Damage Detection in Plate Structures Using Sparse Ultrasonic Transducer Arrays and Acoustic Wavefield Imaging

A methodology is presented for health monitoring and subsequent inspection of critical structures. Algorithms have been developed to detect and approximately locate damaged regions by analyzing signals recorded from a permanently mounted, sparse array of transducers. Followup inspections of suspected flaw locations are performed using a dual transducer ultrasonic approach where a permanently mounted transducer is the source and an externally scanned transducer is the receiver. Scan results are presented as snapshots of the propagating ultrasonic wavefield radiating out from the attached transducers. This method, referred to here as Acoustic Wavefield Imaging (AWI), provides an excellent visual representation of the interaction of propagating ultrasonic waves with the structure. Preflaw and post-flaw ultrasonic waveforms are analyzed from an aluminum plate specimen with artificially induced damage, and the AWI results show the location and spatial extent of all of the defects. INTRODUCTION Structural health monitoring (SHM) is of great interest for critical aerospace and civil infrastructure applications[1][2]. The general goal is to identify, locate and characterize defects as they initiate and grow to a critical size during the service lifetime of a structure. Conventional nondestructive evaluation (NDE) methods such as ultrasonic testing, eddy current testing and X-rays can be used with great effectiveness for identifying and locating defects at the component level, but their use on structural assemblies is limited due to access restrictions and the high cost of field deployable instrumentation systems. The challenge is to develop a viable SHM technique that has the sensitivity of high resolution NDE methods but is cost effective for use on complicated structures. A methodology for such an approach is presented in the paper, and results are shown for an intentionally flawed aluminum plate specimen. In previous work, a sparse array of ultrasonic sensors was used to detect the onset of damage in structures [3], and pre-flaw and post-flaw ultrasonic waveforms were compared using change detection algorithms to detect and classify damage [4] [5]. Similar change detection algorithms are used in this paper to detect damage, and a time-of-flight algorithm is used for locating suspected damage regions. A wavefield visualization tool, referred to here as acoustic wavefield imaging (AWI), is used to snapshot ultrasonic waves as they propagate through a structure. The AWI method produces images