Summarizing, the tests were conducted with the experimental device consists of the combination of Hopkinson bar-chamber, as previously designed. The results of displacement, pressure and projectile speed, were registered thanks to the instrumental system and subsequently analysed and compared. Half of the sample were put in the artificial sea-water bath until the saturation level was reached, but the testing procedure was always the same for all sample types, the only difference is that it is needed to envelop the saturated sample with a plastic film, and to remove them from the bath just on the day of the test, and obviously just when they have reached the saturated level.
Each test is conducted following the next steps:
1. All the sensors to be used during the test are located in the right positions and connected to the oscilloscope, to the signal conditioner and to the digital counter (in the case of the infrared sensors). It is also verified that devices like oscilloscope, counter and conditioner are in the right measure settings.
2. Placement of the specimen and of all the moving wall of the chamber. Therefore, the rubber cover is located on the specimen (keeping attention, in the case of hybrids, to the right side on which it is wanted to conduct the test) is placed in a way that does not
53 obscure the holes, in order to allow the sealing with the framework through the screws. Then is placed the piston and the incident bar is screwed on it, it is important to ensure that this wall, because removable, is perfectly orthogonal. Finally, the top plate is screwed. Obviously, between the chamber and the piston there is a tiny space that is important to close, in order to ensure a perfect sealing of the box, and thus to avoid water leaks especially during the shot, and above all to try to reach a higher pressure in the chamber. To close these narrow gaps, it is manually applied a layer of polymeric insulating material, as it can be appreciated in the figure. Practically, this is the only point where this application is needed, but it was noted that after several tests, the chamber was practically deformed, and then it was needed this manually application also on the sides of the top.
Figure 3.21: Chamber with piston, incident bar, framework and plastiline placed.
3. Pushing of the projectile until the end of the bar using a long thin metallic bar. It is important to not forget this step because otherwise the projectile would not be affected by the pressure generated by the shot.
4. Close the water discharge valve.
5. Filling of the chamber: the box is filled with about 7L of water through the opening on the top of the box. During the filling is important to pay attention to avoid losses of
54 water from any part of the box, especially from the narrow gaps previously manually sealed; if necessary, the leaks must be eliminated by applying other more plasticine.
When the box is filled, the hole is closed with the screw.
6. Sensors verification: it is verified that laser works correctly through a voltmeter. It is also controlled the correct working of the infrared sensors, it is simply done resetting the screen of the digital counter, crossing the chronograph with one hand, if the system registers the time, it works correctly, then the screen is again resetting and considered ready for the measure. It is also verified that the oscilloscope shows all the curves relative to the sensors connected and thus that the trigger is started the measurement.
7. Preparing of the air pressure tank: the valves of the tank are opened allowing to pressurize the tank to the desired pressure, that can be controlled thanks to the manometer. The pressure used for each test is of about 5 bar.
8. Blast: the valve on the top of the Hopkinson bar is opened to shot at the pressure set.
The shot is immediate, as it takes a few milliseconds, therefore it is important to open this valve just when everything is ready for the test. Right after the shot this valve and the valves of the tank are closed.
9. Opening of the valve that allows the evacuation of the water from the chamber.
10. The specimen is removed from the framework in order to verify the damages. Some other parts are disassembled, so that the device is ready to run another test.
Data measured are saved to allow the later analysis.
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Chapter 4 Results
In this chapter, the results of the tests are shown, it is also shown how the data are analysed, how the results are obtained and the comparison between the critical parameters for this application. The measures of the sensors are saved in a “.csv” format file. The data are recorded in a range of 45 ms, meaning that each measure consists of 125000 values. This number is so high that is not possible to work on it through excel, but it is needed another software, thus it is used QtiPlot. This program allows to analyse data and to get the scientific graphing, it is also very intuitive and easy to use.