Velcro is a brand name of fabric hook-and-loop fasteners used for connecting objects.
Velcro was invented in 1948 by George de Mestral, a Swiss engineer. The idea came to him after he took a close look at the seed pod burrs which kept sticking to his dog on their daily walk in the Alps. De Mestral named his invention after the French words velours, meaning 'velvet', and crochet, meaning 'hook'.
Velcro consists of two layers: a "hook" side, which is a piece of fabric covered with tiny plastic hooks, and a "loop" side, which is covered in equally tiny plastic loops. When the two sides are pressed together, the hooks catch in the loops and hold the pieces together. When the layers are separated, the Velcro strips makes a telltale ripping sound. Since the name Velcro is a registered trademark, generic implementations often use the name "hook and loop", though in common usage, "velcro" is used generically.
The strength of the Velcro bond depends on how well the hooks are embedded in the loops and the nature of the force pulling it apart. If Velcro is used to bond two rigid surfaces, e.g. auto body panels and frame, the bond is particularly strong because any force pulling the pieces apart is spread evenly across all hooks. Also, any force pushing the pieces together is disproportionally applied to engaging more hooks and loops. Vibration can cause rigid pieces to improve their bond.
When one or both of the pieces is flexible, e.g. a pocket flap, the pieces can be pulled apart with a peeling action which applies the force to relatively few hooks at a time. If a flexible piece is pulled parallel to the plane of the velcro surface the force is spread evenly like with rigid pieces.
There are three ways to maximise the strength of a Velcro bond with one or more flexible pieces:
ensure that the force is applied parallel to the plane of the Velcro surface, e.g. bending around a corner or pulley.
increase the area of the Velcro bond, e.g. long purse straps.
use a pulley system, e.g. shoe closures.
Shoe closures can resist a large force with little bonding area by wrapping a strap through a slot which reduces the force on the Velcro by ensuring the force is parallel to the plane of the Velcro and by halving the force on the Velcro bond by acting as a pulley system.
Velcro has several deficiencies: it tends to accumulate hair, dust, and fur in its hooks after a few months of regular use and the loops can become elongated or broken. It often becomes attached to articles of clothing, especially loosely-woven items like sweaters. Additionally, the clothing may be damaged when one attempts to remove the Velcro, even if they are separated slowly. The tearing noise made by unfastening Velcro makes it inappropriate for some applications, but is useful against pickpocketing.
The strength of a Velcro bond depends on how much surface area is in contact with the Velcro hooks: full-body Velcro suits have been made that can hold a person to a Velcro-covered wall.