DIGITAL HOLOGRAPHY TECHNOLOGY

Holography has been used since the 1970’s for tire inspection. Before the development of electronic holography or electronic speckle interferometer (ESPI) cameras, film holography cameras were used in combination with vacuum stress. Early film holography cameras were also use to solve a major production issue in the inspection of abradable turbine engine components.
Since the 1970’s turbine aircraft engines have used abradable seals in the compressor stages of the engine to achieve high-pressure ratios per stage, reducing the turbine power required to drive the compressor, engine weight and increasing performance. The loss of this material can affect engine performance and inspection of the bond line in production or engine overhaul is required.
Ultrasonic through transmission C-Scan is capable of detecting disbonds in parts where the shroud geometry is a straight or slightly conical cross section. However, in most engines, the design of the compressor shrouds includes brazed stators, material thickness changes, flanges and other features that obscure or shadow the abatable seal material.
In 1982, a holography NDT technique entered production at Pratt & Whitney, combining time average holography with a low frequency ultrasonic vibration applied to the compressor shroud. Holography provided excellent disbond detection with easily interpreted images essentially identical to UT results, but not affected by part geometry, material thickness changes. Early systems used film holography with a one step chemical process, invented by the author, which produced production quality holograms in approximately 10 seconds. The results were viewed on a video monitor. Electronic holography currently using mega-pixel CCD cameras has radically improved system operation speed and reliability. Since1982, holography has been the inspection standard for Feltmetal and plasma sprayed aircraft abradable seals.