FATIGUE CRACK PROPAGATION GROWTH RATES UNDER A WIDE VARIATION OF DELTA K FOR AN ASTM A517 GRADE F(T-1) STEEL

Fatigue crack propagation experiments on an ASTM A517 F(T-1) steel were carried out within the framework of linear-elastic fracture mechanics. At low stress intensity levels an examination of very slow fatigue crack propagation rates of the order of 0.000000001 in./cycle revealed the existence of a stress intensity threshold level below which fatigue crack propagation rates become diminishingly small (order of 0.000000001 in./cycle). Stress ratio was found to alter the threshold stress intensity with higher stress ratios resulting in somewhat depressed threshold values. The influence of stress ratio was also felt at stress intensity levels above the threshold with increased growth rate being associated with increased values of stress ratio. Low stress intensity level fatigue crack growth rate tests were conducted at room temperature in air, distilled water, and dry hydrogen gas. These environments did not affect the fatigue threshold. Identical values of the threshold stress intensity were recorded in each environment studied. Furthermore, for the range of slow growth rates, order of 0.000000001 to 0.0000001 in./cycle, and frequencies, 120 to 180 Hz, considered, these environments seemed to produce little effect on fatigue crack growth rate results. For higher cyclic growth rates (greater than 0.0000001 in./cycle) and stress intensity levels, the effect of material condition and heat treatment was evaluated. Plate material rated as ultrasonically good and poor was evaluated in two heat treated conditions. Examination of fatigue crack growth rates indicated that ultrasonic ratings do not necessarily give good indications of crack growth resistance. /Author/