The present invention relates to the inspection of components and, more particularly, to an improved method for automatically inspecting gas turbine engine components having complex geometric shapes using eddy current techniques.
Eddy current inspection is a commonly used technique for detecting discontinuities or flaws in the surface of a gas turbine engine component. Eddy current techniques are based on the principle of electromagnetic induction in which eddy currents are induced within the material under inspection. Eddy currents are induced in a test specimen by alternating magnetic fields created in the coil of an eddy current probe when the probe is moved into proximity with the component under test. Changes in the flow of eddy currents are caused by the presence of a discontinuity or a crack in the test specimen. The altered eddy currents produce a secondary field which is received by the eddy current probe coil or by a sensor coil in the eddy current probe which converts the altered secondary magnetic field to an electrical signal which may be recorded on a strip chart. An eddy current machine operator may then detect and size flaws by monitoring and reading the signals recorded on the strip chart. Flaws or defects are detected if the electrical signal exceeds a predetermined voltage threshold.
Present eddy current inspection methods work satisfactorily when the components under inspection have simple geometrical shapes, such as holes, flat plates or the like. However, when the component under test has a complex geometrical shape, such as the dovetail slots of a high pressure or low pressure turbine disk, fan disk, high pressure compressor disk, teeth of a gear or the like, the complex geometry of these components such as edges, transitions between convex, concave and flat regions, produces contributions to the eddy current signals which make it difficult to distinguish between defects and non-defects.
A presently used method of detecting cracks or defects in a complex gas turbine engine component involves scanning a portion of the surface of the complex component with an eddy current probe and converting the received eddy current signals to a two-dimensional digital image. The two-dimensional image is then matched or compared to a chosen template by known image analysis techniques, such as convolution, to detect a defect or flaw. The template is chosen according to the active region of the eddy current probe as well as the size and shape of the defects desired to be detected. The image analysis matching technique will detect a defect only if the size and shape of the defect correspond substantially to that represented by the chosen template. Different templates must, therefore, be used for detecting defects of different sizes and shapes. In order for this detection process to be independent of defect size and shape, too numerous a number of templates would have to be compared to the two-dimensional image to detect all possible flaw sizes and shapes that may be present; such a process would be computationally impractical. Thus, the template matching technique can result in erroneous results if the template does not correspond substantially to the defect and the technique is inefficient because of the large catalog of templates which must be compared to the two-dimensional image.