Load interaction effects on fatigue crack propagation in 2024-T3 aluminum alloy

Variable amplitude fatigue studies have been conducted within a linear-elastic fracture mechanics framework in order to systematically examine the effect of complex loading on fatigue crack retardation in 2024-T3 aluminum alloy. Complex loading conditions were simulated by introducing a second tensile or compressive peak load after the crack had extended various distances, a', into the region affected by a previously applied high load excursion.

Maximum interaction between single peak overloads resulted when the two peak load cycles were separated by a small distance, a' _min, where the fatigue crack propagation rate resulting from a single overload reached a minimum. This behavior was attributed in part to interference of tensile displacements produced during the first peak load cycle which was verified from fractographic observations. Crack growth rate retardation was related also to the development of a favorable compressive residual stress at the crack tip. Peak loads were found to act as completely isolated events only when they were separated by a distance approximately three times the plastic zone size resulting from a single overload. Comparable findings resulted when 10 cycle block overloads were employed in place of single peak excursions.

When a single peak overload was followed by a compressive cycle, retardation was found to decrease to a minimum; however, when the loading sequence was reversed, the effect was less damaging. In addition, as the distance between positive and negative peak loads was increased, the number of delay cycles quickly approached that associated with a single high load excursion.