Fatigue crack detection in thick steel structures with piezoelectric wafer active sensors M. Gresil, L. Yu, V. Giurgiutiu Department of mechanical engineering, University of South Carolina, USA. ABSTRACT This paper presents a set of numerical and experimental results on the use of guided waves for structural health monitoring (SHM) of crack growth during a fatigue test in a thick steel plate used for civil engineering application. The capability of embedded piezoelectric wafer active sensors (PWAS) to perform in situ nondestructive evaluation (NDE) is explored. Numerical simulation and experimental tests are used to prove that PWAS can perform active SHM using guided wave pitch-catch method and passive SHM using acoustic emission (AE). Multi-physics finite element (MP- FEM) codes are used to simulate the transmission and reception of guided waves in a 1-mm plate and their diffraction by a through hole. The MP-FEM approach permitted that the input and output variables be expressed directly in electric terms while the two-ways electromechanical conversion was done internally in the MP-FEM formulation. The analysis was repeated for several hole sizes and a damage index performances was tested. AE simulation was performed with the MP-FEM approach in a 13-mm plate in the shape of the compact tension (CT) fracture mechanics specimen. The AE event was simulated as a pulse of defined duration and amplitude. The electrical signal measured at a receiver PWAS was simulated. Daubechies wavelet transform was used to process the signal and identify its Lamb modes and FFT frequency contents. Experimental tests were performed with PWAS transducers acting as passive receivers of AE signals. The 8-mm thick flange of an I beam was instrumented on one side with PWAS transducers and on the other side with conventional AE transducers (PAC R15I) acting as comparison witnesses. An AE source was simulated using 0.5- mm pencil lead breaks; the PWAS transducers were able to pick up AE signal with good strength. Subsequently, PWAS transducers and R15I sensors were applied to a 13-mm CT specimen subjected to accelerated fatigue testing. The PWAS and R15I transducers signals were collected with PAC data acquisition system using the AE-win software. Comparative results of AE hits and source localization from the PWAS and R15I sensors are given. Active sensing in pitch catch mode was applied between the PWAS transducers installed on the CT specimen and damage indexes were calculated and correlated with physical crack growth as measured optically. The paper finishes with summary, conclusion, and suggestions for further work. Keywords: Acoustic emission, finite element modeling, fatigue test, crack, piezoelectric, structural health monitoring. 1. INTRODUCTION The current stage of bridges in the United States calls for the implementation of a continuous bridge monitoring system that can aid in timely damage detection and help extend the service life of these structures. A typical monitoring system would be one which enables non-invasive, continuous monitoring of the structure. Structural health monitoring (SHM) is an emerging technology that can be used to identify, locate, and quantify damage in a structural member or system before failure occurs. Active SHM systems using interrogative Lamb waves are able to cover large areas from a single location making such systems cost effective and efficient. Another advantage is that Lamb waves provide through-the- thickness interrogation which allows detection of internal defects in materials. Passive SHM monitors acoustic emission (AE) and arriving as guided waves generated by the crack pops. Acoustic emission and Lamb wave are difficult to characterize because of the complex nature of the signals. AE occurs due to stress waves generated when there is a rapid release of energy in a material during a fatigue crack test. Piezoelectric wafer active sensors (PWAS) are used for both active and passive SHM. However, Lamb waves present some difficulties: they are dispersed, at a given frequency, and thus several modes can propagate at different speeds. Work has be done to establish analytically the dispersion curves [1- 6], to validate experimentally [7] and to study the effect of dispersion over long distances [8]. The phenomena of interaction between the ultrasonic wave and the defect and/or the structure, leading to a complex signature (reflection, diffraction, mode conversion, etc.) must be simulated to achieve a specific response signal actually received by sensor. Many authors have already investigated the interaction of Lamb modes with a single defect like crack, notch or circular Nondestructive Characterization for Composite Materials, Aerospace Engineering, Civil Infrastructure, and Homeland Security 2011, edited by H. Felix Wu, Proc. of SPIE Vol. 7983, 79832Y · © 2011 SPIE CCC code: 0277-786X/11/$18 · doi: 10.1117/12.882137 Proc. of SPIE Vol. 7983 79832Y-1