Speaker
Description
Introduction/ Purpose
In contrast to macroscopic fatigue testing, local fatigue experiments on microsamples offer an opportunity to study the crack growth behavior in-situ with correlative microscopy. The basis for this is the combination of microscale sample fabrication by Focused Ion Beam milling and nanoindentation cyclic actuation [1, 2]. The extension of this method by notched samples allows then to use defined stress intensities to initiate cyclic crack growth.
Methods
Square shaped microcantilevers were fabricated by FIB milling with an aspect ratio of 3:3:10 µm (T:W:L) and crack length to width ratio of 0.3 . Based on Gabel & Merle [2], the method was adapted to combine dynamic nanoindentation (CSM) and notched microcantilever bending. The CSM information was used to calculate the crack growth and to analyze the deformation energy in each cycle. This technique makes high cycle fatigue testing accessible for small-scale samples. The method was enhanced by the use of an in-situ nanoindenter inside an SEM, which allowed tracking the crack growth and microstructural changes visually during the loading. Post mortem SEM imaging and TEM cross section analysis revealed the underlying deformation processes.
Results
The evolution of the dynamic parameters was monitored during the in-situ testing and allowed to draw Paris (crack growth vs stress intensity range) plots. The tested samples showed a transition to a region of stable cyclic crack growth and were compared to macroscopic results. The fractured surfaces show stepwise crack growth patterns and exhibit a change from the surface to the inside of the microcantilever. The near-surface region is more ductile, due to the plain stress state. Additionally STEM images close to the crack revealed the formation of typical fatigue dislocation structures.
References
[1] Merle, B. & Höppel, H.W. Exp Mech (2018) 58: 465.
[2] Gabel, S. & Merle, B. MRS Com. (2020) 10: 332.
| Speaker Country | Germany |
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