Krishnan Balasubramanian

Fiber reinforced composite plate-like structures have been used to achieve substantial reductions in the structural weight of both military and commercial aircrafts. For large area and layered structures, damage detection using any conventional testing methods is time consuming. Structural Health Monitoring (SHM) of such structures is seen as a new paradigm that will reduce maintenance costs and increase safety. The aim of the technology is to provide an early indication of physical damage. The early warning provided by an SHM system can then be used to define remedial strategies before the structural damage leads to failure. This paper describes the development and successful demonstration of a SHM system using Smart Flexible Sensor Patch (FSP) that has a built-in network of Piezoelectric Wafer Active Sensors (PWAS) embedded on a thin film of dielectric material. These PWAS are arranged such that conventional cross-hole tomography (CHT) as well as modified cross-hole tomography (MCHT) can be carried out using Lamb waves. Using MCHT, the lateral extent of the damage for barely visible low-velocity impact damages (BVID) on Composite structures like wing and aileron are were imaged. The conventional CHT was deployed in a region of large aspect-ratio such as stiffeners. It was observed that disbonds of stiffener in Eleven and other structures may be effectively monitored by this method. © 2007 American Institute of Physics.

Structural health monitoring (SHM) of plate-like structures used in aerospace industries, using transducer arrays located suitably on the structure, such as the single-transmitter multiple-receiver (STMR) array [Wilcox PD, Lowe M, Cawley P. Lamb and SH wave transducer arrays for the inspection of large areas of thick plates. Review of progress in quantitative nondestructive evaluation, vol. 19A. Melville, NY, USA: American Institute of Physics; 1999. p. 1049-56; Wilcox PD. Guided wave beam steering from omni-directional transducer arrays. Review of progress in quantitative nondestructive evaluation, vol. 22A. Melville, New York, USA: American Institute of Physics; 2002. p. 761-8], has been demonstrated here. The reconstruction of the material state was carried out by utilizing a phased addition reconstruction algorithm. In addition to the signals from damage sites, the ultrasonic guided wave-based reconstruction procedures also need the complete set of elastic moduli as a continuous input throughout the SHM process. In the present study, two flexible printed circuit board (PCB)-based patches: ((i). single-quadrant, double-ring STMR material characterization (MC) array and (ii). Full-ring STMR SHM array) were developed for accomplishing both objectives, i.e. (a) online MC and (b) SHM of anisotropic plate-like structures, respectively. Experiments were conducted on 3.15 mm graphite-epoxy composite plate using PCB-based STMR arrays, the feasibility of accomplishing both objectives was demonstrated. © 2008 Elsevier Ltd. All rights reserved.

Lamb-wave tomography (LWT) offers a powerful nondestructive technique for the health assessment of large structures as their propagation properties depend on the thickness and the mechanical properties of the material. Development of a fast and accurate algorithm for defect detection is of paramount importance in any structural-health-monitoring (SHM) system. The present study explores the prospects of LWT as a SHM technique with an accent on developing a suitable algorithm for real-time inspection. Projection data is collected by electronically scanning an array of ultrasonic sensors arranged in a modified cross-hole geometry. The data thus collected is investigated to extract energy profile of the traveling waves. Multiplicative algebraic reconstruction technique (MART) algorithms are used as a tool for tomographic reconstruction from a set of multiple independent measurements. The performance of algorithms is evaluated from the point of view of the cost of algorithm, achievable resolution, and accuracy of results. Experimental results show that MART is capable of characterizing defects in thin isotropic and composite plates within a reasonable error band (±26% normalized, ±2.6 RMS) and is suitable for application to LWT of large structures such as aircraft skins.

Structural health monitoring of plate like structures, using transducer arrays located suitably on the structure, finds applications in monitoring aerospace structures. These plates are anisotropic, with wave propagation properties varying with direction. Single Transmitter Multiple Receiver (STMR) arrays have been shown before to have the ability to locally characterize the stiffness properties of a composite material with anisotropy. The STMR arrays have also been demonstrated for SHM applications using phase reconstruction techniques. The guided ultrasonic Lamb waves are used where the central piezoelectric wafer-active sensor (PWAS) emits the guided waves, and the other PWAS sensors receive the Lamb wave signals. In the current work, this technique has been extended to the determination of global elastic moduli using the Lamb wave S0 and A0 mode signals that are reflected from features in the structure such as edge of the plate, bolt holes, etc. that are known apriori using STMR array and then reconstruct the unknown defects present on the structure using the same sensor array. The reconstruction of elastic moduli is accomplished using a Genetic Algorithm (GA) based inversion algorithm that optimizes an objective function for a particular configuration of the STMR array and the elastic moduli of the component. The solution to this inversion is the global elastic moduli of the composite which is then used to determine the unknown defects in the test component. Simulations were carried out using S0 and A0 mode velocity data for composite layups such as unidirectional, cross-ply, and quasi-isotropic graphite-epoxy composite layups. The inversion algorithm was tested using the simulated edge reflector data and found to agree well with the expected values. Experimental validation has been performed on 3.15 mm quasi-isotropic graphite-epoxy composite. © 2010 American Institute of Physics.

The capability of embedded piezoelectric wafer active sensors (PWAS) to perform in-situ Nondestructive evaluation (NDE) for structural health monitoring (SHM) of fiber reinforced polymer (FRP) composite plate like structures is explored. The basic principles of Lamb wave transmission and reception with PWAS transducers were verified with simple laboratory experiments, performed on both isotropic and anisotropic plates. In the second case, Noncontact measurements for Lamb wave sensing using Laser Doppler Velocimeter were explored. © 2007 American Institute of Physics.

Feature-guided waves (FGW) have emerged as a promising technique for structural health monitoring as they are well confined in local features such as bends and welds. In this paper, we propose and demonstrate a novel method for identifying defects in 90 transverse bent structures through FGW detection using fiber Bragg gratings. Our experimental results are in excellent agreement with results predicted using 3D-Finite Element (FE) simulations of the bent structure.

The feasibility of imaging low-velocity impact damage in thin multi-layered composite plates using Lamb wave tomography is explored. Low-velocity impact damages (6-20 J) are induced by the drop-weight method on graphite fiber epoxy matrix composite laminates of cross-ply and quasi-isotropic lay-up and the damage assessment is carried out by conventional C-scans and ultrasonic Lamb wave tomography. Lamb wave based tomography based on modified cross-hole sensor configuration is used to image the damage as a function of impact energy for graphite composite plates. The reconstructed tomograms are able to capture the lateral extent of the damage even for impact energies as low as 11 J in a 2 mm graphite epoxy quasi-isotropic lay-up sample that can be classified as barely visible impact damage. Copyright © 2006 SAGE Publications.

Recent literature shows that low-frequency ultrasonic guided waves experience mode confinement and loss of axi-symmetry in pipes with axially uniform features such as eccentricity. Considering extended wall loss as a case of uniform eccentricity, this paper proposes to monitor pipe integrity by measuring changes to the modal structure of low-frequency axisymmetric L(0,2) longitudinal guided waves. Fiber Bragg gratings are shown to be effective in detecting changes to L(0,2) modal characteristics, providing a novel route to health monitoring of pipe assets.