1,721,058 research outputs found
A kinematics-based single-actuator setup for constant-curvature bending tests in extremely large deformations
No full textThanks to the extreme deformability, low weight and high strength, thin elements, such as glass/carbon fiber composites shells, or chemically-strengthened glass laminae, are increasingly used for different engineering applications, ranging from deployable space structures and adaptive surfaces for architecture, to flexible electronics and wearable devices. Since an accurate design must be based on reliable values of the material strength, many research efforts have been made in recent years to propose innovative methods specifically devoted to the evaluation of the bending response of highly deformable elements. One of the most reliable procedures seems to be the clamp bending test, originally proposed for thin glass elements. The test consists in prescribing a rotation on two opposite edges of a rectangular thin plate, while adjusting the distance between the supports so to obtain a deformation into an arc of circle. If, from the analytical point of view, this is very effective because it allows to determine the material strength by using very simple formulae, from the practical point of view, its major limitation is that it requires to synchronize the motors and actuators governing the motion of translational and rotational degrees of freedom. Here, an innovative design is presented, characterized by a mechanical/kinematic interconnection between translation and rotation, so that it is possible to perform a clamp bending test in extremely large deformations by controlling just one degree of freedom, i.e., using only one actuator
Design of a chiral waveguide with mechanically-tunable stiffness for Bloch wave propagation control
No full textShear and flexural deformations in flextegrity segmental beams inspired by Leonardo’s triangular masonry construction
Full text linkIn Tabula XCI verso of Codex Atlanticus, Leonardo da Vinci presents an ingenious masonry structure composed of segments in the shape of inverted triangles. These are assembled by contact in a chain to obtain a lintel or jack arch, where they are pressed together by the thrust of the end constraints. Drawing inspiration from Leonardo's sketches, we show that, by connecting the segments in pair through elastic tendons, this system represents a new type of flextegrity beam. In a classical flextegrity, the contact surfaces of the segments are curved conjugate profiles, imposing a pure rolling motion along properly designed pitch lines: the consequent elongation of the tendon dictates the constitutive response as a function of the relative rotation of the segments. Here, the contact is through plane surfaces, so that the kinematics, besides the relative rotation, is characterized by segmental shearing. This system is theoretically analysed and a continuum model is derived as the length of the segments becomes small. Comparisons with experiments on three-dimensional-printed prototypes confirm the theoretical findings and highlight the possible competition between rotational and sliding types of deformation. Apart from the historical value, this type of construction can be used in innovative structures or metamaterials
Micro-mechanical interpretation of the non-linear tensile response of structured rods. Experiments on prototyped Gedankenmodelle with wavy sub-rods and application to Carbon NanoTube fibers
No full textIn order to demonstrate the wide range of inelastic non-linear tensile responses observed in structured fibers of various types, an ideal phenomenological model (Gedankenmodell) is theoretically analyzed, prototyped and tested. This consists in a rod made of a regular ensemble of sticks in a brickwork -like three-dimensional grid, hooped by elastic bands. The stick lateral faces are wavy surfaces in cohesive -frictional contact; their relative sliding induces a transversal expansion opposed by the bands, providing shear -slip constitutive laws that mimic the complex interaction forces in fibers composed of adherent fibrils. The macroscopic response of the rod to uniaxial loading is theoretically modeled in the framework of generalized standard materials. Experiments conducted on 3D -printed prototypes confirm the theoretical approach. As an example, the Gedankenmodell is applied to the tensile response of Carbon NanoTube (CNT) fibers, composed of CNTs laterally bonded via a spinning process. The model phenomenologically reproduces the tensile response of dry -spun, aerogel-spun and wet -spun CNT fibers, specifically the onset of a yield point, elastic unloading-reloading and inelastic aftereffects. This approach interprets the adhesion of the CNTs within the broad paradigm of cohesive -frictional adherence, combined with conservative forces of attraction
Flextegrity arched structures for Lunar bases built from indigenous materials
No full textFlextegrity is a broad structural concept, represented here by a chain of perforated massive segments in dry contact, coupled by a passing-through pre-tensioned tie-rod (tendon). Contact surfaces realize pure rolling along tailor-designed pitch lines, so that the relative rotation of adjacent segments determines the tendon elongation: the bending constitutive law can be optimized simply by re-shaping the contact surfaces. Flextegrity arches are placed side by side to form a vault, pressurized via an interposed air-tight membrane. The arch follows an almost complete circle (the vault approaches a tube), to minimize the actions on the foundations from internal pressure. The structure also supports a thick radiation shield above, made up of loose Lunar debris. The main characteristic is that the massive segments can be made from cast Lunar regolith, fused into solar-powered devices, whose weight is mitigated by the reduced Lunar gravity. Huge savings on mission budget are expected, because the only components from Earth are the tendons, made of high-performance lightweight carbon nanotube fibers, and the air-tight membrane. Small-scale prototypes are manufactured from cast concrete. The structural response is analyzed through a theoretical model. The concept is applied to a Moon base example, verifying compatibility with the strength of Lunar regolith under various actions, including meteoroid impact
Kinematic and static characterization of everting Möbius kaleidocycles with slightly incongruent links
No full textA Möbius kaleidocycle is a closed kinematic chain of n≥7 identical links connected by revolute joints, forming a linkage with the nonorientable topology of a Möbius band. If its joints are set at a critical, n-dependent twist angle — the smallest that allows closure without forcing — then, despite formally having n−6 internal degrees of freedom, the linkage admits only a single one: a reversible, periodic everting motion. Focusing on the case n=7, we determine the kinematic matrix via the Denavit–Hartenberg construction, under closure and congruence constraints. A geometric mechanism arises alongside the topological one due to a matrix-rank deficiency, accompanied by a corresponding state of self-stress. The geometric mechanism is infinitesimal and stiffened by self-stress, while eversion is enabled by the finite mechanism. Using a variational argument, we confirm that the sum of squared joint rotations remains constant throughout eversion. We further categorize the states of self-stress, identifying conserved quantities — including the sum of twisting moments raised to any positive integer power λ≥1 — which enable estimates of self-stresses in moderately incongruent linkages requiring elastic forcing to close
Flextegrity and sheartegrity demonstration models for the multiscale analysis of structured materials and metamaterials.
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Experimental assessment of the eigenstress state in two-ply yarns and its effect on tensile properties
No full textPropaedeutical to a better understanding of the mechanics of cables, with potential applications in material science and biology, tensile tests were performed on two-ply yarns made of rubber rods, manufactured by transforming the twist on two adjacent straight rods into tortuosity for the resulting double-helix shape. Modeling of the yarn as a pair of Kirchhoff rods in reciprocal contact, fails to provide results in agreement with experiments, especially when the helix slope angle alpha tends to pi/4, representing the geometric limit for material interpenetration. This discrepancy could not be justified by considering the viscosity of the rods, their cross-sectional distortion, or cohesive/frictional contact. Our contribution starts from the experimental observation that the rods are in an eigenstress state already in the reference balanced configuration. This is characterized by a high twist, dictated by the ratio between torsional and bending rod stiffness, which increases as alpha decreases, growing unboundedly as alpha -> pi/4. Experiments confirm that twisting can significantly increase the tensile stiffness of straight rubber rods, in agreement with non-linear elasticity theory. Tensile tests on the yarns can be reproduced excellently only if the twist-induced stiffening is considered
Direct evaluation of the modification coefficient kmod for the static-fatigue design of annealed glass panes under wind load
No full textMost standards reduce the design strength of glass through a modification coefficient that accounts for static fatigue caused by subcritical propagation of surface cracks. For wind pressures on glass plates, the coefficient is derived assuming nominal durations of maximum wind gusts or cumulative winds, which are defined based on tradition and practice. Here we derive closed-form expressions for the modification coefficient using a fracture mechanics approach which incorporates the time histories of wind velocity or their probabilistic distributions. The coefficient depends on the duration of the action and becomes constant for typical design lives. A consistent nominal duration is then defined by referring to a wind with fixed reference velocity which produces the same damage of the actual action. The approach, applicable to the European context, has been applied to data recorded in Italy. The results are compared with prescriptions from various standards showing that they are in general non-conservative
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