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A tripwire-triggered pipe bomb mock-up used to train US military service personnel
A pipe bomb is an improvised explosive device, which uses a tightly sealed section of pipe filled with an explosive material. The containment provided by the pipe means that simple low explosives can be used to produce a relatively large explosion due to the containment causing increased pressure, and the fragmentation of the pipe itself creates potentially lethal shrapnel.
Premature detonation is a hazard of attempting to construct any homemade bomb, and the materials and methods used with pipe bombs make unintentional detonation incidents common, usually resulting in serious injury or death to the assembler.
In many countries the manufacture or possession of a pipe bomb is a serious crime, regardless of its intended use.
Different pipe bombs' appearances, from a bomb awareness report issued by the US Department of State
The bomb is usually a short section of steel water pipe containing the explosive mixture and closed at both ends with steel or brass caps. A fuse is inserted into the pipe with a lead running out through a hole in the side or capped end of the pipe. The fuse can be electric, with wires leading to a timer and battery, or can be a common fuse. All of the components are easily obtainable.
Generally, high explosives such as TNT are not used, because these and the detonators that they require are difficult for non-legitimate users to obtain. Such explosives also do not require the containment of a pipe bomb.
Instead, any sort of explosive mixture the builder can find or make is used. Some of the explosive mixtures used, such as gunpowder, match heads, or chlorate mixtures, are very prone to ignition by the friction and static electricity generated when packing the material inside the tube or attaching the end caps, causing many injuries or deaths amongst builders. Sharp objects such as nails or broken glass are sometimes added to the outer shell, or inside of, the bomb to increase harm and damage.
Pipe bombs concentrate pressure and release it suddenly, through the failure of the outer casing. Plastic materials can be used, but metals typically have a much higher bursting strength and so will produce more concussive force. For example, common schedule 40 1-inch (25 mm) wrought steel pipe has a typical working pressure of 1,010 psi (7.0 MPa), and bursting pressure of 8,090 psi (55.8 MPa), though the pipe sealing method can significantly reduce the burst pressure.
The pipe can rupture in different ways, depending on the rate of pressure rise and the ductility of the casing material.
If the pressure rise is slow, the metal can deform until the walls become thin and a hole is formed, causing a loud report from the gas release, but no shrapnel.
A rapid rate of pressure rise will cause the metal to act as a crystal and shatter into fragments, which are pushed outward in all directions by the expanding gases.
Modes of failure
Pipe bombs can fail to explode if the gas pressure buildup is too slow, resulting in bleed-out through the detonator ignition hole. Insufficiently tight threading can also bleed gas pressure through the threads faster than the chemical reaction pressure can rise.
They can also fail if the pipe is fully sealed and the chemical reaction triggered, but the total pressure buildup from the chemicals is insufficient to exceed the casing strength; such a bomb is a dud, but still potentially dangerous if handled, since an external shock could trigger rupture of the statically pressurized casing.
Minimum evacuation distances
If any type of bomb is suspected, typical recommendations are to keep all people at a minimum evacuation distance until authorized bomb disposal personnel arrive. For a pipe bomb, the US Department of Homeland Security recommends a minimum of 21 m (69 ft), and preferred distance of 366 m (1,201 ft).
As well as users such as criminals, paramilitaries, and militias, they also have a long tradition of recreational use for amusement or mischief with no intention to cause injury to anyone, but due to the dangers of premature ignition and of shrapnel, pipe bombs are much more dangerous than alternatives such as dry ice bombs or spud guns.
This 1886 engraving was the most widely reproduced image of the Haymarket riots. It inaccurately shows Fielden speaking, the pipe bomb exploding, and the rioting beginning simultaneously.
On 4 May 1886, a pipe bomb was thrown during a rally at Haymarket Square in Chicago, Illinois, United States. It reached a police line and exploded, killing policeman Mathias J. Degan. The bomb was made from gas-pipe filled with dynamite and capped at both ends with wooden blocks.
During the preparation of the Columbine High School Massacre, Eric Harris and Dylan Klebold had experimented with pipe bombs. During their testing and experimentation, Eric Harris had posted their results on his website. During the massacre, Eric and Dylan had used their pipe bombs as makeshift hand grenades, alongside various other bombs they had crudely manufactured.
^"Act II: Let Your Tragedy Be Enacted Here, Moment of Truth". The Dramas of Haymarket. Chicago Historical Society. 2000. Archived from the original on 15 March 2008. Retrieved 2008. The details are factually incorrect, because by all accounts Fielden ended his speech before the bomb was thrown, and because the riot did not begin until after the explosion. In [this] depiction, the speech, the explosion, and the riot all take place at once.