Platelike structures, made of composites, are being increasingly used for fabricating aircraft wings and other aircraft substructures. Continuous monitoring of the health of these structures would aid the reliable operation of aircrafts. This paper considers the use of a Lamb wave based structural health monitoring (SHM) system to identify and locate defects in large multilayered composite plates. The SHM system comprises of a single transmitter and multiple receivers, coupled to one side of the plate that send and receive Lamb waves. The proposed algorithm processes the data collected from the receivers and generates a reconstructed image of the material state of the composite plate. The algorithm is based on phased addition in the frequency domain to compensate for the dispersion of Lamb waves. In addition, small deviations from circularity of the slowness curves of Lamb wave modes, due to anisotropy, are corrected for by assuming that the phase and group velocity directions coincide locally. Experiment...

A phase reconstruction algorithm for Lamb wave based structural health monitoring of anisotropic multilayered composite plates

A number of tomographic and phased‐array methods have been proposed for generating two dimensional images of plate‐like structures using sparse arrays of spatially distributed ultrasonic transducers. The phased array differential approach is considered here whereby pulse echo and through transmission signals are recorded before and after localized damaged is introduced, and differenced signals are combined using a focusing rule to produce an image of the plate. The application is structural health monitoring where the transducers are permanently bonded to the structure. The quality of the image is affected by many factors such as the number and location of the transducers, the characteristics of the damage, the signal‐to‐noise ratio, presence of edge reflections, and anything unrelated to damage that may perturb the ultrasonic signals such as temperature changes and transducer bonding variations. Two methods for enhancing image quality are implemented and then evaluated as to their effectiveness. In the first method, the windowing function is changed in width prior to phased signal addition to yield the best image quality. In the second method, signals are envelope‐detected prior to phased signal addition to eliminate phasing artifacts. Results are reported for artificial defects introduced in aluminum plates.

Enhanced Differential Methods for Guided Wave Phased Array Imaging Using Spatially Distributed Piezoelectric Transducers

Ultrasonic methods are widely applied for nondestructive evaluation of structural components during both the manufacturing process and subsequent field inspections. The field inspections often require expensive teardown in order to access the back surfaces of critical components. Active ultrasonic methods are also a subject of ongoing research for structural health monitoring whereby transducers are permanently attached to a structure and signals are monitored to detect changes caused by structural damage. This paper presents a methodology for effectively combining ultrasonic monitoring and inspection. During the monitoring phase, detection and localization of possible damage is demonstrated on several specimens using attached transducers. This detection phase is followed by demonstration of a new inspection method referred to as Acoustic Wavefield Imaging (AWI) which utilizes the attached transducers as sources and a single externally scanned air-coupled transducer as a receiver. The acoustic wavefield images are useful for both checking the viability of the attached transducers and quantifying the extent of damage. The AWI method approaches the sensitivity of conventional through transmission ultrasonic methods but does not require access to both sides of structural components. Thus, it is very well suited for rapid field inspection of structural assemblies.

Integrating monitoring and inspection with attached ultrasonic transducers

Bauschinger effect in unpassivated freestanding metal thin fifilms

Layered composite plate‐like structures are finding an increasing range of applications in the aerospace industry. Structural Health Monitoring (SHM) of such structures is seen as a paradigm that will embrace efficient non‐destructive testing/evaluation techniques. The present study demonstrates two techniques that have the potential for the SHM of multi‐layered composite structures. The first technique is based on multi‐transmitter‐multi‐receiver (MTMR) technique with tomographic methods used for data reconstruction. In the MTMR, the possibility of SHM using algebraic reconstruction techniques (ART) for tomographic imaging with Lamb wave data measured in realistic materials is examined. Commercially available narrow bandwidth PZT crystals were used as sensors on multi‐layered quasi‐isotropic and cross‐ply composite plates with and without defects. Defects (through holes and low velocity impact delaminations) were synthetic and have been chosen to simulate impact damage in composite plates. To achieve reasonable image quality, conventional cross‐hole configuration is replaced by a new modified cross‐hole configuration that also optimizes the number of sensors. The second technique is a single‐transmitter‐multi‐receiver (STMR) technique that is more compact and uses reconstruction techniques that are analogus to synthetic aperture techniques. Here, the phase shifting is performed on the individual signals based on the guided wave dispersion relationships. The reconstruction algorithm uses summation of the phase shifted signals to image the location of defects, portions of the plate edges, and any reflectors from inherent structural features of the component.

B. V. Somasekhar

Papers

Plate waves structural health monitoring of composite structures