Qualitative and Quantitative Fractography of Structural Steel S235

Fatigue fracture surfaces of S235 structural steel were examined using SEM and six complementary three‐dimensional measurement configurations. Six specimens tested under axial constant‐amplitude loading at and were analyzed qualitatively and using the entire fracture surface method with ISO 25178 descriptors. SEM identified surface initiation, stable fatigue propagation, transition, and final ductile‐overload regions. Quantitative measurements showed strong preservation of specimen‐to‐specimen trends across configurations, whereas small configuration‐dependent offsets remained. Sa , Sk , and Vmp exhibited exploratory variations with loading conditions and fatigue life, but the limited dataset does not support predictive relationships. SEM‐based fatigue‐origin assessment was additionally interpreted using modified Murakami and Kitagawa–Takahashi/El Haddad concepts. The results support multiscale postmortem fatigue characterization of S235 while emphasizing the need for material‐specific validation of defect‐based fatigue boundaries.