Corrugated plastic is stiff one way and floppy the other

Anisotropy, and how one sheet has two completely different strengths.

Corrugated plastic is built from two thin, flat sheets held apart by a ridged internal wall running in a single direction, so it resists bending strongly along that direction, where the ridges act like a row of tiny beams holding the two faces apart, and bends far more easily across it, where there is nothing but the thin faces themselves to resist the load.

A fuselage folded from a single sheet of corrugated plastic held its shape confidently along its length, resisting a light push from end to end without any noticeable give, but the same sheet caved in sideways under barely any pressure at all, and the two directions felt like two entirely different materials rather than the same sheet turned ninety degrees.

What is actually happening

A flat sheet resists bending mainly through its thickness, since bending stress is concentrated at the top and bottom surfaces of a sheet and falls away toward the middle, so a thicker sheet resists bending far better than a thin one made from the same material. Corrugated plastic gets a similar advantage without the weight of a genuinely thick solid sheet, by holding two thin outer faces apart with a ridged wall running the length of the sheet, effectively spreading the material that matters most, the part furthest from the centre, across a much greater effective thickness than either face could achieve alone. Bent along the direction the ridges run, the whole structure behaves like a deep, lightweight beam, with the ridges holding the two faces rigidly apart and preventing them sliding past each other, which is what lets the sheet resist bending so strongly in that direction. Bent across the ridges instead, none of that support exists, since the ridges simply act as a series of hinges running parallel to the bend, each one folding slightly, and the whole sheet collapses into a gentle curve with very little resistance, no different in principle from bending a thin flat sheet with nothing behind it at all.

The paper-fan comparison

Folding a strip of paper back and forth into an accordion, the kind of pleated fan made as a simple craft project, produces the same split behaviour in an even more obvious form. Held by both ends and bent along its length, in the same direction the pleats run, the folded strip resists strongly, since each little pleat acts as a tiny rib holding the paper's overall cross-section rigid, exactly the way corrugated plastic's ridges hold its two faces apart. Try to bend or roll the same folded strip the other way, across the pleats, and it offers almost no resistance at all, flexing and curling easily because nothing about the folds does anything to stiffen that direction, the pleats simply hinge open and closed along their own length as the strip bends.

The one number worth remembering

For a given weight of material, a corrugated sheet can be many times stiffer bending along its ridges than a flat, unribbed sheet of the same total weight and thickness could ever be, since almost all of that extra stiffness comes from spreading the material further from the bending centreline rather than from adding more material overall, the ridges doing the same job a thicker solid sheet would do at a fraction of the weight.

What follows from this

Because the two directions behave so differently, a structure built from corrugated plastic needs its ridges oriented to run along whichever direction actually needs to resist bending, and getting that orientation backwards produces a part that looks identical to a correctly oriented one but performs nothing like it. It also explains why corrugated sheet is so easy to fold cleanly along a line running across the ridges, since scoring or creasing along that weak direction simply encourages the collapse that direction was already prone to, while the same sheet strongly resists folding along the ridge direction, requiring a proper cut rather than a crease to bend cleanly that way at all.