A macro- and micro-approach to the anisotropic fatigue behaviour of hot-rolled and cold-drawn pearlitic steel

This paper analyzes the propagation of fatigue cracks in pearlitic steel presented in two forms: as a hot rolled bar and as a cold drawn wire. The experimental procedure consisted of fatigue tests on cylindrical bars under tensile loading, using steps with decreasing amplitude of stress and constant stress range during each step. Results show how the cold drawing process improves the fatigue behaviour of eutectoid steel by retarding the fatigue crack growth rate in the Paris regime. The fracto-metallographic analysis shows how the local microstructural anisotropy of the steel creates a change in the fracture crack path in the form of local micro-deflections (and thus multiaxially-driven fatigue propagation at the very local microscale). The total fatigue fracture surface increases with cold drawing due to the higher angle of crack micro-deflections in the case of the heavily drawn material, thereby allowing a correction of the Paris law curves by considering the real (experimental) fatigue crack path (multi-deflected; locally multiaxial) and the theoretical fatigue crack path (non deflected; globally uniaxial).