arXiv subjects

Sairam Pamulaparthi Venkata

Publications and source records attributed to Sairam Pamulaparthi Venkata.

4 recordsLinked to original sources

arXiv2026

Wrinkling of a bilayered cylinder under tension

We study near-surface tension instabilities in bilayered cylinders composed of an incompressible core coated with a thin compressible film. In theory, circumferential wrinkles with axial wavefronts may appear on the outer surface when the cylinder is stretched. We analyze these instabilities through modelling, numerics, finite element simulations, and controlled experiments. We show that the wrinkle amplitude and wavelength depend on the Poisson ratio of the coating and the ratio of the film and substrate shear moduli. We find that the lower the film's Poisson ratio, the earlier the wrinkles emerge under tension. This observation motivates us to use auxetic films (with negative Poisson ratios) with bespoke microstructures in the finite element simulations and the experiments. For the latter, we fabricated 3D-printed soft cylinders with stiff auxetic outer layers and subjected them to axial loading. Although simulations predicted clear wrinkle formation, experimental observation proved more elusive due to the small amplitude of the wrinkles.

math-ph↗

arXiv2025

Programmable wrinkling for functionally-graded auxetic circular membranes

Materials with negative Poisson's ratio, also known as auxetic materials, display exotic properties such as expansion in all directions under uni-axial tension. For their unique properties, these materials find a broad range of applications in robotic, structural, aerospace, and biomedical engineering. In this work we study the wrinkling behavior of thin and soft auxetic membranes, subjected to edge tractions. We show that spatial inhomogeneities of the Young modulus and of the Poisson ratio can be suitably tailored to produce non-trivial wrinkling patterns, with wrinkled regions that can appear, broaden, merge, and eventually disappear again, as the magnitude of applied tractions is increased monotonically. To model wrinkling in a functionally graded membrane, we employ the mathematically elegant and physically transparent tension field theory, an approximated method that we implement in commercially available software. Beyond unveiling the challenging technological potential to achieve non-standard wrinkling on-demand in auxetic membranes, our study also confirms the potential of using tension field theory to study, analytically and numerically, instabilities in functionally graded materials.

cond-mat.soft↗

arXiv2025

Wrinkling instability of 3D auxetic bilayers in tension

Bilayers, soft substrates coated with stiff films, are commonly found in nature with examples including skin tissue, vesicles, and organ membranes. They exhibit different types of instabilities when subjected to compression, depending on the contrast in material properties between the two components. In this work, we unravel the mechanisms behind wrinkling instabilities in auxetic bilayer systems under uniaxial tension. We find that a soft bilayer in tension can experience significant lateral contraction, and with sufficient contrast in Poisson ratios, compressive stresses may induce wrinkles aligned with the tensile direction. We analytically model the onset of wrinkles and validate our predictions using Finite Element simulations in ABAQUS. Our findings reveal that wrinkles occur when the Poisson ratio of the substrate is greater than that of the film. As the two Poisson ratios converge to a common value, the critical stretch for instability shoots up rapidly and the wrinkles disappear. We also confirm these results through asymptotic analysis. Using inverse analysis, we design film microstructures to achieve desired effective Poisson ratios and further validate the effective properties with the Finite Element code FEAP. We show that the critical stretch ratio for buckling in auxetic structures with microstructural patterns is in strong agreement with the homogenized model predictions. The proposed method has significant potential for controlling surface patterns in auxetic skin grafts and hydrogel organ patches under mechanical loads. Moreover, the asymptotic expressions for compressible bilayers developed in this work can also be applied under finite strain for buckling-based metrology.

cond-mat.mtrl-sci↗

arXiv2024

Designing necks and wrinkles in inflated auxetic membranes

This article presents the potentiality of inflatable, functionally-graded auxetic membranes to produce wrinkles and necks. We obtain elastic instabilities at desired locations in axisymmetric membranes and with prescribed patterns in square membranes. First, we use an analytical approach to obtain a series of universal results providing insights into the formation of wrinkles and necks in inflated, axisymmetric membranes. For example, we prove analytically that necks and wrinkles may never overlap in pressurized, axially symmetric membranes. Second, we implement the relaxed strain energy of tension field theory into a Finite Element solver (COMSOL). By tuning spatial inhomogeneities of the material moduli, we corroborate our universal results, describe the onset of wrinkling in an averaged way, and also generate non-trivial instabilities at desired locations. This study on membranes with morphing or corrugation on demand has potential applications in Braille reading and haptics.

cond-mat.soft↗