flat voltage or flat deformation

[africagia]

Hello.
I would like to expose you a perplexity. how do you see "eye" if a component is in flat tension or flat deformation. I know the difference at the theoretical level but at the practical level it is not very clear to me, I use a lot of time. I wanted something from you that you have a lot more experience than me.
Thank you.

 

[MacGG]

Hi aphrigia,

I imagine you have the results of a structural calculation to the finite elements and then you can view the main tensors of the voltage or deformation state on your fe model.

to this point if, to the eye, you see that a component, always has the same direction and is very small, or better you see that the other two components are always on parallel planes (because the small component can't possibly see if the tensors graphically are proportional to their value), then you should be, to the eye, in a flat state.

I hope I made myself understand.

Hi.

♪

 

[africagia]

Thank you. You made yourself understand. the problem, however, is that I should understand whether it is flat tension or flat deformation before doing the fem analysis to choose whether to use elements with flat tension or flat deformation.

 

[cacciatorino]

I'm sorry to bother you, but if you have five minutes, would you explain to me the difference between these two states of stress?

I would think that since the tension proportional to deformation is indifferent to the one or the other, then what does the need to distinguish?

 

[Franz 2.0]

mah..

 

[africagia]

Are you kidding me? I don't know what you are (I follow the forum and I know you're all outside of an ignorant) I try to explain the difference between these two states of solicitation.

the problem arises to move a structural problem from 3d to 2d. if you can try to always reduce the problem to 2d. It is possible to bring us back to the plane case if for example I have a solid with geometry and asymmetric loads or if I have a solid prismatic loaded evenly along the generators. regarding the last case we can distinguish the cases of flat tension and flat deformation.
I have that sigma_z, tau_xz, tau_yz are null but I have sigma_x, sigma_y and tau_xy non null for which the deformations are: epslon_x=1/e(sigma_x-ni*sigma_y); epslon_y=1/e(sigma_y-ni gammasigma_ the other ranges are obviously null.
when, instead, they are flat deformation I have that epslon_z, gamma_xz and gamma_yz are null. deformations are: epslon_x=1/e(sigma_x-ni*(sigma_y+sigma_z)); epslon_y=1/e(sigma_y-ni*(sigma_x+sigma_z); gamma_xy=1/g(tau_xy); other ranges are null. epslon_z should be epslon_z=1/e(sigma_z-ni*(sigma_y+sigma_x) but being flat deformation it is nothing, so there will be a voltage for polka dot effect on z given by sigma_z=ni*(sigma_x+sigma_y).
I hope I've been clear.

 

[cacciatorino]

the problem arises to move a structural problem from 3d to 2d. if you can try to always reduce the problem to 2d. It is possible to bring us back to the plane case if for example I have a solid with geometry and asymmetric loads or if I have a solid prismatic loaded evenly along the generators. regarding the last case we can distinguish the cases of flat tension and flat deformation.
I have that sigma_z, tau_xz, tau_yz are null but I have sigma_x, sigma_y and tau_xy non null for which the deformations are: epslon_x=1/e(sigma_x-ni*sigma_y); epslon_y=1/e(sigma_y-ni gammasigma_ the other ranges are obviously null.
when, instead, they are flat deformation I have that epslon_z, gamma_xz and gamma_yz are null. deformations are: epslon_x=1/e(sigma_x-ni*(sigma_y+sigma_z)); epslon_y=1/e(sigma_y-ni*(sigma_x+sigma_z); gamma_xy=1/g(tau_xy); other ranges are null. epslon_z should be epslon_z=1/e(sigma_z-ni*(sigma_y+sigma_x) but being flat deformation it is nothing, so there will be a voltage for polka dot effect on z given by sigma_z=ni*(sigma_x+sigma_y).
I hope I've been clear.

Thank you very much for the explanation. so live flat mainly I do not have sigma_z, so I have an epsilon_z. instead in flat deformation I do not have epsilon_z and therefore to have the constant thickness of the element I must assume a sigma_z that contrasts me poisson.
Okay, but if I got it right, then we're talking about shell elements, not 2d pure, right? otherwise if we were really in a 2d domain we would not have this problem.

 

[africagia]

No. are plain stress or plain strain elements (depending on the case). semi-structural elements such as shells are used for thin wall structures.

 

[cacciatorino]

No. are plain stress or plain strain elements (depending on the case). semi-structural elements such as shells are used for thin wall structures.

http://en.wikipedia.org/wiki/stress_(mechanics)#plane_stressHere's something, basically if I understand the flat deformation you have on the cross section of long structures (where z is practically the axis of the curvilinear section).

 

[luigi.paiano]

Excuse me, what do you mean by ''see'' if the stress or deformation status is slow?? ? What do you have at your disposal?what data?

 

[luigi.paiano]

because if, for example, you have the tension field, the resolution is very simple

 

[africagia]

thanks hunting...I realized that if a size is much smaller than the other two we are in plain stress instead, if a size is much larger than the other two we are in plain strain.

hegi.paiano: How have I already said I don't know the field of tension, otherwise what's the point of making a fea? If I already know everything I do to make a fea?

 

[cacciatorino]

thanks hunting...I realized that if a size is much smaller than the other two we are in plain stress instead, if a size is much larger than the other two we are in plain strain.

Thinking about it, I think I'm not, or at least it's not enough.... otherwise even an inflexible beam should be in flat deformation, which obviously absurd.

If you notice, the figure of the link shows that the external forces are transversal to the longitudinal axis of the beam, for that the long z deformation is neglected. Besides, it's a hypothesis similar to what you do with the tension states, where you have to impose as a hypothesis of not having direct external forces along z.

 

[luigi.paiano]

So what are the data? depends on the type of situation.for example in fracture mechanics you can immediately understand in what situation you are in. In other cases there is a need for an estimate of stress, I see no other solution