Load cases in FDM parts

A part almost never fails from one abstract idea of load. It fails from a specific combination of tension, bending, torsion, shear, impact or fatigue, acting on a layered structure that responds differently to each. Naming the actual load case correctly before choosing material or geometry saves a redesign cycle later, because a fix that helps under bending can do nothing for fatigue, and a material chosen for tensile strength can still crack under impact.

Tension and compression

Pure tension pulls a section apart along its axis. In an FDM part, tension in-plane is carried well by the perimeters and by fibre-reinforced grades such as PA12-CF, while tension across the layer stack is limited by interlayer bond strength regardless of material. Compression is more forgiving: most FDM materials handle compressive load well, and infill contributes more here than in any other load case because a supported, distributed pattern resists crushing reasonably even at moderate density.

Bending

Bending puts one face of a section in tension and the opposite face in compression, with the highest stress at the outer surfaces. This is the load case where section geometry matters most: increasing the height of a section in the bending direction, or adding a rib, raises stiffness far faster than adding wall thickness uniformly. Orientation matters too, printing the span so the outer fibres are in-plane rather than crossing layers avoids adding the weak interlayer bond exactly where stress is highest.

Torsion and shear

Torsion twists a part around its axis and produces shear stress that runs at roughly 45 degrees to that axis, which means it inevitably crosses layer lines no matter how the part is oriented. This makes torsion one of the harder load cases for FDM, and it is one reason round or tubular sections under twist benefit from a wall thickness increase rather than an infill increase, plus a material with good shear strength such as PA12-CF or PC. Pure shear, such as a pin loaded across a hole, concentrates stress locally and again favours extra wall material and a generous edge distance over relying on infill.

Impact and fatigue

Impact loads the part suddenly and briefly, and the deciding factor is toughness, the ability to absorb energy before cracking, rather than peak strength. PLA is brittle under impact and tends to shatter along layer lines and at notches. PETG and PA are considerably tougher and absorb impact energy through more ductile deformation before failure. Fatigue is the opposite in timescale: repeated, moderate cycles that grow a crack from a small surface flaw, often a sharp corner or a layer line acting as a stress concentrator. PA and PA12 grades hold up well under cyclic loading because of their ductility, while brittle materials and sharp geometry are a poor combination for anything that flexes or vibrates repeatedly.

Notch sensitivity and creep

Notch sensitivity describes how much a sharp feature, a hole edge, a lettering detail, an internal corner, amplifies local stress compared with the nominal stress in the part. Brittle materials such as PLA are highly notch sensitive, so a small unfilleted corner can trigger failure at a load well below what the bulk material could take. Tougher materials such as PA and PC are more forgiving, but a fillet is cheap insurance regardless of material and should be standard practice at every internal corner in a loaded part.

Creep is a separate concern: under a sustained load, especially combined with elevated temperature, thermoplastics slowly deform even below their normal yield point. This matters for parts that stay loaded for weeks or months, such as a bracket under constant spring tension or a housing near a heat source. PLA and PETG creep noticeably above roughly 50 to 60 degrees Celsius, while PC and PPA-CF retain dimensional stability to significantly higher temperatures. For any sustained-load application, we ask about both the load duration and the ambient temperature before recommending a material.