The electric-current perturbation technique consists of establishing an electric-current flow in the material to be examined and then detecting localized perturbations of this current flow at inhomogeneities such as inclusions or cracks. The current perturbation is sensed by using a small noncontacting probe to detect the associated magnetic-flux perturbation at the surface of the specimen. In the work reported here, analysis of the electric-current perturbation signals was based on an analytic solution for the change in current density caused by a slot of infinite length and finite depth. Comparisons were made with experimental data for electric-discharge machining (EDM) slots of various depths in Incology 901 where the electric current was introduced by ohmic contact. Excellent agreement between theory and experiment was obtained for deep slots, although the model predicts a more rapid decrease in signal amplitude with decreasing slot depth than is experimentally observed. Additional measurements made with fatigue cracks show that cracks as small as 0.45 mm in length are easily detected. It was concluded that the technique is not only a sensitive method for flaw detection, but also offers the possibility of determining flaw characteristics through analysis of signal shapes and amplitudes.

No abstract

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.

No abstract

No abstract