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Air Force-Led Team Develops Synthetic Sand Media and Erosion Test Standard

By Ben Craig

During recent deployments to regions characterized by arid climates, the U.S. military found that the material systems protecting their rotor blades and fixed-wing leading edges were failing prematurely.

Current U.S. military deployments have weapon systems entrenched in extremely sandy environments. Photo courtesy of the U.S. Army.

Deserts and arid climates are known for having loose soil or sand that can be displaced easily by surface winds. During rotorcraft operations, the sand particles collide with the leading edges and rotors, which results in impact-related damage and erosion of materials not designed to withstand such conditions.

What the rotorcraft maintainers found was that the impact of high-velocity sand particles was tearing into the protective appliqué materials, and thus eroding the base rotor material. The damage was considerable, and in certain cases the rotor blades had to be scrapped and replaced. Even when the rotor blades can be salvaged, premature wear can lead to accelerated maintenance schedules and higher maintenance costs.

Rather than looking for ways to temporarily mitigate damage to the rotor blades and leading edges on current weapon systems, a team led by engineers at the Air Force Research Laboratory (AFRL) sought a solution that could resolve the issue for current and future rotorcraft and fixed-wing aircraft platforms.

Defining the Problem

The DoD's Office of Corrosion Policy and Oversight funded a 12-month, $395,000 effort to establish a test standard that would ultimately root out these failures occurring on the military's rotorcraft fleet. Lynne Pfledderer, an AFRL materials engineer, led the effort. "The essential starting point was to define the underlying problem with an isolated focus on the sand erosion aspect of damage occurring with the rotor blade leading edge materials currently used on military rotorcraft platforms," Pfledderer said. The fundamental problem was that no validated test standards exist for the sand erosion of materials. "Without such a standard, any industry claim about improved sand-erosion resistance was relative, forcing the selection of materials to be whatever test 'standard' a material developer believed was appropriate."

The DoD mission was to develop a test standard using a synthetic sand media for screening out materials intended for use in rotor blade applications. "In the case of erosion, you can have multiple factors that contribute to the breakdown of the material." As part of this, it was necessary to determine the factors that influence rotor blade durability. For example, in addition to identifying the material systems used for leading edge protection (e.g., metals, coatings, films, tapes, and protective boots), the sand particles had to be characterized in terms of shape, size, and composition.

Analyzing Different Types of Sand Media

More than a dozen samples of different types of natural and commercially available sand were collected for analysis, including golf sand. "We were able to perform an extensive analysis of natural sand samples and compare those with the synthetic types of material sand media we were using in the laboratory," Pfledderer said. In current military specifications, particles are categorized according to the Krumbein Scale for Particle Roundness/Sphericity, a ratio expressing a particle's sphericity to roundness.

The leading edge of a damaged rotor blade is shown here. Photo courtesy of the Air Force Research Lab (AFRL).

Analysis of the samples led to the conclusion that the test media defined in current military specifications is not representative of the sand that is found in the arid climates in which the U.S. military is currently operating. As a result, Pfledderer's team had to develop its own sand media to use for the effort's testing phase. "We formulated a synthetic media, which we used for erosion testing on rotor blade baseline materials."

Before designers and engineers can refer to a new test standard for down-selecting erosion-resistant materials for rotorcraft and fixed-wing aircraft, the AFRL-led team plans to accomplish a few more objectives. "A post-test analysis will compare damage to the test specimens against damage from actual rotor blades we received from the field," said Pfledderer. "Outside of the scope of this project, the test protocol will need to be verified and validated, before incorporating it into a military test standard. I'm hopeful down the road that we have a few champions that are going to support taking on this effort."

The test standard could end up as a test requirement for military coatings intended for application where sand erosion resistance is needed. "Eventually, this protocol will become a validated military test standard that will impact how we develop and select optical materials, windows and transparent materials, fixed-wing, leading-edge coatings and appliqués, and engine turbine materials," Pfledderer said.

The AFRL team has already noted outside interest in the test standard that will result from the testing program. "In fact, recently I've had quite a bit of interest from the engine materials community," Pfledderer said. "What's evolving out of this program may provide a better qualification standard for their materials and interests."

A Diverse Team

To accomplish the objectives of this project, AFRL needed to bring together experts from several different fields of study. "We had representation in specialty areas such as materials engineering, meteorology, geology, and petrography, as well as technical writers responsible for military-unique test standards." This diverse team had representation from the Air Force, Army, and Navy.

In addition, representatives from each DoD rotorcraft manufacturer participated in the effort. As for how the development of a new test standard will affect the DoD rotorcraft manufacturers (OEMs), it has yet to be determined. "Much to their credit, they are taking a proactive role in recognizing the changes that are yet to come with the sand-erosion characterization of rotor blade materials."

Conservatively Estimating Cost Avoidance

In this image that is doubly magnified, erosion punch-through in polyurethane tape is pictured, from exposure to a golf sand that is commercially available. Photo courtesy of AFRL.

In this image that is doubly magnified, erosion punch-through in polyurethane tape is visIble, caused by exposure to a commercially available golf sand. Photo courtesy of AFRL.

Accomplishing the objectives set forth at the onset of this project will lead to a significant cost avoidance. The actual number will only be revealed in time. But for now, Pfledderer has established some conservative estimates. "In 2003, one of the services reported an annual cost of $189 million, for rotor blades across three platforms, over a 12-month period. With the implementation of a go/no go screening tool for sand erosion, it is assumed that the elimination of poor performing protective schemes will result in a one percent improvement in the removal and replacement costs of rotor blades for that service, or $1.89 million." According to Pfledderer, this one percent improvement in performance is very conservative, especially because the screening tool could be incorporated into internal OEM material specifications and selection protocol, in addition to the military test standards.

If the screening tool is incorporated into the selection of sand erosion-resistant materials for future rotorcraft platforms throughout the DoD, a much greater cost avoidance could be achieved. "With the production of a dependable and enabling screening tool, a more realistic improvement in repair and replacement costs could be in the range of 10 to 25 percent, if not greater. This translates into a significant return on investment, considering the cost of this effort."