Orthotropic rotation-free thin shell elements

A method to simulate orthotropic behaviour in thin shell finite elements is proposed. The approach is based on the transformation of shape function derivatives, resulting in a new orthogonal basis aligned to a specified preferred direction for all elements. This transformation is carried out solely in the undeformed state leaving minimal additional impact on the computational effort expended to simulate orthotropic materials compared to isotropic, resulting in a straightforward and highly efficient implementation. This method is implemented for rotation-free triangular shells using the finite element framework built on the Kirchhoff--Love theory employing subdivision surfaces. The accuracy of this approach is demonstrated using the deformation of a pinched hemispherical shell (with a 18{\deg} hole) standard benchmark. To showcase the efficiency of this implementation, the wrinkling of orthotropic sheets under shear displacement is analyzed. It is found that orthotropic subdivision shells are able to capture the wrinkling behavior of sheets accurately for coarse meshes without the use of an additional wrinkling model.Comment: 10 pages, 8 figure

Wrinkling of Orthotropic Viscoelastic Membranes

This paper presents a simplified simulation technique for orthotropic viscoelastic films. Wrinkling is detected by a combined stress-strain criterion and an iterative scheme searches for the wrinkle angle using a pseudo-elastic material stiffness matrix based on a nonlinear viscoelastic constitutive model. This simplified model has been implemented in ABAQUS/Explicit and is able to compute the behavior of a membrane structure by superposition of a small number of response increments. The model has been tested against a published solution for a time-independent isotropic membrane under simple shear and also against experimental results on StratoFilm 420 under simple shear

Frequency shifts in natural vibrations in pantographic metamaterials under biaxial tests

In this paper a 2D continuum model, thought as the homogenized limit of a microstructured pantographic sheet, is studied. The microstructure is characterized by two families of parallel fibers, whose deformation measures account for bending, elongation and relative rotation of the fibers. The deformation energy density of the homogenized model depends on both first and second gradients of the displacement. Modal analysis is performed in order to assess the peculiarities of the dynamic behavior of higher gradient models, and in particular the difference, with respect to classical laminae, in the dependence of the eigenfrequencies on the stiffness

In vitro bioactivity of novel chitosan/gelatin/halloysite nanostructured coatings on anodised titanium via electrophoresation for bone implant

Electrophoretic deposition (EPD) involves coating densification via matrix micro/nano-filling on a template-assisted substrate. This mechanical interlock technique has recently been used to avoid coating cracking and delamination. This thesis reports that EPD organic-inorganic nanostructured coatings containing chitosan/gelatin hydrogel and halloysite nanotubes (HNTs) produce an ideal mechanical interlock. In the proposed bio-composite, HNTs are used to densify the coating's mixture. The mechanical interlocking of the proposed bio-composite coating defines its net mechanistic bioactivity. Prior to EPD, the study goes through a substrate pre-processing step in which the cp-Ti surface is modified using micro-arc anodic oxidation (MAO) in a CaP-based electrolyte (a mixture of β-glycerophosphate and calcium acetate). The study discovered that anodised titanium (MAT) in donut-shaped morphology (MAO 350 V) has better mechanical stability and osteogenic cellular response compared to the needle-like structure. The findings determined that the donut-shaped MAT microstructure is the best next choice for the EPD substrate in the coating mechanical interlock study. Despite the fact that the EPD processing parameters were varied (10-30 V; 5-20 min), the mechanically interlock nanostructured coating (template-assisted EPD) significantly improved coating adhesion and osteogenic development in this study. In coating fabrication, the weight fraction of HNTs in the hydrogel is critical, and this study determined that the optimal composition of a steric stabilised organic-inorganic EPD suspension for chitosan/gelatin/HNTs is 6:14:12 g/L. Modifying implant surfaces using novel techniques such as varying substrate morphology and/or degraded coatings has become a popular method for improving implant osseointegration. This recent study established that specific surface features influence how bone cells interact with a material and which specific surface features result in optimal bone integration. In this thesis, MAT is designed to be a highly bioactive EPD substrate, resulting not only in a highly stable coating structure but also in improved osteogenic development, specifically osteoblast mineralisation and differentiation