Fatigue Strength of M22 and M24 Bolts under Fluctuating Tensile Load

Historical data shows that some 65% of fatigue failures in bolts occur in the bolt thread adjacent to or just below the nut face (first engaged thread). The remaining fatigue failures occur below the bolt head(15% of failures) and in the first thread adjacent to the bolt shank (20% of failures). The analysis of local stresses in these latter instances are relatively simple, and these failures are alleviated by increasing the bolt head-shank fillet radius, and by reducing the diameter of the bolt shank to that of the thread root, providing a 'plus thread' configuration. In the present study the fatigue strengths of commercially available M22 and M24 high strength bolts with nuts were determined. In each instance of bolt failure, fracture occurred in the threads adjacent to the nut face. Using an assumed Goodman relationship, fatigue strength reduction factors of 8.6 for the M22 bolts with rolled thread profile, and 11.6 for the M24 bolts with machine cut threads, were determined. Modifications made to the M24 nuts to reduce the stress concentration at the bolt/nut interface failed to produce any significant improvement in bolt fatigue strength. The importance of adequate bolt pretensioning to avoid fatigue failure is verified by these results.