A proof test is a form of stress test to demonstrate the fitness of a load-bearing or impact-experiencing structure. An individual proof test may apply only to the unit tested, or to its design in general for mass-produced items. Such a structure is often subjected to loads above those expected in actual use, demonstrating safety and design margin. Proof testing is nominally a nondestructive test, particularly if both design margins and test levels are well-chosen. However, unit failures are by definition considered to have been destroyed for their originally-intended use and load levels. Proof tests may be performed before a new design or unit is allowed to enter service, or perform additional uses, or to verify that an existing unit is still functional as intended.
Applications of proof testing
Industrial tooling Cranes and derricks are proof tested when called on for potentially dangerous load levels or high-valued cargoes. Similarly, items which are smaller and more common (rope and cable, slings, shackles and eyes) are nevertheless in the load path and a failure risk if not tested. Testing generally involves lifting weight or drawing tension equal to or greater than design levels.
Turbomachinery An overspeed proof test involves physically running the machine at high speed up to the test speed. This may be done during manufacture as an initial proof test. Physical overspeed tests may be periodically undertaken on some machines to verify operation of the overspeed protection systems. Operation at speeds above the normal operating speed can greatly increase stress levels in rotating parts. Failing flywheels, rotors, etc. may present a shrapnel risk in case of a failure.
Swords Historically, swords would be proof tested by impact before issuance- the "British test".
Pressure vessels Vessels which may be a failure risk, such as utility-scale water towers, chemical-handling equipment, or very-high-pressure storage tanks, may be proof tested. Rocket stage tankage, being high-valued and often the vehicle's primary structure, are proof-tested when redesigned, and may be tested at the unit level. Testing involves exposure to higher gauge pressures than design levels, or in some cases, pulling harder vacuum.
Firearms
A firearm's chamber and barrel become pressure vessels for brief periods. In firearm terminology, a proof test is a test wherein a deliberately over-pressured round is fired from a firearm in order to verify that the firearm is not defective and will not explode on firing. The firearm is inspected after the test, and if it is found to be in sound condition, then it is marked with a "proof mark" to indicate that it has been proofed (not proven). In many jurisdictions a proof test and valid proof mark are required for the sale of firearms. A "proof round" is an ammunition assembly designed to be used in proof testing; this can use a fixed cartridge, a semi-fixed cartridge, or separately loaded projectile, charge and primer. A "proof shot" is a special projectile used in a proof round or other projectile weapons, electromagnetic guns for example. Small arms proof rounds resemble normal cartridges, though they will typically bear special markings to prevent them from being confused for standard cartridges. Large calibre arms, such as artillery, will in general use an inert solid projectile (the proof shot); although water, sand or iron (powder) filled versions can be found for testing recoil systems.
Testing methodology For both small arms and heavy weapons, the gun is fired remotely and then examined; if undamaged, it is assumed to be safe for normal use and a proof mark is added to the barrel. In the case of revolvers or other multi-chamber firearms, each chamber must be proof tested before the firearm may be marked. Examination of the firearm may be as simple as visually inspecting it (defective components may fail in a spectacular manner, resulting in an explosion of the firearm) or may involve more in-depth examination, at the option of the tester.
Proof marks A proof mark is a mark specific to the manufacturer, importer, or testing facility that is performing the test. It generally takes the form of a stamp that makes an impression in the metal. Since proof marks are unique and nearly universal, they are often used to identify the origins of firearms that lack normal manufacturer's markings, such as military weapons, which are often produced by large numbers of different manufacturers.
Small arms proof testing A small arms proof round is loaded to a higher than normal service pressure, stressing both the gun barrel and breech during firing. This can be due to a heavy projectile fired using the standard propelling charge, the standard projectile fired with a different propellant type or weight, or combinations of charge and bullet weight to give the required proofing pressure. Minimum proof testing pressures are specified by the owner of the cartridge specification, such as C.I.P. or SAAMI for most commercial cartridges or NATO EPVAT testing for appropriate military cartridges. An example proofing round for the .50 BMG (12.7 × 99 mm) is the "cartridge, caliber .50, test, high pressure, M1". This uses the standard-weight .50 BMG M1 round propellant (240 gr of WC860), but a bullet weighing 999 gr (+/- 11 gr). The M1 proof round gives a proofing pressure of ~65,000 psi, 11,000 psi (~17%) above the standard service pressure.
Proof testing in C.I.P. regulated countries
In C.I.P. member states every civil firearm has to be professionally proofed in accredited proof houses before it can be sold to consumers. The proofsign may permit identification of the period of time in which it was used. Some of the signs actually used are:
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