Under the constant potential fatigue crack was propagated in pure titanium polycrystal in NaCI aqueous solution which had very small effect on the fatigue crack propagation rate. The polarization current during the fatigue crack propagation was measured and the electric charge passed during one fatigue cycle was calculated. The amount of fresh bare surface produced at the crack tip during one fatigue loading was quantified by the charge, and this related linearly to the crack propagation rate. This suggests that the shapes of the crack tip show the similar profile through the tested AK range. By comparing with the plastic zone size and the crack tip opening displacement, it is also concluded that the fatigue crack propagation mechanism directly relates to the creation of fresh bare surface at the crack tip, not directly to the plastic zone size or the crack tip opening displacement.
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