To ensure a strong adhesive bond, most standards and adhesive manufacturers specify a maximum adhesive gap of 1 mm when bonding fiber reinforced composite structures. In manufacturing large components, such as joining two halves of wind turbine blades, meeting this gap tolerance specification is impractical; gaps larger than 10 mm are common in large adhesively bonded composite structures using state-of-the-art manufacturing techniques. Currently, there is a lack of fundamental understanding of the failure mechanics of adhesive gaps larger than 3 mm. To create such understanding, glass fiber – acrylic thermoplastic composite panels bonded using different epoxy adhesives within single-lap joint samples with adhesive thicknesses of 0.1 mm, 0.3 mm, 1 mm, 3 mm, 5 mm, and 10 mm were sheared to failure. A transition from cohesive to adhesive failure was observed to occur about 1 mm to 3 mm joint thicknesses. Plotting the shear stress normalized by the ratio of the joint width to thickness as a function of the joint thickness normalized by the joint length is shown to result in the ability to fit simple empirically derived models of the cohesive-to-adhesive failure transition, regardless of the adhesive. Furthermore, using these normalized variables, all the observed cohesively failed specimens collapse to a single master curve, as do the adhesively failed specimens.

This collection of case studies presents a brief introduction of fundamental concepts for four SMA-based applications in the aerospace, energy, and medical fields designed by students and facilitated by professionals. The Consortium for the Advancement of Shape Memory Alloy Research and Technology (CASMART) Student Design Challenge is used as an outreach strategy to promote the implementation of state-of-the-art designs with SMA technology and is meant to inspire the next generation of SMA research. Student design challenge teams address real-world problems facing the SMA community and receive guidance and feedback from CASMART members. Student teams’ hardware and materials deliverables had to meet basic function requirements specific to the application. Key results from seven teams (four hardware designs and three materials designs) highlight the design priorities, processes, and challenges raised during development. The hardware designs used NiTi wires shape set and implemented by the students into prototypes for deployment and reorientation mechanisms in small satellites, linear generators to save energy, and a self-apply tourniquet design. Materials development explored the processability and material properties of CuAl-based and NiTi-based alloys for passive actuators in a deployment and reorientation mechanism for a small satellite, energy recovery from waste heat, and a pseudoelastic spinal curvature correction device.

The project demonstrated how to “close the loop” oncarbon fiber by integrating industrial carbon fiber scrap into new functional components in an automotivelightweighting application. The project serves as a validation of discontinuous recycled carbon fiber in a commercial context, while generating comprehensivematerial data throughout the production chain.To this end, the project exhibited increasing complexity asmaterial evaluation progressed from benchtop to commercial scale through full-scale part production, with key material properties thoroughly characterized throughout the process. Of particular focus was the form of the fiber that was fed into compounding, as recycled fiber has historically been problematic to feedat commercial-scale. Carbon fiber is energy-intensive to manufacture, so reuse of existing fiber materialcan reduce costs and increase sustainability. Additionally, by integrating recovered short fiber into a thermoplastic, regrind processes can be used to providefeedstock for later generations of product. While regrind plastics are not “infinitely recyclable”, reusing themanufacturing scrap over several generations of products can greatly increase material sustainability andlower the fractional embodied energy of each successive product. As such, this project supports the IACMI technical goals of (1) 25% lower carbon fiber-reinforced polymer (CFRP) cost, (2) 50% reduction in CFRP embodied energy, and (3) 80% composite recyclability into useful products. The initial stage of the project involved downselecting surface treatment (sizing) chemistries. Sizing evaluations were performed on Vartega’s chemically recycled intermediate modulus fiber along with standard modulus dry scrap whichwas oversized with sizing provided by Michelman. More dramatic improvements from sizing were found on the standard rather than the intermediate modulusfibers. The strength of the chemically recycled individual fibers were evaluated by Michelman and ORNLthrough single fiber testing and found to be comparable to similarly evaluated virgin fibers. UDRI’s mechanical testing on injection molded test specimens identified similar mechanical properties and fiber distribution relative to benchmark specimens. Additional surface chemistry tests and visualizations were performed by the Colorado School of Mines to confirm closeconformance between the benchmark and recycled-fiber specimens. As the mechanical test results exceeded the 80% threshold established as the go/no-go(GNG), the project scale was increased to use commercial-scale equipment that would both better characterize the manufacturing utility of the target product format and allow qualitative assessment of a complex commercial part.An upscaled compounding evaluation was performed with a 27 mm twin-screw compounding extruding using oversized standard modulus fibers that were formatted to improve bulk solids transfer. The project team anticipated that milestonemechanical benchmarks could be achieved given the favorable performance of the sized standard modulus material identified in the initial micro-compounding trials. While the mechanical performance did meetthe milestone target for that phase of the project, mechanical properties for this standard modulus-basedcompound were still less than those of the Ford specification. To compare the performance, the project team oversized intermediate modulus dry fibers and compounded them with the project resin at BASF using a 40 mm compounder. Test specimen mechanicalperformance exceeded the targets laid out in both the project milestone and the Ford specification. A series of prototype parts were successfully molded, albeit with instances of short shot components due to thehigh thermal conductivity of the carbon fiber compared to glass fiber for which the prototype tool was designed for.The project demonstrated that recycled carbon fiber is a viable option in fiber reinforced compound, providing greatly increased strength and modulus for applications that require them. The “agglomerated” format that facilitated effective bulk solids transfer of recovered fiber showed promise for industrial application.

Recycled carbon fiber has historically proven challenging to integrate into composite manufacturing due in no small part to the low-density, randomly oriented, discontinuous fiber format that results from typical recycling. Discontinuous fiber requires the use of alternative technologies than those traditionally applied to continuous fiber (e.g., hand lay-up, winding). Extrusion compounding is one such applicable technology, but material transfer into the system requires alternative feeding equipment or the use of an altered procedure as trialed in this research. In this study, an injection molding compound for automotive applications was prepared with recycled carbon fiber and compared against an existing commercial compound. Input fibers and molded compound were evaluated for mechanical performance, while relevant variables such as compounded fiber alignment and aspect ratio were compared to the existing baseline material to confirm a like-for-like composite material structure. Analysis indicates that recycled fiber performs similarly to virgin fiber reinforcement, demonstrating that recycled fiber may be a viable drop-in replacement for short-fiber discontinuous applications.

Design and development of innovative material compositions and mechanisms based on shape memory alloys (SMAs) were accomplished as part of the Consortium for the Advancement of Shape Memory Alloy Research and Technology (CASMART) 2nd student design challenge. The challenge consisted of developing an expandable space habitat where the retention and deployment of such structure employs the use of SMAs. Student groups were provided with an initial set of requirements and given an option to design the material or design with the material, within six months. This paper collectively documents the evaluation, alloy selection, fabrication, testing, and implementation processes of three teams, including the tools and information used to successfully design, develop, and implement SMA material systems and habitat technologies. For the mechanism design, students used a combination of superelastic rods and shape memory springs/wires to design collapsible rings to fold and deploy the habitat. Publicly available design tools were used to size the SMA components based on the provided loading scenarios. For the material system development, compositions based on CuAlMn, NiTiFe, NiTi and NiTiSn alloys were explored and designed to satisfy the given set of requirements. Details pertinent to these designs are described in this work, along with lessons learned.