PSI - Issue 68

Mihaela Iordachescu et al. / Procedia Structural Integrity 68 (2025) 1147–1152 Iordachescu M. et al. / Structural Integrity Procedia 00 (2025) 000–000

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Fig. 1. (a) Steel microstructure; (b) Sketch of the surface-notched cylindrical specimen (a, b –semi-axes of the ellipse to which the fatigue cracks are assimilated). 2.2. Fatigue resistance testing procedures The fatigue resistance of the bar steel was assessed by cyclic tensile testing cylindrical specimens of 5 mm diameter, surface notched up to a 0.5 mm depth (Fig.1b). Each specimen was subjected to several cyclic tensile loading steps at a frequency of 8 Hz. The load amplitude was held constant during each step, but decreased by 20% between two successive ones, so that the applied load remained well below the yielding load of the specimen in the current cracked condition. The cracked areas were successively marked by heat tinting at the end of each loading step. In addition, the crack size was monitored after each fatigue step through the specimen stiffness obtained by measuring the slope of the load - crack mouth opening displacement (CMOD) curve in a static unloading process. The CMOD measurements were performed with a resistive extensometer, of 12.5 mm gage length and mounted in front of the notch. At the end of the fatigue testing, each specimen was broken in tension. The failure loads and the final crack sizes were further used to compute the critical stress intensity factor K c for comparison with the fracture toughness value, measured by another testing method.

Fig. 2. Tensile load - CMOD plots resulted from the post-fatigue fracture tests of the cylindrical specimens (F 0 is the maximum failure load in tension of a cylindrical smooth specimen, F 0 = 30.048 kN).