Fatigue crack growth mechanisms in HSLA-80 steels

Fatigue mechanisms of large and small cracks in Cu-bearing HSLA-80 steels were studied at ambient temperature. Fatigue striations were measured for both large and small fatigue cracks by SEM fractography, while dislocation structures adjacent to the fatigue surfaces were characterized by TEM. The results of the fatigue striation and dislocation structure characterization are compared with crack growth data to assess the fatigue mechanisms in HSLA-80 steels and the cause for the lack of a threshold in the small cracks. Comparisons revealed that both large and small cracks propagated via an intermittent growth mechanism. The growth kinetics of small cracks were consistent with the extrapolation of the power-law regime of the large crack data to stress ranges below the large-crack threshold. The absence of a growth threshold in small cracks was discussed in conjunction with the large-crack threshold.