Defects are natural stress concentrators and constitute as such a major cause of fatigue failure in cast aluminium alloys. Most of existing studies focus their efforts on fatigue crack initiating from surface defects as they have been shown to be the most deleterious ones in High Cycle Fatigue (HCF) regime. However in Very High Cycle Fatigue (VHCF), internal crack initiation dominate. Only a few studies on internal fatigue cracks have been published [1, 2]. The aim of this work is to study internal fatigue crack propagation mechanisms and kinetics by the use of in-situ synchrotron micro-tomography fatigue tests. To promote controlled internal crack initiation, specimens containing an internal casting defect artificially introduced are tested at very low stress amplitudes in the VHCF regime. A new and unique ultrasonic (20 kHz) fatigue testing machine was developed in order to perform in-situ synchrotron fatigue tests . This machine is instrumented with real time modal analysis  and infrared thermographic  measurement devices to automatically detect crack initiation. The crack propagation mechanisms show strong interactions between the crack path and the microstructure which is analysed using both diffraction contrast tomography  and EBSD microscopy analysis. Internal crack propagation kinetics are compared to reference crack propagation data resulting from propagation tests at 20 kHz.
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