stress relieving holes for octogonal plate

[moreliagtp]

Hi there!

I have an octogonal plate with a central hole, subjected to both compressive and tensile pressures. I've been trying to add stress relieving holes to the plate to reduce the stress. I've been using FEM to model it through trial and error. This is getting on my nerves - to do it without the understanding. I would like to seek the help from experts regarding stress relieving hole concepts such as size, location and force flow. any exerts able to help? thanks!

The picture of the plate with applied pressure can be seen below.

 

[rb1957]

i'd suggest placing holes in-line with the 150kPa load ... you're trying to create a larger radius in the stress flow-lines to get a lower Kt.

 

[trainguy]

Shouldn't the central hole be elliptical, in line with the larger load? That should help. Also, it's not clear what your pressures are. Are they normal stresses? How are they applied to those edges?

tg

 

[moreliagtp]

Thanks for the replies! I'll do some simulation with the suggestions and analyse the results.

Trainguy - the loads can be considered uniformly distributed forces.

 

[dhengr]

With out any dimensions and some explanation or understanding of what you’re trying to accomplish it’s tough to give you a good answer. The proportions in your sketch are all wrong for relieving holes to be an effective solution. You just don’t have enough length along the axis of the 150kPa stress lines to install the right size and number of holes; because the hole diameter is about the same as the length of the loaded edge, and the hole is to close to the loaded edge. You didn’t answer Trainguy’s most important question, and that was, ‘could the hole be elliptical in shape with its long axis parallel to the 150kPa stress lines.’ That would improve the max. stress a little bit, but be a limited solution too. I’d treat that as a heavy walled ring, whatever its thickness out of the page.

Is this a homework assignment question? Have your Prof. post his comments and suggestions.

 

[hokie66]

If the stress is only 150 kPa, why would you want to relieve it? That's nothing for a structural material.

 

[Ron]

hokie66...I thought we warned you about interjecting common sense into these discussions!

moreliagtp...an elliptical hole will change the stress distribution, but will increase in some areas and decrease in others. You will likely see an increase in shear stress near the "corners" of the ellipse. This will increase the tendency for "oil canning" if the plate is thin.

I would suggest that you first consider an octagonal hole within the plate. Compute the stresses across the "flats" in each direction. Size the interior octagon to get the stresses to levels you want, circumscribe a circle within the inner octagonal...you'll be close.

Be careful of buckling in the compressive direction if your plate is relatively thin.