Executive Summary

           For a number of years, researchers have been developing a technology for repair and reinforcement of airplane structures using bonded boron/epoxy doublers. This technology has been successfully used on military airplanes such as reinforcement of the F-111 wing pivot fitting, repair of stress corrosion on the C-130, and fatigue cracking on the Mirage El and Macchi airplanes. More recently, extensive application has been seen on the USAF C-141 fleet (wing weep-hole riser cracks). The concept involves bonding a boron/epoxy doubler over the metallic structure containing damage. The boron/epoxy doubler has a higher stiffness than aluminum or titanium and thus transfers load through the adhesive to the composite doubler by­passing the damaged metal structure. This reduces the stress level in the metal structure which will prevent or slow continued damage accumulation.

The boron/epoxy doubler reinforcement concept has sparked interest by commercial airlines who are always looking for quicker, more efficient repair techniques. Potential commercial airplane applications of this technology include repairs to damaged secondary and primary structure, and as a preventive stress reliever modification to a problem structural item in the fleet.

The Boeing Company was contracted by Textron Specialty Materials through Boeing Technology Services, contract 6-1171-10A3397R4, to develop a test program which would provide structural data on bonded boron/epoxy repairs to metallic aircraft structures. This proposed test program was developed under a phase I contract. The phase II contract was tasked with accomplishing the test program proposed in the phase I final report. The test plan and tasks performed in phase II were subsequently modified as data was obtained. The report is separated into three sections: boron/epoxy material and doubler installation process specification, nondestructive inspection, and structural analysis and performance tests.

             The writing of the installation process specification for bonding boron/epoxy to metallic aircraft structures (with emphasis on aluminum) was accomplished. This involved chemical, physical, and mechanical characterization tests to understand the boron tape with the 250°F epoxy plus numerous process sensitivity studies. It was determined that the boron/epoxy material met AMS 3 867/4A specification requirements when tested to the AMS specification procedures. The material has similar physical properties to other 250°F curing composite material systems; namely BMS 8-168 materials. The doubler installation cure conditions were actually improved as a result of the process sensitivity studies.