ICAMS / Interdisciplinary Centre for Advanced Materials Simulation

Microstructural influences on growth of short cracks in steel under cyclic loading II

The metallurgical concepts for the improvement of cyclic properties of modern steels are based on the reducing of impurities in a microstructure. A few remaining particles can be responsible for unexpected early failures of components, which work under cyclic loading. On the other hand, small particles like precipitations play a positive role and improve mechanical properties of steel, e.g. its tensile strength. In order to obtain a material with optimal static and cyclic properties the knowledge about the influence of a microstructure on the damage mechanisms is required. Within the frame of this project the fatigue behaviour of very short micro cracks in a structural steel was experimentally characterised. The experimental results shall be used for establishing and calibration of a multi-scale simulation model for quantitatively describing the effect of microstructural features on micro crack fatigue growth and to achieve more robust and reliable fatigue life assessment in the endurance region.

Light optical microscope pictures of microstructure at the notch tip of single-side-notched 4-point-bending specimens before cyclic loading (top left), after two cyclic loading intervals (bottom left) and EBSD picture of crack environment showing grain orientation information (right).

Interrupted multiple-step test loading intervals together with temperature change and direct current potential drop measurements, where metallographic analyses intervene in between, to realize rapid micro crack initiation and to observe crack growth behavior.

The implementation of the extended finite elements method to simulate the cohesive behaviour of fatigue micro cracks enables simulated cracks to grow freely in any favourable direction through microstructure with no mesh dependence. Irreversible fatigue damage accumulation is modeled by gradual user-defined element stiffness degradation in the cyclic damage evolution.

Project Files:

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Incremental increase of load amplitude at 40°C and -20°C and the corresponding temperature in the neck of the specimen. The arrows point at the first occurrence of irreversible mechanisms that result in fatigue.
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Light optical micrograph of the micro-fatigue crack. The microstructure without the crack was digitalized into a FEM mesh and the XFEM method was used to simulate crack initiation at the inclusion. The experimental measurement and the numerical prediction

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