Demonstrating the potential of Accurate Absolute Cross-grain Stress and Orientation correlation using Electron Backscatter Diffraction

We report a first exploration of High-angular-Resolution Electron Backscatter Diffraction, without using simulated Electron Backscatter Diffraction patterns as a reference, for absolute stress and orientation measurements in polycrystalline materials. By co-correlating the pattern center and fully exploiting crystal symmetry and plane-stress, simultaneous correlation of all overlapping regions of interest in multiple direct-electron-detector, energy-filtered Electron Backscatter Diffraction patterns is achieved. The potential for highly accurate measurement of absolute stress, crystal orientation and pattern center is demonstrated on a virtual polycrystalline case-study, showing errors respectively below 20 MPa (or 1e-4 in strain), 7e-5 rad and 0.06 pixels.Comment: Manuscript as accepted for publication in Scripta Materiali

One‐step deposition of nano‐to‐micron‐scalable, high‐quality digital image correlation patterns for high‐strain in‐situ multi‐microscopy testing

Digital image correlation (DIC) is of vital importance in the field of experimental mechanics, yet producing suitable DIC patterns for demanding in-situ (micro)mechanical tests remains challenging, especially for ultrafine patterns, despite the large number of patterning techniques reported in the literature. Therefore, we propose a simple, flexible, one-step technique (only requiring a conventional physical vapour deposition machine) to obtain scalable, high-quality, robust DIC patterns, suitable for a range of microscopic techniques, by deposition of a low-melting temperature solder alloy in the so-called island growth mode, without elevating the substrate temperature. Proof of principle is shown by (near-)room temperature deposition of InSn patterns, yielding highly dense, homogeneous DIC patterns over large areas with a feature size that can be tuned from as small as ~10 nm to ~2 μm and with control over the feature shape and density by changing the deposition parameters. Pattern optimisation, in terms of feature size, density, and contrast, is demonstrated for imaging with atomic force microscopy, scanning electron microscopy, optical profilometry, and optical microscopy. Moreover, the performance of the InSn DIC patterns and their robustness to large deformations is validated in two challenging case studies of in-situ micromechanical testing: (a) self-adaptive isogeometric digital height correlation of optical surface height profiles of a coarse, bimodal InSn pattern providing microscopic 3D deformation fields (illustrated for delamination of Al stretchable interconnects on a PI substrate) and (b) DIC on scanning electron microscopy images of a much finer InSn pattern allowing quantification of high strains near fracture locations (illustrated for rupture of a polycrystalline Fe foil). As such, the high controllability, performance, and scalability of the DIC patterns, created by island growth of a solder alloy, offer a promising step towards more routine DIC-based in-situ micromechanical testing.</p

An integrated experimental-numerical study of martensite/ferrite interface damage initiation in dual-phase steels

Martensite/ferrite (M/F) interface damage is relevant to failure of many dual-phase (DP) steels, but the underlying microscale mechanisms remain unclear. Through an integrated experimental-numerical study, this work examines the recent hypothesis that (lath) martensite substructure boundary sliding triggers and dominates M/F interface damage initiation accompanied by apparent martensite plasticity. The mesoscale morphology and prior austenite grain reconstruction are used as modelling inputs. A multi-scale framework is adopted to predict the interface damage initiation. The M/F interface damage initiation sites predicted by the model based on a sliding-triggered interface damage mechanism adequately agree with those identified from in-situ experiments,confirming the key role of substructure boundary sliding. Moreover, the M/F interface damage initiation strongly correlates with a low M/F strain partitioning rather than the commonly accepted strong M/F strain partitioning. This fundamental understanding is instrumental for the future optimization of DP steel microstructures

Micro-mechanical deformation behavior of heat-treated laser powder bed fusion processed Ti-6Al-4V

Industrial implementation of heat-treated Laser Powder Bed Fusion (L-PBF) processed Ti-6Al-4 V components requires a thorough understanding of the plastic deformation mechanisms to predict the part performance in safety-critical environments. Here, we study the micro-mechanical deformation behavior of a heat-treated L-PBF processed Ti-6Al-4 V by in-situ uniaxial tensile loading, during which high-resolution strain fields were monitored by Scanning Electron Microscope (SEM) based Digital Image Correlation (DIC). SEM-DIC revealed: (i) the transformed beta phase accommodates higher strain than the primary alpha phase; (ii) strain accumulation in primary alpha occurs primarily at the interface regions where the Al content is lower; and (iii) needle-shaped secondary alpha precipitate in the transformed beta creates strain localization pathways that bridge the interfacial strain bands. Based on the in-situ deformation behavior, recommendations are made on microstructure tailoring and alloy design to prevent strain localization and enhance the quasi-static mechanical properties of L-PBF processed titanium alloy components

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