Why things fail: fatigue, creep, and stress concentration
Most parts do not break from one big load — they break from small loads repeated, sustained, or concentrated.
Fatigue: death by repetition
Bend a paperclip once and it survives; bend it twenty times and it snaps at a stress far below its strength. That is fatigue — microscopic damage accumulating under repeated loading. It is the leading cause of mechanical failure in service, which is why aircraft count flight cycles, not just years.
The de Havilland Comet, the first jet airliner, taught this lesson tragically in the 1950s: repeated cabin pressurization grew cracks from the corners of its square windows until wings-off failure. Every airliner window has rounded corners because of it.
Stress concentration and creep
Force flowing through a part behaves like water flowing in a river: any sharp corner, hole, or notch forces the flow to crowd, and local stress can triple. Cracks are born at concentrations — so engineers round corners, polish surfaces, and drill 'crack-stopper' holes at the tips of existing cracks.
Creep is failure in slow motion: under constant load, materials — polymers especially, and anything near its softening temperature — keep stretching over months and years. A plastic shelf sags permanently; a bolted plastic joint quietly loses its clamping force. The viscoelasticity simulation on this site shows the physics behind it.
How composites fail differently
Metals usually fail by one crack growing. Composites fail by committee: matrix cracks appear first (often harmlessly), then plies begin separating (delamination), fibers snap one by one, and finally a whole region lets go. First-ply failure — what the laminate calculator reports — marks the conservative start of that sequence, which is exactly what you want for design margins.
The failure envelope simulation draws the boundary of survivable stress combinations for one ply. Everything inside the ellipse is safe; the shape itself encodes how pulling in one direction changes what the material can bear in another — a genuinely composite phenomenon that has no metal equivalent.
Force flows like water: squeeze it past a hole and it crowds (red lines). That factor-of-three crowding is why cracks are born at holes, notches, and sharp corners — and why airplane windows are round.
The load never changes — the strain grows anyway as chains slide. Run the viscoelasticity simulation to get this curve quantitatively.