Effects of Internal Hydrogen on Fatigue Strength of Commercially Pure Titanium

Commercially pure titanium usually contains a few tens of ppm hydrogen. In order to investigate the effect of such internal hydrogen on the fatigue characteristics, rotating bending fatigue tests were carried out on two kinds of specimen with different hydrogen contents: one contains 34 ppm but the other 3 ppm. The amount of internal hydrogen was controlled by changing the annealing conditions. The fatigue process was successively observed by a replication technique. The higher hydrogen content specimen showed a longer fatigue life, longer crack initiation life and higher fatigue limit than those of the lower hydrogen specimen. No specimen showed any particular fatigue crack growth behavior related to hydrogen content, whereas the crack initiation behavior differed somewhat. The fatigue crack initiation site of the higher hydrogen content specimen with a longer fatigue life preferred the interior of a grain. On the other hand, that of the lower hydrogen specimen preferred the neighborhood of a grain boundary.