Cure monitoring of an epoxy adhesive film for composites structural bonding

Structural adhesive bonding is a growing trend in the aerospace industry. The main benefits of using such technique are elimination of drilled holes and thus micro-cracks and stress concentrations, reducing weight, and having faster assemblies. However, it is still not extensively used in critical components of various structures as inspections may become exhaustive. This is why further structural health monitoring techniques are being developed to monitor adhesive bonded joints. In this work, we monitor, in real-time, the curing cycle of adhesives used for structural bonding of carbon fiber reinforced polymers (CFRP) via guided Lamb waves. The in-situ monitoring is done experimentally inside an oven using piezoelectric transducers on a bonded structure composed of two pre-cured woven CFRP plates adhered together using structural adhesive film. The degree of cure and other cure parameters such as gelation and vitrification of the adhesive are extracted experimentally from the velocity and voltage curves. Then, using a computational finite element model in COMSOL, we further investigate the monitoring results by combining solid mechanics and electrostatics modules, and actuating a single anti-symmetric mode. The computational cure monitoring process of the adhesive is built by importing dynamic mechanical analysis (DMA) results into the numerical model. Furthermore, co-cure monitoring of both uncured CFRP and adhesive film is studied via a reusable flexible PTFE sensing film that was previously designed. Results of the latter experiment show that the A(0) mode amplitude is more sensitive towards the epoxy cure parameters.