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.