Implementation and validation of a cohesive fracture model through contact mechanics with application to cutting and needle insertion into human skin

Understanding the highly non-linear biomechanics of the complex structure of human skin would not only provide valuable information for the development of biological comparable products that could be used for the improvement, restoration or maintenance of the biological tissue or replacement of the whole organ, but would also support the development of an advanced computational model (e.g. finite element skin models) that do not differ (or do not differ very much! ) from experimental data. This could be very useful for surgical training, planning and navigation. In particular, the major goal of this thesis was the development of robust and easy to use computational models of the cutting and tearing of soft materials, including large deformations, and the development of repeatable, reproducible and reliable physical skin models in comparison to in-ex vivo human skin samples. In combination with advanced computational/mechanical methods, these could offer many possibilities, such as optimised device design which would be used for effective and reproducible skin penetration in the clinical setting and for in vivo measurements. To be able to carry out experimental cutting tests, physical models of skin were manufactured in the laboratory using silicone rubber. The mechanical properties of the physical models were examined experimentally by applying tensile and indentation tests to the test models using the Zwick universal testing machine and the Digital Image Correlation (DIC) System. To estimate the mechanical properties of the physical models and calculate the quantities, Poisson’s ratio, Young’s modulus and shear modulus -which were used later in computational cutting models - and inverse analyses were performed for each example of manufactured silicone rubber in the laboratory using the analytical study on indentation method, the curve fitting technique and DIC measurements. Then, the results were compared to the mechanical properties of human skin experimentally obtained in vivo/ ex vivo (from published studies). A new large deformation cohesive zone formulation was implemented using contact mechanics, which allows easy definition of crack paths in conventional finite element models. This was implemented in the widely used open source FE package FEBio through modification of the classical contact model to provide a specific implementation of a mesh independent method for straightforward controlling of (non-linear) fracture mechanical processes using the Mixed Mode Cohesive-Zone method. Additionally, new models of friction and thermodynamically coupled friction were developed and implemented. The computational model for the simulation of the cutting process (the finite element (FE) model of cutting) was reduced to the simplified model for the sharp interaction (triangular prisms wedge cutting), where the Neo Hookean hyperelastic material model was chosen to represent the skin layers for the FEM analysis. Practical, analytical and experimental verification tests, alongside convergence analysis, were performed. Comparison of the computational results with the analytical and experimental results revealed that applying the modified contact algorithm to the fracture problem was effective in predicting and simulating the cutting processes.