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FEA question

11/01/2009 7:53 AM

Hey all, I have a basic question on FEA.

Let's say i want to do an FEA analysis on a rectangular beam (one end of the beam is fixed to a wall) the loads on the beam is its weight and a moment about the beam's longitudinal axis. How does the FEA program go about and determines the value of the high stress region in this problem?

I believe that the stresses will be at its maximum at the connection point between the beam and the wall. Please see if my following prediction is correct.

The stresses will be induced by two thing, the weight of the beam which will create a moment at the beam to wall connection, creating a bending stress (My/I). Second stress will be the shear stress due to the moment about the axis (which will be torque), and this can be calculated by Tr/J. With shear and bending stress, we can use Mohr circle to calculate the principal stress, where sigma x will be the bending stress and Tau will be the shear stress. And the biggest principal stress will be the maximum stress in the system, which will be acting at the connection between the wall and beam.

But how does FEA does the above when the whole beam is split into little elements?? If anyone know where i can find an example of how FEA results are calculated by those programs, it will be very helpful!

Thanks!

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#1

Re: FEA question

11/01/2009 12:58 PM

Another question i have is that how do we do stress averaging (defining membrane stress from normal stress) by hand?

Thanks again!

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#3
In reply to #1

Re: FEA question

11/02/2009 1:51 AM

This is a very simple problem that would be explained in any elementary volume on strength of materials. Roarke's "Stress and Strain" comes to mind. Check out the web site Engineer's Edge for some basic beam formulas...

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#2

Re: FEA question

11/02/2009 1:46 AM

Essentially, the FEA program works element by element from the boundary conditions you set. The elements at the free end will have no stress, but the neighboring elements must support the weight of these elements. This you will have shear stress between the rows. The next row of elements have to support two rows of elements towards the free end, etc., until you get to the supported end, which you have constrained to 0 displacement. Each element will also see a bending moment for the same reason (except at your free end). This will result in the upper elements having a tensile stress, and the lower elements having a compressive stress. Somewhere near the center of the beam will be a region where only sheer stresses apply. The maximum stress will be at the supported end, but you also have distributed stresses throughout the beam. The program actrually uses linearized partial differnetial equations relating displacement to stress.

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#4
In reply to #2

Re: FEA question

11/05/2009 2:58 PM

Great answer. The only thing I would add is that the density of the elements (in this simple beam problem) determines the precision and processing time; ie, higher density results in longer processing time as the program has to iterate through more elements, but the precision of the result is increased.

Hooker <--- not a FEA expert but I trained a lot of people in the modeling aspects on various CAD systems

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