Study on the optimal loading rates in the measurement of viscoelastic properties of hydrogels by conical indentation

It has been reported that the measurement results by the method of indentation for soft viscoelastic materials are affected apparently by experimental conditions, such as strain rate and indentation depth. In order to enhance the effectiveness of the method of indentation and find optimized loading rates in tests, the Lee-Radok's solution was modified with Hay's correction to describe the indentation response for soft viscoelastic materials, further, a pseudo dissipative work was defined in a normalized indentation loop and its variation against the loading rate was obtained. It is found the pseudo dissipative work exists several maximal values related to the viscous properties of materials, and the corresponding loading rates can be regarded as the optimal loading rates in indentation tests. Based on the theoretical analysis, a new method how to determine the optimized loading rates in indentation was recommended, which was verified by tests of indentation and uniaxial compressive creep for a PVA hydrogel.