Piezoelectric-based impedance monitoring: A critical analysis of indirect electromechanical impedance measurements in non-destructive evaluation

Electromechanical impedance (EMI) measurements have revolutionized non-destructive evaluation (NDE) of manufactured parts. By bonding piezoelectric elements to structures, EMI analyzes dynamic responses to identify manufacturing anomalies. Particularly effective in additive manufacturing (AM), EMI detects defects like mass alterations and internal porosity. In structural health monitoring, it continuously compares impedance signatures to baseline, detecting emerging defects over time. This innovative approach addresses limitations of conventional methods, providing a powerful tool for assessing material integrity. The practical application of EMI however faces challenges due to the need for direct instrumentation of each part with piezoelectric elements, which introduces time, cost, and variability issues. To address these, the concept of indirect EMI (IEMI) has emerged, utilizing a secondary structure or instrumented fixture to temporarily couple with the part under test. This approach allows for testing multiple specimens, reduces labor requirements, and facilitates process automation. This paper investigates the sensitivity of IEMI to defects in the specimen, focusing on fixture design, instrumentation process, and clamping force calibration. Various interface conditions and clamping forces are explored to understand their impact on defect detection capabilities. Experimental results indicate that interface conditions significantly influence IEMI measurements, with metal-on-metal contact providing the best sensitivity. Additionally, the clamping force is found to affect the impedance signature, emphasizing the need for consistent force application during measurements. Overall, this study underscores the potential of IEMI as a viable NDE solution, highlighting its ability to detect defects in manufactured parts while addressing practical implementation challenges. Future work will focus on optimizing fixture design for enhanced sensitivity.