1,720,991 research outputs found
Experimental investigation of progressive instability and collapse of no-tension brickwork pillars
Full text linkThe progressive instability behaviour of compressed dry-stone rectangular pillars loaded with an eccentric load is assessed experimentally and compared with the theory. Photoelastic compression tests were designed and executed on polymethyl-methacrylate brickwork pillars to reveal, i) the load-bearing capacity of the structure and the load-lateral displacement relation, ii) the effect of the eccentricity in the stress distribution along the structure, iii) the collapse mode of the system at high eccentricity. By employing a no-tension material model with linear behaviour in compression, new analytical, closed-form expressions for deformed shape of the structure, location of the neutral axis in a generic cross section and axial displacement are provided. The photoelastic stress analysis outcome fully confirms the analytical predictions for both low and high eccentricity loadings
Non-linear double-peeling: Experimental vs. theoretical predictions
Full text linkThe double peeling of detachment of non-linear adhesive tapes from a flat Poly(methyl methacrylate) (PMMA) surface has been investigated from both experimental and theoretical point of view. Double peeling tests show that, as the detachment process advances, the peeling angle stabilizes on a limiting value θlim corresponding to a critical pull-off force Fc above which the tape is completely detached from the substrate. This observed behavior is in good agreement with results obtained following the new theory of multiple peeling and taking into account the hardening-softening non-linear behavior of the experimentally tested adhesive tapes and clarifies some aspects of the experimental data. In particular, the theoretical model shows that the value of the limiting peeling angle depends on the geometry of the adhesive tape as well as on the stiffness properties and on the interfacial energy ∆γ. Finally, theoretical predictions confirm that solutions with a peeling angle lower than θlim are unstable
Design of tunable acoustic metamaterials with periodic piezoelectric microstructure
No full textAn innovative special class of tunable periodic metamaterials is designed, suitable for realizing high-performance acoustic filters. The metamaterial is made up of a phononic crystal coupled to local resonators. Such local resonators consist of masses enclosed into piezoelectric rings, shunted by either dissipative or non-dissipative electrical circuit. By tuning the impedance/admittance of such electrical circuits, it is possible to fully adjust the constitutive properties of the shunting piezoelectric material. This feature paves the way for unconventional behaviours, well beyond the capabilities achievable with classical materials. It follows that the acoustic properties of the periodic metamaterial can be adaptively modified, in turn, opening new possibilities for the control of pass and stop bands. By exploiting a generalization of the Floquet-Bloch theory, the in-plane free wave propagation in the tunable metamaterial is investigated, by varying a certain tuning parameter, to show the efficiency of the proposed shunting piezoelectric system as a wave propagation control device. Particular attention is devoted to the determination of the in-plane constitutive equations of the shunting piezoelectric phase in the transformed Laplace space. Finally, broad design directions of tunable acoustic filters aiming to a changing performance requirement in real-time, is also provided. (C) 2020 Elsevier Ltd. All rights reserved
The deformation of an elastic rod with a clamp sliding along a smooth and curved profile
Full text linkThe design of compliant mechanisms is crucial in several technologies and relies on
the availability of solutions for nonlinear structural problems. One of these solutions is given and
experimentally validated in the present article for a compliant mechanism moving along a smooth
curved profile. In particular, a deformable elastic rod is held by two clamps, one at each end. The
first clamp is constrained to slide without friction along a curved profile, while the second clamp
moves in a straight line transmitting its motion through the elastic rod to the first clamp. For this
system it is shown that the clamp sliding on the profile imposes nontrivial boundary conditions
(derived via a variational and an asymptotic approach), which strongly influence buckling and
nonlinear structural behaviour. Investigation of this behaviour shows that a compliant mechanism
can be designed, which gives an almost neutral response in compression. This behavior could
easily be exploited to make a force limiting device. Finally a proof-of-concept device was
constructed and tested showing that the analyzed mechanical system can be realized in practice
and it behaves tightly to the model, so that it can now be used in the design of machines that
use compliant mechanisms
Structures buckling under tensile dead load
No full textSome 250 years after the systematic experiments by Musschenbroek and their rationalization by Euler, for the first time we show that it is possible to design structures (i.e. mechanical systems whose elements are governed by the equation of the elastica) exhibiting bifurcation and instability (‘buckling’) under tensile load of constant direction and point of application (‘dead’). We show both theoretically and experimentally that the behaviour is possible in elementary structures with a single degree of freedom and in more complex mechanical systems, as related to the presence of a structural junction, called ‘slider’, allowing only relative transversal displacement between the connected elements. In continuous systems where the slider connects two elastic thin rods, bifurcation occurs both in tension and in compression, and is governed by the equation of the elastica, employed here for tensile loading, so that the deformed rods take the form of the capillary curve in a liquid, which is in fact governed by the equation of the elastica under tension. Since axial load in structural elements deeply influences dynamics, our results may provide application to innovative actuators for mechanical wave control; moreover, they open a new perspective in the understanding of failure within structural elements
Rigid inclusions: stress singularity, inclusion neutrality and shear bands
No full textAnalytical solutions in elasticity predict singularities of stress fields at the corners/tips of rigid polygonal/linear inclusions, similarly to the case of void inclusions. On the other hand, a rigid line inclusion is neutral to homogeneous simple shear since a homogeneous stress state is obtained. We show that: (i) photoelastic experimental investigations validate the rigid inclusion model and therefore the assumptions about infinite stiffness of the inclusion and its complete adhesion with the matrix phase; (ii) when perturbations are superimposed upon a homogeneous pre-stress state, analytical incremental solutions display localization of deformation at the tips of rigid line inclusions and along the shear band directions, confirming experimental observations in ductile and quasi-brittle materials
Plastically-driven variation of elastic stiffness in green bodies during powder compaction: Part I. Experiments and elastoplastic coupling
No full textCold compaction of ceramic powders is driven by plastic strain, during which the elastic stiffness of the material progressively increases from values typical of granular matter to those representative of a fully dense solid. This increase of stiffness strongly affects the mechanical behaviour of the green body and is crucial in the modelling of forming processes for ceramics. A protocol for ultrasonic experimental investigation (via P and S waves transmission) is proposed to quantify the elastic constants (Young modulus and Poisson's ratio) as functions of the forming pressure. Experimental results performed in uniaxial strain allow for the introduction of laws that describe the variation of the elastic constants during densification. These laws are motivated in terms of elastoplastic coupling through the simulation of an isostatic pressure compaction process of alumina powder. A micromechanical explanation of the stiffening of elastic properties during densification is deferred to Part II of this study
Plastically-driven variation of elastic stiffness in green bodies during powder compaction. Part II: Micromechanical modelling
No full textA micromechanical approach is set-up to analyse the increase in elastic stiffness related to development of plastic deformation (the elastoplastic coupling concept) occurring during the compaction of a ceramic powder. Numerical simulations on cubic (square for 2D) and hexagonal packings of elastoplastic cylinders and spheres validate both the variation of the elastic modulus with the forming pressure and the linear dependence of it on the relative density as experimentally found in Part I of this study, while the dependence of the Poisson's ratio on the green's density is only qualitatively explained
Hierarchical auxetic and isotropic porous medium with extremely negative Poisson's ratio
Full text linkWe propose a novel two-dimensional hierarchical auxetic structure consisting of a porous medium in which a homogeneous matrix includes a rank-two set of cuts characterised by different scales. The six-fold symmetry of the perforations makes the medium isotropic in the plane. Remarkably, the mesoscale interaction between the first- and second-level cuts enables the attainment of a value of the Poisson's ratio close to the minimum reachable limit of -1. The effective properties of the hierarchical auxetic structure are determined numerically, considering both a unit cell with periodic boundary conditions and a finite structure containing a large number of repeating cells. Further, results of the numerical study are validated experimentally on a polymeric specimen with appropriately arranged rank-two cuts, tested under uniaxial tension. We envisage that the proposed hierarchical design can be useful in numerous engineering applications exploiting an extreme auxetic effect
A 3D Griffith peeling model to unify and generalize single and double peeling theories
Full text linkIt has been shown in recent years that many species in Nature employ hierarchy and contact splitting as a strategy to enhance the adhesive properties of their attachments. Maximizing the adhesive force is however not the only goal. Many animals can achieve a tunable adhesive force, which allows them to both strongly attach to a surface and easily detach when necessary. Here, we study the adhesive properties of 3D dendritic attachments, which are structures that are widely occurring in nature and which allow to achieve these goals. These structures exploit branching to provide high variability in the geometry, and thus tunability, and contact splitting, to increase the total peeling line and thus the adhesion force. By applying the same principles presented by A.A. Griffith 100 years ago, we derive an analytical model for the detachment forces as a function of their defining angles in 3D space, finding as limit cases 2D double peeling and 1D single peeling. We also develop a numerical model, including a nonlinear elastic constitutive law, for the validation of analytical calculations, allowing additionally to simulate the entire detachment phase, and discuss how geometrical variations influence the adhesive properties of the structure. Finally, we also realize a proof of concept experiment to further validate theoretical/numerical results. Overall, we show how this generalized attachment structure can achieve large variations in its adhesive and mechanical properties, exploiting variations of its geometrical parameters, and thus tunability. The in-depth study of similar basic structural units and their combination can in future lead to a better understanding of the mechanical properties of complex architectures found in Nature
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