PSI - Issue 71

Ajay Patel et al. / Procedia Structural Integrity 71 (2025) 196–202

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The EMI technique is an SHM method that utilises piezoelectric materials, such as PZT, to detect damage in various structures via impedance spectra. This technique is based on the principle that the mechanical resonance of a structure is reflected in the electrical characteristics of piezoelectric transducers owing to the electromechanical coupling between the transducer and structure (Parida and Moharana (2023); Park et al. (2003); Bhalla S. and Soh C.K. (2004). The basic equations (1 & 2) for the coupling between electrical and mechanical characteristics is given by the IEEE standard (1987) as follows: 3 = 3 ̅̅̅̅ 3 3 + 31 1 (1) 1 = 1 ̅̅̅̅ + 31 3 (2) In equation (1) & (2), T 1 represents stress, d 31 denotes the piezoelectric strain displacement, S 1 and D 3 indicate the actuating strain along axis (1) and the piezoelectric charge displacement respectively, whilst E 3 signifies the electric field along axis (3). Additionally, ( ) and ( 3 3 ) symbolise the complex Young's modulus at constant electric field and the complex electric permittivity matrix of the PZT material, respectively. This technique involves coupled field analysis, which considers the interaction between electrical and mechanical fields in piezoelectric sensors and the host structure. Moharana and Bhalla (2012) discussed the use of finite element modelling to consider the contributions of active materials, adhesive bonds, and structural damage in EMI analysis. This study modelled the piezoelectric element and host structure, allowing for a comprehensive examination of the coupled electromechanical system with inclusion shear lag parameter. Its effectiveness varies with the type of material and conditions of application, and ongoing research is addressing these challenges to enhance its reliability and applicability across different structural contexts (Dugnani et al. (2016), Patel et al. (2024)). Corrosion is frequently characterised as an electrochemical process, encompassing both oxidation and reduction reactions. During oxidation, the metal surrenders electrons, resulting in the formation of metal ions. Conversely, in reduction, environmental substances like oxygen or hydrogen ions acquire electrons (Harsimran et al. (2021)). The precise equation for corrosion may differ based on the specific metal and environmental conditions. Nevertheless, a simplified general equation representing the corrosion of a metal (M) can be expressed as equation (3): ( ) + 2 ( ) → MO(s) (3) Iron rusting is the most prevalent form of corrosion. The process of iron rusting can be simplified to a basic chemical reaction, which is described as follows in equation (4): 4 ( ) + 3 2 ( ) → 2 2 3 ( ) (4) In this formula, iron is denoted by Fe, oxygen by O₂, and iron oxide (rust) by Fe₂O₃. Mild steel exhibits a high vulnerability to corrosion, especially in severe conditions. Research has demonstrated that mild steel corrodes at a considerably faster rate in coastal areas compared to inland regions (Roy & Ho, (1994), Harsimaran et al. (2021)). The corrosion process in this environment is primarily driven by the presence of chloride ions, which can penetrate protective layers and accelerate the anodic dissolution of iron. (Subash et al. (2023). EMI has emerged as an effective method for monitoring corrosion in mild steel structures. This technique utilises piezoelectric sensors, such as PZT patches, to detect changes in the structural properties (mass, stiffness and damping) caused by corrosion damage (Talakokula et al. (2015)). The ability of the EMI technique to provide quantitative measurements of corrosion-induced thickness loss and its potential for early-stage corrosion warning makes it a valuable tool for SHM in various applications, including pipelines and marine structures (Weije et al. (2019); Subash et al. (2023)). This study involved the experimental immersion of a mild steel coupon in a 3.5% NaCl solution, with mass loss measurements recorded at 10-day intervals. The observed mass loss was then modelled using finite element method and a PZT patch was attached and simulated using coupled field FEM to determine coupled admittance signatures for corrosion progression over time. The methodology section provides a detailed discussion of these procedures. The innovative aspect of this research lies in its combined approach that integrates traditional experimental corrosion monitoring method with PZT patch-based finite element simulation in realistic mode. While mass loss measurements are a conventional method for assessing corrosion, the incorporation of PZT technology offers the potential for non destructive, real-time monitoring of corrosion progression through the use of signatures and statistical indices. 1.3 Corrosion and its Monitoring