The aircraft industry has been at the forefront of the use of adhesives for many years. The industry, however, has also been amongst the first to use adhesives in safety and functionally critical areas to form lightweight, fatigue resistant structures. In other locations the adhesive is used to form lightweight sandwich panels, where adhesive bond stresses are relatively low.
The de Haviland Comet aeroplane, shown above and below, which first flew in July 1949, made very extensive use of adhesives in its construction, and is a living proof of the longevity of adhesive joints, indeed many aircraft are still flying in the form of the Nimrod reconnaissance plane. Phenolic vinyl adhesives were used throughout the aircraft to bond doublers and profiled aluminium alloy stiffeners to the aluminium alloy skins, both in the wings and in the fuselage. Because of the critical nature of the joints, and the anticipated long service life of the aircraft, it was vitally important for a very high quality joint to be made, and this is reflected in the bonding procedures.
The Comet was one of several aircraft developed during the early post-war years which exploited the Redux bonding process. Redux exists in several forms as a bonding system, but essentially consists of a phenol formaldehyde resin and polyvinyl formal (Formvar) toughening agent. In the Comet application the Redux materials were used in a liquid / powder form.
Although there have been some reports of corrosion problems in the belly areas, attributed to aggressive liquids spilled in the passenger compartment being trapped in the top-hat stringers, in general, Redux has proven to be one of the most durable adhesive bonding systems and is still used in essentially the same form today.
This has been confirmed by the forensic analysis of bonded airframe stringers from a 30 year old Comet. Sections from the top wing skin, approximately one third of the way outboard, from one of the converted Nimrod MK2 development aircraft, were made available to the project for analysis of the bonded joints.
Aluminium alloy
Phenolic, Redux Liquid E / Formvar
The bonding was carried out at the factory location.
Etch and chromic acid anodise
In this technique the resin was applied to the surface to be bonded and the coarse powder was then sprinkled onto the wet resin surface. Surplus powder was shaken off and in some process specifications a second liquid resin layer was applied to cover the powder. The two surfaces were then brought together under heat and pressure. The curing process is a condensation reaction and high pressures are needed to prevent the formation of steam from the reaction moisture. In the early years of Redux bonding the complete assembly was carried out in a heated platen press. The manufacturers' literature specifies a glue line temperature of 150°C for 30 minutes at a pressure of at least 0.7 N/mm2 (7 bar). It is reported that de Haviland used curing conditions of 145°C for 20 minutes at 2.8 - 5.0 N/mm2 (28 - 50 bar). In order to prevent a build up of peel stresses within the joint, and to hold the components together during curing, the adhesive joints were backed up by rivets, although a considerably reduced number compared to a purely riveted structure.
The importance of surface treatment of aluminium was well appreciated during the early Redux development period. Initially a chromic/sulphuric acid etch, used for paint pre-treatment, was found to be effective; chromic acid anodizing after etching was introduced by De Haviland to increase resistance against long term ageing and was used on the Comet. On thicker sections the aluminium surfaces were grit-blasted with 120-220 alumina grit at 5.6 bar before etching and anodizing.
145ºC for 20 minutes at 2.8 - 5.0 MPa
The mechanical properties of these joints showed little or no evidence of loss of performance during their 30 year life. Durability tests at 40°C, in which the Comet joints were compared with test material prepared at a similar time and stored in laboratory conditions, as well as similar new materials showed no indication of loss of durability in the old Comet joints. Analytical studies of the fracture surfaces of the old adhesives did not reveal any significant deterioration.
The results of this study have shown that, if care is taken during design and manufacture stages, adhesive joints can be formed which can withstand many years service under aggressive conditions, with relatively little degradation of properties.
If care is taken during design and manufacture stages, adhesive joints can be formed which can withstand many years service under aggressive conditions, with relatively little degradation of properties.
The original adhesive formulation, combined with the surface treatment, provides a high performance bonded joint. This performance should be equalled or bettered by later derivatives of the bonding system.
DTI MTS Project 3 Report No 9 Forensic Studies of Adhesive Joints. Part 1 - General Introduction and Conclusions, Part 2 - De Haviland Comet February 1996 NPL
NPL / ESR Technology Limited
Colour Image courtesy DH Flight Simulation http://www.dmflightsim.co.uk/
