Mass and material

[Jee-Bee]

I was this weekend a bit playing around with the FEM.
What i thought is in the Fem you have to give the material properties to an object.
This is a fine but what i thought is that a lot of time you need to now only the mass of an part or assembly.

Would it not more logic to safe the material properties (and mass) in the part. When you want to change the parameter of one of the materials you can do that in the FEM.
That's more accessible than a long list of materials (without knowing witch one do you need to change)…

A lot of time you need only the total mass (transport, material shop, laws for lifting etc). so a part/ assembly calculation would be nice


Another thing i need an Aluminum (6061-T6) add to the material library so i created that. when i add this to the material library the material assigning doesn't work anymore.. also i found out that in the git a hole bunch of materials is present but not in the build… Why is this?

[bernd]

Material System is not a part of FEMWB. FEMWB just uses the material system of FreeCAD like the ArchWB. Actually for my modells the material is saved in the part. Since I'm structural engineer my modells are made using ArchWB which supports materials Arch_SetMaterial and viewtopic.php?f=23&t=10503&start=10#p85086. The connection between ArchWB and FEMWB is not implemented yet. FEMWB needs to run stand alone before it makes sense to implement a connection to other WBs imho.

... Another thing i need an Aluminum (6061-T6) add to the material library so i created that. when i add this to the material library the material assigning doesn't work anymore.. also i found out that in the git a hole bunch of materials is present but not in the build… Why is this?

If you would give more information what exactly does not work for you and which version of FreeCAD you are runing we may could help to fix the issue you have.

[PrzemoF]

Density is one of the properties we need to add to material files. Fell free to help Currently only a few material files are copied to build directory, so if you want your material to be part of FreeCAD standard material set you have to make some simple changes in the code (*) and send a pull request. Otherwise you can add your material to a user defined location (see FEM preferences for the details)

Just add your file here: https://github.com/FreeCAD/FreeCAD_sf_m ... ts.txt#L16

[bernd]

Density is one of the properties we need to add to material files. ...

An example how to add would be Steel-Generic https://github.com/FreeCAD/FreeCAD_sf_m ... .FCMat#L14 see Material

[PrzemoF]

I just noticed that we mixing specific weight (weight per volume like N/m3) with density also called volumetric mass density (mass per volume like kg/m3). We have SpecificWeight in kg/m3 and I belive we should rename it to Density. Please comment on that, I didn't have my coffee yet

https://en.wikipedia.org/wiki/Density
https://en.wikipedia.org/wiki/Specific_weight

[bernd]

I just noticed that we mixing specific weight (weight per volume like N/m3) with density also called volumetric mass density (mass per volume like kg/m3). We have SpecificWeight in kg/m3 and I belive we should rename it to Density. Please comment on that, I didn't have my coffee yet
...

https://en.wikipedia.org/?title=Mass_versus_weight
Yeah noone realised ...

... I belive we should rename it to Density. ...

+1. Assumed the unit kg in FreeCAD really is the mass! an not the weight.

More comments are really needed on this! Any physicist around ?!

for geman speakers around:
Density --> Dichte
Weight --> Gewichtskraft
Specific Weight --> Gewichtskraft / Volumen = Wichte

[Jee-Bee]

Material System is not a part of FEMWB. FEMWB just uses the material system of FreeCAD like the ArchWB. Actually for my modells the material is saved in the part. Since I'm structural engineer my modells are made using ArchWB which supports materials Arch_SetMaterial and viewtopic.php?f=23&t=10503&start=10#p85086. The connection between ArchWB and FEMWB is not implemented yet. FEMWB needs to run stand alone before it makes sense to implement a connection to other WBs imho.

Why is in ArchWB the material properties part of the part (Sounds weird…) But in the other parts not. In the PartWB/ PartDesign/WB it is in my opinion at least even usefull.

If you would give more information what exactly does not work for you and which version of FreeCAD you are runing we may could help to fix the issue you have.

I run this version of FreeCad

Screen Shot 2015-06-23 at 12.46.25.png (192.6 KiB) Viewed 2636 times

What i mean is that i see in FreeCad-git >> SRC >> Mod >> Material >> Standard Material this:

Screen Shot 2015-06-23 at 12.39.49.png (180 KiB) Viewed 2636 times

But when i want add some materials i see oly:

Screen Shot 2015-06-23 at 10.48.12.png (42.62 KiB) Viewed 2636 times

Except Al 6061-T6. When i add that material i don't have any options to choose a material anymore...

[sgrogan]

More comments are really needed on this! Any physicist around ?!

Not a physicist but,

Mass is in kg. A measure of how much matter something contains. It is a constant.
Weight is in N. N is equal to (kg x m)/s^2 . It is a force and is not constant, think F=ma. On earth a is the acceleration due to gravity in m/s^2. A body in free fall is weightless, but it still has mass.
Density is mass per unit volume kg/m^3.

So, weight[N] = Density[kg/m^3] x Volume[m^3] x Acceleration[m/s^2]

[bernd]

Material System is not a part of FEMWB. FEMWB just uses the material system of FreeCAD like the ArchWB. Actually for my modells the material is saved in the part. Since I'm structural engineer my modells are made using ArchWB which supports materials Arch_SetMaterial and viewtopic.php?f=23&t=10503&start=10#p85086. The connection between ArchWB and FEMWB is not implemented yet. FEMWB needs to run stand alone before it makes sense to implement a connection to other WBs imho.

Why is in ArchWB the material properties part of the part (Sounds weird…) But in the other parts not. In the PartWB/ PartDesign/WB it is in my opinion at least even usefull.

My explanation was bad ...sorry ... I try to do it better ...

short one:
It just needs someone whe develops a material system for Part or PartDesign.

long one:
The material system was developed with background of defining it once for a shape (best place for this would may be Part or PartDesign) But it turned out the there are different requirements according the different use cases. Therefor more than one system to define materials exists. I like the idea to be independent inside FEMWB from any other WB. This way a shape can have different materials for FEM-Analysis, Calculation of Cost and so on. The material system in FEM is designed only to define materials properties for FEM Analysis. As long as the material definition is inside FEM we are tatally indepndent where the geometry the analysis is made with comes from.

For calculation of mass, cost and so on the Material system of Arch fits better for my needs.

[bernd]

More comments are really needed on this! Any physicist around ?!

Not a physicist but,

Mass is in kg. A measure of how much matter something contains. It is a constant.
Weight is in N. N is equal to (kg x m)/s^2 . It is a force and is not constant, think F=ma. On earth a is the acceleration due to gravity in m/s^2. A body in free fall is weightless, but it still has mass.
Density is mass per unit volume kg/m^3.

So, weight[N] = Density[kg/m^3] x Volume[m^3] x Acceleration[m/s^2]

Thanks! According the FreeCAD wiki the unit system is based on the 7 base units ([wiki]Units#Data_model[/wiki] and https://en.wikipedia.org/wiki/SI_base_unit) It would mean kg in FreeCAD really is the mass and therefore the value in the material cards should be for sure the denisty imho. Very good catch przemo !