Research Area One
The ultimate goals of research Area One are to examine various factors influencing friction and wear performance of carbon fiber-reinforced-carbon matrix (carbon-carbon) composite brake materials during dynamometer testing. These factors include: both material properties (mechanical, thermal, structural, and compositional) and the influence of test conditions (energy, rate of energy dissipation, force, force distribution, torque, and surface preparation).

Background: Aircraft Brakes
Under extreme braking conditions, conventional brake materials break down and melt, making additional braking impossible. In the early 1970s, a new generation of brake materials was developed for use on the Concorde supersonic jet. This involved creating a stack of rotor (fixed to the wheel) and stator (fixed to the hub) rings of carbon-carbon composites. Such a stack is housed within most of the wheels of the aircraft. Additionally, carbon is a very light element, so the weight savings of replacing conventional brake materials with carbon allows an aircraft to fly lighter or to carry more fuel, more passengers, or more cargo. There are three primary types of stops that aircraft brakes perform: taxi (such as those from the terminal to the runway), landing, or emergency rejected take-off (RTO). A fully loaded 767 takes-off at 192 mph (310 kph) with a maximum mass of 158 000 kg has 590 MJ of energy that must be dissipated without the use of reverse thrust in stopping during an RTO. Its typical landing speed is 178 mph (287 kph), which is 450 MJ.

Background: Carbon-carbon Composites
Military and large commercial aircraft move at high speeds and stopping them requires brake materials that can absorb large quantities of energy as heat and have sufficient mechanical strength to hold up under large stresses. Some carbon materials have these properties and are combined into composites. Various carbon composites are also employed as structural materials as well. Our center has the full capability and expertise for making carbon-carbon composites.
Carbon fibers are made from polyacrylonitrile (PAN) and pitch precursors and are heat processed to give them their particular mechanical and thermal properties. Fibrous materials can be woven or stitched and felts may be made to create preforms. The matrix materials include phenolic resin, pitch, and chemically vapor deposited (CVD) carbon. The materials may be formed in a mold using chopped fibers or preforms. The materials are hot-pressed and then heat-treated to remove non-carbon elements and compounds. A result of the heat-treatment is a large amount of porosity and poor physical properties. To achieve the final composite, the open porosity is filled to some extent with either CVD or liquid infiltration. The entire fabrication process can be quite complicated and time consuming. Most of the details are considered restricted and therefore we cannot make them public.

Dynamometer Testing
One of the tools that CAFS researchers use in the study of aircraft brake materials is a sub-scale dynamometer. Delivered in 1996, the dynamometer has enjoyed nearly continuous usage on countless projects, including specialized studies for industrial and military organizations.
As indicated above, the energies dissipated in braking an aircraft are quite large. A full-scale dynamometer to simulate such braking is extremely large and expensive and simply isn’t useful for development and comparison studies. Therefore, instead of testing a stack of carbon-carbon composite rings, the system is reduced to two rings (one rotor and one stator). The dynamometer consists of a variable amount of rotational inertia (chosen by adding or removing large steel disks from the rotating axis), a variable speed motor (10 – 5000 rpm), and a pneumatic tailstock that applies the force that pushes the stator into the rotor. The force and torque are measured by independent load cells. The center conducts most tests using the constant torque mode to ensure a constant stop time. The testing chamber is enclosed and the atmosphere can be lab air, inert gas, or humidity-controlled air (0.5 – 90 % RH).

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