Detection of Multiple Damages by Prestack Reverse-Time Migration

An approach to detect and image multiple damages in a platelike structure is presented. A structural health monitoring system (SHMS) with a linear array of actuators/sensors is proposed. The integrated actuators/sensors are used to activate/receive lowest-order antisymmetric A0 mode Lamb waves. A migration technique used in geophysical exploration and seismic prospecting is adopted to interpret the backscatteringwave Ž eld and to image the  aws in the structure. The proposed approach ismodeledby a two-dimensional explicit Ž nite differencemethod both in simulating the re ection waves and in implementing the prestack migration. An analytical solution based onMindlin plate theory (Mindlin, R. D., “In uence of Rotary Inertia and Shear on Flexural Motions of Isotropic, Elastic Plates,” Journal of Applied Mechanics, Vol. 18, No. 1, 1951, pp. 31–38) is derived to verify the accuracy of the numerical algorithm. An excitation-time imaging condition speciŽ cally for the migration of waves in a plate is introduced based on a ray tracing concept, and prestack reverse-time migration is proceeded to propagate the re ection energy back to the damages. The plate is imaged in terms of the velocity of transverse deformation after migration,thus, the locations, dimensions, and seriousness of the  aws can be visually displayed.Numerical results show that multiple damages can be successfully detected and the image of the damages correlate well with the target damages. The conclusion is that prestack migration can be a prospective technique in SHMS applications.