The fatigue strength of internal threads in three materials

Repeated tension fatigue tests have been carried out on $\text{3}\text{4}\text{?16}$ UNF internally threaded specimens in a 60:40 brass, a normalized En 8 steel and a hardened and tempered En 26 steel. Ground threads did not give as high a fatigue strength as tapped threads, despite having a better root form; this is attributed to beneficial cold working effects in thread tapping. Increasing the outside diameter of the specimen from 1.4 to 1.7D, where D is the nominal thread diameter, did not increase in cross-sectional area. Despite having nearly double the static strength of the En 8 specimens with ground threads. A wide range of endurance has been covered and the results contribute to design data in an area where few such data exist.