On Micromechanical Parameter Identification With Integrated DIC and the Role of Accuracy in Kinematic Boundary Conditions

Integrated Digital Image Correlation (IDIC) is nowadays a well established full-field experimental procedure for reliable and accurate identification of material parameters. It is based on the correlation of a series of images captured during a mechanical experiment, that are matched by displacement fields derived from an underlying mechanical model. In recent studies, it has been shown that when the applied boundary conditions lie outside the employed field of view, IDIC suffers from inaccuracies. A typical example is a micromechanical parameter identification inside a Microstructural Volume Element (MVE), whereby images are usually obtained by electron microscopy or other microscopy techniques but the loads are applied at a much larger scale. For any IDIC model, MVE boundary conditions still need to be specified, and any deviation or fluctuation in these boundary conditions may significantly influence the quality of identification. Prescribing proper boundary conditions is generally a challenging task, because the MVE has no free boundary, and the boundary displacements are typically highly heterogeneous due to the underlying microstructure. The aim of this paper is therefore first to quantify the effects of errors in the prescribed boundary conditions on the accuracy of the identification in a systematic way. To this end, three kinds of mechanical tests, each for various levels of material contrast ratios and levels of image noise, are carried out by means of virtual experiments. For simplicity, an elastic compressible Neo-Hookean constitutive model under plane strain assumption is adopted. It is shown that a high level of detail is required in the applied boundary conditions. This motivates an improved boundary condition application approach, which considers constitutive material parameters as well as kinematic variables at the boundary of the entire MVE as degrees of freedom in...Comment: 37 pages, 25 figures, 2 tables, 2 algorithm

Effect of restrained versus free drying on hygro-expansion of hardwood and softwood fibers and paper handsheet

Earlier works in literature on the hygro-expansion of paper state that the larger hygro-expansivity of freely compared to restrained dried handsheets is due to structural differences between the fibers inside the handsheet. To unravel this hypothesis, first, the hygro-expansion of freely and restrained dried, hardwood and softwood handsheets has been characterized. Subsequently, the transient full-field hygro-expansion (longitudinal, transverse, and shear strain) of fibers extracted from these handsheets was measured using global digital height correlation, from which the micro-fibril angle was deduced. The hygro-expansivity of each individual fiber was tested before and after a wetting period, during which the fiber's moisture content is maximized, to analyze if a restrained dried fiber can "transform" into a freely dried fiber. It was found that the longitudinal hygro-expansion of the freely dried fibers is significantly larger than the restrained dried fibers, consistent with the sheet-scale differences. The difference in micro-fibril angle between the freely and restrained dried fibers is a possible explanation for this difference, but merely for the hardwood fibers, which are able to "transform" to freely dried fibers after being soaked in water. In contrast, this "transformation" does not happen in softwood fibers, even after full immersion in water for a day. Various mechanisms have been studied to explain the observations on freely and restrained dried hardwood and softwood, fiber and handsheets including analysis of the fibers' lumen and cross-sectional shape. The presented results and discussion deepens the understanding of the differences between freely and restrained dried handsheets.Comment: 43 pages, 15 figures, 2 table

Experimental Full-field Analysis of Size Effects in Miniaturized Cellular Elastomeric Metamaterials

Cellular elastomeric metamaterials are interesting for various applications, e.g. soft robotics, as they may exhibit multiple microstructural pattern transformations, each with its characteristic mechanical behavior. Numerical literature studies revealed that pattern formation is restricted in (thick) boundary layers causing significant mechanical size effects. This paper aims to experimentally validate these findings on miniaturized specimens, relevant for real applications, and to investigate the effect of increased geometrical and material imperfections resulting from specimen miniaturization. To this end, miniaturized cellular metamaterial specimens are manufactured with different scale ratios, subjected to in-situ micro-compression tests combined with digital image correlation yielding full-field kinematics, and compared to complementary numerical simulations. The specimens' global behavior agrees well with the numerical predictions, in terms of pre-buckling stiffness, buckling strain and post-buckling stress. Their local behavior, i.e. pattern transformation and boundary layer formation, is also consistent between experiments and simulations. Comparison of these results with idealized numerical studies from literature reveals the influence of the boundary conditions in real cellular metamaterial applications, e.g. lateral confinement, on the mechanical response in terms of size effects and boundary layer formation.Comment: 20 pages, 6 figures, Materials & Design, 11 May 202

Transient hygro- and hydro-expansion of freely and restrained dried paper: the fiber-network coupling

The transient dimensional changes during \textit{hygro}-expansion and \textit{hydro}-expansion of freely and restrained dried, softwood and hardwood sheets and fibers is monitored, to unravel the governing micro-mechanisms occurring during gradual water saturation. The response of individual fibers is measured using a full-field global digital height correlation method, which has been extended to monitor the transient \textit{hydro}-expansion of fibers from dry to fully saturated. The \textit{hygro}- and \textit{hydro}-expansion is larger for freely versus restrained dried and softwood versus hardwood handsheets. The transient sheet-scale \textit{hydro}-expansion reveals a sudden strain and moisture content step. It is postulated that the driving mechanism is the moisture-induced softening of the so-called "dislocated regions" in the fiber's cellulose micro-fibrils, unlocking further fiber swelling. The strain step is negligible for restrained dried handsheets, which is attributed to the "dislocated cellulose regions" being locked in their stretched configuration during restrained drying, which is supported by the single fiber \textit{hydro}-expansion measurements. Finally, an inter-fiber bond model is exploited and adapted to predict the sheet-scale \textit{hygro}-expansion from the fiber level characteristics. The model correctly predicts the qualitative differences between freely versus restrained dried and softwood versus hardwood handsheets, yet, its simplified geometry does not allow for more quantitative predictions of the sheet-scale \textit{hydro}-expansion.Comment: 37 pages; 12 figures; 5 table

Revisiting the martensite/ferrite interface damage initiation mechanism:The key role of substructure boundary sliding

Martensite/ferrite (M/F) interface damage plays a critical role in controlling failure of dual-phase (DP) steels and is commonly understood to originate from the large phase contrast between martensite and ferrite. This however conflicts with a few, recent observations, showing that considerable M/F interface damage initiation is often accompanied by apparent martensite island plasticity and weak M/F strain partitioning. In fact, martensite has a complex hierarchical structure which induces a strongly heterogeneous and orientation-dependent plastic response. Depending on the local stress state, (lath) martensite is presumed to be hard to deform based on common understanding. However, when favourably oriented, substructure boundary sliding can be triggered at a resolved shear stress which is comparable to that of ferrite. Moreover, careful measurements of the M/F interface structure indicate the occurrence of sharp martensite wedges protruding into the ferrite and clear steps in correspondence with lath boundaries, constituting a jagged M/F interfacial morphology that may have a large effect on the M/F interface behaviour. By taking into account the substructure and morphology features, which are usually overlooked in the literature, this contribution re-examines the M/F interface damage initiation mechanism. A systematic study is performed, which accounts for different loading conditions, phase contrasts, residual stresses/strains resulting from the preceding martensitic phase transformation, as well as the possible M/F interfacial morphologies. Crystal plasticity simulations are conducted to include inter-lath retained austenite (RA) films enabling the substructure boundary sliding. The results show that the substructure boundary sliding, which is the most favourable plastic deformation mode of lath martensite, can trigger M/F interface damage and hence control the failure behaviour of DP steels. The present finding may change the way in which M/F interface damage initiation is understood as a critical failure mechanism in DP steels

Retardation of plastic instability via damage-enabled microstrain delocalization

Multi-phase microstructures with high mechanical contrast phases are prone to microscopic damage mechanisms. For ferrite-martensite dual-phase steel, for example, damage mechanisms such as martensite cracking or martensite-ferrite decohesion are activated with deformation, and discussed often in literature in relation to their detrimental role in triggering early failure in specific dual-phase steel grades. However, both the micromechanical processes involved and their direct influence on the macroscopic behavior are quite complex, and a deeper understanding thereof requires systematic analyses. To this end, an experimental-theoretical approach is employed here, focusing on three model dual-phase steel microstructures each deformed in three different strain paths. The micromechanical role of the observed damage mechanisms is investigated in detail by in-situ scanning electron microscopy tests, quantitative damage analyses, and finite element simulations. The comparative analysis reveals the unforeseen conclusion that damage nucleation may have a beneficial mechanical effect in ideally designed dual-phase steel microstructures (with effective crack-arrest mechanisms) through microscopic strain delocalization

Three mechanisms of hydrogen-induced dislocation pinning in tungsten

The high-flux deuterium plasma impinging on a divertor degrades the long-termthermo-mechanical performance of its tungsten plasma-facing components. A prime actor inthis is hydrogen embrittlement, a degradation phenomenon that involves the interactions between hydrogen and dislocations, the primary carriers of plasticity. Measuring such nanoscaleinteractions is still very challenging, which limits our understanding. Here, we demonstrate anexperimental approach that combines thermal desorption spectroscopy (TDS) andnanoindentation, allowing to investigate the effect of hydrogen on the dislocation mobility in tungsten. Dislocation mobility was found to be reduced after deuterium injection, which ismanifested as a ‘pop-in’ in the indentation stress-strain curve, with an average activation stressfor dislocation mobility that was more than doubled. All experimental results can be confidentlyexplained, in conjunction with experimental and numerical literature findings, by the simultaneous activation of three mechanisms responsible for dislocation pinning: (i) hydrogentrapping at pre-existing dislocations, (ii) hydrogen-induced vacancies, and (iii) stabilization ofvacancies by hydrogen, contributing respectively 38%, 52%, and 34% to the extra activationstress. These mechanisms are considered to be essential for the proper understanding and modeling of hydrogen embrittlement in tungsten

Attenuated total reflection infrared spectroscopy for studying adsorbates on planar model catalysts : CO adsorption on silica supported Rh nanoparticles

A sensitive method is presented for studying adsorption of gaseous species on metal surfaces in vacuum by attenuated total internal reflection Fourier transform IR spectroscopy (ATR). The method is illustrated by CO adsorption expts. on silica supported Rh nanoparticles. An exptl. setup and a procedure are described in detail to obtain a sensitivity of reflectance change of .apprx.5 * 10-5 absorbance units. Here, a silicon ATR crystal with a 50 nm layer of hydroxylated silica acts as the support for the Rh nanoparticles. These particles are easily prepd. by spincoat impregnation from a RhCl3 soln. followed by H2 redn. XPS before and after redn. shows that rhodium is reduced to Rh0 and that all chlorine is removed. At. force microscope images the distribution of the particles, which are 3-4 nm in height. When the crystal is exposed to pressures up to 1 mbar of CO, a gas which is inert to the silica support, the stretch vibration of linearly adsorbed CO on the Rh nanoparticles is detected at 2023 cm-1, while no bridged CO or geminal dicarbonyl species can be distinguished. The min. detectable coverage is estd. .apprx.0.005 CO per nm2 substrate area or .apprx.5 * 10-4 ML. [on SciFinder (R)