NANOINDENTATION OF BONE - Nanoindentation with Biological Applications - page 191

lengths likely vary with material anisotropy, compositional variations,
and microstructural organization. Scratch testing, also using a cube
corner tip, may also prove to be a useful tool for studying variations in
bone quality. 54

2.1.2. Testing methods

A variety of testing procedures have been used for the nanoindentation of
bone. Three of the most commonly used methods include ramp-and-hold,
constant stain-rate testing with continuous stiffness mode (CSM), and a
partial-unloading technique. Because bone exhibits time-dependent
behavior, properties are sensitive to strain rate and dynamic testing
frequency.
Ramp-and-Hold : While the monotonic ramp-and-hold approach is
likely the most commonly used test for nanoindentation of bone,
variations in its implementation have produced varied results ( Table 7-1 ) .
The viscoelastic nature of bone creates a significant dependence of the
modulus values on the unloading rate and the creep hold at maximum
load. Not permitting the creep displacement to saturate enables further
displacement to occur during the unloading period, causing the unloading
curve to demonstrate a characteristic “bowing out”. In this case, the tip
continues to sink deeper into the test material even while during
unloading. 43 Such behavior increases the elastic recovery rate (d P /d h )
and the apparent depth of contact ( h c ), thereby overestimating the
calculated value for indentation modulus. 41
Approaches to eliminate time-dependence include a fast unloading
rate, at least 10 times of the creep rate in order to elicit a primarily elastic
response on unloading, 43 and the presence of a sufficiently long hold
time at maximum load to allow creep behavior to minimize under
the applied load. 43 The effects of varying the unloading rate and hold
time at maximum load are demonstrated within the broader literature. A
wide range of loading rates (from 0.01 to 10000 N/s) and hold times

(0 to 1000 s) have been reported ( Table 7-1 ) . 43,45,48,50,55-65 The measured
modulus increases with the loading rate using a triangular loading-
unloading profile (0 s creep hold) when using various loading rates
ranging from 10 to 1000 N/s, indicating the significant presence of