Interface Gap Measurement Using Low Coherence Interferometry 2024-01-1920

Large-scale aerostructures are commonly constructed using multiple layers of stacked material which are fastened together using mechanical methods. Ensuring the interface gaps between these materials are kept within engineering tolerances is of utmost importance to the structural integrity of the aircraft over its service life. Manual, right angle feeler gauges are the traditional method for measurement of interface gaps, but this method is tedious and mechanic dependent. A portable hand tool utilizing low-coherence interferometry has been developed to address these issues. The tool uses a right-angle probe tip which is inserted into a previously drilled hole and driven through the depth of the material. A line scan of data is collected and analyzed for the presence of interface gaps. To measure the consistency of the gap around the circumference of the hole, the tool is rotated by the operator and additional scans are collected. The tool is portable, can be operated by a single mechanic, and is capable of measuring different diameter holes by changing a radial index bushing.

Overall system accuracy is targeted at ±0.001” and achievable under most circumstances. A full system cycle, consisting of four scans at various locations around the hole’s circumference, consumes approximately 40 seconds. There is no additional time required for data processing. The system stores the measured gaps and detailed plots of each scan for future analysis.

An extension of this technology has also been developed by automatically rotating the probe tip at several hundred RPM. This provides a 3D point cloud of the hole and enables measurement of additional features such as hole diameter, cylindricity, and detection of surface imperfections. This extended scanning technology is best suited for automated equipment where size and weight are less of a driving constraint.