NANOINDENTATION OF BONE - Nanoindentation with Biological Applications - page 190

( Table 7-2 ) . 43 Thus the selection of tip geometry may be more important
to the underlying contact mechanics and subsequent deformation
mechanisms than to the measured material properties.
Berkovich : The majority of bone indentation experiments have been
carried out with a Berkovich tip, which is supplied with most
commercially available testing systems. The typical Berkovich tip,
possessing a contact radii that is similar in size to a lamella ( R ≈ 100-200
nm), has been effectively used to determine mechanical properties of
such individual structural features in bone. 44,45 Berkovich tips are often
preferred for indentation testing as they are geometrically similar and
thus the indentation process is length-scale independent for a monolithic
material. However, Berkovich tips are “sharp” and cause significant
plastic deformation at the onset of indentation. While the contribution of
plastic (and elastic and viscous) deformation to the overall indentation
depth can be analytically determined, 46 the nature of this plastic
deformation in bone remains unclear. Consideration should be thus given
to the effects of indenting the viscoelastic material of bone with a sharp
indenter tip.
Spherical : Spherical tips of various radii serve as a powerful tool to
study both elastic and viscoelastic properties in bone tissue. 43,47-50 Unlike
Berkovich tips, the geometry of spherical tips is less uniform from one
tip to another and therefore require time-consuming and careful
characterization of the tip area function. For example, spherical tips may
flatten slightly or deviate from a perfect spherical geometry. 51 The radius
of a spherical tip remains the major factor in producing high quality
data. 51
Spherical tips possess the advantage of producing an initial, primarily
elastic response with a detectable transition to plastic deformation. 52 In
contrast, Berkovich tips have an immediate onset of plasticity at very
small displacements due to the high stress concentration near the
material's surface. The use of multiple spherical tips of varying radii,
which also differ in geometric similarity, enables a range of
microstructural features in bone to be explored within a limited load and
depth range. 43,52
Cube corner: Fracture toughness of human tibial cortical bone has
been investigated by using a cube corner tip to induce fracture. 53 Crack