INTRODUCTION - Nanoindentation with Biological Applications - page 17

traditional metal testing, where there can be large hardness variations at
constant elastic modulus due to local microstructural effects associated
with dislocation motion. Hardening mechanisms such as grain size
refinement due to mechanical work are simply unavailable in the non-
metal materials found in the natural world. In the context of instrumented
indentation testing, the hardness is the mean supported contact stress, 17 a
measure of the total resistance to deformation, regardless whether the
deformation is elastic, plastic, 18 viscous, 19 or due to crushing fracture. 20
As such, on average, over a wide range of biological materials, the
hardness is not an independent material property but depends directly on
elastic modulus 21 ( Fig. 1-10 ) .
Figure 1-10. The relationship between elastic modulus and “hardness” in biological
materials.
Although biological materials often do not exhibit elastic-plastic
responses, there are reasons to consider nanoindentation testing as
particularly useful for their mechanical characterization. Nanoindentation
testing is well-suited to study the composite nature of biological