Hardness-dependent surface roughness threshold governing rolling contact fatigue crack initiation

Surface roughness and material hardness strongly influence rolling contact fatigue (RCF) of rolling elements, yet a clear understanding of their combined effect on crack initiation remains limited. This study introduces a systematic approach to quantify the critical surface roughness threshold for fatigue crack initiation across hardness levels from 526 to 762 HV. Experimental characterization capturing hardness-dependent plasticity and surface integrity, including asperity-level hardness, residual stresses, and 3D topography of real surfaces, provides the key inputs to the numerical framework. To achieve computational efficiency and improve accuracy, the approach integrates boundary and finite element methods to simulate rolling contact conditions and predict the risk of surface crack initiation. Results show that material hardness plays a dual role in fatigue performance. Higher hardness limits surface plasticity but increases stress amplitudes near asperities. Depending on surface roughness, increased hardness can either improve or degrade fatigue behavior, shifting low-roughness surfaces toward high-cycle fatigue and high-roughness surfaces toward low-cycle fatigue. A threshold RMS roughness of 0.1–0.3 μ m is identified, below which surface-initiated fatigue becomes improbable, and this threshold saturates at 0 . 3 μ m for hardness levels above 700 HV. The findings suggest that crack initiation can contribute minimally or dominantly to total fatigue life depending on surface roughness and material hardness, and fatigue performance is not necessarily improved by increasing hardness, making it essential to optimize hardness according to service conditions and tribological requirements. • Efficient BEM/FEM framework proposed for modeling RCF in rough contacts. • Coupled effect of surface roughness and hardness on fatigue damage is analyzed. • Critical roughness threshold identified below which crack initiation is unlikely. • Increased hardness does not always improve fatigue life due to asperity stresses.