Investigation of the tensile/compressive residual stresses in AISI 4340 steel under low-cycle fatigue loading

icaf2023 Tracking Number 57

icaf2023 poster

Landing gear fuse pins are notched energy-absorbing components designed to reduce the possibility of aircraft structural damage in the event of a hard landing. The area surrounding a fuse pin notch is in a state of complex stress, causing local yielding under applied loading. This work utilizes the strain-life method to utilize local yielding behaviour to study the low-cycle fatigue life of notched AISI 4340 steel components and to study the effects of complex stresses. Experimental testing involved mechanical property testing, finite element analysis (FEA) simulation, and low-cycle fatigue testing of notched specimens. Control specimens were subjected to quasi-static low-cycle fatigue using the strain-life model. Variable specimens were subjected to either a quasi-static initial overload prior to fully reversed low-cycle fatigue loading, or a mean stress of identical magnitude as the residual stress induced from the applied overload. The magnitude of the residual stress at the notch root was determined using elastic-plastic FEA in ABAQUS-2019x. It was found that the tensile residual stress induced through compressive overload caused a slight decrease in fatigue life, while the compressive residual stress induced by tensile overload caused a slight increase in fatigue life. The residual stress was approximated as a mean stress to determine the suitability of this approximation as a design assumption from the literature. It was found that applying the tensile residual stress as a positive mean stress causes a significant reduction in fatigue life to provide a conservative life estimate. However, applying the compressive residual stress as a negative mean stress causes a significant increase in fatigue life, or a non-conservative estimate. These results are significant in the design of components where plasticity is not negligible in fatigue life assessments. The significance of the residual stress to mean stress approximation is critical in engineering design to ensure conservative estimates are used in engineering applications