PSI - Issue 77

P. Santos et al. / Procedia Structural Integrity 77 (2026) 339–347 P. Santos et al. / Structural Integrity Procedia 00 (2025) 000 – 000

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The scanning electron microscopy (SEM) was used for the metallographic analysis of the steel. According to Fig. 1a, lath martensite is essentially the main phase of the bar steel, which according to Morris (2011) and Morris et al. (2013) is grouped into three hierarchical microstructural levels. The first, namely the basic level, is the martensite lath, the second is the group of several parallel laths which form a martensite block , and the third is the martensite packet that contains several blocks with small differences in the lath orientation.

Fig. 1. a) Steel microstructure; b) Scheck of the SENT specimen used in the fracture tests.

2.2. Fracture testing Fig. 1b shows the basic geometry of the SENT specimens, fracture tested according to the specifications of BS-8571 (2018). The 0.5 mm deep V-notch shown in the specimen middle on one of its faces of 2.5 mm wide, was made by electrical-discharge machining to assure a precise fatigue pre-cracking process. The fatigue loading consisted of several and successive blocks of harmonic tensile loading cycles with constant amplitude during each one, but gradually reduced in between them. A resistive extensometer, attached to the specimen on the notched side face and centered on the notch mouth, allowed the cracking process to be controlled so that the load did not exceed 40% of the plastic failure value corresponding to the progressively reached crack size, as estimated from the extensometer record. Then, the standardized testing method was applied and five fatigue pre-cracked specimens were subjected to tensile loading, three of them up to fracture and the other two up to maximum load. The applied load and the CMOD, at the notch mouth, were respectively recorded by the acquisition system of the servo hydraulic testing machine and by VIC-2D equipment, as reported by De Abreu et al., (2019) and Santos et al. (2024). The CMOD values were measured during tensile loading with a virtual extensometer of 7 mm gauge length (Fig. 1b) provided by the VIC-2D software through the image sequence of captured the speckle painted face of the specimen. Repeated partial un-loading followed by the corresponding reloads were carried out in three of the fracture tests to determine the crack size evolution (Fig. 2a) through the slope of the unloading line, namely, the stiffness R of the specimen. The relationship employed to find the crack size a (Fig. 1b) from R is due to Tada et all (2000) and is given by: R=EB (cos π 2 ã ) 2 4ã(4 . 88−3 . 42cos π 2 ã ) [1] where ã = a/W is the relative crack size, the specimen thickness B and the elastic modulus E of the steel.