Theory 7
Agilent Nano Indenter G200 User’s Guide 7-15
The determination of frame stiffness at one location on fused silica is
sufficient when:
1 Contact stiffness is less than 1 % of frame stiffness.
2 Hardness is the only result of interest.
However, if contact stiffness is greater than 10 % of frame stiffness,
then frame stiffness should be determined using an experimental setup
that is as close as possible to the test set up, even using the test sample,
if possible.
Determining In-Situ Frame Stiffness
As mentioned in the Analysis of Instrument Frame Stiffness, frame
stiffness is determined by finding the y-intercept of the line using
Equation 19 on page 7-12.
Therefore, any material may be used to determine frame stiffness, as
long as the modulus and hardness are uniform, i.e. independent of
applied force and displacement over the domain of included data.
When testing metals, be cautious as the contact stiffness (S) is likely to
be greater than 0.1
K
f
, because:
• Metals are typically soft, leading to relatively large contact
diameter (D) at relatively small force (P).
• Metals typically have high moduli; the combination of large
contact diameter and high modulus leads to a high contact
stiffness, following the relation S
= DE
r
.
Also, hardness is typically not constant with indentation load/depth due
to indentation size effect. This invalidates the automated process for
determining frame stiffness as hardness decreases with indentation size,
approaching its final value only when contact size is much greater than
the grain size.)
Alternate Frame Stiffness Procedure
This procedure may be used for metals, assuming a uniform modulus. If
the sample hardness is non-uniform, then the automated algorithm will
yield errant values for K
f
.
If the modulus is truly uniform (i.e. "bulk" metal), then the value of K
f
which causes the reported modulus to be constant with increasing
force/displacement is the true value of K
f
.
To Next Page
Loading...
Need help?
General
