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A large passenger airplane experienced an inflight rapid decompression and was forced to make an emergency landing. Post-accident inspection revealed that a section of the fuselage skin on the top portion of the airplane (crown) had fractured and flapped open during the flight. The National Transportation Safety Board (NTSB) Materials Laboratory examination of the fuselage structure determined that multiple-site damage (fatigue cracking) originated at the skin panel of a lap joint from areas of improperly drilled rivet holes. Fatigue crack growth analysis on the fracture face of the skin panel via quantitative fractography was performed by scanning electron microscopy revealed that cracking started approximately when the airplane entered service. [ABSTRACT FROM AUTHOR]

Challenges exist when cleaning fracture surfaces to reveal fractographic features without damaging the fracture surfaces further. This technical paper will compare the efficacy of cleaning ferrous-based fracture surfaces with a chelating stripping agent versus inhibited acid solutions. The chelating stripping agent used in this study was commercially available Evapo-Rust. The inhibited acid solutions used in this study were ASTM G1 C.3.1 solution and ASTM G1 C.3.5 solution. In this study, metallographically prepared samples of annealed type 8620 steel were immersed in the ASTM G1 C.3.1 solution, ASTM G1 C.3.5 solution, and Evapo-Rust for different times under inactive and ultrasonic conditions. The extent of damage to the polished surfaces was qualitatively examined using various types of optical microscopy. The sample damage was visually compared to a baseline sample etched with 2% natal. The indices used to qualify the extent of damage were the existence of pits and microstructural etching. It was found that a chelating agent-based oxide remover causes less etching and pitting on metallographically prepared annealed type 8620 steel samples than inhibited acid solutions. [ABSTRACT FROM AUTHOR]