Fatigue crack growth mechanisms in micro/nano specimens

In modern society, the microelectronics industry strives for an ever decreasing size of the used parts. In addition to the ability to produce micron-sized components, scanning electron microscopy and focused ion beam methods can resolve and explicitly test mechanical properties at this scale. Just as in the macroscopic scale, failure of small structures typically occurs through cracks. Describing cracks and their propagation in the microstructure constitutes the field of fracture mechanics. Here, macroscopic specimens are characterized by linear-elastic or elastic-plastic fracture mechanics using standardized tests. However, when the influenced zone in front of the crack tip exceeds the thickness of the specimen, as may be the case for miniature sample sizes, the classical concepts of fracture mechanics may no longer be valid. Therefore, more complex methods have to be developed to evaluate the fracture behavior. In this project, specimen sizes will be varied in the micro- and mesoscale and investigated by in situ scanning electron microscopy investigations. Hence, experiments will be performed under quasi-static loading as well as under cyclic fatigue. Cyclic fatigue is used to introduce natural cracks into the specimens, which serve as realistic notches in the subsequent tests. During the tests, the plastic zone in front of the crack tip can be characterized and images are taken for further evaluation using digital image correlation. In addition to the sample size limitation, the influence zone in front of the crack tip is tailored by means of an adapted layer system. The interaction of this zone with inhomogeneities in the sample can be used to investigate the influence of the change in Young`s modulus or microstructure on the crack propagation. The layer structure will be fabricated from nickel and copper by pulsed electrochemical deposition. The process allows for specific adjustment of the microstructure and thickness of each layer by varying the deposition parameters, as the current density and pulse function control the grain size of the deposited nickel. The influence of the different elastic moduli of nickel and copper will be investigated in nickel-copper-nickel and copper-nickel-copper layer systems. This unique investigation of the microstructural and mechanical properties in the influenced zone in front of the crack tip will allow a better understanding of the crack growth on a microscopic level. Subsequently, these findings can be used to design layered material systems with optimized resistance against cyclic crack growth.