Ratcheting fatigue behaviour of advanced structural materials at their service temperatures.

Ratcheting fatigue behavior is a critical phenomenon in structural materials undergoing cyclic loading, particularly when combined with nonzero mean stress or multiaxial stress states. This manifests as the progressive buildup of plastic strain in one direction with each loading cycle, ultimately leading to material degradation and failure. This behavior is highly relevant in industries such as automotive, aerospace, nuclear, and civil engineering, where components frequently experience complex loading conditions. Structural materials exposed to ratcheting fatigue often exhibit changes in their mechanical properties, such as cyclic hardening or softening, which influence their ability to withstand cyclic stresses. Factors such as the mean stress, stress amplitude, loading path, material microstructure, and environmental conditions play crucial roles in governing the ratcheting response. Understanding the ratcheting fatigue behavior is essential for the design and life prediction of critical components. Comprehensive investigations through experimental testing and computational modelling are required. This investigation presents the asymmetrical stress-controlled (ratcheting) behaviour of 9 C-1Mo modified steel and IN-617 superalloy at service temperatures of 600 and 800(o)C respectively at different tensile mean stresses. When there was change in the mean stress, the amplitude of stress as well as the rate of stress was not changed. The fatigue life of these alloys decreased with an increase in tensile mean stress. The maximum stress increases with the mean stress and leads to a higher inducement of the plastic strain, which results in a lower fatigue life. At elevated temperatures, the fatigue life was found to be higher than that at room temperature (RT) owing to dynamic-strain aging.