Aircraft corrosion is a multi-faceted issue that requires more than a simple, one-dimensional approach. To enable Air Force Research Laboratory (AFRL) personnel to arrive at a complete picture and find out how to best protect valuable military assets, a unique solution was required.
Enter the Accelerated Combined-Effects Simulation test apparatus (ACES). This test chamber, custom-designed under the direction of AFRL through a Small Business Innovative Research effort, allows AFRL researchers to recreate the broad range of simultaneous environmental conditions under which military assets operate, including UV radiation, temperature, humidity, and various gaseous environments. Dynamic fixtures allow for test specimens to be pulled and flexed to further simulate the structural stresses aircraft experience during flight conditions.
According to materials research engineer Dr. Nicholas Wilson, the ability to include all these variables in one testing event is important because it simulates a more realistic operational scenario. To illustrate the difference between the ACES apparatus and traditional chambers, Wilson pointed to a test specimen that was scribed in order to simulate coating damage. Conventional corrosion testing involves placing the scribed specimen in a corrosion test chamber and subjecting the specimen to a period of testing under one specific corrosive condition, such as saltwater spray, for example.
“One problem with this method is that rarely do we actually see simple scribing damage in service,” said Wilson. “Under real-world conditions, corrosion begins around areas such as seams and fasteners. Dynamic stresses cause pulling at fasteners, the paint starts to crack, moisture gets underneath, and a corrosion cell develops under the coating. This type of corrosion is what we are now able to replicate with the ACES chamber.”
Wilson explained that the ACES chamber will enable researchers to test the properties of the entire corrosion protection scheme as a whole, not just the corrosion inhibitor layer. By testing the entire system, researchers can better quantify how the top coat works in conjunction with the primer layer, sealant, and surface preparation methods to provide overall protection.
Improving the test method will help researchers arrive at more accurate standards by which to qualify coating systems for aircraft, thereby enabling more effective corrosion protection for valuable military assets.
Another very significant advantage of using the ACES chamber is that it will allow researchers to test materials in an accelerated manner. Wilson says that by using data from tools such as the AFRL-developed Weather Instrumentation and Specialized Environmental Monitoring Platform, better known as the WISE-MP, they can accurately measure environmental conditions in places where aircraft are based or permanently stored. They can then replicate those conditions in the ACES chamber, using real-world validation to confirm the point at which the chamber accurately simulates long-term corrosion damage. Using this data, the researchers can calculate the rate of testing needed to simulate months’ or years’ worth of corrosion.
“This system will help us better understand how corrosion starts and spreads and how different materials perform under corrosive conditions,” Wilson said. “With this information, we can arrive at the best material solutions for protecting our assets.”
The ACES apparatus itself is an impressive sight. The test chamber includes six dynamic fixtures to hold and apply stress to test fixtures, as well as a lighting system for easy observation of test activities. It is constructed of stainless steel with a thick, glass-paned side door for easy loading and removal of test specimens. Luminaires above the chamber can recreate the full solar spectrum for testing specimens under realistic solar conditions. When the luminaires are not in use, insulating pads can be moved into position to help regulate chamber temperatures, which can be raised to over 150 degrees Celsius and lowered to below -60 degrees Celsius.
A control panel outside chamber allows researchers to program precise environmental conditions under which to subject test specimens. Factors including temperature, humidity, UV levels and much more can be programmed in a variable manner to simulate a real-word mission scenario. Behind the test chamber is a large air compressor that controls the chamber temperature, and compressed gas tanks that provide nitrogen oxides and sulfur dioxide to simulate specific environmental conditions.
The ACES chamber will not replace the more traditional equipment located in the AFRL facility, Wilson said. The simpler test chambers will still be used for tasks that require testing under only one specific environmental condition. However, Wilson says as the team uses the ACES chamber and assesses their needs, they may choose to re-evaluate their current test equipment and make changes as necessary.