Plate Forces - the My and Mx directions seem reversed

[Stazz]

When I view the moments in plate elements it looks like the My and Mx moments are backwards. When I look at the local axis of the plates, I learned that the appropriate moments should spin around/about the axis, for example Mx should twist about the x-axis but according to Risa 3D this Mx would spin in the X-Z plane. This is hard to describe. Anyone else see this?

 

[Stazz]

Ok I just looked at help. Duhh. It makes sense now but it's a dumb way that's probably creating a lot of confusion. Here is what help says...

The plate bending moments (Mx, My and Mxy) are the plate forces that induce linearly varying bending stresses through the thickness of the element. Mx is the moment that causes stresses in the positive x-direction on the top of the element. Likewise, My is the moment that causes stresses in the positive y-direction on the top of the element. Mx can then be thought of as occurring on element faces that are perpendicular to the local x-axis, and the My moment occurs on faces that are perpendicular to the local y axis. To calculate the total Mx or My on the face of an element, multiply the given value by the length of the element that is parallel to the axis of the moment. For example, looking at the 'Plate Moments' figure above, the total Mx moment could be obtained by multiplying the given Mx force by the length of side BC (the distance from joint B to joint C). The total My force can be calculated in the same way by instead using the length of side DC.

 

[JoshPlumSE]

This is always a source of confusion with plate theory. But, it is standard notation for plate theory. All FEM programs should use the same basic notation.

If you go back to a university (Master's level?) class on plates then an Mij moment would be defined as described below.

The first subscript (in this case 'i') would define the plane on which the stress is acting. In this case the plane should be perpendicular to the 'i' axis.

The second subscript designates the direction of the stress (in this case 'j'). Therefore, the stress is then acting in a direction parallel to the local j axis.

Typically plate theory consists of 4 of these moments (Mxx, Myy, Mxy and Myx). But, Mxx and Myy are usually shortend to just Mx and My. Then for isotropic plates Mxy and Myx are always equal. Therefore, we just report Mxy.

 

[Stazz]

Oh Yeah, I remember that now. Its a battle between two different approaches.

The exterior approach wants to keep the moment consistent with rotation about the plate axes.

But the interior approach wants to keep the direction consistent with the equation Mx = SUM Stress in 'X' direction x moment arm x area so that x appears on both sides of the equasion.

And this is where they bump heads and give me a head ache.