Bond Inspection of Bonded Composite Repairs During Depot Level Maintenance

The US Air Force is undertaking process improvements for bonded repairs on C-130 aircraft wings.  These repairs are both composite and metal patches which are bonded to the wing surfaces primarily at points where corrosion, gouges, or other surface damage has been removed.  Patch materials are commonly boron fiber epoxy pre-impregnated composite and titanium with high temperature epoxy film adhesive.  The intention of the patches is to recover the strength lost from material removal. These patch repairs save hundreds of millions of dollars in repair by eliminating the need to complete a full component replacement. Present process control emphasizes surface preparation as the primary means of controlling repair quality and bond strength.  To date, thermographic inspection is the only means, aside from visual inspection, that is used to judge the quality of the bonded repair patch installations.  This is only done at the initial patch installation since this method requires heating of the patch and is perceived as posing a risk for degradation of patches that have seen flight service.  Review of historic inspection data indicates that patch delamination occurs frequently enough that inspection process improvement may result in a net reduction of maintenance hours expended.  The current nondestructive inspection techniques involving ultrasonic inspection are not capable of determining bond strength for certification of bonded structures, limiting bonded joint acceptance for critical structures like composite patch repairs. Two USAF organizations have interest in bonded repair process improvement.  The C-130 SPO is the operational safety, suitability, and effectiveness (OSS&E) authority for the fleet.  Engineers from this organization are responsible for assuring that bonded repair application will result in continued safe aircraft operation and that maintenance on the repaired areas is minimized.  AFLCMC/CMXG is the maintenance organization that employs specified processes for initial damage removal, bonded patch installation, initial and periodic inspections, and rework or replacement when required.  Laser Bond Inspection (LBI) is potentially beneficial to both of these groups since the anticipated gain in initial bonded patch quality would serve to enhance overall aircraft safety and minimize rework at the point of patch installation as well as reducing patch replacement, since patch integrity could conceivably be nondestructively determined. The investments made by Industry and Government to generate data and build the various LBI system components to conduct this inspection process can be leveraged to support this proposed Phase I for maturation and implementation for the inspection of bonded composite patch repairs.  LBI can be used to validate the integrity of repairs that are made to either a metallic or composite aircraft by identifying weak bonded repairs and to detect the presence of “kissing bonds.