As energy producers turn towards decarbonization, hydrogen is being explored as an alternative fuel. Hydrogen is stored at high pressures to achieve an efficient energy density, which requires high performance composite tanks. There is currently no end-of-life (EOL) option for these tanks, other than landfills. If we want to sustainably position hydrogen energy for the future, EOL is an imperative consideration. Herein, the authors present a method of producing these composite overwrapped pressure vessels (COPV), via filament winding, with the ability to later unwind and recycle them. A vitrimer matrix was used, which has the ability to dissolve into its monomeric constituents, allowing for reuse of both the continuous fiber reinforcement and the depolymerized matrix. Neat resin analysis included dynamic mechanical analysis and rheometry, following casting in a custom-built tool. Filament winding was used to produce tubular and flat specimens using custom-designed mandrels. Mechanical testing on both neat resin and reinforced material was completed, including tensile and short beam shear loadings, determining the performance of a vitrimer-based COPV compared to existing materials.

The tensile behavior of repaired composites and the effects of variables that can be realistically modified during on-site wind turbine blade repairs were studied. Using repairs to increase service life is the most economical way to increase blade sustainability. Understanding what variables should be prioritized is therefore necessary. The repair ply thickness was a focus in this work, comparing the tensile performance and failure mechanisms of thick versus thin plies. Results showed that, when time and conditions allow, using double the number of thinner plies is effective in wind turbine blade composite repairs to fully restore tensile strength and modulus.

The methods and approaches used for composite repairs depend on the sector of industry, and exhibit both common elements and distinctions. Here we consider the repair methods used in four exemplary applications: marine, wind, automotive, and aerospace. Repairs are often overlooked as a means of imparting greater sustainability to composite products, but they are generally the least costly route for doing so. Approaching each industry from a common repairs perspective, the similarities are highlighted while the different approaches are compared. The problems associated with current approaches are examined, along with active research methods for each application. Areas for potential to increase efficiency of repairs through automation and introduction of new materials are identified. The review of repair methods is intended to stimulate new approaches and opportunities to transfer the approaches and practices employed across industries.

Cold spray additive manufacturing coupled with the use of a hybrid bond layer is shown to be useful for metallizing polymer composite surfaces to increase durability. However, achieving sufficient and robust metalpolymer adhesion poses a problem. To address this challenge, a hybrid bond layer (BL), which consisted of Al wire mesh embedded in an epoxy film adhesive with Al filler, was co-cured onto the surface of a carbon fiberreinforced polymer (CFRP) laminate, and subsequently metallized by cold-sprayed niobium (Nb). This work describes the evolution of microstructural defects, failure modes, and shear strength (up to 27.1 +/- 1.8 MPa) as a function of particle impact velocity. Microstructural analysis revealed that increasing particle velocity resulted in more severe BL damage, diminishing shear strength and altering failure modes. Numerical simulation of multiparticle impact provided insight into the dynamic response of the BL to particle impacts causing the BL damage. The effects of impact velocity were confined to the bond layer, and these effects diminished at a deposit thickness of similar to 30 mu m. Understanding particle-BL interactions during CS metallization of CFRP-BL is shown to be key to developing BL design principles to increase CS deposit adhesion to non-metallic substrates.

Hydrogen fuel is becoming more popular as an alternative to fossil fuels, particularly for transportation. Hydrogen needs to be stored at high pressures in high-performance composite over-wrapped pressure vessels (COPV) to maintain efficiency. Currently, the only end-of-life option for these tanks is the landfill, which will become more detrimental as the demandfor and thereby number of hydrogen tanks increases. By incorporating a vifrimer matrix to the carbon fiber overwrap, the tanks can be disassembled and unwound. This process allowsfor full recyclability as both the fiber reinforcements and the matrix can be reused. Filament winding was used to produce both tubular and flat specimens for mechanical testing and process characterization. The tensile strength of longitudinally reinforced vifrimer samples was significantly less than the traditional epoxy counterparts. This decrease was consistent with the trend of neat resin tensile strength differences between the two materials. However, the longitudinal strength properties were expected to be similar, as they are fiber dominated. Results indicated that the processing of the vitrimer led to volume fraction and void content variation. While the vifrimer technology is still under development, this work represents the first step towards fully recyclable COPVs and the ability for recover long tows ofvaluable carbon fibers.

Aerospace vehicles that undergo atmospheric re-entry require thermal protection systems (TPS) to protect them from the extreme environment. Currently, TPS production relies on hand layup techniques which are time-, cost-, and energy-intensive. A method to produce composite TPS via additive manufacturing (AM) is demonstrated, which uses a custom-designed formulation. Thermal characterization of the composite system showed that char yield values were comparable to ablative systems presently used in TPS. Printability was demonstrated by in situ deposition onto a rotating/tilting aluminum substrate. A five-axis printer system was developed/adapted to accommodate the contours of TPS substrates and to enable the extrudate to completely cover leading edges of vehicle control surfaces.

Composite laminates were produced by RTM using similar glass and flax fabrics and both vitrimer epoxy and aerospace-grade epoxy, both formulated for liquid molding. Tensile and flexural properties were measured and compared, revealing that the vitrimer composites exhibited equivalent performance in flexural strength and tensile modulus, but slightly lower performance in tensile strength relative to reference epoxy composites. In general, glass–fiber composites outperformed flax–fiber composites in tension. However, both glass and flax–fiber composites yielded roughly equivalent flexural strength and tensile modulus-to-weight ratios. Flax fabrics were recovered from composites by matrix dissolution, and a second-life laminate showed full retention of the mechanical properties relative to those produced from fresh flax. Finally, a demonstration of re-forming was undertaken, showing that simple press-forming can be used to modify the composite shape. However, re-forming to a flat configuration resulted in local fiber damage and a decrease in mechanical properties. An alternative forming method was demonstrated that resulted in less fiber damage, indicating that further refinements might lead to a viable forming and re-forming process.

We report a rapid route to reclaim carbon fiber (CF) fabric and monomeric chemicals from amine-epoxy CF-reinforced polymer (CFRP) composites. We use a reaction that occurs in molten NaOH-KOH eutectic to selectively cleave aryl ether and amine linkages, which involves two temperature-dependant mechanisms. Bisphenol-A is isolated in up to quantitative yields, and recovered CF fabric is remanufactured into 2nd-generation CFRPs.

The welding behavior of prototype vitrimer composites with respect to adjustable parameters and protocols is investigated, and a method for resistance welding of vitrimer composites directly adapted from the welding of thermoplastic composites is described. Adherend laminates are positioned on either side of a matrix-saturated carbon fiber heating element, through which current is driven, and resistance heating welds the adherends and heating element together, forming a single lap joint. Weld strengths matched or exceeded the strength of composite parts produced using the manufacturer-recommended consolidation method (12.0 ± 2.6 MPa vs. 8.4 ± 0.6 MPa). Furthermore, repeating the welding process yielded greater shear strength, withstanding up to five weld–break–reweld cycles with an average increase of 4.6 ± 1.5 MPa or 65% compared to the first weld. The findings from resistance weld experiments highlight the suitability of vitrimer matrix composites for repair. Finally, a process for reversing a welded joint was shown, demonstrating the potential for vitrimers for temporary joining and rejoining.

Carbon fiber reinforced polymers (CFRPs, or composites) are increasingly replacing traditional manufacturing materials used in the automobile, aerospace, and energy sectors. With this shift, it is vital to develop end-of-life processes for CFRPs that retain the value of both the carbon fibers and the polymer matrix. Here we demonstrate a strategy to upcycle pre- and postconsumer polystyrene-containing CFRPs, cross-linked with unsaturated polyesters or vinyl esters, to benzoic acid. The thermoset matrix is upgraded via biocatalysis utilizing an engineered strain of the filamentous fungus Aspergillus nidulans, which gives access to valuable secondary metabolites in high yields, exemplified here by (2Z,4Z,6E)-octa-2,4,6-trienoic acid. Reactions are engineered to preserve the carbon fibers with much of their sizing so that the isolated carbon fiber plies are manufactured into new composite coupons that exhibit mechanical properties comparable to those of virgin manufacturing substrates. In sum, this represents the first system to reclaim a high value from both the fiber fabric and polymer matrix of a CFRP.

We report an electrochemical system for selective deconstruction and re-manufacture of epoxy-carbon fiber composite using a Kolbe-like mechanism for generating methyl radicals from CH 3 COOH. These cleave C–N bonds via hydrogen atom abstraction.

Titanium powders were cold sprayed onto polyetheretherketone (PEEK) to investigate the effects of interface microstructure on bonding. Process guidelines to achieve adherent metallic deposits via cold spray onto thermoplastics are scarce, and mechanisms for metal adhesion onto thermoplastic substrates are unclear. The formation of a transition layer where deposition changes from metal-on-polymer to metal-on-metal deposition can promote adhesion but requires a clearer understanding. The microstructural features of such transition layers provide insights into mechanisms that promote coating adhesion. Select features, including developed interfacial area ratio and gradation of thermal residual stress, correlated to adhesive strength values. The understanding of interlocking mesoscale features formed during cold spray informs the parameter selection for depositing metals onto thermoplastics with strong adhesion.

Contoured laminates were produced by vacuum infusion (VI), and thickness variations were monitored dynamically as a function of process parameters. Process simulations were performed using finite element software (PAM-RTM), and predictions of thickness along the length of the laminate were compared with dynamic measurements. Initial simulations approximated the effects of corners on preform deformation and fabric draping behavior. Subsequent modifications to the simulation geometry and material properties were implemented to increase accuracy and more closely match experimental measurements. User-defined geometry (UDG) simulations were used to predict both the maximum corner deviation and the area of corner deviation with greater accuracy. The present study demonstrates a workflow for use of analytical tools to design and control vacuum infusion processes. The workflow leverages process monitoring and modified process simulation tools to provide insight into parametric effects and to guide process modifications to reduce product variability.

A combination of a metallic mesh and an adhesive layer of metallic particle/epoxy composite was introduced as an intermediate layer to enhance the adhesion between cold-sprayed particles and fiber-reinforced composites (FRCs). Aluminum was considered for both the metallic particles in the adhesive and the metallic mesh. To predict the mechanical characteristics of the intermediate bond layer under a high strain rate, the properties of the adhesive layer needed to be calculated or measured. Therefore, in this study, the Al particle/epoxy adhesive was homogenized by using a rule of mixture. To verify the homogenization, the penetration depth, and the thickness decrease after the cold spray deposition from the undeformed surface, was monitored with FE analysis and compared with experimental observation. The comparison displayed that the penetration depth was comparable to the diameters of one cold spray particle, and thus the homogenization approach can be reasonable for the prediction of the stress level of particulate polymer composite interlayer under a high strain rate for cold spray processing.

Hybrid bond layers (BLs) were designed, fabricated, and evaluated for cold spray metallization of CFRP. The bond layers consisted of metal mesh embedded in a polymer film adhesive co-cured to the CFRP. Efforts were devoted to identifying the critical opening ratio—i.e., the ratio of mesh opening size to powder diameter, for deposition of an adherent coating. Analysis of powder deposited at mesh openings show a transition from erosion (at a mesh opening ratio of 6.4) to mechanical interlocking and formation of a continuous coating with decreasing opening ratio. Selection of opening ratio yielded either (a) a grid of consolidated thin-walled deposits atop mesh wires separated by microchannel openings, or (b) densified coatings of cold-sprayed Ti. The effective opening ratio increased with increasing diameter ratio—i.e., the ratio of wire diameter to powder size, a consequence of eroded wire peripheries at shallow impact angles. These findings inform the design of future hybrid BLs, in concert with the selection of powder size, for cold spray metallization of CFRP.

In this work, we investigate the use of discontinuous resin films in prepregs (semipregs) combined with a semi-permeable (air-permeable, resin-impermeable) release film intended to allow through-thickness air evacuation while simultaneously restricting resin loss. In situ measurements of resin pressure were deployed to test the hypothesis that resin pressure was maintained during prepreg cure when using a semi-permeable release film. Concurrently, visualization of the tool-side surface during cure revealed efficient evacuation of entrapped air. Porosity in laminates formed at high temperatures when using resin-permeable consumables, but did not form when using resin-impermeable (semi-permeable) consumables. To confirm that the observed void growth behavior was due to a loss in resin pressure, experiments were conducted to measure resin pressure during cure with both resin-permeable and resin-impermeable (semi-permeable) consumables. In both cases, resin pressure peaked before decreasing, a finding attributed to resin flowing to fill dry regions in the fabric, present by design. The drop in resin pressure, however, was greater in magnitude and longer in duration when using resin-permeable edge and bag-side surface boundaries, indicating that the observed void growth at elevated temperature was caused by a loss in resin pressure. Use of a semi-permeable membrane was effective in retaining resin content and mitigating such porosity.