Title: Recent Advances in the Use of Piezoelectric Wafer Active Sensors for Structural Health Monitoring Authors: Victor Giurgiutiu, PhD

Piezoelectric-wafer active sensors (PWAS) are small, inexpensive, non-invasive, elastic wave generators/detectors that can be easily affixed to a structure. Piezoelectric-wafer active sensors are broadband non-resonant devices. They can be deployed into sensor arrays and sensor networks. They can be connected to data concentrators and wireless communicators. However, the PWAS development is not yet complete, and a number of issues have still to be resolved. This paper will present recent advancements obtained by the authors in using the PWAS for structural health monitoring. Several directions are explored. The tuned selective excitation of A0 and S0 Lamb wave modes is theoretically predicted using a space-domain closed form solution and experimentally verified using the pitch-catch approach. For crack detection, tuned S0 Lamb waves were used, since this wave type seems to be more sensitive to through-the-thickness cracks. The low-dispersion S0 mode was generated in a PWAS phased array in order to create a virtual sweeping interrogative beam that is able to detect the presence of cracks in a large metallic plate. The embedded ultrasonic structural radar (EUSR) algorithm is used to process the data. The crack detection was performed with the pulse-echo method. Both flat and curved plates are explored. As expected from the theoretical analysis, it is shown that the application of shallow curvature (2m < R < 4 m) does not noticeably modify the crack detection capabilities of the EUSR method. For corrosion detection, tuned A0 Lamb waves were used, since these type of waves seems to be more sensitive to thickness changes experienced during corrosion progression. The high-dispersion A0 modes, which are strongly influenced by the plate thickness changes, are used to detect the presence of corrosion. A 15-mm diameter corrosion pit is generated in a 1-mm aluminum plate with incremental depth ranging from 0.04 to 0.36 mm. The paper ends with conclusions and proposal for further work. Victor Giurgiutiu, Lingyu Yu, University of South Carolina, Department of Mechanical Engineering, Columbia, SC 29208, USA, 803-777-8018, FAX: 803-777-0106, E-mail: victorg@sc.edu James L. Blackshire, Dustin T. Thomas, John T. Welter, Air Force Research Lab, Materials and Manufacturing Directorate, NDE Branch, Wright-Patterson AFB, OH 45433-7817, USA PIEZOELECTRIC WAFER ACTIVE SENSORS (PWAS) In recent years, several investigators[1][2][3] have explored the generation of Lamb waves with piezoelectric wafer active sensors (PWAS)[4]. PWAS are inexpensive, non-intrusive, 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 $15. 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 S s T d E = + , T j jkl kl jk k D d T E ε = + (1) where E ijkl s is the mechanical compliance of the material measured at zero electric field (E = 0), jk ε is the dielectric permittivity measured at zero mechanical stress (T = 0), and dkij represents the piezoelectric coupling effect. For embedded NDE applications, PWAS couple their inplane motion (which is excited through the piezoelectric effect by an oscillatory applied voltage) with the Lamb-waves particle motion on the material surface. Lamb wave modes can be either symmetric (S0, S1, S2, .), or antisymmetric (A0, A1, A2, .). PWAS transducers can act as both exciters and detectors of the elastic Lamb waves traveling in the material. For NDE, PWAS can be used as both active and passive probes. Laser scanning interferometer visualization [5] of the Lamb wave field around a PWAS bonded to a thin aluminum plate (Figure 1) reveals the typical omnidirection circular-crested wave propagation pattern expected from a point source. PWAS operation is different from that of conventional ultrasonic probes: 1) PWAS are coupled to the Lamb waves through in-plane surface strains; conventional ultrasonic transducers are coupled through out-of-plane stresses that acts normal to the material surface 2) PWAS are strongly coupled with the structure and follow the structural dynamics; conventional ultrasonic probes are relatively free from the structure and follow their own dynamics 3) PWAS are non-resonant wide-band devices, while conventional ultrasonic probes are narrow-band resonant devices The main advantage of PWAS over conventional ultrasonic probes lies in their small size, lightweight, low profile, and small cost. In spite of their small size, these novel devices are able to replicate many of the functions performed by conventional 0 5 10 15 20 25 30 35 0 10 20 30 40 50 60 70 80 Position (mm) D is pl ac em en t ( nm ) Figure 1. Laser scanning interferometer visualization of Lamb wave field emanating from a PWAS [5] ultrasonic probes, as proven by the proof-of-concept laboratory demonstrations described further in this paper. TUNED LAMB-MODE EXCITATION WITH PWAS TRANSDUCERS The excitation of Lamb waves with PWAS transducers can be studied by considering the excitation applied by the PWAS through a surface shear stress 0 ( ) i t x e ω τ τ = . Using the space-domain Fourier transform analysis of the basic Lamb wave equations yields the strain and displacement solutions in the form[6][7] ( ) 1 ( , ) 2 2 i x t S A x S A N N i x t e d D D ξ ω τ τ ε ξ π μ ∞ − −∞   − = +     ∫ (2) where τ is the Fourier transform of τa(x), ( ) 2 2 2 2 / L p c ω ξ = − , ( ) 2 2 2 2 / T q c ω ξ = − , while 2 ( 2 ) / L c λ μ ρ = + and 2 / T c μ ρ = are the longitudinal and transverse wave speeds, λ and μ are Lamb constants, ρ is the mass density, and