Determination of viscoelastic-plastic material parameters of biomaterials by instrumented indentation.

Load response of viscoelastic-plastic materials depends on the load magnitude and history, and can be described by rheological models consisting of springs and dashpots. The parameters in these models can be obtained by instrumented indentation. Time-dependent properties are determined best from the time course of indenter displacement under constant load, but one must consider also the initial loading history. For characterization of instantaneous elastic and plastic response, fast loading and unloading are necessary. The paper summarizes the formulae for indentation into elastic-plastic and viscoelastic-plastic materials, and proposes a procedure for testing and data evaluation. Its application is illustrated on the indentation of human enamel.