Fatigue crack detection in thick steel structures with piezoelectric wafer active sensors

Abstract

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 electromechani cal 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 defi ned 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 app lied to a 13-mm CT specimen subjected to accelerated fa tigue 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, stru ctural health monitoring.