Von Mises - Infinite Strength? 1

[DDP]

According to Von Mises if you have a steel cube under hydrostatic stress only, it will never fail no matter how much stress you put on it.

How is this reconciled in real life? Is there a "cap" to Von Mises?

What is the story here?

 

[FeX32]

Sensibly, it may crumble. But I have never looked into it...


Fe

 

[amorrison]

The crystal structure could change - as in carbon to diamond.
Then there is always neutron star theory.

 

[GregLocock]

How could dislocations move if there is no better place for them to be ?

having said that, yes in the fullness of time they would move, probably on about the same timescale as monkeys typing Shakespeare, due to quantum effects.



Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[prex]

Pressure vessel codes use Tresca (stress intensity), but the issue is the same. ASME VIII Div.2 sets a limit of (if I recall correctly) 4 times the allowable stress to the sum of the three principal stresses.

prex

http://www.xcalcs.com

: Online engineering calculations

http://www.megamag.it

: Magnetic brakes for fun rides

http://www.levitans.com

: Air bearing pads

 

[btrueblood]

Do the rocks on the seafloor crumble? Why would you expect ANY solid material (no voids) to yield under pure hydrostatic stress?

ASME VIII does not pertain to bodies under pure hydrostatic states of stress, the limit stresses mentioned pertain to pressure-containing boundaries, not to internal parts.

 

[jhardy1]

@btrueblood:

Yes, but don't forget that the deepest ocean depth is the Mariana Trench, with a depth of 10,911 meters. At this depth, the hydrostatic pressure is "only" 107 MPa (0.1 GPa) - nowhere near enough to test what happens to steel for example. (It doesn't yield, or "collapse in on itself" at this pressure, by the way - we know this, because we have sent submersibles to the bottom of the Mariana Trench - and brought them back!)

Sadly, the pressure at the bottom of the deepest oceans is also several orders of magnitude too small to turn coal into diamonds - you need several GPa at high temperature to synthesise diamonds.

The fact that we can synthesise diamonds using high pressure and temperature (e.g. see

http://en.wikipedia.org/wiki/Synthetic_diamond)

should give some clue to the OP's question - we need devices which can sustain pressures of the order of several GPa (i.e. much, much higher than the yield strength of even the best steels) in order to make artificial diamonds. It follows that some partS of the machine must experience comparable hydrostatic pressures - and they don't fail.

Cheers!

 

[FeX32]

Good one

Fe

 

[btrueblood]

Yeah, but 100 MPa is enough to fracture some carbonate rocks...and they don't fracture on the sea floor...I didn't say anything about steel, only implied it won't yield under purely hydrostatic stresses. I.e. you are saying what I said, Greg said, and amorrison said.