N212-115 TITLE: Additive Fiber Reinforced Composite Repair for Aircraft

RT&L FOCUS AREA(S): General Warfighting Requirements (GWR)

TECHNOLOGY AREA(S): Materials / Processes

OBJECTIVE: Develop a novel additive material deposition system that produces high-quality composite laminate repairs that return parts to original strength.

DESCRIPTION: High-performance composite materials used on current, and future, aircraft construction will exhibit damage and wear throughout their lifetimes. Repair methods usually consist of damaged material removal, surface preparation, and applying a repair patch in order to bring the structure back up to original strength. Although repair systems exist, these can be very labor intensive. Much effort is spent on cutting and handling the repair materials. This handling often produces excess waste as the maintainer must cut circular material to make the patches from stock material that comes in square sheets. These repairs must be carefully cut to fit the damaged area they are replacing, often made from successive circular layers in a step scarf geometry. There is also the struggle with time as the liquid resin systems either pre-impregnated in the fiber or applied from a two-part mixture have a finite shelf life. Finally, there is a great amount of training associated with individuals doing repair to produce consistent and quality products every time.

Recent advances in Additive Manufacturing (AM) have the potential to make automated repairs a reality. Stemming from earlier work on Automated Fiber Placement (AFP), commonly used in industry, AM has moved from printing thermoplastics to fiber-reinforced thermoplastics and thermosets [Ref 1]. It has been shown that the resulting parts are not only stronger, but contain nearly as much fiber content as the high-performance composites used in making aircraft. There have also been advances in resin materials. Recent products featuring higher temperature thermoplastics are becoming commercially available. Thermosetting resins such as epoxies are being developed [Ref 2]. These resins can meet the structural and environmental properties demanded by naval aircraft. Coupling the superior strength of fiber reinforcement with durable, high temperature-resistant resins are the next technological step for performance AM [Ref 3].

An automated repair system will leverage the miniaturization of composite processing through AM to enable fleet maintainers to make faster more consistent repairs to composite aircraft. The user would mount the system over the damaged area and load in the repair materials. Repair patch outer dimensions may reach a maximum size of up to 18 inches and a minimum size of 5 inches. Applying repair material at the point of deposition will eliminate the waste generated by hand cutting pre-impregnated or wet layup composites. It will also speed up repair as the maintainer would not have to handle and mix resin before wetting out the dry fibers before cutting to size. Consistency will also improve as the resin is pre-mixed and uniformly impregnated into the fiber feedstock for the system. Using the aircraft as a tool surface, the proposed system can apply local pressure and heat to improve consolidation of the fibers, as well as, cure. Machine vision can be used to confirm ply orientation, scan for Foreign Objects and Debris (FOD), and provide documentation of the process which is useful for Digital Twin applications. Once complete, the user can remove the system and the leftover repair materials to be stored for future repairs.

The objective is to be able to install, operate, and cure the repair in one shift (8 hr) although longer duration is acceptable depending on technology maturity. Develop and add the capabilities of machine vision to confirm repair ply size, orientation, and if FOD/defects exist. Further refine the intended repair material to be deposited by this system. Utilize a resin system with a cure temperature no greater than 350 �F (177 �C) although lower is desired. Show that the resin is fully cured when the intended system and repair process is complete. Perform additional mechanical testing per ASTM Standard of the repair material to confirm that physical and mechanical properties are suitable for a repair system.

Desired threshold mechanical performance are listed below.

Threshold Composite (0�/90�) Symmetric Laminate Mechanical Properties

• Short Beam Shear Average Strength: 9 ksi Room Temp Ambient, 6 ksi 180 �F (82 �C) wet
• Tension Average Strength: 115 ksi Room Temperature Ambient, 109 ksi 180 �F (82 �C) wet
• Tension Average Modulus: 10.36 msi Room Temperature Ambient, 9.74 msi 180 �F (82 �C) wet
• Compression Average Strength: 69 ksi Room Temperature Ambient, 48 ksi 180 �F (82 �C) wet
• Compression Average Modulus: 7.91 msi Room Temperature Ambient, 7.62 msi 180 �F (82 �C) wet.

The final repair material will have to undergo sufficient mechanical testing and characterization such that strength allowables can be generated and used to perform repair analysis. The proposer should conduct demonstrations of the material being deposited and utilize cross-sectional imaging and Non-Destructive Inspection (NDI) to confirm porosity does not exceed 4% by volume. The proposer should perform acid digestion of repair material to confirm a fiber content.

The goal is to meet or exceed 50% fiber content by volume. The final system should be such that it can be handled, carried, and placed by no more than two personnel.

PHASE I: Develop, design, and demonstrate the feasibility of a portable system that can additively repair a composite material through the automated deposition of thermoset polymer and carbon fiber reinforcement. Identify and prototype a fiber and resin system that will be used by this machine. Fiber may be continuous or discontinuous. The final intended resin system should have a wet Glass Transition temperature (Tg) of no less than 230 �F (110 �C). Demonstrate the system by depositing and consolidating material with a quasi-isotropic fiber orientation on a flat surface. Also, demonstrate that the system is capable of applying subsequent layers in such a manner that the net shape is the stepped scarf normally used in composite scarf repairs. Provide cross-sectional imaging, and additional analysis, of these materials to inspect for porosity and fiber content. The Phase I effort will include prototype plans to be developed under Phase II.

PHASE II: Develop and demonstrate the technology to show that it can deposit material on curved surfaces. Demonstrate that it can deposit material in the scarfed recess left over from removing damaged material from a parent laminate. Integrate and refine the developed system such that it can be handled, carried, and placed by no more than two personnel. Refine the prototype system such that it can be securely mounted in reference to an aircraft's outer surface. Provide refined demo system as a deliverable to be used by NAVAIR.

PHASE III DUAL USE APPLICATIONS: Perform additional work to produce allowables data sufficient for traditional repair analysis per aircraft platform. Determine an adhesive system to be used for this repair. Ideally this would be one currently used in the fleet although the repair�s own resin matrix may be sufficient. Prove this with mechanical testing. Complete full integration of the technology from a prototype to a commercially viable system. Provide this system as well as a comprehensive manual for its operation. Integrate the machine vision system into current aircraft maintenance tracking procedures.

Aircraft repair is common in the aerospace industry including transportation as well as military use. Composites are becoming more and more commonplace for commercial aircraft with the skin and stringers taking up much of the loads as seen with the Boeing 787. Being able to restore material properties to their original strength in an automated and consistent manner will keep aircraft flying longer, reducing costs for air carriers. Additional use can be found within the Army and Air Force for their composite aircraft.

REFERENCES:

1. Brenken, B., Barocio, E., Favaloro, A., Kunc, V. and Pipes, R. B. "Fused filament fabrication of fiber-reinforced polymers: A review." Additive Manufacturing, 21, 2018, pp. 1-16. https://doi.org/10.1016/j.addma.2018.01.002.
2. Nawafleh, N. and Celik, E. "Additive manufacturing of short fiber reinforced thermoset composites with unprecedented mechanical performance." Additive Manufacturing, 33, 2020, 101109. https://doi.org/10.1016/j.addma.2020.101109.
3. Kim, H. J., Kim, H. S., Lee, G. Y., Kim, M. S., Min, S. H., Keller, R., Ihn, J. B. and Ahn, S. H. "Three-dimensional carbon fiber composite printer for CFRP repair." Composites Part B: Engineering, 174, 2019, 106945. https://doi.org/10.1016/j.compositesb.2019.106945.

KEYWORDS: Repair; Composites; Additive Manufacturing; Automated Fiber Placement; Machine Vision; Portable