Pre-IVD Etching Removal and Robotic Shot Peening Retrofit on Aircraft Aluminium Bulkheads

Author:  P. Martin, Z. Hajjar
Source:  ICSP-13
Doc ID:  2017078
Year of Publication:  2017
Abstract:  
Introduction: As a military aging aircraft depot level and in-service support contractor, L-3 MAS have been supporting fleets from many countries over the last 30 years in developing structural retrofit programs tailored to each of these aging aircraft to ensure both airworthiness and logistic risks are maintained to an acceptable level. As part of these retrofit programs, the use of in-situ Shot Peening is one of the structural life enhancement techniques that L-3 has mastered for over 15 years using robotic systems. The advantage of using a robotic system for shot peening is to improve quality,repeatability and allow to reach areas on aircraft where a technician would have no or very limited access. Up to recently, these shot peening retrofits were carried out prior to 70% of the expended fatigue life at each hot spot and assuming a Life Improvement Factor (LIF) of only 1.5. A recent mandate was to provide structural enhancement via shot peening to 9 different Fracture Critical aluminum bulkhead hot spots on a fighter aircraft. Each location had a specific geometry,stress level, grain orientation, blue print (B/P) fatigue life, etc. Based on the most critical areas, a LIF of up to 3.0 was required with a retrofit induction as late as 80% of the location blue print life. Although L-3 did some coupon testing in the past suggesting such LIF should be attainable, the available test data were insufficient to certify that such LIF would always apply with enough confidence for all the specific physical characteristics of the new hot spots. Thus, a new coupon test program was developed specifically to address these bulkhead locations. Moreover, due to the late induction time of these retrofits, a thin layer of material also needed to be removed prior to the shot peening to remove some of the accumulated fatigue damage.