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

Ultra-Stretchable Interconnects for High-Density Stretchable Electronics

The exciting field of stretchable electronics (SE) promises numerous novel applications, particularly in-body and medical diagnostics devices. However, future advanced SE miniature devices will require high-density, extremely stretchable interconnects with micron-scale footprints, which calls for proven standardized (complementary metal-oxide semiconductor (CMOS)-type) process recipes using bulk integrated circuit (IC) microfabrication tools and fine-pitch photolithography patterning. Here, we address this combined challenge of microfabrication with extreme stretchability for high-density SE devices by introducing CMOS-enabled, free-standing, miniaturized interconnect structures that fully exploit their 3D kinematic freedom through an interplay of buckling, torsion, and bending to maximize stretchability. Integration with standard CMOS-type batch processing is assured by utilizing the Flex-to-Rigid (F2R) post-processing technology to make the back-end-of-line interconnect structures free-standing, thus enabling the routine microfabrication of highly-stretchable interconnects. The performance and reproducibility of these free-standing structures is promising: an elastic stretch beyond 2000% and ultimate (plastic) stretch beyond 3000%, with 10 million cycles at 1000% stretch with <1% resistance change. This generic technology provides a new route to exciting highly-stretchable miniature devices.Comment: 13 pages, 5 figure, journal publicatio

Experimental characterisation of the local mechanical behaviour of cellulose fibres: an in‑situ micro‑profilometry approach

The accurate mechanical characterisation of fibres of micrometric length is a challenging task, especially in the case of organically-formed fibres that naturally exhibit considerable irregularities along the longitudinal fibre direction. The present paper proposes a novel experimental methodology for the evaluation of the local mechanical behaviour of organically-formed (aged and unaged) and regenerated cellulose fibres, which is based on in-situ micro-tensile testing combined with optical profilometry. In order to accurately determine the cross-sectional area profile of a cellulose fibre specimen, optical profilometry is performed both at the top and bottom surfaces of the fibre. The evolution of the local stress at specific fibre locations is next determined from the force value recorded during the tensile test and the local cross-sectional area. An accurate measurement of the corresponding local strain is obtained by using Global Digital Height Correlation (GDHC), thus resulting in multiple, local stress--strain curves per fibre, from which local tensile strengths, elastic moduli, and strains at fracture can be deduced. Since the variations in the geometrical and material properties within an individual fibre are comparable to those observed across fibres, the proposed methodology is able to attain statistically representative measurement data from just one, or a small number of fibre samples. This makes the experimental methodology very suitable for the mechanical analysis of fibres taken from valuable and historical objects, for which typically a limited number of samples is available. It is further demonstrated that the accuracy of the measurement data obtained by the present, local measuring technique may be significantly higher than for a common, global measuring technique since possible errors induced by fibre slip at the grip surfaces are avoided

A bulge test based methodology for characterizing ultra-thin buckled membranes

Buckled membranes become ever more important with further miniaturization and development of ultra-thin film based systems. It is well established that the bulge test method, generally considered the gold standard for characterizing freestanding thin films, is unsuited to characterize buckled membranes, because of compressive residual stresses and a negligible out-of-plane bending stiffness. When pressurized, buckled membranes immediately start entering the ripple regime, but they typically plastically deform or fracture before reaching the cylindrical regime. In this paper the bulge test method is extended to enable characterization of buckled freestanding ultra-thin membranes in the ripple regime. In a combined experimental-numerical approach, the advanced technique of digital height correlation was first extended towards the sub-micron scale, to enable measurement of the highly varying local 3D strain and curvature fields on top of a single ripple in a total region of interest as small as approximately 25 microns. Subsequently, a finite element (FE) model was set up to analyze the post-buckled membrane under pressure loading. In the seemingly complex ripple configuration, a suitable combination of local region of interest and pressure range was identified for which the stress-strain state can be extracted from the local strain and curvature fields. This enables the extraction of both the Young's modulus and Poisson's ratio from a single bulge sample, contrary to the conventional bulge test method. Virtual experiments demonstrate the feasibility of the approach, while real proof of principle of the method was demonstrated for fragile specimens with rather narrow ( approximately 25 microns) ripples

A consistent full-field integrated DIC framework for HR-EBSD

\u3cp\u3eA general, transparent, finite-strain Integrated Digital Image Correlation (IDIC) framework for high angular resolution EBSD (HR-EBSD) is proposed, and implemented through a rigorous derivation of the optimization scheme starting from the fundamental brightness conservation equation in combination with a clear geometric model of the Electron BackScatter Pattern (EBSP) formation. This results in a direct one-step correlation of the full field-of-view of EBSPs, which is validated here on dynamically simulated patterns. Strain and rotation component errors are, on average, (well) below 10\u3csup\u3e−5\u3c/sup\u3e for small (E\u3csub\u3eeq\u3c/sub\u3e=0.05%) and medium (E\u3csub\u3eeq\u3c/sub\u3e=0.2%) strain, and below 3×10\u3csup\u3e−5\u3c/sup\u3e for large strain (E\u3csub\u3eeq\u3c/sub\u3e=1%), all for large rotations up to 10° and 2% image noise. High robustness against poor initial guesses (1° misorientation and zero strain) and typical convergence in 5 iterations is consistently observed for, respectively, image noise up to 20% and 5%. This high accuracy and robustness rivals, when comparing validation on dynamically simulated patterns, the most accurate HR-EBSD algorithms currently available which combine sophisticated filtering and remapping strategies with an indirect two-step correlation approach of local subset ROIs. The proposed general IDIC/HR-EBSD framework lays the foundation for future extensions towards more accurate EBSP formation models or even absolute HR-EBSD.\u3c/p\u3