Fatigue and Fracture Mechanics Analysis of Friction Stir Welded Joints of Aerospace Aluminum Alloys Al-2195

Friction-Stir-Welding (FSW) has been adopted as a major process for welding Aluminum aerospace structures. Aluminum (Al-2195) which is one of the new generations Aluminum alloys that has been used for the new super lightweight external tank for the space shuttle. NASA’s Michaud Assembly Facility (MAF) in New Orleans is continuously pursuing Friction-Stir-Welding (FSW) technologies in its efforts to advance fabrication of the external tanks of the space shuttles. The future launch vehicles which will have reusable mandates, for the structure to have good fatigue properties which prompts an investigation into the fatigue behavior of the friction stir welded aerospace structures. The butt joint specimens of Aluminum alloys (Al-2195 and Al-2219) are fatigue tested according to ASTM-E647. The effects of stress ratios, corrosion preventive compound (CPC), and periodic overloading on fatigue life is investigated. Scanning Electron Microscopy (SEM) is used to examine the failure surfaces and examine the different modes of crack propagation i.e. tensile, shear, and brittle modes. It is found that fatigue life increases with the increase in stress ratios; the fatigue life also increases from 30%–38% with the use of CPC; and the fatigue life could increase 8–12 times with periodic overloading; while the crack closure phenomenon predominates the fatigue fracture. Numerical analysis has been used to model fatigue life prediction scheme for these structures, the interface element technique with critical bonding strength criterion for formation of new surface has been used to model crack propagation. The linear elastic fracture mechanics stress intensity factor is calculated using FEA and the fatigue life predictions made using this method; and are within 10%–20% of the experimental fatigue life obtained.