Highlight: AFRL facility brings new capabilities for rain erosion testing
WRIGHT-PATTERSON AIR FORCE BASE, Ohio – Thanks to an impressive set of upgrades and a team of AFRL experts, a new and unique laboratory capability is filling a much-needed role in materials durability testing for the military and beyond.
The Materials and Manufacturing Directorate’s Supersonic Rain Erosion Test Facility, or SuRE for short, answers a vital need for some of today’s most advanced aeronautical and mechanical systems. This highly-specialized apparatus allows scientists to evaluate materials and coatings durability by directing a variable spray of water at test specimens, subjecting them to conditions simulating real-world rain and weather events. The specimens are then evaluated by AFRL specialists who determine the effect of the high-force spray on the materials and can provide expert analysis and recommendations.
Understanding the erosive effects of water is important when considering the durability and performance of a system. An event such as a simple rain shower may not seem on the surface to be a matter of concern, but when fast-moving parts or high speeds are factored in, water erosion can be a major problem.
Prior to the standup of SuRE, water erosion testing at speeds exceeding 650 mph was difficult to accomplish, as there was no specialized facility for such testing. To address this need, AFRL and the University of Dayton Research Institute worked together to deliver this new test capability. Bringing the chamber to full operational capability took several years, as a number of upgrades were needed to achieve maximum performance.
As AFRL erosion team lead Joseph Shumaker explained, to accurately evaluate water droplet speeds as well as confirm drop size, a high-speed camera was added to the system. A tremendous amount of energy is generated to reach the Mach speeds. To ensure the water droplets are uniform and can actually reach their target, a vacuum system and improved water and chamber chilling capabilities were also added to prevent the vaporization of the water droplets.
“When you’re spraying water at such a high force, the energy generated will instantly turn the water droplets into vapor at ambient pressures, and that makes it impossible to conduct a test event,” he said. “By adding the vacuum system to the chamber, you also eliminate shock wave effects that are observed in real-world interactions. This allows for very accurate impact speed determinations.”
Shumaker also noted that the team added improvements to the targeting system, as well as a spray nozzle that spins to ensure the water droplets are evenly dispersed and to prevent hammer effects, or sudden pressure surges, across test specimens. Improvements were made to the lighting system to ensure that it would allow the high-speed camera to visually detect the water droplets moving at the elevated speeds.
In 2016, once all the improvements were in place, the SuRE was ready to take on customer needs. And customers were quick to respond.
According to Shumaker, customers from throughout the Department of Defense, as well as the commercial sector have requested testing in SuRE. Most recently, the team performed propeller blade safety coating tests for NAVAIR Cherry Point. For this test event, a number of candidate blade tip specimens were evaluated to determine the durability of the safety coatings under supersonic rain conditions. Shumaker explained that conducting these types of tests in the SuRE keeps costs to a minimum since blade tip component specimens can be used as opposed to entire blades. Specimen testing such as this eliminates the unnecessary time and expense of constructing blades and testing the product on wing, a costly endeavor that would take an entire aircraft out of service and require nearly optimal weather conditions.
In fact, Shumaker said the SuRE provides a cost-effective and practical solution for most customers. The facility bridges the gap between AFRL’s whirling arm facility, which tests coupon test specimens at speeds up to 650 mph, and the Holloman High Speed Test Track at White Sands Missile Range, New Mexico, which tests at much higher speeds and is extensively more costly to operate.
The SuRE has become a hit with the military and commercial aerospace community since becoming fully operational, and Shumaker said the customer base has expanded significantly. The team will soon be doing test work for several space exploration companies, and a number of other high-profile customers are in the process of arranging tests in the coming months as well. Shumaker says the practicality, low cost, and broad range of test capabilities are what draw in customers. There are no other test facilities outside of AFRL that have this capability.
“We can perform 12 to 15 tests in one day, depending of the needs of the customer,” he said. “The SuRE can accommodate various high-speed components and coupon samples for evaluation such as radomes, leading edge materials, window materials, and safety coatings, for example.”
The facility operates under a “spiral development” concept, meaning that after operational and maintenance costs are met, the remaining funds acquired from testing go back into improvements and added capabilities for the SuRE. This plan ensures that the facility will continue to grow in capability to meet future customer needs. Shumaker says the SuRE’s customer base comes nearly entirely from word-of-mouth, and customer interest continues to grow as more and more tests are conducted.
“Every customer has left happy” he said.
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