Development of fatigue testing system for in-situ observation of stainless steel 316 by HS-AFM & SEM

Abstract

A miniature three-point bend fatigue stage for in-situ observation of fatigue microcrack initiation and growth behaviour by scanning electron microscopy (SEM) and contact mode high-speed atomic force microscopy (HS-AFM) has been developed. Details of this stage are provided along with finite element simulations of the stress profiles of said stage and specimen on loading. The proposed stage facilitates study of the micro mechanisms of fatigue damage evolution when used during SEM and HS-AFM scanning of the sample surface. High amplitude low cycle fatigue tests have been carried out on annealed AISI Type 316 stainless steel to demonstrate the applicability of the system. Characteristic features of surface topography and evolution of slip bands observed have been documented. Images obtained by SEM and HS-AFM are presented for comparison. Finally, to demonstrate the capability of the new facility combined with HS-AFM, the spacing between slip bands and their height at different grains at the centre of the metal sample are measured and compared.