recovered. This is often due to the simple removal of water molecules from the polymer. Physical ageing effects can be swelling of the polymer, decrease in its electrical resistivity and to a certain extent plasticization. This paper is well worth reading. It was concluded that:
a) Dielectric spectroscopy can assess the water penetration inside adhesively bonded composite structure
b) The plasticization effect is the most important
The sixth paper: ‘Behaviour of Bonded Composites’ was given by Bob Adams of Bristol, University.
The paper discussed the production of joints for composites in load carrying situations and how to obtain an efficient design. Bob then went on to discuss the special case of composite adherends. With these materials the transverse strength is very low, being either that of the matrix resin or the strength of the bond, whichever is the lower. This means that peel stresses must be avoided by good design and tapering at the ends of joints and the use of tough adhesives is vital. The whole paper should be read to ensure that all the detail is available.
The seventh and final paper entitled: ‘Applications of Adhesive Bonding in Formula 1 Car Construction’, was given by Brian O’Rourke of Williams Grand Prix Engineering.
This paper gave an interesting insight into racing car design and showed that “make and test” is a good approach. The time to design a new part is about 4 months, which leaves little time for calculation and research. As with aircraft a large range of tests are needed to cover, elevated temperature, fluid contact, impact resistance and fatigue in addition to mechanical stresses. Design requirements, for safety reasons, have become more severe.
A wide range of adhesives and joint designs are used. Metallic inserts are fitted in body panels using honeycomb core. The most vital bonding applications are in suspension components. These tubular components have metal end fittings which are bonded to them. Metal substrates may be titanium, aluminium or sometimes aluminium metal matrix composites. Each component laminated is accompanied by a test joint. On completion the part, test coupon and component use the same vacuum bag and are cured in the same autoclave cure cycle. The coupon is then tested to destruction before clearing the component, which is itself tested to proof load. At set intervals, based on mileage, the component will be statically tested again. This is more severe than for aircraft parts.
This was a good day, which included a fine Christmas lunch, and showed just how far composites have increased their usage in a relatively few years. The whole booklet of papers is well worth reading for anyone connected with composites.