Hydrostatic pressure reading increase during shear testing

[Uly79]

Hi,

This is the first time I am using this forum. Thank you all in advance.

I have a problem with hydrostatic pressure increase in a fluid sack sealed within a rigid support that I am shear testing.

The set-up is quite simple, the fluid sack is filled with an incompressible fluid and a vertical force is applied to it. The sack is an internal part of a rigid structure, both undergo the vertical force. Whilst in this situation a large cyclic horizontal shearing force is applied to the rigid structure. I am measuring the pressure inside the fluid sack with a pressure transducer.

My question is, the pressure inside the sack starts to increase slowly but steadily once the shearing action is started, after 7000 cycles there is roughly a 45% pressure increase. Once the cyclis shearing is stopped, the pressure backs off only a small amount. Why do you think this might be, is there an effect I have not considered?

Thanks again.

 

[IRstuff]

What have you done to rule out anything?

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[Uly79]

We have ruled out temperature and sack size change as neither of these are detectable.

The entire set-up up is within the shearing rig, we are wondering if it ids the effects of putting a hydraulic ram through the shearing action.

 

[btrueblood]

What materials are the fluid sack and fluid?

A little tough to envision what you are doing...could you post a photograph or sketch of the setup?

 

[btrueblood]

Also, how do you know the fluid temperature is not increasing, how did you measure it? And what values did you measure for the pressure change?

 

[Uly79]

Hi,

The fluid sack is actually a ptfe pneumatics hose that is filled with dipropylene glycol.

The vertical load on it increases the internal hydrostatic pressure to 68 Bar, when it is sheared it slowly creeps up to 74 bar, over roughly 2500 cycles.

 

[Woodyjumps]

The 100 ton hydraulic load, there is likely some static friction that is not being overcome either in advance or afterwards.
The "dithering" of the rig may be is allowing settling, so that your 100 ton load becomes something slightly more due to the mass of the hydraulic rod being no longer partially supported by the friction of the seals within.
Heat cannot be ruled out as a contributing factor unless the time between cycles is so great that the disipation of heat is consistent. The ptfe hose will heat with motion. If it heats, so will the glycol, and the glycol will expand.
If the hose is therfore trying to expand, but the friction of the hydraulic cylinder is not allowing it to move freely up, it will not be represented in the pressure behind the cylinder.
Can you measure the force directly, between the ram and the fluid filled hose?
Depending upon the degree of deformation of the structure, and it's material of constuction, there could be two other things going on.
The structure may be becoming taller due to heat (not as likely) or due to work hardening combined with large motions.
Once again, if the ram above is not free to move up due to friction, the pressure within the sack will rise without necessarily seeing an equivalent ratio rise in the ram pressure.

 

[Uly79]

Thank you all for your replies.

Woodyjumps, you were very much on point, I applied a thermocouple and took readings. There was a trend in temperature rise and pressure increase. I did a quick Bulk Modulus calculation, the pressure increase was directly proportional to temperature rise.

Could I ask you to elaborate on the settling of the ram, are we talking about the mass of the internal parts of the ram?

Thanks again.

 

[Woodyjumps]

Sorry Uly79 for the late reply. You are likely already beyond this problem.
Piston seals have friction against the walls.

Imagine yourself in the chimney on american ninga warrier. You can support your whole weight by spreading your legs and pushing your feet against the walls.
Most hydraulic cylinder seals have a sufficient friction when new to sustain the weight of the rod, vertically, with no other force.
Hydraulic pressure increases this force.

Now, imagine yourself in that chimney with your shoes having JUST enough force agains the wall to hold you up.... but then somebody starts to shake the chimney up and down a little. Whoops, suddenly all your weight is not longer supported on the friction between your feet and the wall and you begin to slide down.

That's the basic analogy. The issue of "sticktion" static friction of the seal vs dynamic friction. Breakaway force vs force require for continuous motion.