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SHM Enabled Design: Application to Damage Tolerance Design with Bonded, Composite Joints

P. LIDDEL, P. FOOTE

Abstract


The use of bonded joints in composite structures is limited at present due to lack of any reliable means of verifying the strength and integrity of the joint at the point of manufacture and through its subsequent operational life. Manufacturing defects and accidental damage can degrade joint strength leading to sudden failure at loads lower than the design limits. For this reason damage tolerant, safety critical structures such as airframes use a design assumption of ‘no-growth’ for damage. This is achieved by adding bolts or fasteners to bonded joints as precautionary measures which add weight and complexity to the structures. In this paper the possibility of engineering ‘slowgrowth’ damage growth characteristics into bonded joint design is investigated. This design is enabled by use of SHM as a means of detecting initiation and propagation of disbonds in joints while sufficient residual strength remains to bear design loads. It is proposed that adhesive joints designed to fail in a progressive, recognizable manner can be successfully monitored and operated using intrinsic SHM methods such as surface strain sensors. Experiments demonstrating the benefits of monitored slow damage growth in bonded composite specimens are demonstrated. In addition an entirely novel approach to bonded joint design is proposed based on segmented bondlines which guarantee a slow growth and easily monitored failure mechanism for bonds of any design strength. While not designed to be deliberately weakened, such joints would be forced to fail in a manner that is easily monitored using SHM in the event of degradation from manufacturing defects, environmental damage or operational overload while remaining compliant with current damage tolerant design principles.

doi: 10.12783/SHM2015/139


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