Lamb-wave Embedded Nde with Piezoelectric Wafer Active Sensors for Structural Health Monitoring of Thin-wall Structures

Piezoelectric wafer active sensors (PWAS) are inexpensive, non-intrusive, unobtrusive devices that can be surface-mounted on existing structures or inserted between the layers of new composite structures. The PWAS can be used in both active and passive modes. PWAS generate and detect Lamb waves and enable the development of embedded NDE concepts. This paper will present two embedded NDE concepts based on the PWAS technology and Lamb waves approach. The first concept utilizes traveling Lamb waves and could be described as embedded ultrasonics. It is shown that embedded PWAS are able to reproduce most of the conventional ultrasonic techniques, such as pitch-catch, pulse-echo, and phased array. Several experiments using Lamb waves traveling in thin-wall structures are presented. INTRODUCTION Embedded nondestructive evaluation (E-NDE) is an emerging technology that aims at performing NDE testing with embedded ultrasonic transducers. E-NDE will allow the use of ultrasonics methodology in automated long-term structural health monitoring (SHM). SHM of aircraft structures requires small, lightweight, inexpensive, unobtrusive, minimally invasive sensors to be embedded in the airframe with minimum weight penalty and at affordable costs [1]. Such sensors should be able to scan the structure and identify the presence of defects and incipient damage. Current ultrasonic inspection of thin wall structures (e.g., aircraft shells, storage tanks, large pipes, etc.) is a time consuming operation that requires meticulous through-thethickness C-scans of large areas. One method to increase the efficiency of thin-wall structures inspection is to utilize guided waves (e.g., Lamb waves) [2]. Guided waves propagate along the mid-surface of thin-wall plates and shallow shells. They can travel at relatively large distances with very little amplitude loss and offer the advantage of largearea coverage with a minimum of installed sensors [3]. Guided Lamb waves have opened new opportunities for cost-effective detection of damage in aircraft structures [4]. Traditionally, guided waves have been generated by impinging the plate obliquely with a tone-burst from a relatively large ultrasonic transducer using Snell’s law to achieve mode conversion. Conventional Lamb-wave transducers (wedge and comb probes) are relatively heavy and expensive. They cannot be considered for large-area deployment on an aircraft structure as part of an SHM system. Hence, a different type of sensors must be developed for SHM systems, such as the piezoelectric wafer active sensors (PWAS). PIEZOELECTRIC WAFER ACTIVE SENSORS – PWAS In recent years, several investigators [5,6] have explored the generation of Lamb waves with piezoelectric wafer active sensors (PWAS)[7,8]. PWAS are inexpensive, nonintrusive, un-obtrusive, and minimally invasive devices that can be surface-mounted on existing structures, or inserted between the layers of lap joints or inside composite materials. Typical PWAS weigh around 68 mg, are 0.2 mm thick, and cost $7. PWAS operated on the piezoelectric principle that couples the electrical and mechanical variables in the material (mechanical strain, Sij, mechanical stress, Tkl, electrical field, Ek, and electrical displacement Dj) in the form , E ij ijkl kl kij k T j jkl kl jk k S s T d E