PSI - Issue 70

Anubhav Kumar Singh et al. / Procedia Structural Integrity 70 (2025) 572–579

574

Rogers and his colleagues first proposed the concept of smart materials in 1990 at a US Army Research Workshop. They demonstrated that smart materials have the capability to change certain physical characteristics in a specific way when triggered by specific stimuli. This intelligent behavior is typically embedded in the material during manufacturing by adjusting its composition, adding specific defects, or altering its internal structure so that it will respond in a controlled way. Active and passive are two categories of smart materials. Active smart materials are capable of directly transforming energy from one form to another. For instance, they may alter their shape or stiffness upon the application of electric or magnetic fields. Piezoelectric materials and shape memory alloys are some of the widely used active smart materials. These may be utilized as sensors, actuators, or force transducers. On the contrary, passive smart materials, like fiber-optic sensors, are capable of sensing a change in the surrounding environment but are unable to create mechanical action. 1.2. Electro-Mechanical Impedance (EMI) Technique EMI techniques provide a non-destructive means to conditionally monitor the state of structures. Under such techniques, a piezoelectric (PZT) patch is fixed to a structure ’s surface in strong adhesive form with epoxy. An impedance analyzer (LCR meter) provides a continuous voltage signal to the patch. This signal makes the patch and the material surrounding it oscillate, and the vibrations are transferred to an electric response, two components of which are measured as conductance and susceptance. If these electrical responses are charted over a frequency range, they create a unique signature corresponding to the structure. Provided that the structure itself is not damaged, this signature remains constant. Any variation in the signature means that there has been some damage, which informs us of potential problems that need to be addressed. The admittance signature, over a range of frequencies, varies the dynamic properties of the structure, such as stiffness, damping, and mass, which are affected by corrosion or cracks. One important advantage of the EMI technique is that it has high sensitivity to slight variations in the structure and hence can identify even moderate damage. It provides a non-invasive, low-cost, and real-time method for monitoring structural integrity. However, one of the serious challenges when EMI is applied for damage detection is that the impedance measurement data obtained tend to be affected by various factors, including environmental conditions and operational loads, as well as sensor placement. PZT-structure interactions were initially studied through a static model. Crawley and de Luis (1987) examined the application of piezoelectric sensors, particularly their capacity to regulate vibration in smart constructions. It offers analytic models for estimating structural responses to applied voltages and points out that actuator performance are unaffected by structure size. It further observes that actuators embedded in composite materials can decrease the strength of the material by 20% without much impact on the overall elastic modulus. Liang et al. (1997) introduced the impedance approach for analyzing active material systems, highlighting its advantages over static and dynamic finite element methods. This approach accurately reflected the mechanics of actuator-structure interactions and simplified the analysis of energy consumption in smart materials. J.A. Fairweather et al. (2000) discussed an impedance approach predicting the structural response to piezoelectric patch actuators as a solution to the constraint of needing analytical calculations of structure impedances. Based on finite element analysis, the approach computed the impedances through eigen-vectors, which are applicable to wider use across more structures with limited closed form representations. The method was experimentally verified for two-dimensional structures, proving its ability to analyze multiple responses and actuator locations from one FEA run. The formulated equations for recovering the impedances and responses of the structure proved to have accurate predictions with results within the range of the experimentally derived responses. To overcome the limitations of previous models, Bhalla et al. (2004) gave an impedance-based simplified PZT-structure interaction model and improved the comprehension of the structural coupling with PZT patches. It specified an updated process for the model for structural identification and provided a methodology for the experimental extraction of the mechanical impedance. PZT sensors are lightweight, compact, and fragile and thus suitable for both retrofitting existing structures and new construction work. Due to their low power demands, PZT sensors can function for long periods, even in inaccessible locations or those that are far away. The PZT material, or (Pb(Zr 1-x Ti x )O 3 ), comprises non centrosymmetric crystals with the converse piezoelectric effect, i.e., they deform mechanically upon exposure to electric fields. The fundamental equations for piezoelectric materials are given by Ikeda (1990).