Use of Bonded Crack Retarders to Improve Damage Tolerance of Wing Covers

The trend in aircraft structural design and manufacture has been moving towards lighter structures that present extended lives and improved damage tolerance. Alongside the increasing use of composite materials, metal/composite hybrid structures are an alternative solution to provide weight reduction whilst maintaining damage tolerance. These are typically a combination of advanced alloy parts reinforced by adhesively bonded highly damage tolerant materials.

Previous research has demonstrated the superiority of fibre metal laminates such as GLARE over other reinforcing material such as titanium, aluminium, CFRP and GFRP.  This presentation summarizes an extensive coupon and stiffened panel test program demonstrating that the use of Bonded Crack Retarders (BCRs) can significantly improve the damage tolerance of wing covers.  The coupon test program evaluated various strap configurations and design details and provides guidance for design of the stiffened wing cover test panels with BCRs, and generated data used to develop and validate numerical modelling approaches to predict the crack propagation and delamination development in BCR panels.  Owing to the thermal expansion mismatch between the aluminium substrate and GLARE straps in BCR panels, residual stresses are induced in the coupons and wing cover stiffened panels during the autoclave cure cycles.  Bonding-induced residual stresses were measured on various coupons using neutron diffraction, and compared to the predicted residual stresses from finite element simulations.  The presence of residual stresses can not only introduce distortion in panels, but can also negatively impact the damage tolerance behaviour.  The final demonstration of the benefit of BCRs for stiffened wing covers was the testing of large 5-stringer stiffened wing covers in both baseline and BCR configurations under representative upper and lower transport aircraft wing spectra.  The stiffened panel testing showed that typical crack propagation lives to a 2-bay crack for the BCR configurations are at least double the baseline values.