Improvement of damage tolerance of laser beam welded aerospace structures via local engineering

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Improvement of damage tolerance of laser beam welded aerospace structures via local engineering

Uz, Mustafa Volkan

A significant improvement of the damage tolerance of integrally stiffened metallic structures can provide the aerospace industry with very important advantages as long as this improvement is not accompanied by a noteworthy cost increase. In this study, an innovative methodology for the improvement of damage tolerance of laser beam welded Al-alloy structures was investigated. Newly developed weldable Al-alloy 2139 was used to fabricate laser beam welded 5-stringer fiat panels containing systematic thickness variations (crenellations). A panel failure scenario was adopted with broken central stringer to investigate the fatigue crack retardation phenomenon due to crenellations. Introduction of a selected pattern of thickness variations on a thin plate is a 'new idea' of so-called 'local engineering design concept' to improve damage tolerance performance with tailored stress intensity factor distributions at the crack tip. The aim of placing crenellations between welded stringers was the extension of the time necessary for the central crack to reach an intact stringer. If this should be successful, the overall damage tolerance of the structure would be improved. This novel idea has been systematically investigated in this thesis work. It was shown that by introduction of crenellations, the damage tolerance of aluminum alloy panels can be significantly improved. Under constant amplitude loading conditions, thin crenellated panel attained 42% and the thick ones up to 65% longer fatigue crack growth life compared to their respective reference panels with equal weight. Under spectrum loading conditions, up to 150% lifetime improvements were achieved with thick crenellated panels.