Small turbine blade inspection using laser strain techniques

High-pressure turbine blades in a gas turbine engine are constantly exposed to extremely hostile operating conditions, where temperatures can regularly exceed the melting point of the parent material. These conditions make the blades susceptible to thermal fatigue cracking, corrosion and, in some cases, damage due to particle impact. The required inspection method in aerospace for finding surface-breaking defects is Fluorescent Penetrant Inspection (FPI). However, the blade must be extensively cleaned prior to FPI and this represents a considerable cost in terms of time and resource, particularly if the blade is subsequently found to be unserviceable. Therefore, the ability to inspect such components prior to the cleaning stage is an attractive proposition. This paper describes the use of laser strain techniques to identify cracks in high-pressure turbine blades as a means of conducting inspections in the engine-ran condition. A set of nine blades were cleaned and inspected in the conventional manner using FPI, with all highlighted defects recorded photographically. Next, laser shearography techniques were applied to the test set using a bespoke laboratory system. Distortions in the resultant fringe patterns caused by defects in the material were then used to identify the cracks. All cracks highlighted by FPI were also found using the shearography method. In addition, the laser shearography technique highlighted a region of impact damage on the aerofoil of one blade, not identified at the first inspection.