Biomolecular origin of the rate-dependent deformation of prismatic enamel

Penetration deformation of columnar prismatic enamel was investigated using instrumented nanoindentation testing that was carried out at three constant strain rates (0.05, 0.005, and $0.0005{\mathrm{s}}^{-1}$ ). Enamel demonstrated better resistance to penetration deformation and greater elastic modulus values were measured at higher strain rates. The origin of rate-dependent deformation was rationalized to be the shear deformation of nanoscale protein matrix surrounding each hydroxyapatite crystal rod. The shear modulus of protein matrix was determined. It depends on strain rate in a format $G_p=0.213+0.021\ln \dot{\epsilon }$ .