Experimental and numerical assessment of a thermoformed FEP electret for vibration-based structural health monitoring of composite structures

Driven by the demand for lightweight and efficient structures, composite materials are increasingly used in the automotive, aeronautical, and aerospace sectors due to their high stiffness-to-weight and strength-to-weight ratios. In addition to improving structural performance and reducing fuel consumption, composites enable the integration of embedded sensors for smart structures capable of self-monitoring and adaptive control. Conventional piezoelectric sensors, such as lead zirconate titanate (PZT) and Macro Fibre Composites (MFCs), provide excellent electromechanical performance but are relatively stiff, expensive, and difficult to integrate into lightweight structures. Although thermoformed electrets have emerged as a promising polymeric sensing technology, their applicability to vibration-based damage detection remains insufficiently explored. This study evaluates the feasibility of a thermoformed fluorinated ethylene propylene (FEP) electret as a polymeric piezoelectric sensor for damage detection in a thermoplastic carbon fibre composite beam. Experimental modal analyses under free and forced vibration were performed using impact hammer and shaker excitation. The electret response was compared with measurements obtained from an accelerometer and an MFC sensor. A finite element model was validated against experimental intact and added-mass configurations and subsequently extended to investigate numerical damage scenarios, including delamination and through-thickness cracks. The calibrated model predicted the experimental natural frequencies with errors below 7%. The first natural frequency decreased from approximately 81 Hz in the intact beam to 61.8 Hz after the addition of a 15 g mass. The thermoformed electret successfully identified the first two bending modes and detected progressive structural changes through variations in the frequency response functions (FRFs). Although its damage indices were lower than those obtained with the accelerometer and MFC sensor, particularly for low and moderate damage levels, the electret effectively monitored structural changes while offering advantages such as low weight, flexibility, ease of integration, and low cost.