A question of great interest is:How does the dynamic shearing resistance of metals respond to changes in pressure and applied strain rates? This important question in the dynamic material characterization field, and others, could be addressed by this technique which also mitigates several experimental variation sources that are present in the conventional approach. Though this method was was designed primarily for investigations of high temperature compressive and/or shearing resistance of materials, it can also be applied for studies such as dynamic slip and spall.
The gas gun at Case Western Reserve University is single-stage, with a six meter length and 82.5 millimeter bore. One section houses a custom modification to the gun for this experiment. The modifications are represented on this schematic.
A heating system mates with the existing gun barrel so that the sample held at the front of the custom heat resistant sabot can be heated to desired temperatures in excess of 1000 degrees Celsius. Additional modifications are at the target chamber end. There, a photonic doppler velocimeter probe precisely measures the velocity of the rear surface of the target plate.
Trigger and tilt diagnostic signals come from the voltage-biased pins on the front surface of the target plate. The first step is to create the sample and target for the experiment. Create the samples using high purity polycrystalline aluminum.
The equi-spaced holes are for securing the sample. Now check the surface flatness. Place the sample in contact with an optical flat then position the two under a green monochromatic light source.
Observe light bands that are curved and spaced depending on the surface features. The surface is sufficiently flat if three or fewer light bands are visible across the sample diameter. The experiment also requires a target plate and aluminum ring.
Make the target plates from precipitation hardened alloy rod. Section the ring from an aluminum tube and drill six equal-spaced slots in it before polishing it. Put the target plate and the aluminum ring together to produce an assembly similar to this.
Next mount the assembly on the target holder ring. When done the assembly will include copper pins for connecting to the trigger and tilt diagnostic circuit. Mount the ring on the target holder.
Gather the components of the heat resistant sabot. These include the sample holder, the sample, and the alumina screws. It also includes the alumina silicate lava rock tube and aluminum body tube with its cap.
Now work with the cap. The cap has an eye bolt. It's grooves have a sealing O-ring and PTFE key.
Run a thermo-couple wire through its bottom. Use epoxy to adhere the cap to the back of the aluminum tube. At the front of the aluminum tube epoxy the lava rock tube.
Pull the thermocouple wire from the cap through the tubes to the sample holder and adhere the sample holder to the lava tube. After the cement dries secure the sample to the sample holder with the alumina screws. Before proceeding ensure that the flatness of the front surface of the sample has not changed.
Now begin working with the gas gun at the impact chamber. Mount a three-axis motion stage on a rod above the gun barrel. Attach to the stage a prism holder with a precision optical alignment prism.
Next tape a first surface mirror over the sample plate. Then position the sabot and the gun barrel with the sample facing out. At the target chamber put a first surface mirror on the target and put the target holder assembly in place.
Carefully rotate the prism to be between the target and sample plates and their mirrors. Use a diffused bulb to perform a rough alignment of the plates. Adjust the stage to achieve a single continuous reflected image of the bulb from all surfaces of the prism.
Go on to achieve fine alignment with the aid of an auto collimator. Tighten all the positioning screws on the target holder and remove the motion stage, prism, and mirrors. Move the sabot to the breech end of the gas gun and connect the thermocouple to the temperature monitor.
At this point set up the doppler velocimeter. This requires an optical fiber focuser probe epoxied into an aluminum tube. Connect the optical focuser to an all-fiber-optic NDI TDI interferometer.
Now take the focuser assembly to the target holder. This one is outside of the target chamber for demonstration. Use the attached amount at the rear to support the focuser.
Aim the focuser at the rear surface of the target. Turn on the laser and adjust the focuser probe position using the screws on the focuser assembly. Stop after achieving proper light coupling and signal optimization.
Adjust the variable ratio coupler to match the intensity of the reference and doppler shifted light to optimize the signal. After all preparations are complete seal the breech target chamber and evacuate the gun barrel to less than 100 millitorr. Move the heater into position and turn it on.
Increase its temperature in 100 derees Celsius increments to reach the desired sample temperature. Pressurize the firing dump and load chambers. Also be sure to secure the sabot catcher to the impact chamber.
With everything in place turn off the heater and immediately move it upwards toward the heater-well. Record the temperature of the thermocouple at the sample surface. From here continue with the normal firing sequence.
This is an example of recorded voltage as a function of time during a typical reversed geometry normal plate impact experiment. The red trace is the trigger signal from the voltage based pins. The difference between the trigger times of the first and last pins allows an estimate of the maximum tilt at impact.
A longitudinal wave arrives at the back surface. The black signal is the result of normal motion diagnostics of the free surface using the photon doppler velocimeter. This plot is of the free normal surface velocity for a range of temperatures.
Data symbols go from black to red representing the coolest to warmest initial sample temperatures. The initial sharp rise is related to the stress of the aluminum sample at its interface with the target as the shock evolves. The plateau results from the impedance match between the sample and the target.
Note that the highest free surface velocity is associated with the lowest temperature and velocities decrease with increasing temperature. This suggests possible thermal softening of the sample under the different test conditions. A more interesting result can be seen in figure 16 which shows the normal free surface particle velocity trace obtained from reverse geometry normal plate impact experiments performed on commercial purity polycrystalline magnesium.
The results show monotonically decreasing particle velocities at the shock plateau with increasing temperatures in the range of 23 to 610 degrees Celsius. However at temperatures beyond this level, ie 617 and 630 degrees Celsius, a reversal of this trend can be clearly observed. This increase in particle velocity suggests an increase in the shock impedance of the sample material.
Moreover assuming that the elastic constants of the material decrease as a function of increasing temperature then an increase in the shock impedance in this case suggests an increase in the yield strength and/or plastic modulus of the sample material. It can be seen that the increase in particle velocity at the shock plateau is accompanied by an increase in the particle velocity levels throughout the initial rise in the particle velocity trace, which correlates with the stress levels of the sample target interface during the incipient plasticity of the sample material. Figure 17 shows micrographs of cross sections of the impact surface of post-test specimens.
The images show two noticeable effects on the microstructure as a result of increasing temperature. First is grain-ripening with increasing sample temperature. The images also show a change in twin band formations.
Carefully looking at the images corresponding to temperatures ranging from 23 to 500 degrees Celsius, a clear reduction in twin bands are observed with increasing temperature. However, at higher temperatures, ie 610, 617, and 630 degrees Celsius, a reemergence of these twin bands are observed, which suggests that twin band formation is favored at the latter end of this temperature range. If the test materials are already assembled, once mastered, this technique can be done in about five to six hours.
While attempting this procedure it's important to remember to take proper care in preparing the sample materials so that the flatness and parallelism tolerances can be adequately met. Following this procedure similar methods such as oblique plate impact and pressure shear plate impact can also be performed which enable dynamic material behavior to be probed at various loading rates and modes. After watching this video you should have a good understanding of how to perform an elevated temperature plate impact experiment which enables you to alleviate several experimental challenges present in the conventional approach.
