WORK HARDENING EFFECTS ON THE MECHANICAL BEHAVIOR OF STAINLESS STEEL FASTENERS

Accurate prediction of fastener strength is essential to ensure safe and efficient design. However, current design standards, such as FED-STD-H28/2B and ISO TR/16224, are largely based on data from carbon steel fasteners and do not accurately capture the behavior of stainless steel bolts, which exhibit different behavior due to work hardening during manufacturing. This thesis investigates the mechanical behavior of work-hardened stainless steel fasteners through experimental testing. Twelve lots of stainless steel bolts—spanning three material grades, multiple diameters, and manufacturing methods—were tested in tension, shear, and hardness. The results show that cold-worked stainless steel bolts consistently fail in the shank rather than the threads, confirming that threaded regions experience significant strengthening due to work hardening. Compared to FED-STD predictions, the measured tensile stress areas were on average 35% higher, while external thread shear strength was accurately captured only when using a shear-to-tensile strength ratio of 0.9 rather than the conventional 0.6. Hardness testing further revealed a 10% increase in the threaded region relative to the shank, directly evidencing nonuniform strengthening caused by cold forming. In contrast, bar-stock bolts with minimal work hardening exhibited uniform hardness, lower strength, and premature thread deformation. These findings demonstrate the critical influence of manufacturing-induced work hardening on fastener performance and provide a basis for improved predictive equations and design procedures for stainless steel bolts.