Cylinder stress  In mechanics, a cylinder stress is a stress distribution with rotational symmetry; that is, which remains unchanged if the stressed object is rotated about some fixed axis. Cylinder stress patterns include:  The classical example (and namesake) of hoop stress is the tension applied to the iron bands, or hoops, of a wooden barrel. In a straight, closed pipe, any force applied to the cylindrical pipe wall by a pressure differential will ultimately give rise to hoop stresses. Similarly, if this pipe has flat end caps, any force applied to them by static pressure will induce a perpendicular "axial stress" on the same pipe wall. Thin sections often have negligibly small "radial stress", but accurate models of thicker-walled cylindrical shells require such stresses to be taken into account. The hoop stress is the force exerted circumferentially (perpendicular to the axis and the radius of the object) in both directions on every particle in the cylinder wall. It can be described as: where:  An alternative to "hoop stress" in describing circumferential stress is wall stress or wall tension ("T"), which usually is defined as the total circumferential force exerted along the entire radial thickness: Along with axial stress and radial stress, circumferential stress is a component of the stress tensor in cylindrical coordinates. It is usually useful to decompose any force applied to an object with rotational symmetry into components parallel to the cylindrical coordinates "r", "z", and "θ". These components of force induce corresponding stresses: radial stress, axial stress and hoop stress, respectively. For the thin-walled assumption to be valid the vessel must have a wall thickness of no more than about one-tenth (often cited as Diameter / t > 20) of its radius. This allows for treating the wall as a surface, and subsequently using the Young–Laplace equation for estimating the hoop stress created by an internal pressure on a thin-walled cylindrical pressure vessel:  where  The hoop stress equation for thin shells is also approximately valid for spherical vessels, including plant cells and bacteria in which the internal turgor pressure may reach several atmospheres. In practical engineering applications for cylinders (pipes and tubes), hoop stress is often re-arranged for pressure, and is called Barlow's formula. Inch-pound-second system (IPS) units for "P" are pounds-force per square inch (psi).