Gunfire Vibration — MIL-STD-810

Vibration and shock testing shows whether a product withstands mechanical dynamic loads. The two phenomena are distinct: vibration is a continuous and repeated loading, while shock is a single instantaneous impact at high acceleration. They produce different failure modes and are required separately.

The failure produced by vibration is FATIGUE: a solder joint cracks, a cable sheath abrades, a screw loosens, a connector loses contact. The failure produced by shock is FRACTURE: a brittle component shatters, a bracket breaks, a board comes out of its holder. Testing a product under vibration therefore does not show that it also withstands shock.

Vibration is applied in three forms, each giving different information. Sine vibration sweeps at a single frequency; it is used to find the resonant frequencies of the product and is very valuable at the design stage. Random vibration represents the broadband nature of the real environment and is preferred as evidence of durability. Sine-on-random and random-on-random profiles simulate together the broadband noise superimposed on dominant frequencies from engines and rotors — this is necessary in helicopter and turboprop applications.

RESONANCE is the concept at the centre of the field. Every structure has its own natural frequencies; as the excitation frequency approaches one of them the amplitude grows and the product is exposed to an acceleration many times the input level. A resonance search is therefore carried out before the test, and a design change often takes the form of shifting the resonance rather than lowering the level.

In this family the FIXTURE is part of the result, and this is the most frequently overlooked point. How the specimen is attached to the shaker directly changes the energy transmitted and the resonance behaviour. A poorly designed fixture brings its own resonances into play and imposes a load the product would not otherwise see, or conversely damps the real load and protects the product. Results obtained with different fixtures cannot be compared.

The choice of measurement point also determines the result. The control accelerometer is placed on the fixture and monitoring accelerometers at critical points of the product; the report must document these positions. Accelerations measured at different points in the same test can differ markedly.

Profiles are divided by application area: transport vibration (packaging and shipping), vehicle vibration (by mounting location), rail (EN 61373 categories), aviation (DO-160 sections), military (MIL-STD-810 methods) and battery transport (UN 38.3 T3). The same product passes through different profiles under different programmes.