81 research outputs found
Surprising Instabilities of Simple Elastic Structures
No full textExamples of structures buckling in tension are presented, where no compressed elements are present, slightly different from those previously proposed by the authors. These simple structures exhibit interesting postcritical behaviours, for instance, multiple configurations of vanishing external force are evidenced in one case.
Flutter instability as induced by dry friction is also considered in the Ziegler pendulum, with the same arrangement introduced by Bigoni and Noselli (2011), but considering now the dynamical effects due to the mass of the wheel, which was previously neglected. It is shown that, for the values of rotational inertia pertinent to our experimental set-up, this effect does not change the overall behaviour, so that previous results remain fully confirmed
Experimental evidence of flutter and divergence instabilities induced by dry friction
No full textFlutter and divergence instabilities have been advocated to be possible in elastic structures with Coulomb friction, but no direct experimental evidence has ever been provided. Moreover, the same types of instability can be induced by tangential follower forces, but these are commonly thought to be of extremely difficult, if not impossible, practical realization. Therefore, a clear experimental basis for flutter and divergence induced by friction or follower-loading is still lacking. This is provided for the first time in the present article, showing how a follower force of tangential type can be realized via Coulomb friction and how this, in full agreement with the theory, can induce a blowing-up vibrational motion of increasing amplitude (flutter) or an exponentially growing motion (divergence). In addition, our results show the limits of a treatment based on the linearized equations, so that nonlinearities yield the initial blowing-up vibration of flutter to reach eventually a steady state. The presented results give full evidence to potential problems in the design of mechanical systems subject to friction, open a new perspective in the realization of follower-loading systems and of innovative structures exhibiting 'unusual' dynamical behaviors
Concurrent factors determine toughening in the hydraulic fracture of poroelastic composites
No full textBrittle materials fail catastrophically. In consequence of their limited flaw-tolerance, failure occurs by localized fracture and is typically a dynamic process. Recently, experiments on epithelial cell monolayers have revealed that this scenario can be significantly modified when the material susceptible to cracking is adhered to a hydrogel substrate. Thanks to the hydraulic coupling between the brittle layer and the poroelastic substrate, such a composite can develop a toughening mechanism that relies on the simultaneous growth of multiple cracks. Here, we study this remarkable behaviour by means of a detailed model, and explore how the material and loading parameters concur in determining the macroscopic toughness of the system. By extending a previous study, our results show that rapid loading conveys material toughness by promoting distributed cracking. Moreover, our theoretical findings may suggest innovative architectures of flaw-insensitive materials with higher toughness. ArXI
Localized stress percolation through dry masonry walls. Part II - Modelling
No full textThe highly localized stress distribution found within dry masonry walls through transmission photoelasticity in Part I of this article is explained both proposing a micromechanical model (based on a form of random cascade transmission of forces between bricks, which includes random coalescence additionally to random branching) and applying a phenomenological description (based on the extreme orthotropy of the equivalent homogeneous material)
Localized stress percolation through dry masonry walls. Part I - Experiments
No full textTransmission photoelasticity on scale models is shown to disclose the stress distribution within dry masonry walls. This distribution is found to be complicated by unilateral joints between elements, where ‘randomness constrained within a geometrical scheme’ of contact points occurs, so that stress percolation results highly localized, evidencing ‘unloading islands’ in a ‘stress stream’. These findings are theoretically explained in Part II of this paper from both micromechanical and continuous modelling perspectives
A robotic crawler exploiting directional frictional interactions: Experiments, numerics and derivation of a reduced model
No full textWe present experimental and numerical results for a model crawler which is able to extract net positional changes fromreciprocal shape changes, i.e. 'breathinglike' deformations, thanks to directional, frictional interactions with a textured solid substrate, mediated by flexible inclined feet. We also present a simple reduced model that captures the essential features of the kinematics and energetics of the gait, and compare its predictions with the results from experiments and from numerical simulations
Nonreciprocal oscillations of polyelectrolyte gel filaments subject to a steady and uniform electric field
No full textSoft actuators typically require time-varying or spatially modulated control to be operationally effective. The scope of the present paper is to show, theoretically and experimentally, that a natural way to overcome this limitation is to exploit mechanical instabilities. We report experiments on active filaments of polyelectrolyte (PE) gels subject to a steady and uniform electric field. A large enough intensity of the field initiates the motion of the active filaments, leading to periodic oscillations. We develop a mathematical model based on morphoelasticity theory for PE gel filaments beating in a viscous fluid, and carry out the stability analysis of the governing equations to show the emergence of flutter and divergence instabilities for suitable values of the system’s parameters. We confirm the results of the stability analysis with numerical simulations for the nonlinear equations of motion to show that such instabilities may lead to periodic self-sustained oscillations, in agreement with experiments. The key mechanism that underlies such behaviour is the capability of the filament to undergo active shape changes depending on its local orientation relative to the external electric field, in striking similarity with gravitropism, the mechanism that drives shape changes in plants via differential growth induced by gravity. Interestingly, the resulting oscillations are nonreciprocal in nature, and hence able to generate thrust and directed flow at low Reynolds number. The exploitation of mechanical instabilities in soft actuators represents a new avenue for the advancement in engineering design in fields such as micro-robotics and micro-fluidics
Optimal design of planar shapes with active materials
No full textActive materials have emerged as valuable candidates for shape morphing applications, where a body reconfiguration is achieved upon triggering its active response. Given a desired shape change, a natural question is to compare different morphing mechanisms to select the most effective one with respect to an optimality criterion. We introduce an optimal control problem to determine the active strains suitable to attain a target equilibrium shape while minimizing the complexity of the activation. Specifically, we discuss the planar morphing of active, hyperelastic bodies in the absence of external forces and exploit the notion of target metric to encompass a broad set of active materials in a unifying approach. For the case of affine shape changes, we derive explicit conditions on the body reference configuration for the optimality of homogeneous target metrics. More complex shape changes are analysed via numerical simulations to explore the impact on optimal solutions of different objective functionals inspired by features of existing materials. We show how stresses arising from incompatibilities contribute to reduce the complexity of the controls. We believe that our approach may be exploited for the optimal design of active systems and may contribute to gather insight into the morphing strategies of biological systems
The stress intensity near a stiffener disclosed by photoelasticity
No full textCan the thickness of a thin inclusion (in a matrix material) be made so small (though retaining sufficient stiffness and matrix adhesion) to generate ‘in practice’ a stress state in agreement with the analytical (square-root singular) solution for a rigid line inclusion (so-called ‘stiffener’) embedded in a linear elastic plate? Can this inhomogeneous stress state be generated for tensile loading parallel to the stiffener? We provide a direct and positive answer to these questions, by showing how to produce elastic materials containing thin inclusions and by providing photoelastic investigation of these structures. The experiments fully validate the stress state calculated for an elastic plate containing a rigid (finite-length) line inclusion, until a distance from the inclusion tip on the order of its thickness, corresponding to a stress concentration up to seven
Crawling on directional surfaces
No full textIn this paper we study crawling locomotion based on directional frictional interactions, namely, frictional forces that are sensitive to the sign of the sliding velocity. Surface interactions of this type are common in biology, where they arise from the presence of inclined hairs or scales at the crawler/substrate interface, leading to low resistance when sliding 'along the grain', and high resistance when sliding 'against the grain'. This asymmetry can be exploited for locomotion, in a way analogous to what is done in crosscountry skiing (classic style, diagonal stride).
We focus on a model system, namely, a continuous one-dimensional crawler and provide a detailed study of the motion resulting from several strategies of shape change. In particular, we provide explicit formulae for the displacements attainable with reciprocal extensions and contractions (breathing), or through the propagation of extension or contraction waves. We believe that our results will prove particularly helpful for the study of biological crawling motility and for the design of bio-mimetic crawling robots
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