Composite materials are being used on Naval Aircraft more frequently due to their higher strength to weight ratio over comparable metal materials. As the performance envelope for these materials is expanded, lighter, higher temperature materials are being employed. Unfortunately, the repair development for these materials is lagging, resulting in high scrap rates for high cost components or sacrificial, lower strength epoxy repairs. The purpose of this project was to investigate the use of a high temperature, low volatile organic compounds (VOC), environmentally friendly bisphenol E cyanate ester (BECy or EX-1510) for injection and laminate repairs for bismaleimide/carbon-fiber composite (BMI-cf) repairs on AV-8 and F-35 components to increase the repair envelope of these systems.
EX-1510 generates negligible VOCs and hazardous air pollutants when compared to a benchmark epoxy resin. Furthermore, EX-1510 has lower toxicity than traditional epoxies and is environmentally benign. Additionally, the ability to repair increasingly prevalent BMI-cf aircraft components benefits the environment by reducing the amount of scrapped BMI-cf solid landfill waste and increases fleet readiness.
This demonstration provided a validation of the cyanate ester laminate repair schemes on scrapped AV-8 and F-35 components in a real-world industrial environment. A former Depot Composite Shop artisan performed the repairs, rather than engineers or researchers, to prove the robustness of the process. The performance objectives were: repair of delamination zone (injection repair), overall repair time (injection and laminate repair), overall repair cost (injection and laminate repair), reduction of environmental waste (injection and laminate repair), and repair of damaged area (laminate repair). Half of the objectives related to the laminate repair were met: reduction of environmental waste and repair of damaged area. Unfortunately, the lab-scale injection repair process did not successfully transition to a relevant in-field repair; therefore, objectives related to injection repair were not met. No further research recommended in this area.
Currently, injection and laminate repairs were performed using epoxy resin systems. Epoxy resin systems can be cured at room or elevated temperatures, but their true adhesive service temperatures are rarely above 350°Fahrenheit (°F). EA9396, a common laminating and injection epoxy, has a glass transition temperature (Tg) of ~200ºF. Comparatively, EX-1510, the cyanate ester demonstrated here, requires an elevated temperature cure but has a published Tg of 380ºF. The elevated temperature cure is 350ºF which is acceptable for on-aircraft repairs.
EX-1510 is part of a new class of extremely low viscosity, high Tg resin systems which results in a cured polymer with excellent mechanical properties and a high degree of cross-linking. Through initial Strategic Environmental Research and Development Program (SERDP) funding, this system has been shown to be an excellent candidate for the repair of high temperature, polymer matrix composite (PMC) based structural parts used on advanced Department of Defense (DoD) air vehicle systems. The use temperature limit for the BECy system is high because of the polymer’s Tg of greater than 450°F (260°Celcius [°C]) and onset of decomposition above 750°F (400°C). The prepolymer also has very low toxicity, contains negligible VOC, and has extended room temperature stability, facilitating reduced wastes due to spoilage compared to traditional thermosets.
Although adequate for most epoxy parent repairs, epoxy resin systems are inadequate for repair of BMI-cf structures due to their limited service temperature. EX-1510 is a perfect candidate due to its high Tg, low viscosity at room temperature and acceptable elevated cure temperature. During the course of this project, novel procedures for lab-scale injection repairs and in-service repair laminates were developed using EX-1510. Laminate repairs on AV-8 and F-35 components were demonstrated for the engineering community.
As described in the joint Test Protocol (JTP), a full round of mechanical tests were conducted on injection repaired laminates and solid cyanate ester laminates. In most cases, the cyanate ester laminates were considered stronger or comparable to the corresponding epoxy panels, making the cyanate ester an acceptable repair alternative for the demonstrations.
Cyanate ester repair laminates were successfully demonstrated on scrapped F-35 and AV-8 BMI-cf components. Non-destructive inspection (NDI) was performed after cure to determine success of repair.
No implementation issues foreseen on AV-8 applications. F-35 will be implemented on an as needed basis once the airframe is inducted into the Depot for repair.