Biomedical Engineering Reference
In-Depth Information
viscoelastoplasticity and energy dissipation of the material, which are
necessarily a function of these conditions. Finally, the depth range over
which many biological materials are probed and analyzed is not at the
nanoscale, according to International Organization for Standardization
(ISO) 14577 definitions specified originally for metallic materials ( h <
200 nm).
Note that stability of these signals over time is also a valid concern,
particularly for experiments of extended duration such as contact creep
compliance measurements. The signal stability (in terms of thermal drift
[N/s] or [m/s]) stated by the vendors is typically at the level of electronic
noise. The actual signal stability also depends on operating environment,
so this should be measured via comparable experiments on a rigid
material for each new experiment.
2.1.2. Frame compliance
All instrumentation that applies loads to a sample within a load train has
the potential to dissipate some of that applied energy through the load
frame itself. The magnitude of this dissipation during sample loading is
quantified as the frame compliance C f in units of [m/N], the distance over
which the frame deflects per Newton of applied load during quasistatic,
load-controlled extension or compression.
The same concerns apply to instrumented indentation of biological
materials. In fact, when stiff biological materials are indented on
universal testing frames at the macroscopic scale ( P ~ N and h ~ mm), C f
is inferred from a reference sample that is subject to the same loading
conditions, and is thus typically a hardness standard for which both
E and H are independently verified. For commercial instrumented
indenters, two approaches are possible: indirect estimation of C f from
micro- or nanoscale indentations on a reference sample 4 or direct
measurement of C f via compression of the indenter holder against the
rigid indenter stage. 5 The former approach was included as an iterative
procedure in the seminal paper of Oliver and Pharr, 4 but this iteration
(coupled to estimation of the indenter contact area A c ( h c )) can be
challenging to implement and relies on several assumptions and
idealizations regarding the reference material's response to contact
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