Imaging of flaws in composite honeycomb aircraft structures using instrumented tap test

This paper describes the development of an instrumented tap test and imaging system for the inspection of defects and damages in composite aircraft structures, especially honeycomb sandwich constructions. The resulting tap test images based on the impact duration and displayed in a C-scan format can readily reveal the shape and extent of honeycomb core damages. The images also reveal considerable detail of internal substructures such as core splices, ply build-up, and changes of foam core density. Based on a grounded-spring mechanical model, the tap scan images can be converted into images that show the quantitative changes of the local stiffness. A manual tap scan imaging system was assembled for laboratory experiments. Using actual composites parts from airplanes, impact damages and substructures were imaged. In addition, effects of the tapper mass, impactor radius, tap velocity and operator dependence were studied. A compact fieldable system is being built and an automated tapping and imaging system is also under development. The technique was compared to other similar devices currently available.