Date of Award

Spring 2021

Document Type

Open Access Thesis

Department

Mechanical Engineering

First Advisor

Michael Van Tooren

Abstract

The use of carbon fiber composites is rapidly increasing in the aerospace industry. Fiber reinforced plastic composites can primarily be classified as thermoplastic and thermoset, both offer unique advantages over one another. Thermosets cure at relatively low temperature and in general, are lower priced than thermoplastic composites. However, the cost of autoclave for the curing of thermosets is a major drawback. On the other hand, thermoplastics are showing higher damage tolerance. They can be re-melted and re-consolidated and have the potential to be re-used / recycled. The repair of TP composites is not developed yet. Therefore, both the types attract various applications in the aerospace industry. Ideally, it could be advantageous to have zones of thermoplastic and zones of thermoset composite in a single structural element. Joining thermoplastic composites to thermoset composites is challenging and the prevailing methods employed obstructs from realizing the complete potential of these materials, this has led to a greater emphasis on the improvement and development of specific joining approaches allowing to get effective mechanical joint properties even when the thermoplastic carbon fiber composite panel is bonded to a thermoset carbon fiber panel.

This research aims to consolidate thermoplastic and thermoset (epoxy) based carbon composites in a single structural element by grafting the thermoplastic surface with Poly(Glycidyl Methacrylate) (PGMA) which would act as a surface activator/intermediator and facilitate reliable joint of thermoplastic and epoxy. This would also, in turn, enable fusion bonding-based assembly of such structures. For successful application of this concept in primary aircraft structures, a good understanding of what yields reliable and predictable strength of the bond between the thermoplastic zones (implants) and the epoxy zones (substrate) in such structural element is required.

To test the hypotheses in this work, PEKK thermoplastic substrate were polymer grafted with PGMA polymer and were joined (mono grafted co-cured) to TS epoxy plate. The surface topology and dispersion properties were evaluated using water contact angle (WAC) measurements before and after each surface treatment of Atmospheric Pressure Plasma Jet (APPJ). The WCA was used as a measure of surface energy. A design of experiments approach was used to find an optimum process parameters of plasma treatment required to achieve consistent surface energy. Using dip coating, PGMA was grafted on the PEKK based thermoplastic composite panels which then was co-cured with epoxy to form Single Lap Shear (SLS) coupons and Double Cantilever Beam (DCB) coupons. Lap shear strength and Mode I Interlaminar fracture toughness G1c were calculated for each surface preparation configuration. To further examine and understand the bond, optical digital microscopy was performed on the joints.

A significant increase in surface energy was observed after the APPJ surface treatments. The plasma and polymer treated “mono-graft co-cured” specimens showed higher lap shear strength and fracture toughness energy over the untreated specimens. Digital microscopy unveiled the bond line cannot be determined and the bond is seamless, even at the microscopic level. The study demonstrated that the combined effect of APPJ and PGMA surface treatment yields higher bond strength for thermoplastic-epoxy interface joint.

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