Bg Bart Vossen

Electron Micrographic Digital Image Correlation: Method Optimization and Microstructural banding Case Study

Segregation-induced microstructural banding is commonly encountered in commercial steels, yet its effect on the global mechanical behavior is disputed in the literature due to the difficulty of designing clean control experiments. This work locally compares the deformation of banded phases with unbanded regions in the same microstructure from in-situ electron micrographs analyzed with (optimized) micrographic digital image correlation. To this end, first, the employed electron micrographic digital image correlation (EMDIC) methodology is optimized in terms of its experimental parameters: specimen surface preparation settings, scanning electron microscopy imaging settings (contrast modes, magnification, resolution, and contrast-brightness), and image correlation settings (facet size and facet step size). Subsequently, the strength of the (optimized) EMDIC methodology is demonstrated on a case study on segregation-induced microstructural banding in steel, in which the influence of the banded phase and its morphology is probed by comparing the mechanical behavior of two carefully chosen, extreme cases of banded microstructures: a microstructure containing a continuous, hard band (the martensitic-ferritic system) and a microstructure containing non-continuous, softer bands (the pearlitic-ferritic system). The obtained micro-scale strain fields yield clear insight into the influence of band structure, morphology and band material properties.

Advances in Delamination Modeling of Metal/Polymer Systems: Continuum Aspects

Adhesion and delamination have been pervasive problems hampering the performance and reliability of micro- and nano-electronic devices. In order to understand, predict, and ultimately prevent interface failure in electronic devices, development of accurate, robust, and efficient delamination testing and prediction methods is crucial. Adhesion is essentially a multi-scale phenomenon: at the smallest scale possible, it is defined by the thermodynamic work of adhesion. At larger scales, additional dissipative mechanisms may be active which results in enhanced adhesion at the macroscopic scale and are the main cause for the mode angle dependency of the interface toughness. Undoubtedly, the macroscopic adhesion properties are a complex function of all dissipation mechanisms across the scales. Thorough understanding of the significance of each of these dissipative mechanisms is of utmost importance in order to establish physically correct, unambiguous values of the adhesion properties, which can only be achieved by proper multi-scale techniques.

From Fibrils to Toughness: Multi-Scale Mechanics of Fibrillating Interfaces in Stretchable Electronics

Metal-elastomer interfacial systems, often encountered in stretchable electronics, demonstrate remarkably high interface fracture toughness values. Evidently, a large gap exists between the rather small adhesion energy levels at the microscopic scale (‘intrinsic adhesion’) and the large measured macroscopic work-of-separation. This energy gap is closed here by unravelling the underlying dissipative mechanisms through a systematic numerical/experimental multi-scale approach. This self-containing contribution collects and reviews previously published results and addresses the remaining open questions by providing new and independent results obtained from an alternative experimental set-up. In particular, the experimental studies on Cu-PDMS (Poly(dimethylsiloxane)) samples conclusively reveal the essential role of fibrillation mechanisms at the micro-meter scale during the metal-elastomer delamination process. The micro-scale numerical analyses on single and multiple fibrils show that the dynamic release of the stored elastic energy by multiple fibril fracture, including the interaction with the adjacent deforming bulk PDMS and its highly nonlinear behaviour, provide a mechanistic understanding of the high work-of-separation. An experimentally validated quantitative relation between the macroscopic work-of-separation and peel front height is established from the simulation results. Finally, it is shown that a micro-mechanically motivated shape of the traction-separation law in cohesive zone models is essential to describe the delamination process in fibrillating metal-elastomer systems in a physically meaningful way.