Using embedded polyvinylidene film sensors and metal foil strain gages to monitor the resin curing process in thick fiberglass-resin composite laminates

Abstract

The composite manufacturing process is a complex one, with many factors affecting the integrity of the finished components. Most current composite laminate manufacturing processes involve injecting liquid matrix into the fiber laminates. The matrix material then undergoes a curing process, which can create residual stresses and strains. In order quantify these effects, a method of embedding strain gages into the laminate was developed. Metal foil strain gages (MFSGs) and polyvinylidene film (PVDF) sensors were characterized and embedded into the composite laminate. These sensor circuits were designed to compensate for the temperature fluctuations seen during the cure process, and placed so that they could be used for structural health monitoring purposes after the cure process. To facilitate this, a double bagging technique was developed for extracting the sensor wires from the mold that allowed for easy egress of the wires, and enabled lead wire access. PVDF sensors were investigated for usefulness in cure and health monitoring applications. It was determined that, when used with simple charge amplifier circuits, the charge decay of the circuits prevented useful long-term results from being obtained; however, they show promise as shock-loading, short-term sensors. The composite laminate strains were monitored throughout the cure process, and the MFSGs indicated that residual strains were apparent in the laminate as a result of the matrix cure.