EXPERIMENTAL AND NUMERICAL-ANALYSES OF INDENTATION IN FINITE-SIZED ISOTROPIC AND ANISOTROPIC RUBBER-LIKE MATERIALS

Indentation tests perpendicular to the major plane of a material have been proposed as a means to index some of its in-plane mechanical prop erties. We showed the feasibility of such tests in myocardial tissue a nd established its theoretical basis with a formulation of small inden tation superimposed on a finitely stretched half-space of isotropic ma terials. The purpose of this study is to better understand the mechani cs of indentation with respect to the relative effects of indenter siz e, indentation depth, and specimen size, as well as the effects of mat erial properties. Accordingly, we performed indentation tests on slabs of silicone rubber fabricated with both isotropic, as well as transve rsely isotropic, material symmetry. We performed indentation tests in different thickness specimens with varying sizes of indenters, amounts of indentation, and amounts of in-plane stretch. We used finite-eleme nt method simulations to supplement the experimental data. The combine d experimental and modeling data provide the following useful guidelin es for future indentation tests in finite-size specimens: (i) to avoid artifacts from boundary effects, the in-plane specimen dimensions sho uld be at least 15 times the indenter size; (ii) to avoid nonlineariti es associated with finite-thickness effects, the thickness-to-radius r atio should be >10 and thickness to indentation depth ratio should be >5; and (iii) we also showed that combined indentation and inplane str etch could distinguish the stiffer direction of a transversely isotrop ic material.