1. What is the difference between hydrostatic and quasi-hydrostatic pressure?
2. Are all SPD techniques providing this pressure?
As Zhilayev et al. mentioned, Multi-directional forging (MDF) not providing quasi-hydrostatic condition in www.scientific.net/DDF.385.302
Dear Ali Alyani
- Hydrostatic pressure is usually obtained in a diamond anvil cell where the specimen studied is put between two diamonds, in a pressure medium: which is a liquid (as Silicon oil) or a gas (as He gaz) sealed by a special ring.
- Hydrostatic pressure is obtained when the strain is isotropic, i.e. the pressure is distributed in the same way in all space directions. This holds in the low pressure range and up to a critical value Pcr depending on the pressure medium (1)
- Quasi-hydrostatic pressure appears at high pressures above this critical value Pcr which depends on the type of pressure medium used.
- The point is that at P>Pcr begins the pressure-induced solidification of the pressure medium which is no more fluid. Pcr also depends on temperature: By decreasing temperature, Pcr decreases.
- Change in fluidity of the pressure medium often starts well below Pcr (2)
See following references for better insight :
(1) https://iopscience.iop.org/article/10.1088/0022-3727/42/7/075413/meta
(2) Article Pressure distribution in a quasi-hydrostatic pressure medium...
Best regards,
Prof. A. Kadri
Mr. Abderrahmane Kadri, thank you for your response; it helped me to a certain extent! But I should emphasize that there is neither liquid nor special gas in the SPD techniques, unlike the diamond method. So, it remains unanswered for me, and more importantly, Zhilayev et al. allegations about MDF.
SPD processes which are done in a closed die usualy provide hydrostatic pressure . For example all the SPD methods that are based on extrusion will provide a high amount of hydrostatic pressure .
Ali Alyani
I did answer to your question because I meant that there is no way to obtain any real hydrostatic pressure without using an adequate hydrostatic transmitting medium with high hydrostaticity which can be:
1- either a fluid gas (and not anyone) and the best one is Helium gas,
2- or a liquid (and not anyone) and more particularly some special hydrocarbons like very light and refined oils.
Otherwise you can use any other trick you want, you will not obtain any hydrostatic pressure to any extent.
The term ''quasi-hydrostatic'' pressure is usually used for well known hydrostatic transmitting media when compressed at pressures higher than the critical pressure limit of their hydrostaticity which decreases with decreasing temperature below 0°C.
Obviously the paper you are referring to is using this term ''quasi-hydrostatic pressure in the purpose of confusing a multidirectional pressure (which is not necessarly a hydrostatic pressure unless it is infinite multidirectional pressure and this can hold only in a hydrostatic fluid) with a true hydrostatic pressure.
Best regards,
Professor A. Kadri
I've tried to answer your third question . Not all SPD methods but some of them may provide heydrostatic pressure .
For more details o reffer you to a book written by Dr. Gh. Faraji production of nabostructure tubes by severe plastic deformation
The main role of hydrostatic pressure is to increase the workabillity of material , by closing voids and microcracks . Back pressure is a main parameter to increase the hydrostatic pressure .
Almost every stress that we put on the material is divided into two groups : hydrostatic stress and deviantoric stress . The portuon of each group can be calculated using plasticity theory
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