Fault Detection and Localization Using Laser-Measured Surface Vibration

Structural health monitoring techniques have become increasingly important to the Navy of the 21st century whose strategy is to emphasize advanced designs and new material technologies in its modern high performance structures while utilizing existing aging structures beyond their planned lifetimes. At the same time, the Navy would like to reduce manning levels on Naval platforms, reduce time in repair and total ownership costs, and increase survivability. Among other things, these trends have driven the need for the development of reliable, automated, structural health assessment methodologies. In response to this need, we have been addressing the feasibility of structural acoustic techniques for monitoring the mechanical condition of structures. The focus of our structural acoustic development efforts thus far can be summarized by the following question: Given sufficient but readily accessible displacement information over the surface of a vibrating structure, can we develop and implement corresponding local inversion algorithms for mapping material parameter variations, detecting and localizing flaws (such as cracks, voids, and delaminations), and uncovering the depth profiles of such? In this article, we present the details of the structural acoustic approach to fault monitoring, describe various “inversion” algorithms for extracting the fault information, show the results of numerical feasibility studies, discuss the associated measurement technologies, and then present applied studies we have carried out using this methodology. These include both laboratory-based studies on simple structures and spin-off work we have done on art-laden walls at the U.S. Capitol building.