1,402 research outputs found
Enhanced Piola–Hencky discrete models for pantographic sheets with pivots without deformation energy: Numerics and experiments
No full textThe problem of the synthesis of second gradient (meta)materials, via architectured microstructures made of micro-lattices, has been solved (Alibert et al., 2003; Seppecher et al., 2011) by choosing ideal pivots as preferred constraints. The obtained homogenized macro-equations (Boutin et al., 2017; Eremeyev et al., 2017) show some pathologies that reflect the exotic behavior of the considered metamaterials, even if they are of interest by themselves (Eremeyev et al., 2017). The theoretical issues that they raise not only represent an intellectual challenge but also means for disclosing potentially interesting new phenomena. To make such disclosure evident, the related technological demand arose, namely, to find an innovative design and production process to construct (Golaszewski et al., 2018), by using additive manufacturing, some pantographic sheets (made in this instance of polyamide but hopefully later also using metals or alloys) whose pivots do twist practically without deformation and with negligible dissipation. Remarkably the specimen could be printed as a monolith and required no post-assembly but only an easily standardized run-in procedure. In this paper, in order to introduce a mathematical description for pantographic sheets with perfect pivots and to avoid to face the aforementioned pathologies, a discrete, finite dimensional, Lagrangian model is formulated. Moreover, in order to include the case in which the beams interconnecting the pivots are long enough to store non negligible bending energy between the closest pairs of pivots, an enhanced Piola–Hencky discrete model is introduced. Two types of nodes are distinguished, the first one interconnects two pantographic fibers, the second one simply interconnects two different segments of the same fiber. The Vietnam long neck peculiar deformed shape experimentally observed in standard extension bias test is obtained with very short computing time, so that the innovative code that has been elaborated can be used as subroutine in more complex computation schemes. A preliminary digital image correlation analysis (Sutton et al., 2009; Hild and Roux, 2012b) is performed and shows that a remarkable agreement between theoretical predictions and experimental evidence can be obtained. This circumstance is easily explained by observing that the numerical code is based on a discrete model directly inspired by the mechanical properties of pantographic sheets and that, therefore, the passages to a continuum model via homogenization (dell'Isola et al., 2016) and then to the subsequent re-discretization, via the introduction of more or less suitable finite elements, are avoided. In our opinion a theory driven formulation of a directly discrete numerical model presents many advantages and it seems suitable for tackling future structural optimization problems
Βιβλιοκρισία: F. HILD, Karien in Portulanen und Portulankarten von der Antike bis in die frühosmanische Zeit, Wien 2019
No full textF. Hild, Karien in Portulanen und Portulankarten von der Antike bis in die frühosmanische ZeitWien 201
Multiscale DIC Applied to Pantographic Structures
No full textBackground: Since the mechanical behavior of pantographic metamaterials depends upon the properties of their microstructure, accurate descriptions of unit cells are needed. Objective: The present effort is motivated by this requirement to characterize the detailed deformation of unit cells formed of two orthogonal sets of 3 beams. Methods: Their deformations in a bias extension test were measured via digital image correlation performed at different scales. Results: Thanks to the gray level residuals, the microscale results were found in better agreement with the experiment than mesoscale and macroscale analyses. Fine analyses around the hinges showed that relative displacements occurred between the two beam layers. Conclusions: Such experimental analyses supply full-field data to validate models (e.g., as a starting point in homogenization procedures) for describing the mechanical behavior of pantographic metamaterials
Identification of adhesive properties in GLARE laminates by Digital Image Correlation
No full textOne page abstrac
Assessment of interface properties and boundary conditionsby Digital Image Correlation and Kalman filter
No full textAbstract 2 pag
Una metodologia basata sulla Correlazione di Immaginiper l’identificazione del comportamento a fratturadi giunti ed interfacce
No full textIn questa comunicazione si propone una metodologia inversa per la caratterizzazione meccanica di
giunti/interfacce alla microscala, basata su misure senza contatto, a tutto campo, mediante correlazione di
immagini digitali. Seguendo un approccio locale, le misure cinematiche raccolte durante una prova di frattura
non-convenzionale vengono impiegate, da una parte, per guidare le simulazioni ad elementi finiti che riguardano
esclusivamente il sotto-dominio monitorato, quali condizioni di Dirichlet imposte sul bordo, e dall’altra parte
come termini di confronto a fini identificativi, da includere nella funzione obiettivo
Strain and damage interactions under plane strain conditions: 3D in situ measurements for different microstructures
No full textInternational audienceThe link between microstructure, strain heterogeneity, strain localization and damage under plane strain conditions is very complex. Yet, it is important to understand as forming limit diagrams exhibit the lowest strain level under plane strain conditions.Here, the interactions of strain and damage are assessed ahead of notches of thin sheet specimens where a plane strain condition is found, with no strain in crack propagation direction [1,2]. This region is investigated via in situ synchrotron laminography and digital volume correlation (DVC). Synchrotron laminography is a technique specifically developed for three-dimensional (3D) imaging of regions of interest inside laterally extended sheet specimens with micrometre resolution. DVC is carried out using the natural 3D image contrast caused by iron rich intermetallic particles present in the alloy [3].The study is carried out for different 2XXX series alloys with different tempers and associated work hardening, grain shapes and sizes as well as textures [4-6]. The development of early slant strain concentration bands is found for the majority of these conditions and further discussed using FE simulations.[1] A. Buljac, T. Taillandier-Thomas, T.F. Morgeneyer, L. Helfen, S. Roux, F. Hild, "Slant strained band development during flat to slant crack transition in AA 2198 T8 sheet: in situ 3D measurements", International Journal of Fracture, 200 (1) (2016) 49–62[2] F. Bron, J. Besson, "Simulation of the ductile tearing for two grades of 2024 aluminum alloy thin sheets”, Engineering Fracture Mechanics, 73 (2006) 1531-1552[3] T.F. Morgeneyer, L. Helfen, H. Mubarak, F. Hild, "3D Digital Volume Correlation of Synchrotron Radiation Laminography images of ductile crack initiation: an initial feasibility study", Experimental Mechanics, 53 (2013) 543-556[4] T.F. Morgeneyer, T. Taillandier-Thomas, L. Helfen, T. Baumbach, I. Sinclair, S. Roux, F. Hild, "In situ 3D observation of early strain localisation during failure of thin Al alloy (2198) sheet", Acta Materialia, 69 (2014) 78-91[5] T.F. Morgeneyer, T. Taillandier-Thomas, A. Buljac, L. Helfen, F. Hild, "On strain and damage interactions during tearing: 3D in situ measurements and simulations for a ductile alloy (AA2139-T3)", Journal of the Mechanics and Physics of Solids, 96 (2016) 550–571[6] A. Buljac, F. Hild, L. Helfen, T.F. Morgeneyer, "On deformation and damage micromechanisms in strong work hardening 2198 T3 aluminium alloy", Acta Materialia, 149 (2018) 29-4
Large in-plane elastic deformations of bi-pantographic fabrics: asymptotic homogenization and experimental validation
No full textBi-pantographic fabrics are composed of two families of pantographic beams and correspond to a class of architectured materials that are described in plane as second-gradient 2D continua. On a discrete level, a pantographic beam is a periodic arrangement of cells and looks like an expanding barrier. The materialization of a bi-pantographic fabric made from polyamide was achieved by additive manufacturing techniques. Starting from a discrete spring system, the deformation energy of the corresponding continuum is derived for large strains by asymptotic homogenization. The obtained energy depends on the second gradient of the deformation through the rate of change in orientation and stretch of material lines directed along the pantographic beams. Displacement-controlled bias extension tests were performed on rectangular prototypes for total elastic extension up to 25%. Force–displacement measurements complemented by local digital image correlation analyses were used to fit the continuum model achieving excellent agreement
Rescaling and mesoshear deformations of lattice metamaterials
No full textExperimental results are presented for a lattice metamaterial with a complex mesostructure composed of parallel fibers aligned at different levels. The experimental evidence suggests that the fibers, in addition to their own deformations, such as elongation and bending, and to the relative rotation between two adjacent layers, may also slide relative to each other under certain conditions. This deformation mechanism is taken into account and used as a key hypothesis in the development of a new continuum model. A remarkable rescaling property possessed by the considered metamaterial is also discussed
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