Symmetry- stupid question

[lavecchiasignora]

Hi all,

I have a structure and I only model half of the structure due to symmetry. The structure is exposed to an impact of an sphere in the middle of the structure, this is modeled by a prescribed displacement of the sphere. . Due to this I only model half the structure and half the rigid sphere. If I want to know the reaction force on the symmetry line of the structure .. will the reaction force be twice the plotted reaction force that Workbench says because it is only half of the model? E.g. if workbench says 50 kN as the reaction force on the symmetry line will the actual reaction force on the whole structure be 100 kN?

Thank you

 

[corus]

Yes. Just the same as if you applied a force on a symmetry line it would be half the total.

Tara

 

[lavecchiasignora]

Are you sure? Because when I look at the stresses on that line it wont be twice the stress?! I thought symmetry was only used to reduce calculation and stuff like that. For example if I have a roof which can be divided into 4 quadrants and in the middle of the roof a point force of 10 kN is applied. If I use symmetry condition and only model one fourth I still use 10 kN and not 10kN/4=2.5kN?

 

[rb1957]

if you've got 1/2 a model, the force required to deflect it is 1/2 that of the full model (which would have double the strain energy). and, yes, both the 1/2 model and the full model would have the same stress levels (the full model has 2x the volume under strain.

force of P on a 1/2 model is equivalent to 2P on the full model, 'cause the force is being applied along the plane of symmetry.

 

[flash3780]

Is it appropriate to apply a displacement to an impact problem? It seems as though an initial velocity would be appropriate.

 

[rb1957]

"If I use symmetry condition and only model one fourth I still use 10 kN and not 10kN/4=2.5kN?" ... 1/4 would be appropriate IMHO ...

 

[lavecchiasignora]

Thank you all for your answer.

So, if the stresses of the full model and the half model is the same I suppose the strains are the same as well.. ? I dont have to multiply the strains of the half model by 2 in order to get the strains for the full model if I got it correctly ?

 

[rb1957]

yep

 

[bkal]

Just one clarification, what do you mean by the 'reaction force on the symmetry line'. Is it a force between the sphere and the structure, or is it a force between two halves of the structure? Sorry if this is obvious.

 

[rb1957]

the load applied is along the plane of symmetry, so when you cut the model in two (along the plane of symmetry) 1/2 the load (and 1/2 the reaction) stay with either 1/2.

the internal loads between the two 1/2s are different (to the external load and reaction).