Mechanical behavior of polymer-based materials and structures under extreme conditions: experiments and modeling

Polymer-based materials, due to their versatile properties, are widely applied in various aspects of daily life, such as packaging (plastics), 3D printing, industrial engineering (spaceships, airplanes, boats, sports equipment, etc.), civil engineering (e.g. polymer concrete), and medical sector (orthopedics, dental, hard and soft tissue replacements, cardiovascular devices, etc.). The study of these materials spans many scientific areas, such as solid mechanics, material science, polymer chemistry and physics, and structural engineering. The study of the nonlinear mechanical behavior of polymeric materials and structures is a critical area of research, driven by the ever growing demand for advanced materials capable of withstanding peculiar environmental and loading scenarios. However, understanding and predicting their performance requires robust modeling approaches coupled with comprehensive experimental investigations. This Thesis is organized into six main chapters, each addressing a specific aspect of the research. The complete list of chapters, excluding the final remarks, is presented below. • Chapter 1 delves into the mechanics of polymer-based materials. It begins with an overview of their characteristics and properties and then provides a background on the basic concepts of finite strain nonlinear elasticity. • Chapter 2 presents experiments on Polymers of Intrinsic Microporosity (PIMs) and hydrogels. The auxeticity of a synthetic molecular auxetic polymer obtained by embedding an expandable cavitand into a PIM is validated by means of Digital Image Correlation (DIC) analyses. Finally, compression tests are performed on dry and swollen hydrogels and, by means of Hertzian contact theory, their stiffness is derived, validating the results of a microscopically derived theoretical model. • Chapter 3 deals with the mechanics of puncturing of soft solids, investigated by means of experiments and theoretical modeling. In particular, experiments involve puncturing of bulk silicone samples with needles of various diameters. A theoretical model based on linear elastic fracture mechanics (LEFM) is introduced to describe the crack propagation during puncturing, with finite element analyses validating the model. Moreover, the influence of friction and adhesion is explored by conducting tests on samples with pre-formed cavities, allowing for the determination of adhesive frictional shear stresses at the needle-solid interface. Chapter 4 investigates the mechanical behavior of soft elastomeric membranes under indentation by a rigid spherical object, with particular focus on the failure mechanisms leading to puncture. An analytical model is developed to describe the axisymmetric nonlinear deflection of the membrane up to failure, validated by experiments on silicone membranes. The chapter finally investigates how pre-existing flaws, such as cracks, affect the resistance and failure of the membranes. Both lubricated and non lubricated conditions are studied. • Chapter 5 examines the mechanical behavior of 3D printed lattice metamaterials under compression, focusing on the influence of geometrical defects and how these affect structural performance. It is demonstrated that incorporating a secondary filling material, like silicone rubber, significantly increases load-bearing capacity, compressive strength, and energy absorption, almost compensating the presence of manufacturing defects even if infilling is introduced only in a few lattice cells. • Chapter 6 investigates the wrinkling behavior of compressed bilayer structures, constituted by a rigid isotropic skin laying on an orthotropic substrate. By varying the orientation of the orthotropic substrate, fine-tuning of wrinkling properties such as wavelength and amplitude can be achieved. The study is conducted by means of FE models and experiments on FDM 3D-printed specimens.