AFRL materials engineers resolved a potentially serious problem that affects the operational life of F-15E vertical stabilizers. Working with engineers from Warner Robins Air Logistics Center (WR-ALC), Georgia, they successfully developed, demonstrated, and transitioned an adhesive bonding process and nondestructive inspection (NDI) technique that ensure the stiffening doublers attached to both sides of the aircraft's two vertical stabilizers remain adequately secured (see figure on next page). Successful transition of these innovative methods restores the operational life of the vertical stabilizers.
Several F-15E aircraft have lost doublers during takeoff and/or while in flight. To remedy this situation, AFRL adhesives experts created a new bonding process and nondestructive evaluation specialists developed and fine-tuned a thermography-based NDI procedure. Their combined efforts ensure the stiffening doublers remain a reliable and cost-effective structural reinforcement solution.
The F-15E's vertical stabilizers require composite stiffening doublers to reduce in-flight vibrations, which can decrease the stabilizers' life span up to 25%, by preventing crack growth in underlying substructure. WR-ALC engineers found the bonding process currently used for securing the doublers to the stabilizers was inadequate and resulted in extensive rework to replace damaged or detached panels. After receiving multiple reports of incidents where the stiffening doublers separated or partially debonded from the aircraft during takeoff or flight, they made the decision to cease further modifications until a more reliable bonding process could be implemented.
In response to this urgent need, AFRL engineers expeditiously developed, demonstrated, and provided a new adhesive bonding process and NDI procedure to resolve the problem. To verify the new bonding method's reliability, they affixed two doublers to an F-15E undergoing maintenance at WR-ALC. After the doublers had accrued approximately 250 flight hours on the aircraft, which was ultimately allocated to Elmendorf Air Force Base (AFB), Alaska, AFRL engineers went to Elmendorf to carefully inspect them. They found the doublers to be in excellent condition and, using the new NDI procedure, verified that the adhesively bonded surfaces had almost no detectable disbond areas (>0.25 in. diameter) and that no disbond areas were evident near the critical zone at the edge of the doublers. The F-15 Systems Group subsequently acknowledged the improvement and will consider implementing the new bonding process and thermography-based inspection procedure throughout the entire F-15E fleet. AFRL engineers initiated the process transition to the logistics centers by installing the remaining two doublers on the test aircraft. The final transition to depot teams occurred at Seymour Johnson AFB, North Carolina, on a second F-15E aircraft.
Specific improvements over the original bonding process include (1) incorporated holes in the doublers to allow air to escape during the vacuum/cure process, (2) reduced number of bond line voids (from >15% to <3%) to increase strength, (3) improved bond line thickness control (from >20 mils to <14 mils), (4) improved surface preparation (using grit blasting versus hand sanding), (5) simplified adhesive preparation (using premeasured kits versus bulk mixing kits in the field), and (6) enhanced adhesive curing (applying heat versus curing at room temperature). Overall, this new process reduces repair time and improves both surface wetting and adhesive properties.
AFRL engineers estimate the new bonding and inspection techniques will generate substantial savings for the Air Force. The projected cost of restoring the vertical stabilizers to their original design life is substantially lower than that of replacing them, and the new processes will eliminate the need to replace the doublers during the stabilizers' design life.
Mr. Kenneth J. LaCivita, Dr. Brett A. Bolan, and Dr. Peter S. Meltzer (Anteon Corporation), of the Air Force Research Laboratory's Materials and Manufacturing Directorate, wrote this article. For more information, contact TECH CONNECT at (800) 203-6451 or place a request at http://www.afrl.af.mil/techconn/index.htm . Reference document ML-04-28.