Quantitative Scale‐Invariant Analysis of Fracture Surface Roughness in Very‐High‐Cycle Fatigue of LPBF‐Manufactured 316L Stainless Steel
V. A. Oborin, M. V. Bannikov, B. S. Voloskov, J. V. Bondareva, S. A. Evlashin, A. I. Morkovkin, I. V. Sergeichev, O. B. NaimarkABSTRACT
The very‐high‐cycle fatigue behavior of 316L stainless steel produced by laser powder bed fusion was investigated using a multitechnique approach combining infrared thermography, nonlinear acoustic monitoring, X‐ray computed tomography, and quantitative fractography. The primary aim of this study was to establish quantitative links between process‐induced defect characteristics, damage accumulation mechanisms, and fracture surface morphology in the very‐high‐cycle fatigue regime. The fatigue limit was determined via the Risitano–Luong thermographic method (285 ± 10 MPa) and validated by conventional S–N curve analysis (260 MPa). Tomography identified elongated fusion‐related defects (mean size 26 μm, maximum 157 μm) that may have acted as internal crack nucleation sites, leading to subsurface‐initiated cracking with characteristic «fish‐eye» morphology. Scale‐invariant analysis of the fracture surface revealed three distinct zones with progressive increases in the Hurst exponent H (0.40 → 0.66) and in the characteristic scales l sc (0.2 → 1.9 μm) and L pz (11.5 → 27.4 μm), indicating increasingly correlated roughness during crack propagation. These findings demonstrate that quantitative fractography combined with scale‐invariant parameters can effectively capture the transition from short‐range, disorder‐dominated fracture initiation to long‐range, correlated crack growth in additively manufactured metals under very‐high‐cycle fatigue loading.