Microscale Testing and Characterization Techniques for Benchmarking Crystal. . .
95
benefits are that it is nondestructive and can be used to capture 3D microstructural
information in situ rather than from a postmortem sample. For example, Marguiles
et al. demonstrated the use of HEDM to investigate deformation of a single grain
within a Cu sample during incremental loading and scanning [38]. Oddershede et
al. performed an HEDM experiment on a steel sample in a similar manner, but
oriented the tensile axis to be vertical with respect to the beam [39]. In addition,
this experiment did not simply study a single grain during testing, but rather about
200 grains, demonstrating a marked improvement in this characterization technique.
More recently, Schuren et al. and Shade et al. developed a load frame for in situ
HEDM experiments, known as the rotation and linear axial motion system (RAMS)
[40, 41]. This load frame allows for the use of μ-CT, far-field HEDM, and near-field
HEDM concurrently during a tension or compression experiment, while also being
able to rotate the sample 360 degrees. In terms of ICME efforts, in one instance, this
tool was utilized to collect a HEDM dataset for a Ti-7Al sample, which was then
used to instantiate a CPFEM simulation [42]. The model showed a good correlation
with experimental results, and this work also highlighted the importance of the stress
states of buried grains.
In addition to the determination of global mechanical response and the capturing
of an explicit microstructure representation of tested samples to be used for
benchmarking, the local deformation behavior needs to be determined for direct
comparison with simulation results. Hemker and Sharpe summarized numerous
efforts to measure the mechanical response of materials for microscale microelectromechanical systems (MEMS) in their review on small-scale mechanical testing
[43]. Espinosa et al. performed tensile tests on very thin films by pushing an Au
freestanding film that was fixed at each end, measuring the vertical deflection of the
film with interferometry and converting the deflection into elongation. While this
is a novel concept, this technique is only able to be applied to a very specific thin
film geometry [44, 45]. Haque and Saif constructed a micro-machined test system
that combines the load frame and the thin film specimen into a single part that can
be tested in an SEM and elongation determined by tracking the displacement of
markers deposited on the samples [46].
For a geometry as small as a microtensile specimen, traditional contact methods
for measuring strain (such as strain gauges or extensometers) are insufficient, and
other methods have developed over time. Attempts to determine strain from grip
displacement are made difficult by the compliance of a test machine relative to
the small sample geometry being tested, particularly with the large machines used
in macroscale testing. Greek and Johansson proposed a method for removing the
effects of sample compliance, but use of this technique requires large differences in
stiffness and it is generally accepted that strain is better measured in the gage of the
specimen rather than corrected grip displacement measurements [47]. As described
in the review on small-scale mechanical testing [43], one of the first noncontact
methods developed for measuring strain in microtensile samples employed an
interferometric strain displacement gage (ISDG). In this technique, two reflective
markers are placed on a sample and illuminated with a laser, generating fringe
patterns. As the sample changes shape, the motion of the fringe pattern can be
95
benefits are that it is nondestructive and can be used to capture 3D microstructural
information in situ rather than from a postmortem sample. For example, Marguiles
et al. demonstrated the use of HEDM to investigate deformation of a single grain
within a Cu sample during incremental loading and scanning [38]. Oddershede et
al. performed an HEDM experiment on a steel sample in a similar manner, but
oriented the tensile axis to be vertical with respect to the beam [39]. In addition,
this experiment did not simply study a single grain during testing, but rather about
200 grains, demonstrating a marked improvement in this characterization technique.
More recently, Schuren et al. and Shade et al. developed a load frame for in situ
HEDM experiments, known as the rotation and linear axial motion system (RAMS)
[40, 41]. This load frame allows for the use of μ-CT, far-field HEDM, and near-field
HEDM concurrently during a tension or compression experiment, while also being
able to rotate the sample 360 degrees. In terms of ICME efforts, in one instance, this
tool was utilized to collect a HEDM dataset for a Ti-7Al sample, which was then
used to instantiate a CPFEM simulation [42]. The model showed a good correlation
with experimental results, and this work also highlighted the importance of the stress
states of buried grains.
In addition to the determination of global mechanical response and the capturing
of an explicit microstructure representation of tested samples to be used for
benchmarking, the local deformation behavior needs to be determined for direct
comparison with simulation results. Hemker and Sharpe summarized numerous
efforts to measure the mechanical response of materials for microscale microelectromechanical systems (MEMS) in their review on small-scale mechanical testing
[43]. Espinosa et al. performed tensile tests on very thin films by pushing an Au
freestanding film that was fixed at each end, measuring the vertical deflection of the
film with interferometry and converting the deflection into elongation. While this
is a novel concept, this technique is only able to be applied to a very specific thin
film geometry [44, 45]. Haque and Saif constructed a micro-machined test system
that combines the load frame and the thin film specimen into a single part that can
be tested in an SEM and elongation determined by tracking the displacement of
markers deposited on the samples [46].
For a geometry as small as a microtensile specimen, traditional contact methods
for measuring strain (such as strain gauges or extensometers) are insufficient, and
other methods have developed over time. Attempts to determine strain from grip
displacement are made difficult by the compliance of a test machine relative to
the small sample geometry being tested, particularly with the large machines used
in macroscale testing. Greek and Johansson proposed a method for removing the
effects of sample compliance, but use of this technique requires large differences in
stiffness and it is generally accepted that strain is better measured in the gage of the
specimen rather than corrected grip displacement measurements [47]. As described
in the review on small-scale mechanical testing [43], one of the first noncontact
methods developed for measuring strain in microtensile samples employed an
interferometric strain displacement gage (ISDG). In this technique, two reflective
markers are placed on a sample and illuminated with a laser, generating fringe
patterns. As the sample changes shape, the motion of the fringe pattern can be