- Elastic Membrane Analogy
The

**elastic membrane analogy**, which was first published by pioneering aerodynamicistLudwig Prandtl in 1903, [*Prandtl, L.: "Zur torsion von prismatischen stäben", Phys. Zeitschr.,*] [**4**, pp. 758-770 (1903)*Love 1944, article 224, page 322.*] describes the stress distribution on a long bar in torsion. The cross section of the bar is constant along its length, and need not be circular. Thedifferential equation that governs the stress distribution on the bar in torsion is of the same form as the equation governing the shape of a membrane under differential pressure. Therefore, in order to discover the stress distribution on the bar, all one has to do is cut the shape of the cross section out of a piece of wood, cover it with a soap film, and apply a differential pressure across it. Then the slope of the soap film at any area of the cross section is directly proportional to the stress in the bar at the same point on its cross section.**Application to thin-walled, open cross sections**While the membrane analogy allows the stress distribution on any cross section to be determined experimentally, it also allows the stress distribution on thin-walled, open cross sections to be determined by the same theoretical approach that describes the behavior of rectangular sections. Using the membrane analogy, any thin-walled cross section can be "stretched out" into a rectangle without affecting the stress distribution under torsion. The maximum shear stress, therefore, occurs at the edge of the midpoint of the stretched cross section, and is equal to $3T/bt^2$, where T is the

torque applied, b is the length of the stretched cross section, and t is the thickness of the cross section.**Notes****References***cite book | last = Bruhn | first = Elmer Franklin

title = "Analysis and Design of Flight Vehicle Structures

publisher = Jacobs Publishing | date = 1973 | location = Indianapolis

pages = | url = | isbn = 0-9615-2340-9

*cite book | last = Love | first = A. E. H.

authorlink = Augustus Edward Hough Love

title = A Treatise on the Mathematical Theory of Elasticity

publisher = Dover | date = 1944 | location = New York

pages = | url =

isbn = 0-486-60174-9. Especially Chapter XIV, articles 215 through 224. "This Dover edition, first published in 1944, is an unaltered and unabridged republication of the fourth (1927) edition."**External links***cite web | last = Lagace | first = Paul A.

title = Unit 10 St. Venant Torsion Theory

work = Structural Mechanics Fall 2002 | publisher = MIT

date = 2001

url = http://ocw.mit.edu/NR/rdonlyres/Aeronautics-and-Astronautics/16-20Structural-MechanicsFall2002/088DEC8D-4ACB-4E3E-9911-F3BB5EB6AF89/0/unit10.pdf

format = pdf | accessdate = 2008-07-03*cite web | last = Lagace | first = Paul A.

title = Unit 11 Membrane Analogy (for Torsion)

work = Structural Mechanics Fall 2002 | publisher = MIT

date = 2001

url = http://ocw.mit.edu/NR/rdonlyres/Aeronautics-and-Astronautics/16-20Structural-MechanicsFall2002/4BA039AA-752A-47C0-B037-8D379615F602/0/unit11.pdf

format = pdf | accessdate = 2008-07-03

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