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tube analysis

tube analysis

tube analysis

(OP)
can someone tell me how to calculate the best value (ie lowest weight for a given stress) value for t/r for a straight tube loaded: in pure tension, and pure bending?

My background is medical and I need this info for an assignment in Biomechanics.

RE: tube analysis

Not sure what t/r is (tube thickness to tube radius ration?), but here's what I would do:

In tension: Stress(s) = Axial Load(P)/Area(A), so if you know your stress level, solve for A. This is the area of the tube and would be equal to 0.25*PI(Do^2-Di^2). You will need to pick the inside or outside diameter and then solve for the other value.

For Bending, the thot process is similiar with just a different equation for stress. In Bending:
Stress = Applied Moment(M)*C/Moment of Area(I)

Moment is the force being applied to the tube at some distance. You will want to pick the worst case since it is a function of distance or they may have already specified the bending moment.

C is the distance the neutral axis to extreme fiber. In your case, it is a fancy way of saying the outside radius of the tube (Do/2).

I is the moment of area and for a tube cross section would be equal to PI*(Do^4-Di^4)/64.

Again, with a known stress, do some algebra and solve for one of the diameters.

If you need to solve the combined loading case, you can assume the stress are additive and get something like:

Total Allowed Stress = Axial Stress + Bending Stress

or another way:
S = P/A + M*c/I
Solving for a dia. will get alittle messy, but will work.

If you need some references, goto the library and look for Machinery Handbook or any undergrad engineering text on strength of materials

Post back if more questions.

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