Shock testing is essential for shipboard machinery, equipment, systems, and structures, in order to verify the ability of these installations to withstand shock loadings, which may be incurred during wartime service due to the effects of explosive weapons. Military shipboard items are classified in accordance with several standard grades, classes, and types, each of which defines the unit’s importance to essential ship operations, method of installation within the ship structure, and required performance during shock testing, respectively. Military specification MIL-S-901D details shock testing requirements for shipboard components, including the following classifications:
Grade A: Items that are essential to the safety and continued combat capability of the ship. The unit must survive shock testing fully intact and functional.
Grade B: Items whose operation is not essential to the safety and combat capability of the ship, but could become a hazard to ship operations as a result of exposure to shock. The unit is allowed to malfunction as a result of shock, but must remain fully intact due to the possibility of debris causing harm to personnel or other equipment.
Class I: Equipment required to meet shock specifications without the use of resilient or isolation mountings.
Class II: Equipment that meets shock requirements with the use of resilient mountings.
Class III: Equipment which may be mounted on a ship with or without the use of resilient mountings installed between the item and the ship structure; therefore required to meet both class I and class II requirements.
Shock Test Types:
Type A: Test of a principal unit, which is directly supported by the ship structure.
Type B: Test of a subsidiary component or item, which is a major part of a principal unit.
Type C: Test of a subassembly, which is a part of a principal unit or subsidiary component.
Medium and Lightweight Shock Testing:
Depending on the combined weight of the equipment being tested and the fixture used to secure it, either lightweight, medium weight, or heavyweight shock testing must be selected. Medium and lightweight high intensity shock testing machines operate by using a winch or pulley system to raise a massive steel hammer, which is subsequently released, swings like a pendulum, and strikes the anvil plate to which the equipment being tested is mounted. The height to which the hammer must be raised is determined by the weight supported by the anvil plate (i.e. the test unit and support fixture). Lightweight shock machines have a maximum capacity of 550 lbs, while medium weight shock machines have a capacity of 7,400 lbs. Lightweight shock testing consists of three increasingly energetic blows applied parallel to each of the three geometrical axes of the item being tested. However, medium weight shock testing requires a minimum of two blows in three orientations, but includes a third blow depending on the operating conditions required to be simulated during testing.
Heavyweight Shock Testing:
Equipment that is beyond the weight limits of medium and lightweight shock machines (i.e. heavyweight) must be tested on a barge. A water-filled former rock quarry is used as the venue for barge testing. The units being tested are mounted to the barge in a fashion similar to their intended shipboard mounting, and high explosives are detonated at specified depths and distances from the barge to simulate the required shock levels. The heavyweight shock test usually consists of four shots, each one closer to the shock platform than the last. The number of blasts depends on the mounting orientation of the equipment being tested. Four shots are used when the orientation of the equipment is unrestricted, however if the equipment is designed for use in only one specific orientation aboard the ship, only one blast is needed. The explosive used in heavyweight shock testing is HBX-1, and when a standard sized floating shock platform is used the weight of the explosive charge is 60 lbs. When four blasts are needed the first charge is detonated at a depth of 24 feet, 40 feet from the barge. Subsequent blasts occur at 30, 25, and 20 feet from the floating shock platform respectively, all at the same depth of 24 feet. The image at the top of this article depicts a heavyweight shock test with two standard sized floating shock platforms or barges. Observation of the size of the water plume resulting from the explosion begins to explain the enormous amount of energy which military shipboard equipment must be able to withstand.
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