Carbon fibre is strong in one direction only
Why fibre orientation is the design, and what happens across the fibres.
Carbon fibre is strong in only one direction because its strength comes from thousands of individual fibres running the same way through a resin that merely holds them in place, so a laminate resists a pull running along those fibres enormously well and resists very little running across them, where the resin is doing all the work alone.
Laying up carbon fibre makes it obvious that the material does not exist until the moment you make it. A sheet of dry fibre cloth arrives as something closer to fabric than to anything that could be called strong, draping over a curved mould the way a piece of cotton would, and only once resin has been worked into it and cured does a rigid, load-bearing part exist at all. Before that point, every decision about which way the fibres run is still reversible, and after it, that decision is permanent.
Tearing parcel tape along and across
Pulling hard lengthwise on fibre-reinforced parcel tape, the kind with thin strands embedded inside the plastic, shows the behaviour in an object almost anyone has handled. Lengthwise, the tape resists a strong tug without stretching or tearing, because the strands run exactly along the pull and do essentially all the work. Tear the same tape sideways, across its width, and it gives way almost as easily as ordinary paper, because none of the strands run in that direction and the thin film left to resist on its own is too weak for the job. Nobody expects that tape to resist a sideways tear, since its one job, holding a parcel shut against a lengthwise pull, only ever asks it to work in the direction its strands run.
Fibres carry the load and resin holds them in line
A single carbon fibre is extremely stiff and strong along its own length, but it is only a few thousandths of a millimetre wide, and on its own it offers almost no resistance to being bent or pushed sideways, the way a single human hair resists almost nothing sideways despite being hard to snap end to end. A laminate bundles enormous numbers of these fibres into a sheet, all aligned in a chosen direction, and binds them with resin so that a load applied along that direction is shared across every fibre at once. The resin adds little strength along the fibres. Its job is to keep them aligned, pass load between them, and hold the part in the shape it was moulded into. Pull across the fibres instead, and the resin, a much softer and weaker material, is the only thing resisting the load.
Ten times stiffer one way than the other
A typical sheet of carbon fibre and epoxy with every fibre running one way is roughly ten to fifteen times stiffer along the fibres than across them. A sheet of aluminium or steel has the same stiffness whichever way it is pulled. That gap is the reason the material is chosen, since it lets a designer put stiffness and strength exactly where a load needs to travel and save weight everywhere else, instead of paying for uniform properties in every direction as a metal sheet does. It also means a laminate's datasheet value cannot be treated as one figure for the whole part, because the number on the sheet almost always describes performance along the fibres and says very little about what happens across them.
Stacking layers at different angles
Laying up a composite part is therefore an exercise in predicting every direction a real load will travel through it and making sure fibres run that way to meet it. A single sheet with every fibre running one way handles a load from one direction extremely well and almost any other load badly, so real structural laminates are built from several layers, each with fibres at a different angle, so that whichever direction a load arrives from, some layer in the stack is oriented to resist it. A common stack uses layers at zero, ninety and plus and minus forty-five degrees, trading some of the peak stiffness in any single direction for a part with no badly weak direction.
A finished carbon part can look identical from the outside while behaving completely differently under load, depending on what is hidden beneath its surface. Two panels with the same thickness, the same resin and the same visible weave can carry fibre stacks oriented in entirely different directions, invisible to anyone who has not seen the drawing that specified how each layer was meant to be laid, and the parcel tape would look just as convincing held up against a sideways tear as a lengthwise one, right up until someone pulled it.