Federico Bosi

Thermoviscoelastic modelling and yield domain of inflatable thin films

Structural membranes are ubiquitous due to their ultralow weight and ability to undergo large deformations, with applications ranging from novel civil constructions to advanced aerospace systems, and including biomedical devices, soft robotics, stretchable electronics, and tissue engineering. As the highly deformable thin films employed in these lightweight structures are difficult to characterise and control, accurate thermoviscoelastoplastic models must be developed to predict their responses at different temperatures and loading rates.

In the present talk, I will show how to model the time-dependent behaviour of an anisotropic polyethylene thin film that constitutes the lobes of NASA stratospheric balloons. Starting from an extensive experimental campaign, a constitutive nonlinear thermoviscoelastic relation that involves the free volume theory of viscoelasticity and the time-temperature superposition principle is established to describe the response of the elastomer. Furthermore, I will propose a molecular model to assess the development of plasticity in the membrane. The yield criterion is based on the strain rate-temperature superposition principle and the cooperative theory of yielding, resulting in very good agreement with experimental measurements. Finally, I will present a new experimental method to characterise the yield strength of elastomeric membranes, thus overcoming the limitations of the available empirical techniques. The proposed approach allows identifying the material yield domain through the abrupt change in strain distribution that develops at the onset of plasticity when soft membranes are inflated.