Tracking down a crack

Automated inspection procedure for engine components

During flying operations engine components are exposed to constantly varying thermal and mechanical loads due to the variety of operating conditions encountered. The damage that arises as a result of normal wear and tear is therefore different for every component. Cracks feature prominently in the typical damage patterns and these are repaired during engine maintenance.

At the start of the repair every component undergoes crack testing and measurement. The challenge lies in reliably detecting even the smallest hairline cracks event though these never appear in the same place and branch off in different ways. This stage of the work is performed manually and requires a lot of knowledge, experience and care on the part of the operator.

One major goal of Lufthansa Technik´s Engine Services unit is to continually improve engine component maintenance, repair and overhaul processes (MRO) for customers. In this connection the automation of MRO processes - from automated inspection and component measurement to repair - is increasingly the subject of attention. To implement the first stage in what will eventually be an automated process chain, the AUTOINSPECT research project was launched in April 2011 with the aim of fully automating crack detection and assessment in combustor components.

In collaboration with strong partners and with the support of the Department of Business, Transport and Innovation of the City of Hamburg, Lufthansa is developing a novel crack test method that in future will constitute an alternative to the established dye penetration test. The innovative procedure is based on the familiar principle of white light interferometry. Under this optical measurement procedure the interference effects of light waves are used to conduct a high-resolution scan of the component surface.

This is the first time that the white light interferometer has been operated by an industrial robot. This procedures the flexibility necessary to ensure that components exhibiting a wide variety of geometric characteristics can be accommodated. In the course of these measurements large volumes of data that make heavy demands on subsequent image processing are generated. Any damage identified is automatically digitally marked and further processed, enabling the data to be transferred directly to the AUTOREP research project in the combustor area, which is currently running  in parallel, without any problems.

The dye penetration test that is currently used is an industry-wide established procedure that is employed for crack-testing a number of (engine) components. It involves several stages. The first step is to thoroughly clean and dry the test objects. The penetrant, a penetrating oil containing fluorescent paint pigments, is then applied to the surface of the test object. Capillary action forces it into even the finest cracks. Excess penetrant is now removed and the test object is dried again. Next the test object is sprayed with a developer (a special chalk-based powder), which sucks up any penetrant contained in the cracks so that it can be seen by the human eye. The evaluation is then performed under ultraviolet lighting, which accentuates the contrasting fluorescent paint pigments, making them easier to identify.

The aim of the AUTOINSPECT research project is to reduce this multi-stage, energy-intensive and in part ecologically harmful inspection process to a single, automated "clean" process step. Again, because everything is automated under the innovative procedure, the measurement results are totally reproducible. As a result, process reliability and productivity can be increased further.

Due to the high degree of innovation and the considerable potential offered by use of the novel procedure, an application for an international patent has been filed so that AUTOINSPECT is already protected in selected countries. To ensure that as many customers as possible can benefit from the new inspection procedure, there are plans to obtain approval from the engine manufacturers. Once the research project phase is over at the end of 2014 industrialization of the system is expected to start. During the industrialization phase the question of whether the procedure can be transferred to other products from the engine area and also other business areas of Lufthansa Technik will then be investigated and, if appropriate, implementation will follow.

In combination with the AUTOREP research project for the first time a fully automated MRO process - inspection followed by repair - is being developed using th example of highly complex engine components.