Suleman, Khurram; (2023) Mechanics of inflatable membranes and measurement of the yield domain in soft materials. Doctoral thesis (Ph.D), UCL (University College London).

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Text PhD Thesis - Mechanics of inflatable membranes and measurement of the yield domain in soft materials (submitted).pdf - Accepted Version Download (65MB) | Preview

Abstract

The use of structural membranes in engineering is rapidly growing, with a wide range of applications in civil construction, advanced aerospace systems, biomedical devices, soft robotics, stretchable electronics, and tissue engineering. Their success is attributed to their extremely lightweight nature and the ability to undergo large deformations while withstanding severe hygrothermal and mechanical loading conditions. However, membrane plasticity characterisation remains an open issue due to the lack in theoretical understanding of the nonlinear and viscous mechanisms inherently related to the constituent material and the inappropriateness of traditional experimental techniques. Therefore, it is of paramount importance to develop a novel method to assess the onset of plasticity in thin films in order to equip engineers with innovative tools that guarantee a safe and reliable structural design. In this work, a new experimental imaging technique has been proposed and validated to predict the onset of irreversible deformations in isotropic structural membranes. The new method is based on an abrupt change in strain localization observed at the onset of plasticity at the apex of pressurised thin films. An analytical framework for the mechanics of inflated elastoplastic circular thin films has been developed to reveal the phenomena associated with the occurrence of plasticity and to lay out the foundation of the proposed approach. The analytical results have been validated by finite element simulations, showing an excellent agreement and proving the mechanism of plastic strain localisation during diaphragm inflation. Based on the theoretical results, a material-independent criterion to predict the onset of plasticity has been defined and experimentally tested to find the yield strength of Polyether ether ketone (PEEK), Ethylene tetrafluoroethylene (ETFE), Thermoplastic polyurethane (TPU), Linear low-density polyethylene (LLDPE) membranes and aluminium foils. The yield stresses obtained from the proposed method were in very good agreement with the values measured from the existing procedures, hence ensuring the feasibility of the new technique and highlighting its advantages in terms of accuracy and reduced experimental time. The determination of the equibiaxial yield strength via film inflation has been extended to obtain a large portion of the yield locus of thin materials by applying the proposed method to the inflation of elliptical membranes, which results in non-equibiaxial stress states at their apex. Unlike circular membranes, the inflated elliptical shapes from the bulge tests are arbitrary and statically indeterminate, which usually require known constitutive behaviour to obtain the stresses associated with the instant of strain localization at the onset of plasticity. However, since the proposed technique aims to determine the onset of plasticity for the material in the absence of a known material response, a forward linear elastostatic shell formulation is developed and validated to directly obtain the stresses during the inflation of the elliptical membrane without prior knowledge of constitutive behaviour of the material. Using linear shell formulation, the proposed method is applied to the inflation of elliptical membranes made of ETFE to demonstrate the ability of the new technique to get the yield locus of the material. The predicted yield locus of ETFE is found very close to that reported in the literature, thus proving the ability of the newly proposed imaging method to obtain the yield behaviour of the materials for a wide range of multiaxial stress states.

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