Reading Shear Loads on BAR elements

[legersalazar]

Hi,

I'm analyzing some fasteners modeled as bar elements, therefore I need to determine the tension and shear on the fastener.

FEMAP has output vectors shear forces (in both plane1 & plane2), this is confusing to me. Is this in both directions perpendicular to the bar?

I mean there is only one plane that goes across a cylinder, the other two are axial (not shear).

Thanks,
W

 

[rb1957]

plane1 and plane2 are both transverse planes ... one direction is along the beam and the other one of the transverse axes.

the tird plane is of course the cross-section, yes?

Quando Omni Flunkus Moritati

 

[legersalazar]

This is confusing, let me try to clarify.

Let's assume the cross-section of the bar is a circle, hence the whole thing is a cylinder.

Axis 1 runs through the cylinder's axis. 2 and 3 are perpendicular to the axis of the cylinder.

Since this is a fastener, I'm interested in reading tension and shear. The shear loads should be in directions 2 and 3 which form a plane of its own that cuts perpendicular to Axis 1.

FEMAP has output vectors shear forces (in both plane1 & plane2), I wish these were the loads in directions 2 and 3 from my example, but I'm not sure. Does anyone has a good understanding of these output vectors?

Thanks,
W

 

[jotunn]

If I'm understanding correctly, I think you've got your shear directions a bit mixed up. Don't confuse shear directions with the cross section of the beam that the shear plane would create.

In your example, to get the maximum transverse shear, you want the shear forces from both planes 1-2 and 1-3. Simple vector math will give the combined magnitude...from which max. shear stress is easy. There is no 2-3 shear for a 1-D beam/bar element...

Stated another way, ditch the computer, get out the pencil and paper, and draw the simple cantilever beam example problem from statics 101. Label your axes... 1 along the length of the beam, 3 up. The shear force due to the cantilever load is in plane 1-3.

Hope this helps...

 

[rb1957]

axis 1 is tension.

the shear in axis 2 and 3 is what FeMap (and NASTRAN) call plane 1 and 2. think of shear as in a panel, it affects the plane, not just a single direction (like tension). its the same with you fastener ... the shear is causing displacements in a plane. yes?

Quando Omni Flunkus Moritati

 

[legersalazar]

ok so if I understand correctly the shear force in plane 1 means the force direction is perpendicular to plane 1?

That's what I understood from your explanations and it would make sense as to why there is no such thing as shear in plane 3.

Hence, total shear would be SQRT( shear-plane1^2 + shear-plane2^2)???

Thank you

W

 

[rb1957]

no ... i think it's like this ...

plane 1 is defined by two axes, lets say 1 (axial) and 2 (transverse, up).
now a moment can cause deflection in plane 1 (ie the moment vector is normal to plane 1)
than moment can be a couple of shear forces, and the shear force direction would be along axis 2.
so plane 1 shear is along axis 2 and plane 2 along axis 3 ... clear as mud ??

look up the NASTRAN element library reference book, it shows the positive sign convention.

Quando Omni Flunkus Moritati

 

[jotunn]

Although your understanding was incorrect (as rb pointed out...with a neat explanation and way of thinking about it), your conclusion was correct. The total magnitude of the shear force vector is the square root of the sum of the squares.

 

[legersalazar]

ok, let me see if I get it right this time....

shear is caused by a couple (isnt that ironic?)

hence, the plane defined by FEMAP (1 or 2), is the plane where that couple is acting.

Is that it?

 

[rb1957]

again, not quite what i had in mind. i think about the plane definition in terms of bending, and then i tried to relate to shear (hence the couple comment).

maybe try this, typically shear distorts a square into a rhombus, yes? the square is defined by a plane ...

or your fastener will deflect due to the shear force, in the direction of the shear force, defining a plane ...

btw, why use a bar for a fastener ? why not an RBE and a CBUSH ?

Quando Omni Flunkus Moritati

 

[legersalazar]

yeah I think we're saying the same thing. In the case of a cylinder shaped fastener, your square would be the cross section of the cylinder if you cut it along the axis, that would be your square/plane that becomes a rhombus when deforming under shear.

si?

I'm using BAR elements because the customer is always right

 

[rb1957]

hear your pain, brother ... been there done that.

bars are a bit nasty, in that they fix the ends making it harder to get the shear stiffness you want

Quando Omni Flunkus Moritati