Flexible Packaging for High Pressure Treatments: Delamination Onset and Design Criteria of Multilayer Structures

Multi-layer flexible polymeric films employed for high pressure treatments of food packaging for pasteurization and sterilization frequently display delamination phenomena. This problem limits packaging reliability used for this treatment technology. This contribution is aimed at understanding the delamination phenomena of packaging structures under high pressures. Development of interlaminar stress fields, which promote localized delamination events, is here addressed by considering the case of mechanical failure of bi-layer structures. Analytical models and Finite Element based numerical simulations are exploited to this purpose. The theoretical and numerical results, that highlight the crucial role played by the mismatch of Young moduli and Poisson ratios of the laminated film sheets, are in full agreement with experimental findings on high pressure-treated food multilayer packages realized coupling different polymeric materials (i.e. polypropylene-polyethyleneterephthalate, polypropylene-cast polyamide and polypropylene-bioriented polyamide)

Stability Analysis of Circular Beams with Mixed-Mode Imperfections under Uniform Lateral Pressure

The elastic-plastic collapse of circular beams under uniform lateral pressure with an initial imperfection represented by a combination of different modes and amplitudes and with varying material properties is analysed from a computational viewpoint. The work is stimulated by a number of accurate experimental tests recently performed and it is found that both the initial imperfection and the material inhomogeneity along the beam axis can affect the collapse and produce a sensible variation in the carrying capacity of the structure on account of the changes between the underlying buckling modes. This can give reason for some apparently anomalous observed experimental results

An Analytical Approach to the Analysis of Inhomogeneous Pipes under External Pressure

Pipes for deep-water applications possess a diameter-to-thickness ratio in a region where failure is dominated by both instability and plastic collapse. This implies that prior to failure the compressive yield strength of the material must be exceeded, followed by ovalisation and further local yielding. This paper presents an investigation into the mechanics of this specific problem and develops an analytical approach that accounts for the effects of geometrical and material data on the collapse pressure of inhomogeneous rings under external hydrostatic pressure. The analytical expressions have been correlated to numerical and experimental test data, proving their accuracy

An Analytical Approach to the Analysis of Inhomogeneous Pipes under External Pressure

Pipes for deep-water applications possess a diameter-to-thickness ratio in a region where failure is dominated by both instability and plastic collapse. This implies that prior to failure the compressive yield strength of the material must be exceeded, followed by ovalisation and further local yielding. This paper presents an investigation into the mechanics of this specific problem and develops an analytical approach that accounts for the effects of geometrical and material data on the collapse pressure of inhomogeneous rings under external hydrostatic pressure. The analytical expressions have been correlated to numerical and experimental test data, proving their accuracy

Wrinkling prediction, formation and evolution in thin films adhering on polymeric substrata

Wrinkling has recently attracted an increasing interest by suggesting a number of unforeseeable applications in many emerging material science and engineering fields. If guided and somehow designed, wrinkles could be in fact used as an alternative printing way for realizing complex surface geometries and thus employed as an innovative bottom-up process in the fabrication of nano- and micro-devices. For these reasons, the prediction of wrinkles of films adhering on flat as well as on structured substrata is a challenging task, genesis and development of the phenomenon being not yet completely understood both when thin membranes are coupled with soft supports and in cases where the geometry of the surfaces are characterized by complex three-dimensional profiles. Here we investigate the experimental formation of new intriguing and somehow unforeseeable wrinkled patterns achieved on periodic structures, by showing prediction through a new hybrid analytical-numerical strategy capable to overcome some common obstacles encountered in modeling film wrinkling on flat and 3D-shaped substrata. The proposed approach, which drastically reduces the computational effort, furnishes a helpful way for predicting both qualitative and quantitative results in terms of wrinkling patterns, magnitude and wavelength, by also allowing to follow the onset of film instabilities and the progressive evolution of the phenomenon until its final stage. Keywords: Thin film, Wrinkling, PDMS substrates, Lithium niobate crystals, FEM simulation

TENSILE INTEGRITY ACROSS THE SCALES OF THE LIVING MATTER: A STRUCTURAL PICTURE OF THE HUMAN CELL

Tensile integrity principle governs the existence of stable constructs in which sets of pre-tensed cables and pre-compressed struts mutually interconnect according to specific topological rules and exchange forces in a way to guarantee the structure’s overall self-equilibrium. Starting from the simplest form of 2-element bow-like system, several structural components can be arranged together to assemble increasingly intricate tensegrity architectures where bars levitate sustained by a precise interplay with tensed cables, whose peculiar organization balances the vector field of axial forces. Modulation of the internal pre-stress tunes tensegrity systems towards disparate forms with different rigidities and stored elastic energies, while the floating arrangement of the compressed elements and the possible chirality confer to the whole structure pronounced deployability. This makes tensile integrity a persuasive structural paradigm for explaining and reproducing some underlying mechanisms at the basis of several dynamics experimentally observed in single cells as well as at different scales of biological architectures. In particular, by deeply exploring the intra-cellular environment, one discovers that the cytoskeleton mechanically sustains the cell’s membrane, structurally integrates cellular sub-constituents and steers migration, adhesion and division activities by behaving as a dynamic tensegrity lattice, hierarchically assembled by protein filaments, in turn made of continuously reacting polymeric tensegrity-chains at the lower nano-scale

Best to clarify to avoid misunderstandings in the biomechanics of Ross Operation: parentheses matter

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Hexagrid-Voronoi transition in structural patterns for tall buildings

In this paper, a first insight into the role that non-conventional structural patterns might play in the design of tall buildings is presented. The idea is to explore the mechanical properties of selected non-conventional structural patterns, in the form of both regular (Hexagrid) and irregular (Voronoi tessellation inspired) arrays, in order to assess their actual applicability in tall building design. For this aim, the concept of Representative Volume Element (RVE) and a classical homogenization-based micromechanical approach are employed for identifying the pattern units and deriving the relevant generalized stress-strain relationships. In the case of irregular patterns based on Voronoi diagrams, obtained by perturbing prescribed key geometrical features of hexagrids, a statistically significant sample of RVEs is defined on the basis of sensitivity analyses, and the related mechanical characterization is developed in statistical terms. Finally, a preliminary stiffness-based design procedure is proposed and applied to a tall building model with Voronoi exoskeleton. In conclusion, a discussion on the effectiveness of the design procedure and on the structural efficiency of the Voronoi patterns for tall buildings is presente

Topology optimization-guided stiffening of composites realized through Automated Fiber Placement

The paper proposes a mixed strain- and stress-based topology optimization method for designing the ideal geometry of carbon fibers in composite laminates subjected to either applied tractions or prescribed displacements. On the basis of standard micromechanical approaches, analytical elastic solutions for a single cell, assumed to be a Representative Volume Element (RVE), are ad hoc constructed by involving anisotropy induced by fiber orientation and volume fraction, also taking into account inter-laminar stresses and strains. The analytical solutions are then implemented in a Finite Element (FE) custom-made topology optimization-based procedure rewritten to have as output the best curves the reinforcing fibers have to draw in any composite laminate layer to maximize the overall panel stiffness or to minimize the elastic energy. To verify the effectiveness of the proposed strategy, different structures undergoing either in-plane or out-plane boundary conditions have been selected and theoretically investigated, determining the optimal fibers’ maps and showing the related results in comparison to standard sequences of alternate fibers disposition for the same composites. Two optimized panels were at the end actually produced using an innovative Automated Fiber Placement (AFP) machine and consolidating the materials by means of autoclave curing processes, in this way replicating the fiber paths obtained from theoretical outcomes. As a control, two corresponding composite structures were also built without employing the fiber optimization strategy. The panels have been tested in laboratory and the theoretical results have been compared with the experimental findings, showing a very good agreement with our predictions and confirming the capability of the proposed algorithm in suggesting the arrangement of the fibers to obtain enhanced mechanical performances. It is felt that the hybrid analytical-FE topology optimization strategy, in conjunction with the possibilities offered by AFP devices, could pave the way for a new generation of ultra-lightweight composites for aerospace, automotive and many industrial applications