Mechanical impedance inspection of aluminum honeycomb structures

This paper describes an evaluation of the capability of a mechanical impedance instrument, the MIA 3000, for detecting discontinuities in aluminum honeycomb structures. Good repeatability has been found. The system damping was measured using the half power method and discontinuities were found to produce an increase in damping due to their presence. This technique can aid the detection of discontinuities in aluminum honeycomb structures. The resonant frequency was found to decrease while system damping increased as the center of a discontinuity was approached. Also, the amplitude at the resonant frequency was found to have no correlation with distance as the center of the discontinuity was approached. Discontinuity sizing using the single frequency test was investigated and some difficulties involved in this procedure an presented. The discontinuities were found to be reasonably accurately represented by a model for a vibrating plate which is rigidly clamped at its edges. It was also possible to use resonant frequency to determine the size of discontinuities in the range 40 mm (1.6 in.) to 90 mm (3.6 in.) in diameter.