Early stages of fatigue damage in riveted Alclad 2024-T3 aluminum alloy lap joint specimens

The nondestructive study of short fatigue cracks in riveted lap joints has long been a dilemma due to cracks originating in the subsurface of the joint not being visible on the outside surface. The scanning acoustic microscope may be used to quantitatively investigate subsurface fatigue cracks such as at and near countersunk rivets in riveted lap joint specimens even when they are very small. When combined with optical and electron microscopic examination of the surface and with fractography of fractured specimens, the formation and growth of subsurface cracks near rivets may be characterized in detail. Such an investigation was made on specimens fabricated from two panels of Alclad 2024-T3 aluminum alloy riveted with 2017-T4 aluminum alloy flathead chamfered rivets. These specimens are similar to the riveted lap joints found in the fuselages of many aircraft. The fatigue testing was interrupted periodically for examination and some specimens with just detectable and larger subsurface cracks were fractured in tension to reveal the fatigue fracture surfaces for analysis. A detailed study of crack formation and microcrack growth kinetics near such rivets is presented including location of the initiation sites as affected by applied load.