Stick-slip synchronization in stack of elastically coupled frictional interfaces

We perform physical and numerical experiments to study the stick-slip response of a stack of slabs in contact through dry frictional interfaces driven in quasistatic shear. The ratio between the drive's stiffness and the slab's shear stiffness controls the presence or absence of slip synchronization. A sufficiently high stiffness ratio leads to synchronization, comprising periodic slip events in which all interfaces slip simultaneously. A lower stiffness ratio leads to asynchronous slips and, experimentally, to the stick-slip amplitude being broadly distributed as the number of layers in the stack increases. We interpret this broadening in light of the combined effect of surface disorder, complex loading paths of the asynchronous slips, and creep. Consequently, the ageing rate can be readily extracted from the stick-slip cycle. The extracted aging rate is found to be of the same order of magnitude as existing experimental results on a similar material. Finally, we discuss the emergence of slow slips and an increase in creep-rate variations when more slabs are added to the stack

Elasticity and tremors of knitted farbics

Les propriétés mécaniques d’un tricot diffèrent drastiquement de celles du fil dont il est constitué. Par exemple, une étoffe tricotée d’un fil inextensible présente une étonnante propension à la déformabilité. À l’instar des systèmes mécaniques où la géométrie joue un rôle prépondérant, tels les origamis, la réponse mécanique d’un tricot va être déterminée par le chemin imposé au fil. Lors du tricotage, le fil est contraint de se courber et de former des points de croisement suivant un motif répétitif, figeant de cette manière sa topologie. Les trois ingrédients sur lesquels repose la réponse mécanique d’un tricot sont l’élasticité du fil, sa topologie et le frottement aux contacts. Une sélection des nombreux phénomènes qui émergent du couplage entre ces ingrédients fait l’objet de cette thèse. Premièrement, l’intérêt a été porté sur l’élasticité du tricot. En se basant sur une expérience de traction d’un tricot-modèle, une théorie, qui vise à décrire cette réponse mécanique, a été construite en tenant compte de la conservation de la topologie, l’énergie de flexion et l’inextensibilité du fil. Dans un second temps, l’accent est mis sur les fluctuations de la réponse mécanique. Ces fluctuations ont pour origine la friction du fil qui empêche sa répartition dans la maille jusqu’à ce qu’un contact glisse brusquement, déclenchant alors une succession de glissements. La mesure de la réponse en force et du champ de déformations montrent que ces évènements suivent une dynamique d’avalanches. Enfin, l’action de la topologie et de la métrique du tricot sur sa forme tridimensionnelle, ainsi que la transition de configuration spontanée de la structure d’un tricot, ont été examinés.Knits mechanical properties are fundamentally different from those of its constitutive yarn. For instance, a fabric knitted with an inextensible yarn demonstrates a surprising inclination for deformability. Like mechanical systems where geometry plays a preponderant role, such as origami, the mechanical response of knitted fabrics is governed by the pattern imposed on the yarn. In the process of knitting, the yarn is constrained to bend and to cross itself following a periodic pattern, anchoring its topology. The three factors which determine the mechanical response of a knit are the elasticity of the yarn, its topology, and friction between crossing strands. This thesis explores several phenomena that arise from the interplay of these factors. First, we focused on the elasticity of a knit. Working from experimental data, we developed a theory to decipher the mechanical response of model knits under traction, taking into account the unaltered topology, bending energy, and inextensibility of the yarn. Next, we explored fluctuations in the mechanical response of a knit. Those fluctuations originate from yarn-yarn friction, preventing free yarn redistribution in the stitch until a contact slides and triggers propagative slips. Measures of the force response and deformation fields reveal that those events follow an avalanching dynamic, including a power law distribution of their size. Finally, the impact of topology and metric on knit three-dimensional shapes, along with spontaneous configuration transitions in a knit structure, are studied

Élasticité et tremblements du tricot

Knits mechanical properties are fundamentally different from those of its constitutive yarn. For instance, a fabric knitted with an inextensible yarn demonstrates a surprising inclination for deformability. Like mechanical systems where geometry plays a preponderant role, such as origami, the mechanical response of knitted fabrics is governed by the pattern imposed on the yarn. In the process of knitting, the yarn is constrained to bend and to cross itself following a periodic pattern, anchoring its topology. The three factors which determine the mechanical response of a knit are the elasticity of the yarn, its topology, and friction between crossing strands. This thesis explores several phenomena that arise from the interplay of these factors. First, we focused on the elasticity of a knit. Working from experimental data, we developed a theory to decipher the mechanical response of model knits under traction, taking into account the unaltered topology, bending energy, and inextensibility of the yarn. Next, we explored fluctuations in the mechanical response of a knit. Those fluctuations originate from yarn-yarn friction, preventing free yarn redistribution in the stitch until a contact slides and triggers propagative slips. Measures of the force response and deformation fields reveal that those events follow an avalanching dynamic, including a power law distribution of their size. Finally, the impact of topology and metric on knit three-dimensional shapes, along with spontaneous configuration transitions in a knit structure, are studied.Les propriétés mécaniques d’un tricot diffèrent drastiquement de celles du fil dont il est constitué. Par exemple, une étoffe tricotée d’un fil inextensible présente une étonnante propension à la déformabilité. À l’instar des systèmes mécaniques où la géométrie joue un rôle prépondérant, tels les origamis, la réponse mécanique d’un tricot va être déterminée par le chemin imposé au fil. Lors du tricotage, le fil est contraint de se courber et de former des points de croisement suivant un motif répétitif, figeant de cette manière sa topologie. Les trois ingrédients sur lesquels repose la réponse mécanique d’un tricot sont l’élasticité du fil, sa topologie et le frottement aux contacts. Une sélection des nombreux phénomènes qui émergent du couplage entre ces ingrédients fait l’objet de cette thèse. Premièrement, l’intérêt a été porté sur l’élasticité du tricot. En se basant sur une expérience de traction d’un tricot-modèle, une théorie, qui vise à décrire cette réponse mécanique, a été construite en tenant compte de la conservation de la topologie, l’énergie de flexion et l’inextensibilité du fil. Dans un second temps, l’accent est mis sur les fluctuations de la réponse mécanique. Ces fluctuations ont pour origine la friction du fil qui empêche sa répartition dans la maille jusqu’à ce qu’un contact glisse brusquement, déclenchant alors une succession de glissements. La mesure de la réponse en force et du champ de déformations montrent que ces évènements suivent une dynamique d’avalanches. Enfin, l’action de la topologie et de la métrique du tricot sur sa forme tridimensionnelle, ainsi que la transition de configuration spontanée de la structure d’un tricot, ont été examinés

Geometry and Elasticity of a Knitted Fabric

International audienceKnitting is not only a mere art and craft hobby but also a thousand-year-old technology. Unlike weaving, it can produce loose yet extremely stretchable fabrics with almost vanishing rigidity, a desirable property exhibited by hardly any bulk material. It also enables the engineering of arbitrarily shaped two- and threedimensional objects with tunable mechanical response. In contrast with the extensive body of related empirical knowledge and despite a growing industrial interest, the physical ingredients underlying these intriguing mechanical properties remain poorly understood. To make some progress in this direction, we study a model tricot made of a single elastic thread knitted into a common pattern called stockinette. On the one hand, we experimentally investigate its tensile response and measure local displacements of the stitches during deformation. On the other hand, we derive a first-principle mechanical model for the displacement field based on the yarn-bending energy, the conservation of its total length, and the topological constraintson the constitutive stitches. Our model solves both the shape and mechanical response of the knit andagrees quantitatively with our measurements. This study thus provides a fundamental framework for theunderstanding of knitted fabrics, paving the way to thread-based smart materials

Crackling Dynamics in the Mechanical Response of Knitted Fabrics

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Bending Response of a Book with Internal Friction

International audienceWe study the bending of a book-like system, comprising a stack of elastic plates coupled through friction. The behavior of this layered system is rich and nontrivial, with a non-additive enhancement of the apparent stiffness and a significant hysteretic response. A dimension reduction procedure is employed to develop a centerline-based theory describing the stack as a non-linear planar rod with internal shear. We consider the coupling between the nonlinear geometry and the elasticity of the stacked plates, treating the interlayer friction perturbatively. This model yields predictions for the stack's mechanical response in three-point bending that are in excellent agreement with our experiments. Remarkably, we find that the energy dissipated during deformation can be rationalized over three orders of magnitude, including the regimes of a thick stack with large deflection. This robust dissipative mechanism could be harnessed to design new classes of low-cost and efficient damping devices

Homogenized yarn-level cloth

We present a method for animating yarn-level cloth effects using a thin-shell solver. We accomplish this through numerical homogenization: we first use a large number of yarn-level simulations to build a model of the potential energy density of the cloth, and then use this energy density function to compute forces in a thin shell simulator. We model several yarn-based materials, including both woven and knitted fabrics. Our model faithfully reproduces expected effects like the stiffness of woven fabrics, and the highly deformable nature and anisotropy of knitted fabrics. Our approach does not require any real-world experiments nor measurements; because the method is based entirely on simulations, it can generate entirely new material models quickly, without the need for testing apparatuses or human intervention. We provide data-driven models of several woven and knitted fabrics, which can be used for efficient simulation with an off-the-shelf cloth solver