The automotive industry is a sector which, in recent years, has made an ever increasing use of adhesives in order to reduce production costs, facilitate assembly, and improve performance through the reduction of weight. The particular case considered here is that of the Foden Series 4000 truck cab which has made extensive use of adhesives since 1987.
The cabs are constructed from a number of GRP and aluminium alloy sections, most of which are bonded together. The particular area under investigation in this forensic study is the cab doors, which are formed from several aluminium alloy pressings. This is an area where the bonding performs in a more structural role, and is not backed up by mechanical means. Two aluminium alloy stiffeners are attached to the outer skin of the door, one to resist bending of the door around the handle area as the door is opened and slammed closed, and the other to damp vibrations in what is otherwise a fairly large flat panel. Both stiffeners are attached to the skin using adhesives, as the use of conventional spot welding techniques would have resulted in marking of the door skin, which would have affected the finished appearance of the cab, or required additional trim to be applied to cover the affected area. The use of adhesives has also simplified the assembly of the doors, compared to other more conventional assembly methods, resulting in a reduction of assembly costs.
A truck assembled in 1987 was made available for forensic study. The truck from which the door was obtained had been in regular service until 1993, when, following an accident, the door was removed. The door was then stored in a maintenance depot, until retrieved for use in the study. The door had thus been exposed to some six years of natural ageing, under full exposure to the environment, followed by six months in a more protected environment prior to testing.
The forensic analysis included non-destructive testing NDT, lap shear and tensile butt tests and accelerated ageing tests on the same adhesive (but on steel adherents). The fracture surfaces were examined using a range of physico-chemical techniques.
Aluminium alloy
Adhesive
Surface treatment
Cure cycle
Toughened acrylic
Degrease using 1.1.1-trichloroethane
A simple bonding procedure, coupled with a relatively fast curing, contaminant tolerant adhesive was adopted in order to minimise stock levels and assembly tooling. Both mating surfaces were degreased using 1.1.1-trichloroethane, and a toughened two part acrylic adhesive applied. The surfaces were brought together and clamped for around 6 minutes, while the adhesive cured. After this time, work on the door could immediately be continued, thus saving time compared to the use of slower drying, but possibly stronger adhesives.
Room temperature cure, clamped for approximately 6 minutes
The bonding was carried out at the factory location.
Prototype testing of cab doors using adhesive technology, prior to general release, indicated that this level of construction, relatively simple surface pre-treatment, and a rapid curing acrylic adhesive was fit for purpose, and more expensive improvements to the construction were not warranted. Now, with some 15 years in service, and no known cases of the adhesive failing under normal conditions, this compromise has been shown to have been valid. Indeed the door used in the detailed forensic study programme was involved in an accident in which the door was damaged. Although locally to the incident the adhesive had peeled back, more than 60% of the joint had remained intact.
Non-destructive testing of the aged joints indicated that the simple assembly procedures had resulted in intermittent bonding of the surfaces. However, given the contact areas of the bonded surfaces, the strength of the completed joint was more than adequate to meet the performance requirements specified for the door.
Lap shear and tensile butt tests indicated that there had been some degradation of the adhesive properties over the six years since the door was assembled. Comparison of these figures with those obtained from accelerated ageing tests on the same adhesive (but on steel adherends) suggest that this level of degradation is, if anything, lower than predicted.
Observation of the fracture surfaces using a range of physico-chemical techniques has shown that the locus of failure was in fact in the oxide layer of the aluminium alloy, rather than a truly interfacial failure of the adhesive joint. The oxide layer was identified, using an electron microprobe, as being approximately 10 nm thick.
A small region was observed in which cohesive failure had occurred. Inspection of the failure surfaces indicated that this was probably due to localised poor mixing of the two components of the adhesive, resulting in less good mechanical properties of the adhesive in this region.
Direct comparison of the accelerated ageing test results with the naturally aged samples suggests that six years of natural ageing is equivalent to approximately 4,250 hours of accelerated ageing at 60°C. An alternative view point is that the accelerated ageing tests can introduce additional degradation, not seen in practice. From the results obtained from the forensic study it is not possible to make this distinction, as this would require an extensive, carefully controlled experimentation programme.
The use of adhesives in this application has clearly been very successful, with no known problems with the adhesive joints in that time. In the case of the sample used for this study, even after an accident where the joints were loaded considerably beyond their normal working load, much remained intact. It illustrates that, even with a relatively simple, low cost bonding procedure, high performance joints can be formed, which meet the demands of the application.
MTS Project 3 Report No 9 Forensic Studies of Adhesive Joints. Part 1 - General Introduction and Conclusions Part 3 - Foden Series 4000 Truck Cab February 1996
NPL / Foden http://www.foden.com/
Images courtesy Foden ltd.
