Geometry effect in the fatigue behaviour of microscale 316L stainless steel specimens

Abstract

Advances in engineering fabrication technology permit the manufacture of microscale components and devices. These tiny products are used in a wide range of applications, from medical devices to smart phones. There are occasions when the speed with which these new fabrication technologies are developed outpaces the scientific research that seeks to understand the behaviour of microscale components. The behaviour of traditional engineering material is generally expressed in terms that relate to bulk material. However, it is known that bulk material behaviour does not apply at the microscale and that the physical characteristics of tiny components are modified by size effects. These size effects influence every aspect of a material's behaviour; mechanical, electrical and thermal properties all differ from the properties normally associated with a particular material. This thesis focuses on the microscale fatigue properties of 316L stainless steel. This material is commonly used in microscale medical devices such as stents, which, once implanted, are subjected to hundreds of millions of fatigue cycles. Fatigue testing was carried out on 50 um, 75 um, 100 um and 150 um 316L specimens. A size effect was shown to exist in the fatigue performance of the 50 um specimens. In order to determine the cause of this size effect, the outer surfaces and fracture surfaces of failed test specimens were examined using a scanning electron microscope. These SEM studies suggested that the process of strain localization was occurring and was most pronounced in the 50 um specimens. The surface roughness of the test specimens was measured using white light interferometry and this confirmed the SEM observation. Finally, finite element models, designed to approximately represent the grain structure and the anisotropic material behaviour of the grains were developed. These models shed further light on the process of strain localization. Taken as a whole, the thesis shows that a size effect exists in the microscale fatigue behaviour of stainless steel, and that this size effect is caused by the process of strain localization.