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Currently, adhesive bonding cannot be used to join primary aircraft structures, such as fuselage components and load-bearing wings. To be acceptable to aerospace manufacturers and users, adhesive use must fulfil one of the three defined means of compliance (MoC) set out in international air-safety guidelines.
The main challenge for the EU-funded BOPACS project was to reduce the weight and cost of joining primary aerospace structures, often made from light composite materials, by developing metal bolt-free, adhesive-bonded joints that comply with stringent airworthiness and safety requirements. The use of adhesives would considerably reduce both the weight and cost of making aerospace structures.
“A thorough programme of research was performed into the propagation of cracks in the bond line area of airframe joints bonded with adhesives,” explains project coordinator Jan Halm of the Netherlands Aerospace Centre NLR. “Subsequently, the project investigated developing features that can be implemented to arrest or slow down the propagation of cracks.”
Adhesives vs. bolts
State-of-the-art metal fasteners, such as lock bolts, were selected as the benchmark for the adhesive bonding. This meant that any adhesive solution had to compare favourably with the metal bolts in terms of the bonding’s static and fatigue strength, its crack-stopping capability, and the cost and lead time for manufacturers.
Bolt-free adhesive-bonded joining methods were developed to meet stringent European Aviation Safety Agency (EASA) airworthiness requirements. Project results were reviewed on a regular basis with the certification department at Airbus – a project partner – as regards certifiability and acceptance by the authorities.
BOPACS focused on obtaining certification for adhesive-bonded joints by developing methods for analysing crack growth and disbond propagation mechanisms in adhesively bonded joints. It provided solutions involving a range of disbond stopping features (DSFs) which can prevent cracks or disbonds from growing above a predefined acceptable size. This means the joint is still capable of carrying its design load.
Crack stoppers
A range of different types of DSFs were developed and tested. The most promising features were then tested in a full-scale demonstration using a fully bonded aileron and a large lap-joint panel for high-load applications. The tests showed that, due to integration of the DSFs, any initial cracks, caused by impacts or manufacturing defects, did not grow to a critical length and were stopped by the DSFs.
Methods were also developed to predict the propagation of cracks in the bond line. They were validated by comparing the analysis with test data and showed that the BOPACS methods are capable of predicting how the cracks were propagated.
“The results from BOPACS show that potentially adhesive bonding can be used safely in major airframe structures although additional research work has to be done,” says Halm. “This will bring economic benefits to aerospace manufacturers in terms of cost savings and, in addition, social and economic benefits should arise from the reduced weight of aircraft resulting in lower fuel consumption and lower environmental impact.”
BOPACS results are very promising and form the basis of further research into the industrialisation of crack-stopping solutions. The project’s research also contributed to the awarding of two PhDs.