The study of surface in early stages of fatigue failure on carbon steels using a confocal microscope and a scattered light sensor

• The scattered light sensor is an interesting device to implement for measuring the tiny amount of the alteration on the surface • The high-carbon steel provides lower surface roughness evolution during cyclic loading on roughness than the medium-carbon steel at the same level of applied stresses. • The formations of extrusion and intrusion can modify the original surface roughness of the component during fatigue testing. The cyclic plastic strain generates persistent slip bands (PSBs) concentrated in thin bands during the fatigue loading and revealed on the surface as markings called persistent slip markings (PSMs). PSMs consist of extrusions and intrusions that are the key factors of surface alterations and of early stage of crack initiation. There are two main goals to achieve in this study. Firstly, the use of the scattered light sensor was expressed, and the results are shown on the different surface qualities (as-machined and as-polished). Secondly, the surface development during fatigue testing using the specimens with rough surfaces (as-machined), a confocal microscope was conducted to measure the initial and developed surface. The specimens were prepared by the carbon steels of C45E and 115CrV3. Fatigue tests were carried out in a low cycle fatigue regime (LCF) on a servo-hydraulic push–pull testing machine with selected stress amplitudes (σ a ) of 30 % and 50 % of their ultimate tensile strength (UTS), a load ratio ( R ) = -1, and a frequency ( f ) = 10 Hz. As a result, the scattered light sensor provides dissimilar roughness through measured gradient values. Moreover, the surface roughness development during the fatigue test on as-machined surfaces is confirmed. However, more details of the use of a scattered light sensor on the fatigue test, deformed microstructures, and characterization of extrusions and intrusions are still required for future study.