Indentation rheometry for glass-forming materials

Theoretical considerations are made on the time-dependent viscoelastic deformation and flow during penetration by a flat-ended, spherical, conical, or a pyramidal indenter into glass-forming materials at temperatures near the glass-transition. The concept of representative stress and strain combined with the hereditary integral leads to principal constitutive equations for linear viscoelastic indentation. An experimental study of the viscoelastic behavior of soda-lime silica glass, as a representative of glass-forming materials, is conducted with the use of a trigonal sapphire indenter (Berkovich indenter). The self-consistency of the present theoretical framework for viscoelastic indentation is demonstrated on the basis of experimental results. It is emphasized that the indentation technique provides the most efficient micro/nano-probes for examining viscoelastic behaviors of glass-forming materials at temperatures near the glass-transition; testing specimens with extremely small dimensions and thin films with no special preparation, as well as testing with compressive loading for elevated temperatures.