N221-006 TITLE: Room-Temperature Filler for Honeycomb Repairs

OUSD (R&E) MODERNIZATION PRIORITY: General Warfighting Requirements (GWR)

TECHNOLOGY AREA(S): Air Platforms;Materials / Processes

OBJECTIVE: Develop and demonstrate a lightweight honeycomb core filler for repairs with a short, room room-temperature cure.

DESCRIPTION: Current composite honeycomb repairs rely on either epoxy fill, higher temperature cure (150-160� F; 65.56�71.11 �C) syntactic foams, or 7-day room-temperature cures of syntactic foam. These options have drawbacks, including long times to complete the repair, additional required equipment for controlling/monitoring the cure, and increased resultant weight for epoxy fill repairs. In addition, syntactic foams were designed for battlefield or non-aviation applications and lack the target room-temperature cure duration and required mechanical performance. Composite honeycomb repairs need to be simplified and streamlined to reduce repair turnaround time and necessary support equipment. Reducing the time to cure can drastically improve the rate at which repairs are completed, increasing fleet readiness. As an added benefit, development and qualification of a new repair material will offer an additional solution for repairs.

A new lightweight, streamlined composite honeycomb repair material and process using a novel lightweight, structural filler formulation is sought to reduce the time required for honeycomb repairs. The proposed material can take advantage of various matrix chemistries and fillers [Ref 1]. The repair material would be expected to meet threshold requirements, threshold mechanical properties, and target requirements.

Threshold requirements include, but are not limited to:

• Maximum of 24 hour cure at room temperature 70 �F (27 �C)
• Compatibility with aluminum, Nomex, and polyurethane foam cores
• Maximum exothermic temperature during cure not to exceed 200 �F (93.3 �C)
• Minimum 15 minute pot life
• Maximum density of 0.8 g/cc
• Shelf life of 12 months at 77 �F (25 �C)

Threshold mechanical properties include, but are not limited to:

• Compressive strength of 8 ksi at 77 �F (25 �C) by ASTM D695 [Ref 2]
• Lap shear of 700 psi at 77 �F (25 �C) by ASTM D1002 [Ref 3]
• Retention of 50 % of compressive strength at 180 �F (83 �C)

Target requirements include, but are not limited to:

• 1 hr cure before sanding at room temperature 70 �F (27 �C)
• Retention of greater than 15% compressive strength at 350 �F (177 �C)
• Ability to cure at temperatures down to 32 �F (0 �C) without added heat
• Greater than 15 min pot life
• Density 0.5 g/cc or lower

PHASE I: Develop novel composite honeycomb filler formulations and screen candidate formulations for feasibility. This assessment can include cure kinetics (maximum exothermic temperature, cure time, pot life) and density to identify the most promising candidate formulations. The Phase I effort will include prototype plans to be developed under Phase II.

PHASE II: Develop and provide a prototype repair process (suitable for a Depot or I Level repair) using the developed repair material in Phase I. Complete a repair with a minimum size of 3 inches in diameter and 1.5 inches in thickness on a representative part with the developed honeycomb filler. Conduct, in coordination with the Government, mechanical and physical testing that includes a limited set of screening tests sufficient to ensure acceptable properties. Demonstrate reduction in overall cure time and compatibility with common naval aviation honeycomb materials.

PHASE III DUAL USE APPLICATIONS: Generate data for full qualification and validate the repair filler material and the repair technique. Repair instructions for Depot or I Level honeycomb repairs will be developed and provided. Transition the technology to applicable Navy Depot.

Honeycomb repair is applicable to composites outside of the aerospace industry. The automotive, as well as the marine industry, use honeycomb or filled-composite structures. A repair material that is effective across multiple industries is possible.

REFERENCES:

1. Afolabi, L. O., Ariff, Z. M., Hashim, S. F. S., Alomayri, T., Mahzan, S., Kamarudin, K. A., & Muhammad, I. D. (2020). Syntactic foams formulations, production techniques, and industry applications: a review. Journal of Materials Research and Technology, 9(5), 10698-10718. https://doi.org/10.1016/j.jmrt.2020.07.074.
2. Subcommittee D20.10 on Mechanical Properties. (2015). ASTM D695-15: Standard test method for compressive properties of rigid plastics. ASTM International. https://doi.org/10.1520/D0695-15.
3. Subcommittee D14.80 on Metal Bonding Adhesives. (2019). ASTM D1002-10: Standard test method for apparent shear strength of single-lap-joint adhesively bonded metal specimens by tension loading (metal-to-metal). ASTM International. https://doi.org/10.1520/D1002-10R19.

KEYWORDS: Composites; Honeycomb; Repair; Lightweight; Room temperature; Filler