Investigations were conducted on surface entering fatigue cracks, 0.13 to 1.27 mm (0.005 to 0.05 in.) long, in rod-type tensile specimens of HY 180 and AISI 4340 steels using surface wave ultrasonics and magnetic perturbation. Pulse-echo, surface wave ultrasonic results indicate that at 10 MHz the minimum flaw size detectable in HY 180 specimens is two to three times larger than that in AISI 4340. Also, fatigue cracks as long as 0.76 mm (0.030 in.) in HY 180 specimens could not be reliably detected with 10 MHz surface waves unless load was applied to open the crack. Extensive magnetic perturbation measurements on AISI 4340 suggest a “hinge” model for the opening of small surface entering fatigue cracks.

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