Elasto-plastic fatigue life improvement of bolted joints and introducing FBI method

The failures of bolted joints, which are the basic and popular fasteners in industry, are mostly due to fatigue. This paper presents an effective method to improve the elasto-plastic fatigue life of a bolt. A complete bolted joint is considered and modeled with the finite element approach to calculate stress and strain. The code based on crack initiation theory is developed to calculate the fatigue life of the bolted joint. Local plasticity and mean stress correction are both applied in this analysis. Among the many factors that influence failure and fatigue behavior in bolt-nut fasteners, geometry has the most important effect on fatigue life. The geometry of a bolt is classified to four decisive parameters: Thread Flank Type, Thread Root Radius, Thread Run-out, and Head Fillet Radius. However, sensitivity studies are performed to investigate the effect of each of the four geometrical parameters on fatigue life. Finally, a new bolt design configuration is proposed and the results are compared with the conventional bolt. This new design is named FBI which stands for "Fatigue-Bolt-Improvement method. This modification in design and fabrication of bolts will greatly reduce the stress concentration at the critical area, and will increase the fatigue strength of the bolt as compared to the classic standard bolt.