Effect of specimen geometry on low-cycle fatigue behavior of high-performance TMCP steel

This study investigates the influence of specimen geometry on the low-cycle fatigue (LCF) and extremely low-cycle fatigue (ELCF) behavior of high-performance TMCP (thermo-mechanical controlled processing) steels. Uniaxial tension–compression cyclic loading tests were conducted using cylindrical specimens with a diameter of 8.5 mm and three parallel lengths (10, 15, and 20 mm), corresponding to parallel length-to-diameter ratios ( L e / D ) of 1.18, 1.76, and 2.35, respectively. Strain amplitudes ranging from 0.5% to 15% were applied, covering both LCF and ELCF regimes. Two steel grades were examined: TMCP385B, with nominal yield and tensile strengths of 385 and 550 N/mm 2 , and TMCP440C, with corresponding strengths of 440 and 590 N/mm 2 . The influence of specimen geometry, particularly parallel length, was evaluated in terms of (1) the evolution of peak stress, (2) the shape of the stress–strain hysteresis loops, and (3) the resulting LCF/ELCF life. The results show that when the parallel length is extremely short ( L e / D = 1.18), a geometry commonly used in ELCF testing, both the stress–strain hysteresis behavior and the fatigue life are modestly affected, with specimens exhibiting marginally higher peak stresses and longer fatigue lives. In contrast, such geometric effects were not observed for specimens with longer parallel lengths ( L e / D = 1.76 and 2.35) under LCF conditions. These findings provide practical guidance for selecting appropriate specimen geometries in LCF and ELCF testing to ensure accurate evaluation of material fatigue behavior.