Bulletproofing on a budget

May 5, 2005
Accumulators in aircraft store considerable amounts of energy.

For ballistic testing the accumulators mounted to heavy concrete stands and were shot with a .50-caliber M-2 armorpiercing bullet per SAE ARP4378 specs. The bullet entered the accumulator, but there was no exit hole.

No surprise, then, that their behavior when struck by a bullet is of great interest to designers of military planes.

There are stringent ballistic fragmentation specs that dictate how aircraft accumulators must perform when hit by live fire. Designers routinely meet these specs, but only by liberal use of heavy steel and Kevlar in the accumulator housing. The downsides to this design strategy are the extra cost and weight of the Kevlar. The extra layers also can take up appreciable space.

A new approach from Senior Aerospace Metal Bellows, Sharon, Mass. (metalbellows.com), is to use lightweight composite housings. Their structural integrity is such that the Kevlar overwrap is unnecessary. And composite accumulators can weigh up to 40% less than older versions. The housings consist of a metallic liner wrapped with a composite. The allwelded metallic liner provides a hermetic seal for the precharge gas, while the lightweight composite gives the housing strength. A CNC machine precisely wraps the composite in multiple layers around the metallic liner. A high strength-to-weight ratio comes from varying fiber orientation between layers in both the hoop and longitudinal directions.

A detailed netting analysis lets designers calculate stresses in the composite under specific pressure loadings. For analysis, designers assume the composite carries the entire load without the liner. During final design, however, the composite housing with metallic liner gets a detailed 3D finite-element analysis.

The FEA model includes each layer of the composite wrap with the specified wrap angles as well as calculated orthotropic material properties. Iterative element-byelement, ply-by-ply analysis takes place for mechanical and pressure loads. The analytical and test data helps establish failure criteria to predict the maximum allowable loads with appropriate safety margins.

The symmetry of the design along the axis of the housing was a consideration in creating and meshing a 1/8 model of the housing. The complex nature of the composite dictated that the FEA model consider the actual fiber orientation that varies between elements.

The ballistic fragmentation test took place with the accumulator charged to 2,000 psig and mounted to a heavy concrete test stand using two ratchet straps. The fluid side of the accumulator was pressurized with water to an operating pressure of 5,000 psig.

The accumulator took a hit from a .50-caliber M-2 armor-piercing bullet with a muzzle velocity of 2,800 ± 100 fps. The bullet was fired from 25 yd. ARP4378 specs call for the bullet to "tumble," producing an entry hole of at least 0.5 X 1.3 in. (13 X 38 mm). An obstacle in the path of the bullet before it hits the accumulator initiates the tumbling action.

The test concluded successfully. The bullet entered the accumulator but there was no exit hole. The composite wrap contained the damage and the housing surface opposite the bullet entrance showed no damage.

A detailed netting analysis lets designers perform preliminary calculations on the stresses in the composite under specified pressure loading. FEA gives stresses in the cylindrical region of the composite while the housing sees internal pressure. A 1/8 slice shows the stresses in the fiber direction (top) and perpendicular to the fiber direction (bottom).

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