A review of the fundamentals are in order here. At any one station along the wing, you have lift accumulating from the tip towards the station as shear, lift outboard accumulating as bending moment, and pitching moment of the foil accumulated from the tip to the station as torsion.

When you apply bending, it bends the entire cross section of the wing. The strain is directly proportional to how far you are above or below the neutral axis of the cross section. Everything sees the strain and the stress from this, with the strain growing linearly from zero at the neutral axis to max strain at the place on the cross section where you are furthest from the neutral axis. Likewise, the torsion accumulated twists the cross section about the centroid of the cross section, and the max strain from torsion is seen at the biggest radius from the centroid. Lesser trains are seen at smaller distances from the centroid. These two effects at every point around any cross section are superimposed.

Now the load goes where the stiffness is, so a beam sized to carry the load with a rather thin wing skin should still carry a large fraction of the bending, but the torsion is largely carried by the skins. And the skins are also seeing significant stresses. Our standard airplane design textbooks talk about crippling stresses when thin elements are under compression and have ways of calculating if those then elements will buckle.

I suggest that you do some research and then some calculations on your skins. Calculate the total stress state in the skins at a couple places and see if the crippling predictors say you skins should be buckling at this load, and is crippling is likely at full load. If yes, please do not get tempted to increase load and see what happens. If the calcs say you need more stiffness in the skins to get to full load, you might as well know it and get on it now, then resume test.

If you do the crippling checks and every thing checks out OK, but you are getting these wrinkles anyway, you might be OK. Just know that the wrinkled skin is now carrying a smaller fraction of the wing bending that it was, putting more strain and thus stress on the other parts that are still in shape. So, if your design passes the crippling check, I would look into how high the crippling stresses are in the various bays of the wing. Then think about stiffening all of the panels that calculate out as having stresses at similar fractions of the crippling strength before you put any more load on your test article.

Billski