Tech Briefs

Research could lead to development of a composite material that can be processed at a low temperature and still be used at 1000°F.

The objective of this research was to hydraulically pressurize the internal diameter of one 102mm Polymer Matrix Composite (PMC) over-wrapped cylinder up to 25,000 pounds per square inch (psi) and during pressurization, in real time, collect and store pressure and strain data simultaneously. Strain data must be captured from the inside diameter of the oil filled metallic cylinder and from the outside diameter of the composite over-wrap material.

Composite Cylinder Testing Conducted in 2004

To do this, the test specimen was machined to a 15” height with seal pockets on each end. The seal pockets house the top and bottom enclosures. A 12” undercut was machined on the outside diameter for PMC over-wrapping. The test specimen evaluated had an average OD of 5.1366” which resulted in a nominal composite wall thickness of 0.2183”.

Four strain gages were placed 90 degrees apart from each other on the test specimen bore to measure strain in the hoop direction. Axial location of the interior strain gages was in the center of the test specimen. On the exterior surface of the test specimen, four rosette strain gages were placed in the same radial and axial location on the outside PMC material.

Each interior hoop direction strain gage contained a three-wire set-up for ease of balancing the bridge. Each internal strain gage wiring system was insulated from the test specimen due to grounding loops, which cause noise in the test data. Polyethylethylketone (PEEK) material was utilized for this purpose.

1080 series steel piano wire of 0.040” in diameter was silver soldered to cylindrical 4340 steel connector pin housings. Harris brand “Stay-Clean” liquid flux and “Stay-Brite” silver solder was used by bringing the wire and pin housing to 500°F and soldering the two together. The silver soldered sub-assembly was placed inside the cylindrical shaped PEEK insulating seal.

The entire assembly was placed inside a counterbore in the top and bottom enclosures. Located at the bottom of the counterbores was a disc of PEEK material separating the silver soldered assembly from the enclosure. O-ring seals were used between the connector pin and the inside diameter of the PEEK material. Another o-ring was used for sealing pressure between the outside diameter of the PEEK material and the counterbore in the top and bottom enclosure.

All wires for the respective internal strain gages were connected to piano wires. As a result, there were six wires running through the top and bottom end closures in order to successfully read the four internal strain gages. Placed over the top and bottom end closures were cover plates, which accept and protect the strain gage wires. The set-up enables the wires to be connected to the computerized data acquisition system, located outside the test cell.

For testing conducted in 2005 for similar composite cylinders, strain data was successfully collected on the interior strain gages and correlated well with the exterior gages. This test was conducted several times around the 2002 - 2005 timeframe as a screening test for different polymer composite overwrapped cylinders, but the data was never published. The goal of the test was to see if there was a lag between the internal and external strain gages. Any lag between the gages would indicate that there was a gap between the steel substrate and the composite overwrap.

The cylinder under study this time was produced under the Phase I of the “Low-Cost Low Temperature Processed Polyorganosiloxane Armament Composites with High Temperature Durability” SBIR, the goal of which is to develop a composite material that can be processed at a low temperature and still be used at 1000°F. Normally a composite cannot be used above its cure / melt temperature. This causes issues as thermoset composites become soft during cure and don't assume their final shape until after cure and they have a very low coefficient of thermal expansion (CTE) compared to metals. So, during cure of a thermoset composite over a steel substrate, the steel would expand as the temperature increased, the composite would soften allowing the expansion, and then set its final shape at the cure temperature. As the steel cools it shrinks, but the composite doesn't, forming a gap. The material developed under this ILIR can be moisture cured at room temperature so the difference in CTE between the steel and composite should not result in a gap forming after cure.

This work was done by Lucas B. Smith and Andrew G. Littlefield for the Armament Research, Development and Engineering Center. ARDEC-0002