DON27BZ02-NV018 — Rapid Thermoplastic Patching Capability for Legacy Thermoset Airframe Sustainment

Deliver a rapid, high-strength thermoplastic patching capability for legacy thermoset composite airframes by developing the necessary interfacial enabling technologies to fuse the incompatible polymer systems.

Current composite airframe repair methodologies rely on a combination of wet-layups, B-staged epoxy prepregs, and bonded pre-cured thermoset patches. While pre-cured patches are the standard for high-quality depot repairs due to their autoclave-consolidated strength and “inspectability”, they present significant operational and logistical limitations. All these traditional methods require secondary adhesive bonding (using paste or film adhesives) to attach the repair to the airframe. This secondary bond is the inherent weak link of the repair, demanding long thermal cure cycles (often 4 to 24 hours), strict environmental controls, and burdensome cold-storage logistics (freezers) for the B-staged resins and adhesives. Furthermore, these out-of-autoclave epoxy repairs often struggle to meet the high service temperature requirements of advanced tactical aircraft. This SBIR topic seeks to evolve the current pre-cured repair methodology by transitioning from secondary adhesive bonding to high-speed fusion welding. The objective is to develop the capability to use pre-consolidated, high-temperature thermoplastic patches (e.g., Carbon/PEEK, Carbon/PEKK) on legacy thermoset aircraft (epoxy/Bismaleimide [BMI]). By replacing the lengthy adhesive cure cycle with a rapid welding process, this technology aims to achieve flight-ready repairs in expeditionary and carrier-based environments in a fraction of the time. Because thermoplastics melt and thermosets do not, achieving a structural weld between these dissimilar materials is the primary barrier to fielding this capability. The Department of War seeks complete thermoplastic patching solutions that incorporate the requisite chemical or mechanical interfacial enablers—such as plasma surface activation, novel chemical tie-layers, or laser-ablated mechanical interlocking. These enablers must allow the thermoplastic patch to successfully fuse with a fully cured, legacy thermoset airframe using portable ultrasonic, resistance- or -induction welding equipment. The outcome must be a deployable repair process that eliminates cold-storage logistics, restores full structural integrity, meets high service temperature demands, and drastically reduces Aircraft on Ground (AOG) time.

Develop a conceptual thermoplastic patching process and demonstrate the physical viability of the proposed enabling interfacial technology (e.g., tie-layers, surface functionalization). Establish the feasibility of the conceptual process. Perform coupon-level mechanical testing (such as lap-shear and peel-strength) to empirically prove that the thermoplastic-to-thermoset weld achieves a structural load transfer that is comparable to, or exceeds, baseline epoxy adhesive repairs. Deliver a detailed theoretical model explaining the fusion mechanism, an initial assessment of the repair's thermal limits (ensuring the parent thermoset is not degraded), and a conceptual workflow for field-level application that eliminates the need for depot-level infrastructure or cold-storage consumables.

Develop, optimize, and scale a patching capability prototype to representative legacy airframe structures, such as contoured wing skins or stiffened panels. Perform the proposed thermoplastic patch repair using field-representative heating equipment (e.g., portable induction or resistance welding). Validate the repair through rigorous structural testing, including fatigue spectrum loading, thermal cycling, and damage tolerance assessments, to prove airworthiness. Deliver a prototype repair kit and process documentation. Target metrics must include a complete return-to-flight repair cycle of under 4 hours, restoration of > 115% of the parent structure's Design Ultimate Load, and thermal isolation ensuring the thermoset parent material remains below its degradation threshold during the welding process. Prepare a Phase III commercialization/transition plan.

Transition the validated thermoplastic patching capability to Naval Air Systems Command (NAVAIR) and Air Force Life Cycle Management Center (AFLCMC) for integration into legacy platforms. Incorporate the materials, tooling, and process parameters into standardized Structural Repair Manuals (SRMs) for platforms such as the F/A-18, F-35, or V-22, providing expeditionary units with a rapid, out-of-autoclave, room-temperature-storable repair solution. Commercial dual-use applications include rapid repair for the commercial airline industry, structural maintenance for composite wind turbine blades, and automotive crash repair for high-performance composite vehicles.