Material Approaches to Mitigate Gap Filler Cracking

Development of crack resistant conductive gap-fillers fits within the Air Force"s goal of investing in low observable maintenance improvements to reduce sustainment costs and aircraft downtime. Commercially available gap-fillers are initially effective, but performance degrades under thermal and mechanical fatigue resulting in micro- and macrocracks within the matrix. Phase I showed the feasibility of formulating more durable elastomeric matrix resins capable of delivering the required conductivity and fatigue resistance. A 50-fold increase in fatigue crack resistance at 73oF was obtained with the materials developed compared to the commercial gap-fillers tested. The same gap-filler material tested a -65 degrees F showed improvement of 75 percent in crack resistance. The combination of mechanical and thermal fatigue generates the most aggressive stresses for gap-fillers on aircraft, where temperatures within the range of -65 degrees F to 250 degrees F are experienced. These stresses occur because of differential thermal expansion and modulus changes in the gap-filler material. Additional testing will be conducted in Phase II to replicate these stresses in the laboratory to determine durability over a range of temperature-stress profiles. The elastomeric gap-filler matrix developed during Phase I will be further refined and fully tested for qualification. The project team assembled includes a major gap-filler producer and airframe manufacturers. BENEFIT: Mitigation of cracking within gap-fillers will benefit many modern fighter/bomber platforms by reducing maintenance costs and downtimes. Commercial aircraft could potentially benefit as well from the development of high endurance sealant matrix materials.