Technical note | F-111 Adhesive Bonded Repair Assessment Program (FABRAP) - Phase I Testing, Preliminary Results

ABSTRACT

Adhesive bonded repairs are being used for the through-life-support of secondary and tertiary aircraft structure. This technology has not been accepted for application to primary aircraft structure due largely to the lack of a non-destructive inspection technique for, and uncertainty regarding the environmental durability of, adhesive bonds. Over the last twenty five years a large number of adhesive bonded repairs have been applied to the Royal Australian Air Force F-111 and its retirement in December 2010 represented a unique opportunity to evaluate these bonded repairs. The F-111 Adhesive Bonded Repair Assessment Program (FABRAP) was created to generate statistically valid data regarding the efficacy of the tap-test as a non-destructive-inspection technique and the environmental durability of adhesive bonds. In FABRAP Phase I Testing, over 300 repairs were tap tested and 820 residual strength tests conducted using a pneumatic adhesion tensile testing instrument. This report details the FABRAP process and summarises the results from the Phase I testing.

EXECUTIVE SUMMARY

Adhesive bonded repair technology (ABRT) has been used extensively by the Australian Defence Force (ADF) for the through-life-support of secondary and tertiary aircraft structures, where failure of the repair would not result in structural failure of the aircraft. This has resulted in significant cost savings and increased aircraft availability. Wider adoption of ABRT, particularly on primary aircraft structure that is critical to the safety of the aircraft, has the potential to compound these benefits.

A major impediment to the adoption of ABRT for primary aircraft structure is the difficulty in obtaining airworthiness certification. The two major reasons for this are (i) the lack of a non-destructive inspection (NDI) technique that can assess the in-service integrity of a bonded joint, and (ii) uncertainty regarding the environmental durability of adhesive bonds.

ABRT has been used to reinforce and repair parts on the Royal Australian Air Force (RAAF) F-111 for over twenty five years. It is estimated that over 5,000 ABRs have been applied, mainly to honeycomb sandwich panels. Retirement of the RAAF F-111 in December 2010 represented a unique opportunity to evaluate the integrity of a large number of airworthy ABRs.

The F-111 Adhesive Bonded Repair Assessment Program (FABRAP) was created to (i) generate statistically valid data on the efficacy of the "tap-test" NDI method and (ii) evaluate the environmental durability of the adhesives and processes used in these repairs. This report details the FABRAP program and a summary of the raw results from the first phase of testing (Phase I).

FABRAP – Phase I Testing occurred in November 2010. It consisted of a team of DSTO scientists travelling to RAAF Base Amberley and identifying the ABRs on eleven aircraft. Tap-test NDI was performed on over 300 ABRs followed by over 820 tensile tests on the ABRs using a pneumatic adhesion tensile test instrument (PATTI). FM300 adhesive was found on approximately 170 of these ABRs (500 PATTI tests), FM300-2K adhesive on 60 ABRs (190 PATTI tests) and other adhesives on the remaining ABRs. A search was made of the aircraft records in order to acquire the service history of the tested ABRs.

Additional phases of testing are planned. These will consist of (i) tap and PATTI testing additional ABRs, and (ii) stripping the ABR doublers off to ensure that the adhesive under the PATTI stub was representative of the condition over the entire ABR. When the testing phases have been completed the (i) NDI and mechanical test results will be correlated in order to evaluate the efficacy of the tap-test to detect failed bonds, and (ii) service history and mechanical test results correlated in order to quantify any environmental degradation of the adhesive bonds.