About the development of the FEM module/workbench.

Moderator: bernd

bernd
Posts: 10814
Joined: Sun Sep 08, 2013 8:07 pm
Location: Zürich, Switzerland

HarryvL
Posts: 1071
Joined: Sat Jan 06, 2018 7:38 pm
Location: Netherlands

bernd wrote:
Thu Jul 05, 2018 5:53 pm
https://forum.freecadweb.org/viewtopic. ... 10#p184850 this may help too
Bernd, I don't believe they are all available in the equation interpreter at the moment and those that are available can only be accessed by the names I mentioned.
HarryvL
Posts: 1071
Joined: Sat Jan 06, 2018 7:38 pm
Location: Netherlands

bernd wrote:
Thu Jul 05, 2018 5:55 pm
issue #2918

1. The issue description mentions 9 stress components, but you only need 6 due to symmetry.

2. In my Concrete / StressPlots project I have also added reinforcement ratios (rx, ry, rz) and Mohr Coulomb stress (mc) for display in the equation editor and export to VTK.
nquantum
Posts: 22
Joined: Fri Aug 03, 2018 1:29 am

OK, I see this user defined plot text box now , but what does it do? What I'm intended to put in there. Could you give an example?
Lets say I want to know what my safety factor on my design is. I can divide the maximum Principal stress by the yield of the material.
P1/ Yield
If the yield of your material is 100 MPa at temperature
P1/100
According to this the use of "user defined equation" is interesting for use in determine safety factor just like in the Autodesk Fusion360 in simulation. Make FreeCAD more flexible.

However I've two question.
1. According to web below it should be 100/P1 instead P1/100, that say "factor of safety = yield stress/working stress" am I correct?
https://en.wikipedia.org/wiki/Factor_of_safety

2. I've try in Freecad 0.17 windows10 64bit the result seem to be wrong. How?
Let see:
pic1 is value of max principal stress
pic2 is value of P1 which is same for pic1 that ok
pic3 I try 1/P1 the value look wrong isn't? why 1/0.852548=1.62763e+07? why 1/0.0543474=302.263? why 1/-0.317716=-2.57587e+07

the reason is I try to put 100/P1 the result look strange so I try P1, 1/P1 the show that maybe something wrong.
Attachments
pic1.jpg (171.13 KiB) Viewed 393 times
pic2.jpg (168.28 KiB) Viewed 393 times
pic3.jpg (169.25 KiB) Viewed 393 times
test.jpg (168.52 KiB) Viewed 393 times
HarryvL
Posts: 1071
Joined: Sat Jan 06, 2018 7:38 pm
Location: Netherlands

nquantum wrote:
Fri Sep 14, 2018 5:33 am
OK, I see this user defined plot text box now , but what does it do? What I'm intended to put in there. Could you give an example?
Lets say I want to know what my safety factor on my design is. I can divide the maximum Principal stress by the yield of the material.
P1/ Yield
If the yield of your material is 100 MPa at temperature
P1/100
According to this the use of "user defined equation" is interesting for use in determine safety factor just like in the Autodesk Fusion360 in simulation. Make FreeCAD more flexible.

However I've two question.
1. According to web below it should be 100/P1 instead P1/100, that say "factor of safety = yield stress/working stress" am I correct?
https://en.wikipedia.org/wiki/Factor_of_safety

2. I've try in Freecad 0.17 windows10 64bit the result seem to be wrong. How?
Let see:
pic1 is value of max principal stress
pic2 is value of P1 which is same for pic1 that ok
pic3 I try 1/P1 the value look wrong isn't? why 1/0.852548=1.62763e+07? why 1/0.0543474=302.263? why 1/-0.317716=-2.57587e+07

the reason is I try to put 100/P1 the result look strange so I try P1, 1/P1 the show that maybe something wrong.
@nquantum, some considerations:

S_eq/f_yield is indeed a measure for stress utilization (I tend to avoid the term "safety factor", because it tends to lead to a discussion about what safety means, e.g does it actually matter that S_eq/f_yield>1). I write something about that here: https://forum.freecadweb.org/viewtopic.php?f=24&t=27431

I presume that the 100 you quote is the yield stress? If so, then that agrees with the above formula. For S_eq it is better to use the von Mises stress (Von in the equation editor) rather than P1.

On the topic of "funny" results for 100/P1: Please keep in mind that max(100/P1) is not necessarily the same as 1/max(P1/100), because it may refer to different stress points.
Also, the huge numbers in max(100/P1) can easily be explained. It comes from points where P1 is close to zero.