Stiffness, strength and toughness
Three words get used as if they mean the same thing, and mixing them up is behind a lot of parts that fail in ways their maker didn't see coming. Stiffness, strength and toughness are three different properties, they don't come bundled together, and a material can be excellent at one while being poor at another. PLA is the classic trap: it feels strong because it's stiff, and then it shatters the first time it's dropped. Separate the three cleanly and you stop being surprised.
Three different questions
Stiffness is how much a part resists bending under load — the modulus from the datasheet. A stiff part barely moves when you push it and springs back; a flexible one gives. Stiffness says nothing about breaking, only about deflection.
Strength is the load it takes before it actually fails — the point where it tears or cracks rather than just bends. A part can be stiff but weak, or flexible but strong.
Toughness is the energy it absorbs before it fractures — how much abuse it soaks up. This is the one PLA lacks. A tough material bends, dents, or stretches and stays in one piece; a brittle one holds its shape right up to the instant it snaps clean.
Picture the stress–strain curve. Stiffness is the steepness of the line at the start. Strength is how high it climbs. Toughness is the area under the whole curve — and that's why PLA and PETG can trade places: PLA's line is steep and tall but stops abruptly, so it's stiff and reasonably strong but the area underneath is small. PETG and nylon climb less steeply but keep going, so the area — the toughness — is far larger.
Choose by the mechanical demand
Match the property to what the part does. A bracket that must hold a sensor perfectly still wants stiffness — pick a high-modulus material and it won't sag. A hook that carries a heavy load wants strength. A phone case, a drone arm, a snap-fit clip — anything that gets dropped, flexed or shock-loaded — wants toughness, and there PLA's stiffness is a liability, not a virtue. "Strong" isn't a single axis; decide which of the three you actually need before you pick the spool. Strength and structure goes deeper on turning that demand into walls, ribs and infill.
The anisotropy reminder
There's a catch that ties straight back to the datasheet: every one of these three properties is lower across the layer lines than along them. An FDM part is glued together in Z, and that glue is the weak direction. A material that's genuinely tough in the lab becomes brittle the moment you load it across its layers, because the beads peel apart before the plastic itself gets a chance to stretch. So the material number and the print orientation are not separable decisions — a "strong" material loaded the wrong way is a weak part. Orient the part so the load runs along the layers, not across them; How FDM Shapes Your Design is the whole story of designing around that grain.
Stiffness, strength and toughness cover how a part behaves under force. But force isn't the only thing the world throws at it — heat and sunlight quietly defeat parts that were mechanically perfect, which is where Temperature, UV and environment picks up.