Mechanical anisotropy is an important property of fibrous tissues; for example, the anisotropic mechanical properties of brain white matter may play a key role in the mechanics of traumatic brain injury (TBI). The simplest anisotropic material model for small deformations of soft tissue is a nearly incompressible, transversely isotropic (ITI) material characterized by three parameters: minimum shear modulus (μ), shear anisotropy (ϕ = μ1/μ − 1) and tensile anisotropy (ζ = E1/E2 − 1). These parameters can be determined using magnetic resonance elastography (MRE) to visualize shear waves, if the angle between the shear-wave propagation direction and fiber direction is known. Most MRE studies assume isotropic material models with a single shear (μ) or tensile (E) modulus. In this study, two types of shear waves, “fast” and “slow”, were analyzed for a given propagation direction to estimate anisotropic parameters μ, ϕ, and ζ in two fibrous soft materials: turkey breast ex vivo and aligned fibrin gels. As expected, the speed of slow shear waves depended on the angle between fiber direction and propagation direction. Fast shear waves were observed when the deformations due to wave motion induced stretch in the fiber direction. Finally, MRE estimates of anisotropic mechanical properties in turkey breast were compared to estimates from direct mechanical tests.

We explore the foam extrusion of expanded polypropylene with a long chain branched random co-polypropylene to make its production process simpler and cheaper. The results show that the presence of long chain branches infer high melt strength and, hence, a wide foamability window. We explored the entire window of foaming conditions (namely, temperature and pressure) by means of an ad-hoc extrusion pilot line design. It is shown that the density of the beads can be varied from 20 to 100 kg/m3 using CO2 and isobutane as a blowing agent. The foamed beads were molded by steam-chest molding using moderate steam pressures of 0.3 to 0.35 MPa independently of the closed cell content. A characterization of the mechanical properties was performed on the molded parts. The steam molding pressure for sintering expanded polypropylene beads with a long chain branched random co-polypropylene is lower than the one usually needed for standard polypropylene beads by extrusion. The energy saving for the sintering makes the entire manufacturing processes cost efficient and can trigger new applications.

The effect of extrudate swell on extrusion foam of thermoplastic polymers is presented using an experimental approach supported by a modelling of the phenomenon. Its understanding is fundamental in designing the geometry of a die for extrusion foam and to predict foaming. The extrudate swell is the swelling of a viscoelastic material due to a fast elastic recovery after being subjected to stresses. We show that there exists a link between the extrudate swell and foaming, performing experiments with simple and complex extrusion dies to measure the expansion ratio. It was found that the expansion ratio is anisotropic and the anisotropy in the expansion of the foam is due to the extrudate swell that affects strongly the final shape of the product and it cannot be neglected in standard application for extrusion foam. A simple heuristic model was developed to predict the extrudate swell from geometrical parameters and rheological characterization of the fluid. It was found that the foaming mechanism of polyethylene terephthalate, blown with cyclopentane, changes as function of extrudate swell and the lowest density foam is achieved using the die that has the bigger extrudate swell.