Optical inspection of holes in jet engine blades

The need for improved thermal efficiency of jet engines has led to changes in the design of combustor turbine blades. Modern turbine stage inlet temperatures now exceed the melting point temperatures of turbine blade materials. Super alloys, based on nickel, have been developed for use as blades, guide vanes, afterburners etc. To combat and avert blade failure caused by excessive operating temperatures, film cooling has been incorporated into blade design. In film cooling, cool air is bled from the compressor stage, ducted into internal chambers of the turbine blades, and discharged through small holes in the blade walls. This provides a thin, cool, insulating blanket along the external surface of the turbine blade, and large numbers of shaped holes have allowed designers to maximize the cooling effect. This paper explores a new design for measuring the presence and depth of blind holes in turbine blade. In the paper, we examine the inspection techniques currently in use and present a novel optical technique as an alternative. To precisely locate and measure the holes on the turbine blade, an XYZ translation stage is employed. Using a small collimating tube, a micro-beam illuminates each hole in a pre-programmed fashion. Depending on the level of reflected intensity and when it occurs, the presence of a hole bottom is determined. The optical inspection system consists of a laser, motorized micro-positioning stage, collimating tube, optical detector/amplifier, data acquisition software and a customized fixture for manipulating the samples.