GregK wrote: ↑
Fri Jun 28, 2019 2:19 am
... US army researchers investigated the bending stiffness of inflated tubular structures, measured it, and confirmed a theoretical analysis of a pressurized thin-wall structure subjected to bending : the stiffness is independent of inflation pressure, both in theory and in practice. ...
CORRECTION - I went back and reviewed the test results and simulation/modelling work reported in these articles, and my summary above is not correct.
While their analysis and test results confirmed the linear dependence of the onset of buckling ( or wrinkling as they called it ) with respect to inflation pressure, their test results also clearly showed that increased inflation pressure does increase the bending stiffness of woven fabric inflated structures.
It was not a linear relationship, in the first test reported ( a 6-inch diameter un-coated plain-woven Vectran fabric air beam with a 2:1 ratio of weft-to-warp fiber density ) in the low-load region of the test below the onset of wrinkling, bending stiffness increased with pressure with slightly less than a linear dependence, about a 0.7 order or power relationship.
In their second test series using a smaller 2-inch diameter air beam with everything else similar to the first series, the bending stiffness increased with very close to a linear relation to inflation pressure, about 0.9 order.
And in their third series of tests, only two test pressures were used, so not enough data from which to determine a relationship, but definitely an increase in bending stiffness with the higher inflation pressure.
It is an over-simplification to assume plain-woven fabric will act with isotropic in-plane properties. When inflated, the axial or hoop load in the fabric will be twice the axial load, creating shear stress, to which the woven fabric's response is complex ( weft-warp fiber slip & friction, weft-warp fiber angular shift ) and non-linear, not a function of material properties, but rather a system property. Also increased inflation pressure alters the weave crimp, the undulating pattern of fibers running over and under adjacent fibers in the weave. Alterations in crimp geometry, termed crimp interchange " is a source of nonlinear load-extension behavior for fabrics " - a direct quote from the first article.
So go ahead and pump harder, your kite does bend less with higher inflation pressure.