Semi in-situ observation of crack initiation in compacted graphite iron during thermo mechanical fatigue

Compacted Graphite Iron is a suitable material for the engine cylinder heads of heavy duty trucks. In the base of the cylinder head, the Valve Bridge is subjected to Thermo-Mechanical Fatigue as a result of daily start-up and shut-down operational cycles. With the aim to investigate crack initiation, a semi in-situ experiment was conducted by employing a disassemblable dog-bone sample. The procedure consisted of iteratively interrupting a thermo-mechanical fatigue test to observe the evolution of the microstructure by Scanning Electron Microscopy, and Electron Backscatter Diffraction. Finishing the first thermo-mechanical fatigue cycle, edge delaminated graphite particles were observed to be connected by cracks. Cyclic plastic strain computation showed that edge graphite particles can interact by overlapping plastic zones that arise at the tip of the delamination cracks and thus provide a crack propagation path. Finally, the electron backscattered diffraction technique allowed to observe the competition between recovery and plastic strain during the first few thermo-mechanical fatigue cycles.