Sharp internal corners break parts

Stress concentration, and why cracks start where lines meet.

A sharp internal corner is where a part is most likely to crack first, because whatever load the part carries has to bend suddenly around that corner just as sharply as the part's own outline does, and squeezing the same amount of force through a smaller and smaller area, right at the point of the corner, raises the stress there far above what the rest of the part is carrying.

Every moulded object within reach turned out to be slightly tapered, and once that habit of looking became hard to switch off, the corners on the inside of most of the same objects turned out to be gently rounded too, never sharp, which was the first hint that neither feature was an accident.

The notch that opens a crisp packet

A small notch cut into the edge of a plastic packet, the kind built into the top of a crisp packet or a bag of sweets, lets the packet tear open cleanly from exactly that point the moment it is pulled, because the pulling force gathers at the sharp tip of the notch instead of spreading along the whole edge. The same plastic pulled from an edge with no notch resists tearing far more strongly, since the force has nowhere to concentrate and has to work across a much wider stretch of material. A sharp internal corner on a loaded part is an unintended version of that notch, giving stress a sharp point to gather at exactly where the part can least afford it.

Load crowding through a narrow point

A part under load carries force through its own material the way traffic moves through a road network, spreading out to use the whole width available wherever the shape allows it. At a sharp internal corner the outline suddenly changes direction, and the force running through it has to make the same sudden turn, so a disproportionate share of the load is forced through the small region right at the point of the corner. The material there carries far more than its fair share, and for an idealised corner with no radius at all, the calculated peak stress grows without limit as the corner gets sharper. Real materials give a little before that, but the material at the corner is still the first to reach the stress it can bear.

None of this requires the part to be loaded unusually hard. An ordinary load, well within what the rest of the part could shrug off, can still start a crack at the one point where it was never spread out, which is how a part designed with a healthy margin everywhere else manages to fail first at the one unrounded corner nobody treated differently. Repeated loading makes it worse, since a crack that starts at a corner grows a little with each cycle, long before the part as a whole looks anywhere near its limit.

Softening the turn

Because the concentration comes from the sharpness of the corner and not from the size of the load, the fix is nearly always the same whatever the part is for or what it is made from: replace the sharp corner with even a small rounded transition. It is the sudden change in direction that creates the problem, and softening that change, however slightly, brings the peak stress back down towards the level the rest of the part is carrying. This is also why so many everyday objects, handled and dropped and knocked about far more than any drawing accounted for, rely on rounded internal corners to survive treatment their designers never explicitly tested for. The rounded inside corners on a plastic storage box, a moulded chair or the housing of a kettle are there for the part's strength as much as for the look of it.