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

Damage tolerance of an aerospace grade aluminum alloy was studied using 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 center cracked flat panels to modify the stress intensity factor (SIF) distribution and hence to improve fatigue life. 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 “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 efficiency of component design with crenellations to improve damage tolerance behavior of welded Al-alloy and steel structures. Issues including microstructural examinations, numerical investigations, fitness-for-service (FFS) analysis and residual strength aspects of this program will be topics of another communication.