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Question on mechanics of material

It has been several years since i last worked on strength of materials, and i can't be certain of what seem to be a simple problem:

Suppose i have a rectangle block. Inside the block is a rectangle cavity with the long side horizontal. Consider the secanarios whereby the interior cavity is rotated 30, 45 , 60 and 90 degrees. In which case is the compressive strength the highest?

Since the max shear is at 45 degrees, the weakest block should be the one with the cavity align at 45. Right? And 30 and 60 should be the same??

philip

Reply to
Philip
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There is only an axial compressive load, right?

If so, the block will be strongest when the cavity runs lengthwise and is the narrowest, e.g. 90 deg. The net section axial stress will be the lowest then. The block will be weakest when the cavity is horizontal.

Reply to
Jeff Finlayson

The block with the cavity oriented at 90 degrees provides the thickest twin beams to carry compressive load. The face between the 'beams' might fail by crushing though.

Brian W

Reply to
Brian Whatcott

To add on, The only applied force is acting on the top surface in a downward direction so that there is only axial compression and no shear.

I was assuming that the failure mode would be shear failure. What I really want to ask if its stupid to put the cavity at a 45 degree angle if tilting it up or down a little will prevent premature shear failure.

I understand the reasoning of jeff and brian. Actually, the cavity runs all the way through the block in the x-dir (i left this detail to keep it simple) and most of the compression is taken by material in the out-of-plane direction. In this geometry, tilting the cavity 90 degrees of course dosen't make sense.

Reply to
Philip

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