1,721,209 research outputs found
Optimised Design of a Foldable Structure Bio-Inspired by the Structure of Pollen Grains
No full textIn this paper, a new foldable structure has been studied and designed drawing inspiration from the structure of a pollen grain of angiosperm. This structure is made by two materials: a more rigid upper part which works only in compression and a less rigid lower part which undergoes tensile stresses during operation and can be easily folded after its use. Numerical optimisation has been used in the design to reduce the volume of the structur
Materiali strutturali biologici e bioinspirati
Full text linkI materiali sviluppati dall'uomo e utilizzati per applicazioni tecnologiche, in generale non sono multifunzionali, né tolleranti ai difetti, né auto-riparanti, né auto-pulenti, né gerarchici. Il contrario è vero per i materiali naturali, che manifestano queste proprietà nonostante siano "costruiti" usando un numero limitato di componenti di base estremamente comuni. Una comprensione approfondita del perché sia così potrebbe fornire la chiave per accelerare l'avvento di una nuova era basata su nuovi materiali.Questo è l'obiettivo della ricerca sui materiali bioispirati, che è in continua evoluzione e ha già fatto passi significativi verso questa meta
A generalized Paris' law for fatigue crack growth
No full textAn extension of the celebrated Paris law for crack propagation is given to take into account some of the deviations from the power-law regime in a simple manner using the Wohler SN curve of the material, suggesting a more general "unified law". In particular, using recent proposals by the first author, the stress intensity factor K(a) is replaced with a suitable mean over a material/structural parameter length scale Delta a, the "fracture quantum". In practice, for a Griffith crack, this is seen to correspond to increasing the effective crack length of Delta a, similarly to the Dugdale strip-yield models. However, instead of including explicitly information on cyclic plastic yield, short-crack behavior, crack closure, and all other detailed information needed to eventually explain the SN curve of the material, we include directly the SN curve constants as material property. The idea comes as a natural extension of the recent successful proposals by the first author to the static failure and to the infinite life envelopes. Here, we suggest a dependence of this fracture "quantum" on the applied stress range level such that the correct convergence towards the Wohler-like regime is obtained. Hence, the final law includes both Wohler's and Paris' material constants, and can be seen as either a generalized Wohler's SN curve law in the presence of a crack or a generalized Paris' law for cracks of any size. (c) 2006 Elsevier Ltd. All rights reserved
Gas Adsorption and Separation in Realistic and Idealized Frameworks of Organic Pillared Graphene: A Comparative Study
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A model for hierarchical anisotropic friction, adhesion and wear
Full text linkDue to a widespread number of examples, biological hierarchical structures have been studied for years, founding that surface properties are strictly correlated to the surface roughness. Since friction appears to be a multiscale phenomenon and hierarchy optimizes the macroscopic mechanical properties of bodies, it may be of great interest to develop a model to predict friction of structures with hierarchical surfaces. Thus, we propose an analytical model to describe anisotropic friction, adhesion and wear of hierarchical surfaces. The model describes friction between two generic rough surfaces in contact, sliding one against the other. Then, it has been extended to adhesion, wear and finally to multiple hierarchical levels to obtain the global frictional response
Self-healing of hierarchical materials
Full text linkWe present a theoretical and numerical analysis of the mechanical behavior of self-healing materials using an analytical model and numerical calculations both based on a Hierarchical Fiber Bundle Model, and applying them to graphene- or carbon-nanotube-based materials. The self-healing process can be described essentially through a single parameter, that is, the healing rate, but numerical simulations also highlight the influence of the location of the healing process on the overall strengthening and toughening of the material. The role of hierarchy is discussed, showing that full-scale hierarchical structures can in fact acquire more favorable properties than smaller, nonhierarchical ones through interaction with the self-healing process, thus inverting the common notion in fracture mechanics that specimen strength increases with decreasing size. Further, the study demonstrates that the developed analytical and numerical tools can be useful to develop strategies for the optimization of strength and toughness of synthetic bioinspired material
Investigating the dynamic influence of passive effects on undulatory locomotion in viscous environment and unleashing the potential of hybrid friction
No full textThis study encompasses the passive interdependence of body material (internal damping, stiffness, and geometry) and environmental (frictional anisotropy and frictional coefficients) properties on the performance and adaptability of undulatory locomotion. Undulatory locomotion is modeled in viscous and hybrid environments. The body’s ability to adapt passively results in different locomotion gaits, such as swimming and crawling. Results show that, in swimming gaits, body stiffness dominates frictional forces, whereas in crawling gaits, frictional forces dominate body stiffness. Our result showed that the interdependence between endogenous and exogenous dynamics can be utilized to achieve the desired speed. The hybrid frictional environment is modeled as the combination of dry and viscous frictions and recognized as a valuable asset in the modeling of undulatory locomotion in granular media. In the hybrid environment the respective ratios of dry and viscous frictions play an essential role in the speed optimization. The optimal combination of dry and viscous frictions can be found depending on the characteristics of the body and environment. We validate the viscous based frictional model using the properties of the body of Caenorhabditis Elegans from the state of the art, in a viscous environment. The results can be employed to design robotic solutions to perform optimally in different environments and with customized body properties
On unified crack propagation laws
No full textThe anomalous propagation of short cracks shows generally exponential fatigue crack growth but the dependence on stress range at high stress levels is not compatible with Paris’ law with exponent m=2. Indeed, some authors have shown that the standard uncracked SN curve is obtained mostly from short crack propagation, assuming that the crack size a increases with the number of cycles N as [Formula presented]=HΔσha where h is close to the exponent of the Basquin's power law SN curve. We therefore propose a general equation for crack growth which for short cracks has the latter form, and for long cracks returns to the Paris’ law. We show generalized SN curves, generalized Kitagawa–Takahashi diagrams, and discuss the application to some experimental data. The problem of short cracks remains however controversial, as we discuss with reference to some examples
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