The feasibility of optical interference-based NDE methods to inspect helicopter rotor blades

Currently, the NDE procedure for Alouette helicopter rotor blades consists of a visual inspection followed by a manual acoustic inspection procedure by trained personnel using purpose manufactured tapping hammers. The former inspection is aimed at detecting surface cracks and corrosion whereas the latter is intended to inspect the rotor blade for possible areas of delamination between the alloy skin and the leading edge spar or blade root reinforcing strip. This paper investigates the feasibility of using either the authors locally developed Portable Digital Shearography or Electronic Speckle Pattern Interferometry in conjunction with Mechanical Impedance Analysis in order to determine the possible presence and extent of defects more accurately and reliably. Both optical inspection techniques are discussed, their theory and apparatus presented and the inspection procedure described. The principle of Mechanical Impedance Analysis is also outlined and the inspection method described. The successful results of the study as well as defects detected are presented and discussed. Outlining the potential of using this alternative NDE method as an on-site, in-situ inspection procedure concludes the paper.