Proposed through-life management approaches for adhesively bonded repair of primary structures. (December 2018) - Baker, Alan
Abstract: The inability of current non-destructive inspection (NDI) procedures to confirm bond integrity has greatly limited the application of adhesively bonded repairs to primary aircraft structure, especially in applications where failure of the repair would lead to safety in flight concerns. Given these concerns, applications to primary structure are generally limited to situations where the residual strength of the parent structure in the absence of the repairs can exceed the design limit load by an acceptable factor, most conservatively as high as 1.5, which is the design ultimate strength. This paper proposes technologies and associated strategies that could lead to some relaxation of the current residual strength requirements. This makes possible the wider application of adhesively bonded repairs to primary metallic structure suffering fatigue cracks and composite airframe structure suffering visible impact damage. The detection of weak, and even absent ("Kissing"), adhesive bonds generally requires the application of a significant stress to the region of the adhesive bond which cannot be achieved by conventional NDI – ultrasonic techniques for example. Since it is generally not feasible to stress the actual repair patch, a "Proof Test" has been developed. This test requires the application of shear stress to a bonded repair coupon (BRC) made of the patch material. The BRC is bonded to the parent structure concurrently with the patch. This test can confirm both the initial and through-life structural integrity of the repair bond. However, it must be agreed by the appropriate authority that the BRC is fully representative of the patch system. In addition to the proof test more critical repairs also require through-life structural health monitoring (SHM) with a focus on detecting patch disbonding and a secondary focus in the case of repairs to metals of monitoring crack growth. It is concluded that to detect patch disbonding, a simple approach using resistance strain gauges, or more robustly optical fibre sensors, holds the most promise, at least in the short term. Disbonding is detected from measurements of reductions in strain transfer from the parent structure into the patch, an approach previously demonstrated during full-scale fatigue testing of a repaired F111 wing. Based on use of the Proof Test together with SHM, as appropriate, decision charts are presented for the management of adhesively bonded repairs to fatigue cracks in metallic structure in which the crack is not removed and to composite structure following removal of visual impact damage. A simple approach to the management of stress-reducing reinforcements for metallic components is also presented.