As known, a shock wave is induced when the plastic stress wave becomes faster than the elastic one. Can anybody suggest to me how it's possible to make shock waves in metals like steel using an impact?
If you have a high energy laser available, laser shock processing is a well proven method to introduce shock waves inside steels and other methods. You need a few GW/sqcm of energy density introduced on the steel surface, under restricted plasma expansion conditions (that is, for instance, underwater), and you will have a shock wave due to the strong pressure of the ablation plume, which expands at ultrasonic speed against the .
One advantage of this method is that the "projectile" used is just a bunch of photons, so you can monitor the workpiece through it and study precisely the behaviour of front and bottom surfaces. Normally high power Q-Switched nanosecond lasers are used.
Using a Split Hopkinson Bars Pressure device working in compression (or traction also).
Dear Pablo
Thanks to your reply. But,we have not such facilities. We have a one-stage gasgun in our lab and must produce shock wave using a projectile impacting the target.
Dear Adinel
We have both gasgun and Hopkinson bar, but they are not able to produce shock waves...
Even if the impact velocity of the projectile is 50-100 m/s? You must regards especially the specimen length and eventually the lenghts of the bars.
Dear Adinel
I work on dynamic consolidation of Al powder using shock wave (via single stage gasgun). I could reach up to 90 m/s velocity for a 1 kg projectile. It seems no shock waves are induced because the powder was not consolidated. How do you say 50-100 m/s? Is there any equation to evaluate needed velocity?
http://history.nasa.gov/sp4801-chapter8.pdf
See attached paper. PAGE 130 Ref 26
Over 700 books articles and reports for a comprehensive overview, see D J Mynors and B. Zhang ‘ Applications and Capabilities of Explosive Forming’ Journal of Metal Processing Technology’ 125/126 (2002) PP1-25
The sound velocity in a (metal) powder is very low, so it would be easy to get a shock wave in it. Especially using a gasgun. If youi reduce the mass of the projectile even higher impact velocity can be obtained.
If have experience with shock compaction of powders using explosives (cylinder configurations). You did not obtain a solid compact probably due to a too low starting density of the powder. Gass in the pores is highly compressible, but tends to heat and expand after pressure release, which expands the compact back into a loose powder.
You need at least 60% dense samples to start shock compaction, in fact the higher the starting density the better.
To have a real plastic shock wave, beside a sufficiently high impact velocity (eventually low mass of the projectile) it is necessary to have same impedencies between the tested material and the bars or projectile materials. See theory of Hopkinson Bars or of Taylor impact test.
Would you please explain what is your purpose to make shock waves in metals like steel ?
If Adinels requirement would be true, not a lot of materials could be tested using a Hopkinson Bar (of one material, hence impedance).
All materials will create a shock wave when impaceted by another material at sufficiently high velocity.
Qualitatively you can testing all type of materials with a SHPB system and if the projectile energy is sufficiently great as the required plastic energy of the tested material we have a plastic wave shock. You deformed the material but you can not already extract a correct quantitative variation of plastic wave. For quantitative purposes concerning evaluation of stress-strain curves it is necessary to have same relationships of impedancies.
Dear Adinel
Thank you for your discussion. I agree with Erik. How can I have same impedance for steel punch and Al powder? if mechanical impedance is (density*sound velocity), it will be a big difference between these two materials impedance.
Baoling Zhang, we are going to make shock wave in Al powder to consolidate it and reach to cold sintering...
Thank you Erik, the initial density was about 55%. Are you sure with increasing the starting density, the full compaction would be reached? And what about the needed velocity?
Did you have any experience with gasgun?
Normally steel SHPB system and aluminium specimens works very well if specimen shape is correctly chosen, even if same differences exists between the impedancies (50% of differences). If necessary you can use other material for the bars and projectile (with high yield stresses) in order to have approximately the same impedancies values.
I do not have experience with gasguns, only with explosive compaction.
Apart from initial density (as high as possible) the powder should be well confined in order to prevent radial expansion.
The minimal projectile velocity depends on the mass (kinetic energy) of the projectile, as well as the strength of the powder particles (aluminum alloy and temper).
You need a minimal compressive stress for full consolidation, this pressure is not possible to obtain when the impedance of the powder is too low (that is why the starting powder should have an as high as possible starting density. The sound velocity will increase with density as well, hence the impedance rises fast with increasing powder density. You also reduce the influence of gass in the powder if the starting density is high (smaller gass volume fraction). Gasses tend to heat very much my shock compression (in fact they can only be compressed to 1/6=1/4 their oroginal volume by a single shock wave (no matter how much pressure you apply with the shock!!).
The heated gass will expand after pressure release and may reduce the final density of the compact (even back to the powder state).
If you are interested Icould send you a copy of an article that I wrote on powder compation (I did this for my PhD in 1998)....
regards,
Erik
Thanks a lot for your full explanation Erik. I'd be really appreciated if you could send me your paper.
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