SPECIALTY MATE R IALS, INC.

1 SPECIALTY MATERIALS, of Boron and SCS Silicon Carbide Fibers and Boron Nanopowder EVALUATION OF BORON/ epoxy DOUBLERS FOR REINFORCEMENT OF COMMERCIAL AIRCRAFT METALLIC STRUCTURES Prepared by: Boeing Company March 1996 Appendices available from SPECIALTY Materials upon request. D658-10401-1 1 Boeing s involvement with Boron/ epoxy doublers is limited to the scope of this document. This document does not constitute Boeing s endorsement of Boron/ epoxy doublers. CAGE CODE: 81205 THIS DOCUMENT IS: CONTROLLED BY: Structures Research and Technology, 9-5514 PREPARED UNDER: CONTRACT NO. 6-1171-10A3397R4 DOCUMENT NO: D658-10401-1 MODEL: All TITLE: Evaluation of Bonded Boron/ epoxy Doublers for Reinforcement of Commercial Aircraft Metallic Structures THE INFORMATION CONTAINED HEREIN IS NOT PROPRIETARY PREPARED BY: Paul Rutherford 9-5514 3/15/96 Steve Berg 9-5576 3/15/96 CHECKED BY: Chris Mazur B-YH12 4/2/96 APPROVED BY.

2 Matt Miller B-YH12 3/27/96 SIGNATURE ORGN DATE D658-10401-1 2 Boeing s involvement with Boron/ epoxy doublers is limited to the scope of this document. This document does not constitute Boeing s endorsement of Boron/ epoxy doublers. 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.

3 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.

4 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.

5 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. D658-10401-1 3 Boeing s involvement with Boron/ epoxy doublers is limited to the scope of this document. This document does not constitute Boeing s endorsement of Boron/ epoxy doublers.

6 Nondestructive inspection techniques were evaluated for the detection of: disbond or delamination of the doubler, and, crack growth under the doubler. Textron took the lead role in this effort with the development of the NDI procedures while Boeing contributed a consulting role with design of the reference standards and evaluating the NDI procedures. The structural analysis and performance tests were separated into material property testing, stress analysis, and field repair testing. The stress analysis was performed to support all the specimen designs and is referenced in the material property testing and the field repair testing sections. The material property tension testing was accomplished on three batches of boron/ epoxy with two different specimen configurations: unidirectional and bow-tie (0 45 ).

7 This was done to verify differences between the autoclave process and the heat blanket/vacuum bag process and differences between the 350 F and 250 F cure epoxy resins. The bulk of the testing was performed on the field repair specimen which was a boron/ epoxy doubler bonded to a damaged 4 x 1 6 inch 7075-T6 thin aluminum sheet. Field repair testing was divided into two separate sections: baseline geometry determination tests and parametric tests. The baseline geometry tests examined different configuration variables relating to the design of the boron/ epoxy doubler. The intent in this testing is to culminate in selection of one design for further evaluation in the parametric tests.

8 The parametric tests were designed to examine different variables on the selected baseline design. Some of the variables investigated were doubler ply reduction, aluminum thickness changes, crack length, environmental exposure, different doubler anomalies, doubler impact events, and disbonds. From the parametric tests performed the following were found: the -65 F exposure had a significant impact on the fatigue life of the specimen with a doubler; restraining the lateral bending during the -65 F and room temperature fatigue tests significantly increase the fatigue life of the specimen; effects of fasteners under the doubler and in-line with the flaw had a significant impact on the fatigue life where as inch simulated disbonds had little effect on the fatigue life of the specimen; when impacted inches from the stop drill the doubler seemed to show an increased fatigue life with impact levels less than 300 in-lbs; fatigue stress levels at 0-18 ksi have a greater impact on fatigue life than fatigue stress levels of 3-20 ksi.

9 And doubler applications on thicker metallic substrates have a large impact on fatigue life. D658-10401-1 4 Boeing s involvement with Boron/ epoxy doublers is limited to the scope of this document. This document does not constitute Boeing s endorsement of Boron/ epoxy doublers. Abstract This document reports the work accomplished on contract 6-1171-10A3397R4 which evaluated boron/ epoxy doublers for the application to repair or reinforce commercial aircraft. 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 an installation process specification for bonding boron/ epoxy to metallic aircraft structures (with emphasis on aluminum) was accomplished.

10 Nondestructive inspection techniques were evaluated for the detection of: disbond or delamination of the doubler, and, crack growth under the doubler. The structural analysis and performance tests are separated into material property testing, stress analysis, and field repair testing. The material property tension testing was accomplished on three batches of boron/ epoxy with two different specimen configurations: unidirectional and bow-tie (0 45 ). This was done to verify differences between the autoclave process and the heat blanket/vacuum bag process and differences between the 350 F and 250 F cure epoxy resins. The bulk of the testing was performed on the field repair specimen which was a boron/ epoxy doubler bonded to a damaged 4x16 inch 7075-T6 thin aluminum sheet. Field repair testing was divided into two separate sections: baseline geometry determination tests and parametric tests.