The goal of the following experiment is to develop a method for studying the dynamic fracture and fragmentation of a material under high strain rate, tensile loading for a range of initial sample temperatures. This is achieved with a gas gun driven expanding cylinder geometry, where the sample takes the form of a hollow cylinder containing a steel O jive shaped insert within, which is the temperature control apparatus. As a second step, the target cylinder is then mounted ally to the end of the gas gun barrel, and laser-based v symmetry Diagnostics are installed to measure the expansion velocity at several points along the cylinder and high speed imaging is used to track fracture, initiation and growth.
Next, the cylinder is brought to the desired temperature and the polycarbonate projectile is launched into the target. The resulting impact between the projectile and the OJ insert causes the polycarbonate to flow outwards driving the cylinder into uniform radial expansion, which creates a tensile stress state around the circumferential direction. The vela symmetry and high speed imaging can then be used to determine the deformation failure and fragmentation characteristics of the material with temperature, along with fragment analysis to ascertain the fracture mechanisms.
The main advantage of this technique over other existing methods, such as the classical winter technique or the use of explosives or tensile split Hopkins and bars so that we can study the effect of the initial sample temperature without affecting experimental factors such as the loading or the strain rate in the sample. Begin preparations for the experiment with a target cylinder machined from the titanium alloy titanium, 6%aluminum, 4%vanadium percentage by weight or tie six four. The cylinder is 150 millimeters long with an inland diameter of 50 millimeters and a wall thickness of four millimeters.
The surface finish should be free from machining marks and left slightly diffuse for use With photon doppler vela symmetry, proceed to bond thermocouples along the cylinder. Start to assemble the target. First place the OJ insert into the rear of the cylinder and fix it in place with the grab screws.
Next, place the cylinder and O jive assembly into the mounting sleeve. Secure it with grub screws. At this point, install the heating or cooling apparatus.
For this experiment, place a microme resistive load into the rear of the O jive insert. Here is the final assembly with the thermocouples to allow measurement of temperature along the cylinder and the probes prove symmetry here. The final target assembly is shown next to the gas gun projectile for comparison.
After cleaning the probe connectors roughly align the probes with a visible laser, our 660 nanometer laser light through a probe and onto the cylinder for approximately normal alignment. Adjust the probe until reflected light falls back on the probe. Proceed to a more thorough alignment using a 1, 550 nanometer laser and an optical circulator.
To measure the amount of reflected light collected, adjust each probe until the maximum value is achieved. Once the target is ready, begin work with the gas gun. Place a carefully machined alignment plug inside the end of the barrel.
Next, install the target assembly over the plug and adjust the target mount until the cylinder is aligned. Coaxially with the barrel. With the target aligned, connect the heating coil to high current cables and the optical fibers to the photon Doppler Vela symmetry system system.
Return to the barrel to install a trigger for the timing hardware. Place a trigger pair at the end of the barrel such that a projectile will complete a circuit on exiting the barrel. Then measure the distance from the trigger to the point of impact on the O jive to allow an estimate of the time of impact.
At this point, set up the high speed imaging system. Use mirrors to relay images through the gas gun's optical port. Place flash guns to point at the cylinder from the front and above.
Mount a high speed camera outside the target tank aligned and focused on the cylinder. The cylinder should almost fill the frame, leaving room for cylinder expansion. When everything is in order, take appropriate safety precautions for the lasers in the photon doppler vela symmetry system.
Then turn on the vela symmetry system and the oscilloscopes, and check the vela symmetry. Probe power levels and alignment. Next, use an infrared viewing card to determine the distance from the rear of the cylinder to where the vela symmetry probes are focused.
Turn on the reference laser to check the signal For each channel. Set the zero velocity beat to five gigahertz. After a final test of all diagnostics, remove the alignment plug and close the tank.
Install the projectile prepare to fire by evacuating the target tank to around 50 milli to and setting the timing hardware to have high speed imaging and oscilloscope. Zero time coincide with the estimated time of impact. Turn on the lasers and arm the diagnostics before securing the room and moving to the control room.
From the control room. Begin to heat the target to the desired temperature. When the temperature is reached, charge the gun to the desired firing pressure.
Once it is at the firing pressure, do a final check. Turn off the heating power supply. Proceed with a countdown and fire 3, 2, 1.
After firing, bent the gas gun, save the camera data as the gun returns to atmospheric pressure. When it is safe to proceed, return to the gun room, turn off the laser and save the oscilloscope data. Continue by opening the target tank to collect the debris.Debris.
The debris will consist of Tie six four and other metal fragments. Remove all metallic fragments from the tank for later extraction and analysis of tie six four. This high speed video is of a titanium alloy cylinder at 150 K.The projectile speed was 1000 meters per second.
The framing camera collected one image every 10 microseconds with a 0.7 microsecond exposure. Botone Doppler vela symmetry provided the data for this plot, which gives the expansion velocity as a function of time. The two solid curves are for the 150 K cylinder.
The black curve is for the pro position. At about the tip of the O jive, the red curve is for a point further back along the O jive. For comparison, the dotted curves give data for a cylinder at 800 K.Again, the black curve is at the O jive tip, and the red curve is further back.
The called cylinder has less deceleration after the peak velocity suggesting fracture initiated earlier. Here, the solid colored curves correspond to the radial strain accumulated at four points along the length of the cylinder held at 150 K.The dots and connecting dotted line are the number of visible cracks using the axis at, right. Well, this images from the high speed video allow gathering information on the temporal activation of fracture.
Here, multiple longitudinal fractures are evident in the 150 K cylinder Following the procedures we've presented here such as the high speed imaging and the velocity measurements. Careful analysis of the recovered fragments can reveal the fracture mechanisms occurring in the sample responsible for the fragmentation.
