Detection of Hidden Fatigue Crack Growth in Single-Sided Fastened Joints

Aircraft structural elements are usually assembled using single-sided fasteners— pins with flush heads on one end and threads on the opposite end coupled with collars designed to ensure specific torque levels. Common examples include Hi-Lok and Hi- Lite pins. Unfortunately, fastened joints also provide unique opportunities for hidden damage, including fatigue cracks, corrosion and fretting, due to stress concentrations at the hole, friction between fasteners and surrounding material, and moisture that can ingress and become trapped between layers. The complex multi-layered geometry at these joints also forces disassembly to inspect using conventional techniques. Many proposed structural health monitoring (SHM) approaches tackle this problem by modifying the installed condition of the fasteners by placing a sensor between the structure and the collar or fastener head. This paper presents a novel SHM approach that can detect hidden fatigue cracks in multilayer fastened structures without disassembly or modification to the installed condition of the fastener. Piezoelectric elements are integrated post-installation to turn the fasteners themselves into impedance sensors. The coupled electro-mechanical resonance changes in the presence of local damage, which softens the adjacent boundary condition constraining the fastener. Experimental data illustrates a detection capability for finding hidden fatigue cracks of at least 1.5 mm.