Identifier

etd-08272009-003127

Degree

Master of Science in Mechanical Engineering (MSME)

Department

Mechanical Engineering

Document Type

Thesis

Abstract

Friction-Stir-Welding (FSW) has been adopted as a major process for welding Aluminum aerospace structures. Al-2195, which is one of the new-generation Aluminum alloys that has been used on the external tank of the new super lightweight external tank of the space shuttle. The Lockheed Martin Space Systems (LMSS), Michoud Operations in New Orleans is continuously pursuing Friction-Stir-Welding technologies in its efforts to advance fabrication of the external tanks of the space shuttle. The future launch vehicles which will have to be reusable, m, an dates 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 Al-2195 and Al-2219 are fatigue tested according to ASTM-E647. The effects of: (i) Stress ratios, (ii) Corrosion Preventive Compound (CPC), and (iii) Periodic Overloading on fatigue life are investigated. Scanning electron microscopy (SEM) is used to examine the failure surface, and examine the different modes of crack propagation i.e. tensile, shear, and brittle modes. It is found that fatigue life increases with increase in stress ratio; the fatigue life increases from 30-38% with the use of CPC, the fatigue life increases 8-12 times with periodic overloading, , and crack closure phenomenon predominates the fatigue facture. Numerical Analysis in FEA has been used to model a 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 (LEFM) stress intensity factor is calculated using FEA, and the fatigue life predictions made using this method are within acceptable 10-20% of the experimental fatigue life obtained. This method overcomes the limitation of the traditional node release scheme, and closely matches the physics of crack propagation.

Date

2009

Document Availability at the Time of Submission

Release the entire work immediately for access worldwide.

Committee Chair

Wahab, Muhammad A

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