[ASME ASME 1999 International Gas Turbine and Aeroengine Congress and Exhibition - Indianapolis, Indiana, USA (Monday 7 June 1999)] Volume 4: Manufacturing Materials and Metallurgy; Ceramics; Structures and Dynamics; Controls, Diagnostics and Instrumentation; Education; IGTI Scholar Award; General - Application of Perturbation Methodology and Directional Filtering for Early Rotor Crack Detection