Dr.-Ing. Francesca Concas

Closed-cell polyvinylchloride (PVC) foams are widely used as core for sandwich composites for applications, in which multiaxial loads are involved. In the present work a wide range of uniaxial (tension, compression and torsion) and multiaxial experiments (both simultaneous tension-torsion and compression-torsion) were conducted on a high performance PVC foam. Failure data for each experiment were collected and depicted in the invariants plane. The whole cylindrical surface of the specimen was monitored by means of an 8-camera-system, strain fields were obtained by 3D-DIC. Hence, the occurrence and the evolution of deformation bands were inspected. The usage of an 8-camera system was essential for the observation of the deformation mechanism, especially for pure compression, pure torsion and combined axial load-torsion, in which the arising of deformation bands is affected by the occurrence of buckling and the orthotropy of the foam.

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Liquid Crystal Elastomers (LCEs) are a class of materials which respond with large deformations to external stimuli, such as mechanical loading, heating and the application of electric fields. Hence, LCEs are ranked among the most promising materials for the development of artificial muscles. In nematic LCEs, rodlike mesogenic molecules are linked to the polymer backbone. The mesogenic rods are aligned toward a unique nematic director and their linkage with the polymer backbone influences the radii of gyration and hence the anisotropy. Furthermore, nematic LCEs are manufactured as thin film in order to keep the alignment of the nematic director through the whole thickness of the specimen. This work focuses on a new finite element formulation based on the principle of virtual power for simulating the behaviour of nematic LCEs, in which the nematic director is described by means of drilling degrees of freedom. We consider different linear momentum balances and angular momentum balances for the deformation mapping of the monolithic material and the nematic director, which are taken as independent variables in our formulation. All balances are satisfied by applying an energy-momentum scheme.

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Liquid crystal elastomers (LCEs) are a class of materials which exhibit an anisotropic behavior in their nematic state due to the main orientation of their rod-like molecules called mesogens. The reorientation of mesogens leads to the well-known actuation properties of LCEs, i.e. exceptionally large deformations as a consequence of particular external stimuli, such as temperature increase. Another key feature of nematic LCEs is the capability to undergo deformation by constant stresses while being stretched in a direction perpendicular to the orientation of mesogens. During this plateau stage, the mesogens rotate towards the stretching direction. Such characteristic is defined as semi-soft elastic response of nematic LCEs. We aim at modeling these material behaviours in a bespoke dynamic finite element method based on a variational-based mixed finite element formulation. The reorientation process of the rigid mesogens relative to continuum rotations are introduced by micropolar drilling degrees of freedom. Responsible for the above mentioned stress and temperature characteristics is an appropriate free energy function. Starting from an isothermal free energy function based on the linear continuum theory, we aim to widen it into the framework of large strains by identifying tensor invariants and thermo-mechanical coupling effects. In this work, we analyze the isothermal influence of the tensor invariants on the mechanical response of the finite element formulation and show that it space-time discretization preserves mechanical balance laws in the discrete setting.

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