Holographic FLI (Fringe Linearization Interferometry) for detection of defects

This interim report describes the work performed during the first half of Phase 2 on the two step holographic fringe linearization interferometry study. The FLI process consists of deflecting the object beam between holographic exposures to create linear fringes and spatial filtering of the image reconstructed from the hologram about the linear fringe carrier frequency. This filtering is meant to discriminate between subsurface defects and random fringe noise. During this phase a loading limitation for the FLI process (of one quarter wave/linear fringe period for the out of plane deformations) was demonstrated. To circumvent this limitation two modifications to the FLI process were investigated: four-exposure FLI - a moire technique and laser pulse separation control with dynamic loading. With the former method linear fringes have been recovered from the random noise in a simulated laboratory experiment. Experiments to demonstrate the fringe shifts at defect locations with differential loading are still in progress. Preliminary experiments performed on the NADC holographic system indicate that it should be adequate, albeit cumbersome, to demonstrate the laser pulse control method. The finite element analysis is predicting the experimental fringe patterns obtained with static loading and the modeling effort for the dynamic loading experiments is discussed. Plans for the work to be done during the remainder of Phase 2 are given.