Identification and modelling of damaging mechanisms in cast Al-Si-alloys during high-cycle- and very-high-cycle fatigue

  In-situ observation of fatigue crack growth in hypo-eutectic cast Al-Si-alloys Copyright: © IEHK Figure 1: In-situ observation of fatigue crack growth in hypo-eutectic cast Al-Si-alloys

The scientific project funded by DFG “Identification and modelling of damaging mechanisms in cast Al-Si-alloys during high-cycle- and very-high-cycle fatigue” aims to show the correlation between local microstructure and fatigue-mechanisms for a very high number of cycles up to 109 cycles. For this purpose, resonance frequency fatigue testing machines and a high-resolution thermography– and microscopy-system are used (Fig. 1). To monitor the interaction between fatigue crack-growth and the dendritic-eutectic cast Al-Si-microstructure, specimens are equipped with shallow-notches for in-situ observation. Based on this experiments mechanisms of fatigue crack initiation and short-crack-growth can be examined and identified.

Additionally to this Wöhler-like experiments single edge notch bend specimens are tested under pure but cyclic bending to observe the crack propagation behavior under varying loadings. Due to this the relation between the crack propagation rate da/dN and the range of the stress intensity factor ΔK can be investigated (Paris-law). Furthermore, with high importance for the identification of possible microstructure components with an increased barrier-effect against crack propagation, the threshold value against technical crack initiation ΔKth can be determined.

By combining the results from fatigue and crack propagation investigation a so-called Kitagawa-Takahashi-diagram is produced. Such diagrams are working as threshold-diagrams for a defined limiting number of cycles to pass before failure. They are showing the bearable cyclic loading amplitude σa for a given size of internal defects a.

The overall aim of the scientific project in cooperation with the TU Dortmund is to generate a better understanding for the fatigue crack initiation and propagation mechanisms in cast Al-Si alloys and to focus this findings in a material model for short-crack-propagation-simulation. For this purpose, an existing simulation tool based on the boundary element method will be developed in the long term.