The way I read the research it was meant that force of bending a tube doesn’t change on internal tube pressure and it makes sense.GregK wrote: ↑Fri Jun 28, 2019 2:31 amLet me answer this in another way :
Two identical kites, one pumped to 4 psi, the other 12 psi.
Fly them in a very light breeze, the load on their airframes is quite light, both will deflect ( bend ) under that light load exact the same amount.
Now fly them in a strong wind. The load on the airframes is much higher, but not high enough for the highest load point on the 12 psi kite to reach the buckling point ( buckling is like kinking, excessive bending all in a small area versus the gradual deformation of bending deformation ).
Different story with the 4 psi kite, that highly-loaded point has exceeded the buckling point and the LE will fold or kink.
So depending on how you interpret "resistance to bending" the 12 psi kite will seem more resistant. Yes it can resist higher bending loads & deflection without kinking or buckling, but the rate of deformation, which is the true definition of stiffness, is the same.
Techin' out on ya, eh ?
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.
But amount of initial force required to start bending it would depend on the pressure and stiffness would change. And larger tube diameter takes less pressure to be more stiff.
When I had 2012 Rallys and started boosting on them it took higher pressure to prevent eartips folding on me in the middle of very powered jumps. Then I stopped abusing poor kites and foiled on them replacing 2012 Rally with more structured later model.
Nice discussion, thanks for coming back to the topic!