Improvement of damage tolerance of laser beam welded stiffened panels for airframes via local engineering

Abstract

Damage tolerance of aerospace grade aluminum alloys was studied in relation to a new design philosophy in skin and stringer geometries. Systematic thickness variations (crenellations) were introduced onto the skin and stringers of the laser beam welded (LBW) stiffened Al2139-T8 large flat panels in order to modify the stress intensity factor (SIF) distribution on center cracked panels in such a way that fatigue life can be improved. Fatigue crack propagation (FCP) tests (on panels with crenellations) with crack growing perpendicular to the welded stringers were conducted under constant amplitude and spectrum loading conditions. Results were compared with the results of “classical” LBW stiffened panels (with no crenellations) having equal weight and tested under the same conditions. The new panel design with crenellations showed substantially longer fatigue lives under constant amplitude loading. This gain significantly improved under spectrum (Mini-Twist) loading fatigue tests. This paper presents the first FCP test results of a comprehensive ongoing program which investigates the benefits and potential role of crenellations on welded Alalloy and steel structures. Further issues including microstructural examinations, numerical investigations, fitness for service analysis and residual strength aspects will be topics of another communication.