Gears made from Case-carburised SAE8620H steel were subjected to contact fatigue testing at different contact stress levels in the Gear Test Rig facility at Newcastle University. The High frequency Magnetic Barkhausen Noise (MBN) measurements were made at different intervals of fatigue cycles. The MBN signal level gradually increases with progressive fatigue cycles and the rate of change in MBN level systematically increases with the increase in contact stress level. However, at higher contact stress levels, the MBN level quickly reaches a maximum value and then starts to decrease with further fatigue cycles. This maximum MBN level may be considered as an indication of the critical stage of fatigue life beyond which a gear may be expected to fail soon. With increasing numbers of fatigue cycles, the large MBN activity occurs at lower magnetic field strength which clearly indicates the formation of soft magnetisation regions near the surface. This may be attributed to the deformation induced transformation of paramagnetic retained austenite into ferromagnetic martensite.
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