Post-failure fracture surface analysis of notched steel specimens after bending-torsion fatigue

Abstract

This paper contains analysis of fatigue fracture surfaces parameters of circumferential v-notched 10HNAP (S355J2G1W) steel specimens. Fatigue tests were performed under bending, torsion and combinations of bending MB and torsional MT moments. Stationary and ergodic random loadings had normal probability distribution and wide-band frequency spectra from 0 to 60 Hz.Thirty years after fatigue tests, the fracture surfaces of the specimens were subjected to observations with the use of a focus variation microscope (FVM), which allows data sets with a large depth of field to be obtained. Surface topography analysis was performed on the entire fracture using height parameters and functional parameters (volume) in accordance with ISO 25178, as was the fractal dimension.Tendencies of changes in height, functional and fractal fracture surface texture parameters for three types of loadings were analysed. The examined fracture surfaces showed the dependence of the loading history on their topography. Parameters that were responsive to surface irregularities were selected, because they can be used, for example, to investigate the causes of material damage. Regression analysis of the relationship between values of areal parameters was also carried out.