Fatigue Inspection of Carbon Steel Due to Extrusions and Intrusions Using an Angle-Resolved Scattered Light Sensor

An angle-resolved scattered light sensor is a non-contact surface inspection technique that is robust and has a high resolution for roughness measurement at approximately 1 nm. Typically, during the early stage of cyclic loading in metals, persistent slip bands (PSBs) emerge on the surface as persistent slip markings (PSMs) which consist of extrusions and intrusions. It is commonly reported that the initiated crack is generated at the tip of one of those intrusions. Thus, the PSM formation is an important key for explaining the crack initiation behaviors in the early stage through surface development that was encouraged by the local plastic strain deformation. In this study, the optical-based measurement methods using the confocal microscope and the angle-resolved scattered light sensor were used to inspect and describe the crack formation during cyclic loading due to extrusion and intrusion formations. The specimens were fine-polished by diamond suspension (until 1 µm) and tested on a servo-hydraulic push–pull testing machine with the selected stress amplitude of 492 MPa (70% of the ultimate tensile strength), frequency (f) = 10 Hz, and load ratio (R) = −1. The extrusions and intrusions on the specimen were reliably observed and detected by our surface measurement techniques. The FIB–SEM results reported the emergence of PSMs and crack formations on the modified surface during fatigue loading. Moreover, the scattered light sensor has presented a remarkable result, which could be a potential technique for a new approach to study fatigue life estimation through the formation of extrusion and intrusion.