Identifying the limitation of Oliver and Pharr method in characterizing the viscoelastic-plastic materials with respect to indenter geometry

Nanoindentation tests are widely used in recent years to characterize the mechanical properties of viscoelastic-plastic materials like polymers and biomaterials at the micro or nano-scale using the analysis method proposed by Oliver & Pharr (OP). However, recent studies revealed that the mechanical properties of viscoelastic-plastic (polymeric) materials determined using the OP method does not lead to a correct evaluation of Young's modulus. A systematic experimental study is performed with different indenter geometries like spherical and Berkovich geometries using various polymers in order to identify the limitations of the OP method.