Relationship of torsional constant to polar moment of inertia.
Relationship of torsional constant to polar moment of inertia.
(OP)
Does anyone know the relationship of the torsional constant to the polar moment of inertia for other than circular cross sections? I am trying to compare several different cross sections using a software package that only gives me polar moment of inertia. Is it safe to say that as the polar moment increases the torsional constant increases some proportional amount?
Thanks!
Aaron W.
Thanks!
Aaron W.






RE: Relationship of torsional constant to polar moment of inertia.
T = QKG/l, where Q = angle of twist, G is shear modulus of material, and l is the length of the bar
"For a circular section, K is the polar moment of inertia J; for other sections K is less than J and may only be a small fraction of J."
Primarily this (reduction in torsional stiffness) is due to the warping of plane sections that occurs when non-circular bars are twisted.
RE: Relationship of torsional constant to polar moment of inertia.
RE: Relationship of torsional constant to polar moment of inertia.
There are analytical approaches which may work, the membrane analogy is one, adding up the K for each plate in an I beam is another, conservative one.
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Relationship of torsional constant to polar moment of inertia.
RE: Relationship of torsional constant to polar moment of inertia.
Rentrant sections, or even closed ones with slightly silly shapes, can be very poor in torsion.
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?