Surface factor assessment in HCF for steels by means of empirical and non destructive techniques

Abstract

The fatigue limit value in steels is strongly influenced by many factors, among them the surface finish. In particular, the fatigue limit decreases with increasing the surface roughness, referring to standard grinded specimen.Technical literature provides an empirical correction factor, named surface factor, to be used if surface roughness is different from standard specimen conditions. This factor is traditionally lower than 1 and it reduces the fatigue limit value corresponding to the material in standard conditions. This coefficient may be obtained from literature graphs and it can be identified by means of two parameters: materials ultimate tensile strength and surface finish Ra.Aim of the present paper is to evaluate the effectiveness of fast procedures to assess the surface factor. The reference is the Murakami model, which estimates the fatigue limit by means of roughness parameters other than Ra.In the present paper the fatigue limit estimations related to specimens with sanded Rahave been obtained by utilizing empirical destructive and nondestructive methods and then have been compared each other.Experimental testing was carried out on a structural steel specimens by means of axial alternate fatigue testing with two different surface roughness.The results obtained referring to Murakami model have been compared with those obtained by means of both thermographic and Staircase method.The Murakami model results to be easy to use and non destructive.The corresponding fatigue limit estimations match with the thermographic ones above all when surface roughness is elevated.