Automated high-precision crack detection in airplane combustion chamber liners using white light interferometry

The maintenance, repair, and overhaul (MRO) industry plays an important role in increasing the long-term profitability and safety of large investments, especially in the aviation industry. MRO is, therefore, an important economic factor in its own right. The core of any successful MRO procedure is an appropriate inspection technique, which is capable of detecting all relevant damages in a given part. The requirements for these inspection procedures can be very high, especially for safety-critical parts. Furthermore, the inspection processes can be very challenging to automate, especially for complex and used parts, because they often require a high degree of flexibility. As a consequence, of the day of this writing, many of the inspection procedures in the aviation industry are still performed fully or at least partially manually.

In this dissertation, an overview of the most important non-destructive inspection (NDI) technologies in aviation MRO is given, followed by examples of their use in automated inspection systems. One technology that is not currently in automated use, but has a very high potential for the automatic high-resolution inspection of aviation parts, is white light interferometry. Unfortunately, this technology has several attributes that make it very challenging to integrate in automated processes. In general, a white light interferometer's field of view (FOV) is much smaller than the part under test, while simultaneously having a small depth of view. This means that relative movements between the part and the sensor are required. Furthermore, the technology is very sensitive to vibrations, necessitating special considerations for the sensor integration. In addition, the sensor has a relatively large parameter space, with partially interdependent settings, making it difficult to automatically find suitable parameter sets. For these reasons, white light interferometry is not currently used in automated MRO processes.

This dissertation bridges the gap between the well-established white light interferometry sensor technology and its practical use for automated inspection processes. In order to fully understand the technology's implication, an overview of the theory behind white light interferometry was derived and serves as the basis for developing solutions for the problems that arise when white light interferometers are used in practice.

Even though the novel inspection process itself can be used with any part, it was developed with the combustion chamber outer liner of a CFM-56 aircraft engine in mind, in order to verify the validity of the approach on a relevant practical example. Since cracks represent the majority of flaws in that particular part, the developed inspection method focuses on the detection of those damages. The description of the novel inspection approach focuses, in particular, on the following aspects: the automation of white light interferometry, the three- and two-dimensional data processing required to detect cracks and the verification of the potential crack indications.