Fatigue Fracture of a Carbon Steel Pawl Spring That Originated at a Delamination at a Rivet Hole
The pawl spring which was part of a selector switch used in telephone equipment failed. The springs were blanked from 0.4 mm (0.014 in.) thick tempered 1095 steel and then nickel plated. Numerous pits around the rivet holes were revealed by microscopic examination of longitudinal specimens. Delaminations that were formed at inclusion sites during punching of the rivet holes and that were filled with nickel during the plating operation were revealed by microscopic examination of the rivet hole. These delaminations were interpreted to have acted as stress raisers and initiated the fracture. Long, narrow sulfide stringers which were the probably the cause of delamination in this spring material were revealed in the raw material used to make the springs. It was concluded that fracture of the springs was caused by fatigue that had originated at delaminations around the rivet holes.