The FWF project entitled “Mechanisms for fatigue crack growth in meso/micro and nano specimens - crack initiation and short crack growth under geometrical and mechanical constraints” aims for the quantification of cyclic crack growth in miniaturized specimens at various length scales. Upon small scale testing, the obtained experimental data may show an inherent dependency of specimen size from a material perspective, termed size effect, which appears superimposed with consequences of undersized specimens. Thus, discriminating between those two effects is not straightforward and can lead to misinterpretation of the materials mechanical response to the external load scenario. This issue is even more severe if the actual intrinsic size effect behavior is unknown in any related micro-mechanical investigation.

To tackle this issue in the framework of fracture mechanics, the project deals with nickel and nickel-copper composite materials, obtained through pulsed electro deposition. Due to the extensive knowledge of the German project partners regarding the correlation of deposition parameters (and rates) and microstructural features (mainly grain size), a well-defined model material is available. The employed pulsed electro deposition strategy allows for a change in deposition parameters during film growth and even the exchange of the electrolyte during deposition to facilitate a layered microstructure and layered metal-metal composites. The mechanical experiments mainly aim for an investigation of cyclic crack growth characteristics starting from a predefined notch, complemented by static miniaturized fracture mechanical experiments. 

The specific mechanical loading case triggers certain plastic effects within the material to different extents and with closely related relevant zone sizes. The growth characteristics and spatial extent of this specific zone is crucial for interpretation of miniaturized experiments and to define validity criteria. For example, the ultra-fine grained or nanocrystalline microstructure of the material exhibits grain growth upon cyclic loading, which continuously changes the material condition in front of the growing crack and thus represents a nonlinear effect. By artificially limiting this and related effects trough geometric (specimen dimension) or mechanical (through composite materials) constraints, the effects can be indirectly quantified. Therefore, related specimen geometries of different sizes are manufactured and tested on the one hand, while on the other hand specimens with the crack running parallel to or perpendicular towards shielding interfaces of a different material are examined. Since the miniaturized nature of the investigated specimens requires testing within a scanning electron/ion or transmission electron microscope, valuable in situ images and videos can be continuously obtained throughout the entire experiments, which are helpful for the interpretation of the testing data. Furthermore, high resolution post testing investigations, such as 3D electron backscatter diffraction imaging, allow for characterization of microstructural changes around the cyclic fracture.

Complementary nanoindentation mapping experiments have been performed on multilayered thin film cross sections to spatially resolve hardness and elastic modulus variations across Ni sublayers featuring grain size variations. In addition, nanoindentation-based strain rate jump tests are conducted within individual sublayers to probe rate-dependent mechanical behavior, enabling the determination of parameters such as strain rate sensitivity and the associated activation volume. These measurements provide localized insight into the deformation mechanisms governing the distinct microstructural regions and support the interpretation of the observed fracture behavior.

Both project partners contribute experiments in specific sample size regimes, but also overlapping regions are investigated to assure comparability of results obtained on different testing machines in different labs. In a later stage of the project, finite element simulations and discrete dislocation dynamics computations are planned to validate the mechanical material model for such ultra-fine grained to nanocrystalline materials. Ultimately, the combined work will contribute to an advanced methodological approach and a better mechanistic picture of the cyclic failure of high strength nanostructured materials.

The figures show preliminary results, which were presented in a poster exhibition at the conference IC-MPPE 2024 conference. The corresponding experimental data will be published in future scientific articles.