Prediction of the endurance limit taking account of the microgeometry after finishing milling

Abstract

The aim of this study is to investigate the influence on the high cycle fatigue behaviour of high strength steels of the surface micro-geometry induced by their machining. The machining process considered herein is high speed finishing milling with a ball-end tool. The micro-geometry of the machined surface is characterized by 2- and 3-D roughness parameters. The studied material is a 25MnCrSiVB6 resulphurized steel with a bainitic matrix. The high cycle fatigue behaviour of this material is characterised by its endurance limit evaluated from symmetric tension-compression (Rσ = −1) tests performed according to standard staircase method. This endurance limit was compared to the results of so-called self-heating tests as well as rupture tests performed according to Locati approach. It appears that the difference between the endurance limits measured on the machined or the polished specimens is not fully accounted for by the self-heating measurements. Furthermore, the Arola–Ramulu model also appears as being unable to predict the influence of the surface integrity induced by the milling process on the high cycle fatigue life of this material. Last, a sensitivity analysis of the lead angle of the tool on the anisotropy of the machined surface micro-geometry proves that the roughness of surfaces milled with ball-end tools must be characterized by surface—and not linear—parameters.