PART ONE IN A SERIES: FAILURE MODE Fracture Mechanics Applied to Tensile Fasteners

摘要

Fracture Mechanics is a technology that has evolved over the last 65 years. The purpose of this article is to illustrate how this new technology can be applied to material selection and design of tensile loaded fasteners. Detailed analyses are not presented, but instead, simplifying equations are used to provide a tour through the methodology used to apply Fracture Mechanics to tensile loaded fasteners, and more importantly, the physical significance of the conclusions derived from the analysis. Fracture Toughness: Fracture Mechanics is used as a basis for material selection, that can explain the size effect in bolts, define the failure mode, quantify the significant levels of Non-Destructive Testing as related to a specific fastener, quantify Charpy V-notched impact toughness and other quality control requirements, quantify installation torques that can prevent fatigue and stress corrosion cracking in service by the proper selection of coatings to lead to the ultimate goal of infinite life design. In the absence of a crack, the strength of a material is measured on a smooth round bar in tension by the yield strength (YS) and fracture stress (FS) or ultimate tensile strength (UTS) in accordance with ASTM Standard E8. In the presence of a crack, the strength of a material is defined as the Fracture Toughness (KIc) (Ref #1) evolving through ASTM Standards E399/E812/E1820. For the tensile loaded threaded fastener (bolt), which is a helically threaded round bar in tension, the strength is measured as the notched tensile strength (NTS). The yield strength is estimated to be at 85% NTS.