How does FEM software compute inertial forces?

[zoltrix]

TL;DR

FEM for machine design

Hello

I have alwayes seen FEM applications for static structures while each " finite element" of a bar linkage mechanisms should be subjected also to a variable inertial force besides its constant weight
how do FEM softwares compute inertial forces ?

 

[pasmith]

Start with the equation of conservation of momentum. Apply the same steps as in the static case; the result is a system of ordinary differential equations for the evolution of displacement over time.

 

[zoltrix]

in principle I know how to take into account also the inertial forces
The point is that , to my knowdlege, all FEM softwares consider only static structures

 

[berkeman]

Paging @FEAnalyst

 

[gmax137]

I was curious so I googled

'non inertial reference frame in ansys'

Some interesting info

 

[FEAnalyst]

The point is that , to my knowdlege, all FEM softwares consider only static structures

Exactly what kind of structures and operating conditions do you want to consider ? With a proper level of model simplification and dynamic analysis (supported by almost all FEA programs) it should be possible to account for all such effects. And then there's software meant specifically for multibody dynamics.

 

[A.T.]

I have alwayes seen FEM applications for static structures while each " finite element" of a bar linkage mechanisms should be subjected also to a variable inertial force besides its constant weight
how do FEM softwares compute inertial forces ?

The option you are looking for is usually called "inertia relief". It allows to analyse non-inertial bodies by introducing inertial forces.

 

[FEAnalyst]

The option you are looking for is usually called "inertia relief". It allows to analyse non-inertial bodies by introducing inertial forces.

I'm not sure what the OP is looking for (and thus I asked him for some more details) but it seems that he needs a dynamic analysis or MBD since he mentioned mechanisms. Inertia relief is used to stabilize unconstrained (or not fully constrained) models - it balances applied loads to avoid rigid body motions.

 

[zoltrix]

yes I had in mind a dynamic analysis
you can study a mechanism in motion same as it were a static structure provide you add also the inertial forces
a kind of trick
I went through several FEM examples but it seem that they deal only with true static structures
Apparentely there is no mention of the inertial forces even in case of analysis of machines
As I said, each finite element should be subjected also to a local inertial force proportional to its mass and acceleration

 

[FEAnalyst]

yes I had in mind a dynamic analysis
you can study a mechanism in motion same as it were a static structure provide you add also the inertial forces
a kind of trick

This is commonly done for rotating structures. Instead of running a transient dynamic analysis, one can apply centrifugal force load (available in most FEA programs) in a static analysis. And there are more loads like this for rotating structures. For example, Abaqus offers also Coriolis forces, rotary acceleration and rotordynamic loads in a fixed reference frame. Of course, gravity load is also available in every FEA software to apply various accelerations (not necessarily only those caused by gravity but also the deceleration of a braking car, for instance) in static analyses.

 

[zoltrix]

given a body you can distinguish :

- local forces -surface forces and body forces

the body forces are distributed overe the whole volume
in case of a static structure there is one only body force : the weight
in a machine you should add also the inertial forces changing over time and point to point

do you mean that FEM software can apply specific acceleration only such as centrifugal load or linear acceleration/deceleration or can they handle a general dynamic situation

are you familiar with Fusion 360 ?

 

[zoltrix]

in other words, take an assembly :

piston-rod-crankshaft

pston is subjected to linear acceleration/ deceleration so FEM analysis is possible
crankshaft is subjected to centrifugal forces again , it is ok
what about the rod ?
Can a FEM software calculate stress and displacement of the rod ?

 

[FEAnalyst]

With transient analysis using direct implicit time integration, it shouldn't be a problem to solve something like this. Not all FEA programs support such analyses but the standalone ones (like Abaqus or ANSYS) should handle it easily.

 

[zoltrix]

Impressive