Depth sensing indentation of linear viscoelastic–plastic solids: A simple method to determine creep compliance

Viscoelastic solids may deform plastically under indentation. This leads to an overestimation of the creep compliance when the analytical solution for indentation of linear viscoelastic materials [Lu H, Wang B, Ma J, Huang G, Viswanathan H. Measurement of creep compliance of solid polymers by nanoindentation. Mech Time-Depend Mater 2003;7(3–4):189–207] is used for its determination. Using finite element analysis, in this work it is shown that the plastic and viscoelastic deformation processes occur simultaneously, even during holding of a constant indentation load. A simple procedure to separate the viscoelastic response from the plastic response is proposed, which involves spherical indentations at different loads. To illustrate the proposed method, indentation tests are conducted on a polymer, polymethylmethacrylate (PMMA), and its viscoelastic properties are determined.