Environmental and cost-saving advantages derived from the use of composites attract aerospace and automotive industries as these materials offer significant structural and aerodynamic advantages over traditional metal structures. The composites industry, however, is concerned with the manufacturing processes as they cannot provide fast enough cycle time to match metal alloy processes. Our research aims to develop a sensing technology in the form of a reusable in situ cure monitoring and assessment system that can predict the formation of manufacturing defects and monitor the degree of cure. Thin-film material is chosen from various PTFE-based material by prioritizing the debonding effect and signal transmission through the composite part. Then, the film is used to sandwich piezoelectric actuators and sensors to monitor out-of-autoclave carbon fiber composite plates using ultrasonic Lamb waves by temporarily adhering to the manufactured part creating an effective electromechanical coupling between the sensing film and the laminate. Initial results, through the analysis of the fundamental antisymmetric A0 mode at low frequencies, indicate that analyzing the velocity and amplitude of these waves over cure time determines gelation and vitrification points. Experimental results have also proved the feasibility of using such a reusable film for different curing cycles, always determining certain cure parameters.

Composite materials are incorporated in various applications and their industry is widely growing. They offer cost savings and are more environmentally friendly than conventional metal structures. Some of the concerns this industry faces are the energy and time spent on long curing cycles to achieve permanent bonding between the matrix and fibers. In our previous work, a reusable sensing polytetrafluoroethylene (PTFE) system that can monitor the degree of cure of the composite while curing was developed and tested through Lamb waves analysis. This thin film is now used to monitor the same cure parameters for a shorter curing cycle than that suggested by the CFRP manufacturer. The results show that the three cure parameters: Minimum viscosity, full gelation, and vitrification are offset by the same time deducted from the cycle, highlighting the feasibility of using such technology. To verify the viability of this approach, tensile testing and dynamic mechanical analysis are performed on these composites. Tensile testing results show that the average tensile modulus for the shortened cycle is of similar values if not slightly higher than that of the normal cycle. Dynamic mechanical analysis (DMA) results verify both previous conclusions: Time shift of cure parameters and enhanced mechanical properties of the shortened cycle.

Advanced carbon fiber composites are renowned for their great tenacity as, although being thin, they provide great strength, keeping structures light in weight. The composites industry struggles with longer cure times when compared to other traditional material production. In this study, a computational model for a carbon fiber reinforced polymers (CFRP) plate is developed to imitate experimental monitoring of its cure cycle and degree of cure. The CFRP storage modulus is measured during the curing cycle with the aid of dynamic mechanical analysis, and its trend is incorporated into COMSOL combined structural and electrostatics multiphysics to replicate the same mechanical fluctuations during oven curing. Then, Lamb waves are excited and sensed via sandwiched piezoelectric transducers in a reusable Polytetrafluoroethylene sensing film to monitor the structural health of the structure. Minimum viscosity, gelation and vitrification are cure parameters observed from analyzing voltage and velocity curves of the A0 mode of the sensed signal. The cure cycle is trimmed, and the same cure parameters are shown offset by the 1 h deducted, proving that the numerical model is valid. Further analysis of the numerical voltage and velocity curves suggests an additional cure parameter defined as “gelation initiation” when compared directly to the experimental trends. Additionally, the decomposition of different wavefield modes is scrutinized to describe their scattering throughout the layered structure. Results show a new entrapped antisymmetric mode appearing inside the CFRP laminate at the start of the cure, which suggests that the previously studied A0 mode had been initially converted from the CFRP S0 mode.

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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 A0 mode amplitude is more sensitive towards the epoxy cure parameters.