Joining of composites can be a challenging issue. If adhesives are used, the joints are permanent and cannot be undone. If they need to be undone, inserts are often used and these inserts increase cost and weight. Additionally, fibers can be cut in the process leading to a part with weakened mechanical properties.

Schematic representation of the concept

Even with these drawbacks, most components that need to be joined are threaded together, allowing for removal of the parts at a later time. However, threaded connections are costly to design and manufacture, and are often the location of fatigue failures due to their inherent stress concentrations. From an environmental and cost standpoint, there is a large waste associated with the removal of material to form the threads, even more so in sectored threads, where up to half of the machined thread is then cut away. Additionally, threaded joints often require grease to seal out environmental contamination and ensure that they can later be disassembled. This effort is aimed at replacing seldom-used threaded connections with a reusable thermoplastic adhesive.

The use of a thermoplastic makes the joining reversible, allowing any connection to be treated almost like a threaded joint, only one that uses heat instead of torque for activation. Recently, using thermoplastics as reusable adhesives has been researched by DoE for application in automobiles. However, that work is focused on thermoplastics for room temperature applications, with no work being conducted on high temperature thermoplastics such as PEEK and Polyimide. The use of localized microwave radiation to heat the thermoplastic will eliminate the need for large furnaces which consume large amounts of time and energy, and ultimately heat other parts of the system that do not need to be heated. Therefore, an investigation was begun into activating high temperature thermoplastics using microwave radiation.

The accompanying figure shows the basic concept. Two adherends are brought together with a graphene-doped thermoplastic between them. The assembly is then subjected to microwaves which excite the graphene nanoplatelets in the adhesive generating heat and causing the adhesive to melt. When the microwave source is removed, the adhesive solidifies joining the two materials. Since the adhesive is a thermoplastic, subsequent applications of microwaves can be used to remelt the adhesive and disassemble the assembly. For adherends that block microwaves, a wave guide would be needed to direct them to the bondline.

While various carbon species can absorb microwaves, nanospecies such as carbon nanotubes and graphene have been investigated most recently because of their highly effective absorption at low weight loadings and ability to improve mechanical properties as well. Microwaves, when incident on an absorptive material, create heating by the interaction of the electromagnetic fields with the molecular and electronic structures of the molecules in the material exposed to the microwaves. The amount, and rate, of heating can be a function of microwave power, frequency, absorption, etc.

This work was done by Andrew Littlefield, Joshua A. Maurer, and Stephen F. Bartolucci for the Armament Research, Development and Engineering Center. ARL-0207

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This article first appeared in the December, 2017 issue of Aerospace & Defense Technology Magazine.

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