Axial fatigue specimen fractured at threaded ends

[DuncanGibbons]

Hi all,

I recently performed some axial fatigue tests (ASTM E466) on a metal alloy. A handful of the specimens fractured in the threaded grip sections. Is this a common failure type for axial fatigue?

Could it be due to poorly machined threads or that the thread pitch (1mm pitch was used) was too small resulting in "play" in the grips?


Thanks