Monitoring growing cracks in aircraft lugs by means of the electro-mechanical impedance method

Abstract

Lugs are common connecting elements in many fields of engineering, e.g., aircraft and automotive. High stress concentrations at the inner edge of the bolt hole lead to early crack initiation and rapid fatigue crack growth under cyclic loading. However, a complete failure of such components is in most cases not acceptable. This contribution presents a model-based methodology to monitor the crack propagation in aircraft lugs by means of the electro-mechanical impedance method. For this purpose, piezoelectric sensors are permanently attached to the monitored structure. The evaluated structural components are necked, straight and tapered lugs. Crack propagation is analyzed numerically by coupled-field finite element models and experimentally using an impedance analyzer. Simulated and measured frequency spectra of pristine (no crack present) and damaged (a crack of various lengths is present) structures show significant deviations in resonance frequencies which are considered as clear indicators of the present crack. Furthermore, the length of artificially introduced cracks is determined by specific resonance frequency shifts for each lug shape. Crack length estimation based on resonance frequency shifts has already been shown in a recent study for necked double shear lugs, which is now extended for straight and tapered lug shapes. Finally, a discussion on the applicability of the presented crack analysis methodology for structural health monitoring of aircraft lugs under fatigue loading is presented.