Understanding stress (pressure) simulation resuls

[Anonymous]

Hello everyone. Short story: I am begginer with Inventor 2011. I have applied internal pressure on cilinder made of PVC-U with wall thickness 4.8 mm. One of results in figure with Van Mises Stress with values ranging from 0 to 19.22 MPa. I have modified cilinder wall thickness down to 0.48 mm and now stress values are ranged from 0 to 176.2 MPa.

Question: how to understand such data? Does is say, that cilinder with thiner wall can hold bigger pressure values? It's noncense however... Or maybe I miss some important point in interpreting the results??? Please advise. Thanks in advance. And sorry if if Q sound like a n00bish as it is.

[henderh]

Hi berniokas,

  When decreasing the wall thickness, the stress in the material will be higher (with the same loads and constraints).

  Activating the Safety factor results is a different way to interpret what is happening.  A lower thickness will have a lower safety factor.

Hope this helps, -Hugh

[blair]

Just like life, nice a relaxed, low stress, get spread out thin, same load produces high stress

[JDMather]

You probably want to take a class on this, but -

the Safety Factor is = Yield Strength/Calculated Stress. 

See attached material properties for PVC-U

your Yield Strength is 46.53 in the numerator

you want a caculated stress in the denominator less than the Yield Strength for a Safety Factor >1

In general you want a SF > 2 for a well understood system or larger if less well understood.

[Anonymous]

In fact what I need is to determine what wall thickness should be so cilinder could hold 10 atmospheres internal pressure with safety factor 1.5. Thanks again for all your replies.

[Anonymous · Accepted solution]

One thing I believe that's been missed above is what the numbers actually mean.  A stress of 19.22 MPa indicates that the pressure applied to the interior of the pressure vessel is generating a 19.22 MPa (or 2787 psi) peak stress in the pressure vessel walls.  You will need to compare this to the yeild stress of the material you are planning on using (for example 302 Stainless steel has a yield stress of 275 Mpa (39900 psi).

As the walls of the pressure vessel get's thinner the stresses will increase since the same load is having to be carried across the smaller area.  That's the reason the your seeing the stress values increase to 176.2 MPa.

Think of it this way.  If you pull on two ends of a rod that has a cross-sectional area of 1 square meter with 200 kN of force the rod will have stresses in it equal to 200 kPa.  If you pull on the same rod with 400 kN of force the stresses will increase to 400 kPa.  The stresses don't mean the rod is stronger when you pull on it with 400 kN.  The stress is simply a measure of the forces being transmitted through the Rod through a given area.

Alan