In situ high speed imaging study and modelling of the fatigue fragmentation of dendritic structures in ultrasonic fields

The fatigue fragmentation of dendritic structures of a SCN-5wt.% Camphor transparent alloy in an ultrasound field was investigated systematically using high-speed imaging. Dynamic interactions between oscillating ultrasonic bubbles and the primary and secondary dendrites were observed and recorded. High speed images show that when an ultrasound bubble was trapped inside an interdendritic region, the oscillation of the bubble can cause cyclic bending of the dendritic arms. Consequently, a fatigue type crack initiated at the arm root and propagated through the dendrite, causing the dendrite to fragment, i.e. dendritic fragmentation. From the recorded videos, the cycle numbers for the fatigue fragmentation were extracted, then, the fatigue strength and fatigue life of the SCN-5wt.% Camphor transparent alloy were calculated. Results showed that the cyclic fatigue load cause a crack initiating on a SCN dendrite at a much lower stress level than its tensile strength under high frequent oscillation.