Fatigue behavior of the aeronautical Al–Li (2198) aluminum alloy under constant amplitude loading

Abstract

Tensile and fatigue mechanical behavior of wrought aluminum alloy 2198-T351 is examined and compared against 2024-T3 that is currently used in aerostructures. Experimental fatigue tests were carried out under constant amplitude stress ratio R=0.1 and respective stress–life (S–N) diagrams were constructed for both alloys. Fatigue behavior of both alloys is described with varying parameters being the percentage of fatigue life as well as the effect of maximum applied stress as a function of ultimate tensile strength. It was found that fatigue endurance limit of AA2024-T3 is approximately 40% below its yield stress, while only 9% below for the AA2198-T351. The latter was found to be superior in the high cycle fatigue and fatigue endurance limit regimes, especially when considering specific mechanical properties. Absorbed energies per fatigue cycle as well as dynamic stiffness of the fatigue hysteresis loop were calculated and plotted against the number of fatigue cycles and with varying maximum applied stress; both parameters are continuously decreasing due to the combination of hardening effect and micro-cracking in AA2024-T3, while this was the case only for the high applied stresses regime in AA2198-T351. Cyclic stress strain (CSS) curves were constructed and proved that work hardening exponent of AA2198-T351 is substantially decreasing with increasing fatigue life.