Fractographic analysis of fatigue crack growth in lightweight integral stiffened panels

Purpose - The purpose of this paper is to complement available macroscopic fatigue crack growth measurements in flat stiffened panels with scanning electron microscopy (SEM) measurements of striation spacing. Design/methodology/approach - The paper's approach is fatigue testing of two-stiffener flat panels manufactured using three different processes, with a central initial crack perpendicular to the stiffeners and load, in order to identify striation spacing during crack growth up to final fracture, using SEM. Findings - An increase of striation spacing as cracks grow was quantified. Although when cracks approach the stiffeners the stress intensity factor decreases, there is no clear decrease of striation spacing in that region. Striation spacing is roughly similar to macroscopic crack-propagation rate da/dN measured in the panels testing. This observation is no longer valid once the stiffeners are reached; this stage is characterized by fast acceleration of the cracking process until final complete rupture is verified, and macroscopic crack growth measurements are made difficult because of the "T" geometry in that region. Originality/value - A complete picture of the striation spacing during the fatigue crack growth up to final fracture of a two-stiffener flat panel is provided for three different manufacturing processes: high-speed machining, laser beam welding and friction stir welding.