Abstract

This paper studies the effect of fatigue crack growth on the propagation characteristic of multi-frequency anti-symmetric Lamb wave modes in a C-Channel type aerospace structure through a Damage Response Factor (DRF) approach based on experimental testing with numerical verification. The approach is based on an algorithm for analyzing guided wave data excited at different frequencies, providing a method for identification of damage along an actuator-sensor path in a pitch-catch scenario. Computation of the DRF is based on the changes in the wave amplitude ratio, energy ratio, and cross correlation coefficient between the current and a baseline signal taken at the beginning of the test or at lowest load level. In addition to the commonly used threshold value, the DRF approach uses the upper and lower bounds on the average DRF value along with the separation of the DRF based on excitation frequency. By combining these criteria, a method for damage detection was developed. The approach presented here was experimentally and numerically verified for a C-Channel manufactured from aluminium alloy 7075-T651. A constant amplitude fatigue cyclic loading was applied to the C-Channel in order to grow a crack from a 3.2 mm diameter hole with a ~0.4 mm saw-cut starter notch. Multiple piezoelectric sensors were utilized to generate and capture a series of guided Lamb waves at different excitation frequencies to evaluate the presence of damage along each sensor path. Both the experimental measurements and the Finite Element Method (FEM) provide good agreement on the application of the DRF as a means for detecting damage. A “cycle-free baseline” damage detection method is also presented, which is based on interrogating the structure during the same cycle, but at different loading conditions.

Keywords

multi-frequency guided waves, structural health monitoring (SHM), damage detection, damage response factor (DRF), fatigue crack, cycle-free baseline