A study was made on the effects of stress increasing time or stress rise time T1, maximum stress holding time T2, stress decreasing time T3 and minimum stress holding time T4 in a wave on fatigue crack growth for a low alloy carbon steel (σu=91.8kg/mm2, σy=81.5kg/mm2) in 3% NaCl solution. Measurements of the effective stress intensity range ratio U and observation of crack tip were performed to clarify the cause of waveform effect.
The results were summarized as follows.
(1) T1 had a strong accelerating effect due to corrosive dissolution of fresh surface of the crack which was formed during T1. The crack growth rate was enhanced as T1 increased and reached to a constant value (about 3 times than that in air) after T1=10sec. The crack growth rate at low ΔK, however, decreased as T1 increased (more than T1=1sec).
(2) T2, T3 and T4 decreased the crack growth rate, respectively. The extent of decrease not only depended on the period of T2 (or T3, T4), but also on ΔK and T1.
(3) The crack growth law which was derived by considering the effects of waveform and frequencies in the previous paper, is also valid to first approximation for the present results.
