concept

Hoop stress

The tension running circumferentially around the wall of a pressure vessel — internal pressure times radius, divided by twice the wall thickness. The formula is short and its biological consequence is brutal: as an animal grows, the volume of the cavity it pressurises rises with the cube of length while the thickness of its muscular wall rises only linearly, so hoop stress climbs until it meets the maximum a muscle fibre can generate, around 2.5×10⁵ Pa in invertebrate tissue. At that point a giant squid has two options and both are surrenders: contract more slowly, or widen the siphon. Either way peak thrust falls.