70
As with miniature compression bars, several authors have developed small scale versions of the tensile bar. In contrast to
the compression bar development, the motivation here is to test small samples at high rates, not necessarily to reach extraordinarily high rates. For example, [14] used miniature tensile bars to test thin films, and [15] has used a small scale bar to test
individual fibers. The latter used a quartz crystal load cell in place of the output bar, further simplifying the setup.
13.2 Current Design
The objective of the work discussed in this chapter is to develop a miniature tensile Kolsky bar to study ductile fracture in
metals at the mesoscale, i.e., to study the influence of microstructural defects on the ductile fracture process. To date, only
basic prototypes have been investigated, so this chapter documents what is currently planned. Like [14, 15], the goal is not
to achieve rates beyond those commonly achieved; rates in the range of 1000–5000/s are adequate. However, sample sizes
will be quite small. Typical gage lengths will be on the order of 500 μm, with a cross-section of ~100 μm × 100 μm. With a
typical metal, like aluminum, failure might occur at a load of only 10 N, and perhaps 20% strain. Thus, the current design,
shown in Fig. 13.1, has been selected to meet these requirements. The concept is basically the classic design, except at least
initially a compression spring will be used to accelerate the striker, as done by [15]. This avoids the need to build a small gas
gun. In our design, the use of a spring will limit the impact speed of the striker to about 2 m/s. However, this will be sufficient
for the short gage length specimens that will be studied.
The bars and striker are all steel but could be changed to a lower impedance material (e.g., aluminum) if needed. Similarly,
a smaller diameter output bar can be used to gain more sensitivity with smaller samples. A normal displacement interferometer (NDI) at the free end of each bar is sufficient for instrumentation. This is adapted from the miniature compression bar
described in [5]. Note that in the compression arrangement (Fig. 13.2), there is no free end on the input bar and a transverse
Fig. 13.1 A proposed optically instrumented miniature tensile bar. Not to scale
Fig. 13.2 A miniature compression bar instrumented with optics. A diffraction grating at the midpoint of the input bar is used in a TDI. An NDI
is used to measure the motion of the free end of the output bar
D. T. Casem
As with miniature compression bars, several authors have developed small scale versions of the tensile bar. In contrast to
the compression bar development, the motivation here is to test small samples at high rates, not necessarily to reach extraordinarily high rates. For example, [14] used miniature tensile bars to test thin films, and [15] has used a small scale bar to test
individual fibers. The latter used a quartz crystal load cell in place of the output bar, further simplifying the setup.
13.2 Current Design
The objective of the work discussed in this chapter is to develop a miniature tensile Kolsky bar to study ductile fracture in
metals at the mesoscale, i.e., to study the influence of microstructural defects on the ductile fracture process. To date, only
basic prototypes have been investigated, so this chapter documents what is currently planned. Like [14, 15], the goal is not
to achieve rates beyond those commonly achieved; rates in the range of 1000–5000/s are adequate. However, sample sizes
will be quite small. Typical gage lengths will be on the order of 500 μm, with a cross-section of ~100 μm × 100 μm. With a
typical metal, like aluminum, failure might occur at a load of only 10 N, and perhaps 20% strain. Thus, the current design,
shown in Fig. 13.1, has been selected to meet these requirements. The concept is basically the classic design, except at least
initially a compression spring will be used to accelerate the striker, as done by [15]. This avoids the need to build a small gas
gun. In our design, the use of a spring will limit the impact speed of the striker to about 2 m/s. However, this will be sufficient
for the short gage length specimens that will be studied.
The bars and striker are all steel but could be changed to a lower impedance material (e.g., aluminum) if needed. Similarly,
a smaller diameter output bar can be used to gain more sensitivity with smaller samples. A normal displacement interferometer (NDI) at the free end of each bar is sufficient for instrumentation. This is adapted from the miniature compression bar
described in [5]. Note that in the compression arrangement (Fig. 13.2), there is no free end on the input bar and a transverse
Fig. 13.1 A proposed optically instrumented miniature tensile bar. Not to scale
Fig. 13.2 A miniature compression bar instrumented with optics. A diffraction grating at the midpoint of the input bar is used in a TDI. An NDI
is used to measure the motion of the free end of the output bar
D. T. Casem