Modal Characterization of Glass–POE–Glass Laminate Specimens with Different Gel Contents for Non-Destructive Curing Assessment in Photovoltaic Applications

The curing state of polyolefin elastomer (POE) encapsulants influences the mechanical behavior of laminated photovoltaic structures, while conventional gel-content (GC) determination requires destructive testing. This study evaluates vibration-based modal characterization as a potential non-destructive screening approach for distinguishing curing conditions in simplified glass–POE–glass laminates. Five GC levels (25%, 35%, 49%, 68%, and 75%) were investigated, with five independent laminate specimens tested at each level and five repeated modal measurements performed per specimen. The first six vibration modes were characterized using Frequency Domain Decomposition (FDD) and Enhanced Frequency Domain Decomposition (EFDD). The mean natural frequencies exhibited mode-dependent differences across the investigated curing conditions. Mode 1 varied by 2 Hz (1.34%), whereas Modes 2–6 showed peak-to-peak variations of 7–17 Hz (3.07–4.33%), with the largest relative variation observed for Mode 5. Modes 2–5 exhibited an apparent partial frequency recovery at GC = 75% following a decrease toward GC = 68%. However, the absence of retained individual frequency estimates prevented quantitative assessment of experimental variability and statistical confirmation of these differences. Finite element analysis supported modal correspondence, while a calibrated partial-composite-action model provided a structural interpretation of incomplete interlayer shear coupling. The results suggest potential for comparative non-destructive screening, but quantitative prediction of unknown GC requires further validation and uncertainty analysis.