Investigation on repairs of aircraft composite structures made with various joining techniques

Abstract

In the race for developing aircraft with lighter airframe, aiming at increasing aircraft payload and/or fuel efficiency, the shift from conventional metal structures to light-weight composite structures has attracted attention and competition among the aircraft manufacturers. Apart from fuel consumption, other operating costs such as maintenance costs play an important role in the aviation industry. Hence, since composites are increasingly used in light-weight airframes, understanding the behaviour of these materials under various load conditions is essential. Maintenance costs of aircraft include general maintenance and repairs, which are commonly seen as time-consuming, due to identification and installation of the repair. Repairs on damaged composite structures must be reliable and durable for a certain period with maximum confidence. In this study, structural repairs on Carbon Fibre Reinforced Epoxy (CFRE) structures are investigated with experiments and numerical analysis. Particularly, the substrate composite plates are repaired with twin CFRE doublers (case-1: like doublers, i.e., same material as substrate) and with aluminium alloy (AA) 2024-T3 doublers (case-2: unlike doublers, i.e., different material to substrate). Investigated joining techniques are pure riveting, pure bonding, and hybrid joint. Riveting is the most common technique in joining or repairing composite structures. This has the advantage of shorter installation time, but then composites are vulnerable to the high stress concentrations due to the rivet holes. Other techniques such as adhesive bonding and hybrid joining are relatively efficient, but installation is more time-consuming. Experiment results show, there is no significant difference in the average repair static strength between pure bonded and hybrid joints. Repairs with adhesively bonded composite doublers have highest average static strength (24.8 kN), whereas the lowest is observed for repairs with riveted metal doublers (5.5 kN). Standard deviation of specimens is significant for bonded joints but low for riveted and hybrid joints. Numerical analysis is complemented with experiments and the observed difference in the results is smaller than 1.5%.