Cracking of S355J2+N steel in the high-cycle and very-high-cycle fatigue regimes

The aim of this paper is to present the results of HCF and VHCF tests of the S355J2+N steel performed on the ultrasonic testing machine at 20 kHz load frequency. The test method consists of three parts: preliminary tests aimed at identification of basic properties of the tested material, specimen geometry analysis using the finite element method (FEM), main fatigue test using ultrasonic system. The fatigue tests consisted of two stages. In the first one, the fatigue limit was determined using the Staircase (Up-And-Down) Method. In the second stage, the fatigue tests were carried out above and below the fatigue limit. Based on the fractographic analyses of the fatigue fracture surfaces, three types of them were distinguished. The analyses of the cracking made it possible to determine the upper load limit for tests of this material in both VHCF and HCF regimes on the ultrasonic testing machine. This level is almost 20 % lower than the range of the linear relationship between stress and strain in tensile test.