Holes, little circular ones. There are billions of them in engineering machines and structures. There are more than a million in a jumbo jet alone. Some of them are filled with fasteners, such as bolts and rivets, others are empty to allow fluids to flow through a surface. Load passing through a structure has to flow around holes, especially when they are empty, and the contours of stress bunch up around a hole to form a stress concentration. For a small hole in a very large plate, the stress on the circumference of the hole is three times the level found in the absence of the hole. This concentration increases for bigger holes or smaller plates, so that holes are a potential source of failure – that’s why sheets of stamps are perforated with lines of holes.
A hole can also stop a failure. For instance a crack extending under repeated loading will often stop when it grows into a hole because the ‘sharpness’ of the crack tip is blunted by the roundness of the hole. Engineers sometimes deliberately drill a hole at a crack tip to arrest its progress. So, holes can be both an engineer’s friend and foe.