Life Extension Techniques for Aircraft Structures – Extending Durability and Promoting Damage Tolerance through Bonded Crack Retarders

Abstract

This paper explores the viability of the bonded crack retarder concept as a device for life extension of damage tolerant aircraft structures. Fatigue crack growth behaviour in metallic substrates with bonded straps has been determined. SENT and M(T) test coupons and large scale skin-stringer panels were tested at constant and variable amplitude loads. The strap materials were glass fibre polymer composites, GLARE, AA7085 and Ti-6Al-4V. Comprehensive measurements were made of residual stress fields in coupons and panels. A finite element model to predict retardation effects was developed. Compared to the test result, predicted crack growth life had an error range of -29% to 61%. Mechanisms and failure modes in the bonded strap reinforced structures have been identified. The strap locally reduces substrate stresses and bridges the crack faces, inhibiting crack opening and reducing crack growth rates. In the absence of residual stress, global stiffness ratio accounts for effects of both strap modulus and strap cross section area. In elevated temperature cure adhesives, retardation performance was best in aluminium and GLARE strap materials, which have the closest thermal expansion coefficient to the substrate. Strap materials of high stiffness and dissimilar thermal expansion coefficient such as titanium had poor retardation characteristics.

Effect of impact damage on fatigue performance of structures reinforced with GLARE bonded crack retarders

“…The principle is to adhesively bond a stiffening ''strap'' in regions where crack propagation can be expected. The local increase in stiffness owing to the strap will reduce the stress intensity factor at the crack tip, so reducing crack growth rates thereby resulting in longer life so allowing for longer inspection intervals [3,4]. GLARE is a promising fibre-metal laminate to improve damage tolerance by application as a bonded crack retarder [5].…”

A comparison of fatigue crack growth performance of two aerospace grade aluminium alloys reinforced with bonded crack retarders

“…Another solution is to incorporate adhesively bonded straps at selected locations acting as crack retarders, thus enhancing the fatigue performance. [2][3][4][5][6][7] Both solutions build crack retarders at the manufacturing stage before any fatigue crack is developed. They are different from other crack arresting techniques, such as stop hole at crack tips and bonded repair patches.…”

“…A doubling of the fatigue life is obtained. Another solution is to incorporate adhesively bonded straps at selected locations acting as crack retarders, thus enhancing the fatigue performance . Both solutions build crack retarders at the manufacturing stage before any fatigue crack is developed.…”

“…In application, curing at elevated temperature is a requirement based on aircraft operation conditions. Among various materials, the fibre‐metal laminate GLARE has been shown to be most effective owing to its low density, low residual stress arising from curing straps at elevated temperature, and improved fatigue and impact performance . In previous work, application of GLARE bonded crack retarders showed large benefit in AA2024 and AA7085.…”

Fatigue modelling of aluminium plates reinforced with bonded fibre metal laminates

“…Nevertheless, to overcome the inherent lack of fail safety and to satisfy the damage tolerance requirements of such type of structures, reinforcement technologies have been developed, e.g. the selective reinforcement (Heinimann et al, 2005), bonded crack retarders (BCR; Zhang et al, 2007;Irving et al, 2011), and crenellations (Ehrström et al, 2005).…”