Joining techniques for internal bonding of components into carbon fibre-reinforced polymer structures with limited access

Abstract

Hall spars, a leading innovator in the composite mast-building industry with a long history of successful projects, provided a challenge which is the inspiration of this thesis. This thesis aims to contribute to the challenge: Joining techniques for the internal bonding of carbon fibre-reinforced polymer components into carbon fibre-reinforced polymer structures. ”Internal bonding” can also be called ”bonding.

Currently, the epoxy adhesive film is used for these types of joints. Adhesive co-bonding raises challenges due to the limited access to the mast. Challenges concerning surface preparation, low mechanical strength, reliability and labour intensity. Other techniques will be reviewed for the internally bonding limited access hollow tubes. The aim is that no new tooling is required, and the currently used materials and process parameters can be used. The thermoset resin used is an epoxy carbon fibre prepreg.

According to the literature study, potential joining techniques for this application are joining with partially cured thermosets and joining with fusion welding of thermoset composites. Differential scanning calorimetry measurements are performed to analyse the thermoset resin. Material characterisation is required to apply the joining methods with the currently used material and process parameters of Hall Spars. Process parameters for the curing model, and glass transition temperature model are derived. Using this material model allows creating the insight that partial cured joining with the thermoset resin of Hall Spars is not feasible. The structural integrity of the joined parts can not be guaranteed in the curing cycle.

Polyetherimide is used as thermoplastic material to apply the fusion welding technique on the thermoset. This thermoplastic film is co-cured with the thermoset and used as a coupling layer to join the two thermoset parts. Performing an interphase analysis with the scanning electron microscope indicates interphase formation at the PEI/epoxy interface. The higher the isothermal curing temperature, the thicker this interphase. Gelation causes the interphase formation to slow down. The interphase formation at the interface is not been observed as expected based on the literature review. This required a test if the interphase formation would have occurred at a smaller scale than the scanning electron microscopy could observe. This can be achieved by performing single-lap shear experiments. These joints are processed according to Hall Spars process parameters. Joints created with the fusion coupling technique with the PEI film show promising results. However, due to high void content, a lower ultimate lap shear strength is observed. Therefore it is advised to investigate a curing cycle with a second dwell phase. This changes the viscosity profile of the resin. Therefore allows the trapped air at the PEI/epoxy interface to leave the joint. This could potentially lead to more reliable joints.