Crack initiation in delayed failure under repeated loads of various stress wave forms was investigated using the notched specimens of SNCM 439 quenched and tempered at 473K. The results obtained are as follows:
(1) The crack initiation time ti and the threshold stress σSCC were decreased by superposition of repeated load on the sustained load, the degree of which increased with an increase of stress increasing period at constant frequency, i. e. ti and σSCC decreased most under repeated loads with positive saw tooth wave, followed by those with sinusoidal, negative saw tooth and trapezoidal.
(2) The reason why the crack initiation time ti and σSCC decreased when the stress wave form had a long stress increasing period seems to be that the corrosive reaction is activated during the stress increasing period, since the protective film on notch root surface is destructed mechanically by repeated stress. In the stress decreasing period, on the other hand, the protective film destructed mechanically by repeated stress is reformed, so that hydrogen might invade less into material when the stress wave form has a long period of stress decreasing or stress holding.
