May 25, 2006 Mechanical engineers at Purdue University have teamed up with the U.S. Army to design a new portable test system ensuring the safety and readiness of the eight-wheel "Stryker" vehicle, the newest ground combat vehicle deployed in Iraq.
The system uses sound waves to detect damage to a key component in the vehicles' wheel assemblies.
"Excess dynamic forces can cause cracks to form in a critical component of each wheel assembly called the spindle, which supports the wheel," said Douglas E. Adams, an associate professor of mechanical engineering. "The cracks can grow large enough to cause the spindles to break apart. As with any wheeled vehicle, if the supporting spindle fails, the wheel might fall off. The inspection system looks for these cracks so that damaged wheels can be replaced."
The testing, which is expected to be introduced later this summer, will be part of routine maintenance procedures.
"The Army has worked with Purdue to develop a proactive approach to manage the health of spindles in the field," Adams said. "Although this work has resulted in some important research findings, this is more than a research project. This is an opportunity to keep vehicles in service and reduce the costs of operating a tremendously important asset in the Army's arsenal.
"The suspension of this vehicle is an engineering wonder, and its complexity makes detecting cracks especially challenging."
Several test kits initially will be shipped to Iraq by the Army's Stryker Program Management Office at the TACOM Life Cycle Management Command.
A recent research paper described the mathematics behind the method and includes data from tests on vehicles demonstrating that the system works. The data includes information gathered from tests at Fort Lewis in Washington state, the Yuma Proving Ground in Arizona and the Aberdeen Proving Ground in Maryland.
"To develop our software algorithm for the test kit, we had eight assemblies that were cracked, plus a bunch of other assemblies that weren't cracked," said Adams, whose research is based at Purdue's Ray W. Herrick Laboratories. "We tested all of the assemblies, cracked and not cracked, and used those results to develop our algorithm. Then we tested our algorithm on assemblies in which we did not know cracks existed."
Strykers are used in a variety of roles, including infantry carrier, commanders' vehicles, medical evacuation, reconnaissance, anti-tank guided missile delivery, fire support, engineering squad vehicle and mortar carrier.
The "fault-detection method" developed at Purdue uses a sensor called an accelerometer to detect acoustic energy, or sound waves, passing through the spindle. Data collected with the sensor are fed to a computer, where software interprets the information to analyze a part's performance.
An Army technician or mechanic must first remove the wheel and attach the accelerometer to the spindle with a plastic zip-tie fastener, tightening it with a ratchet wrench. Then a "modal impact hammer" is used to tap the hub on the outside of the wheel assembly, sending sound waves through the spindle. Sound flows through the metal differently depending on whether the spindle is cracked. The sound waves not only reveal the presence of cracks, but how large they are, Adams said.
"It's like comparing the difference between the sound of a cracked bell and a bell that is undamaged," Adams said. "Like the Liberty Bell, the spindle is going to sound differently when it's cracked. Of course, we can't hear the spindle because it's buried deep within the assembly, so we need a high-sensitivity sensor to listen to the sound waves."
The same principle governs a common annoyance in everyday life: loud car stereos. Only the bass portion of music is heard from a nearby car's booming stereo system.
"You can't hear the words or melody, but you can hear the bass thumping away," Adams said. "That's because the car's body is very good at blocking out the sound in the higher frequency range, where the voice and melody are, but it's terrible at blocking out the lower frequency range, the 'thump, thump.'
"The same exact thing happens in the spindle. When no crack is present, the lower frequency 'music' from the impact on the hub of the wheel is quieter than when a crack is present. In other words, the spindle gets louder when it is cracked."
The spindles cannot be removed for testing in the field and then reassembled because doing so would expose gears inside the assembly to the elements.
The Purdue engineers developed a touch-screen display that guides mechanics through the testing process. A software algorithm converts data from the sensor readings, eliminating the need for mechanics to interpret complicated graphs.
"It's very user friendly, so that they can quickly do the task," Adams said. "Basically, you press some buttons, you hit once with the hammer, and it comes up with a red light or a green light. A green light is good, a red light is bad, meaning cracked. It usually takes 50 minutes or less to test all eight wheels of the vehicle.
"The Stryker Program Management Office is taking a real proactive approach. They are going to be doing this at maintenance intervals, so when they bring in the vehicle for routine brake inspections, they will remove the wheels and run this test. If a crack is detected, the wheel assembly can be shipped out and replaced. Otherwise, the vehicle continues operating."
The testing not only prevents a safety hazard by detecting cracks, it reduces the need to do needless "scheduled maintenance" by ensuring that the vehicle is ready for action and does not need overhauling.
The engineers tested 35 vehicles that had recently returned from Iraq from a single brigade.
"About 10 percent of the assemblies we tested turned out to be significantly cracked, meaning cracks longer than a quarter inch, which are most likely to grow and cause the spindles to break," Adams said. "The majority of the cracks were found in the rear wheels, where there is increased weight."
Details about the method were contained in the research paper, which researchers presented in February during an International Society for Optical Engineering conference called Non-Destructive Evaluation for Health Monitoring and Diagnostics. The paper was written by Adams, doctoral students Harold Kess and Timothy Johnson, graduate students Spencer Ackers and Jonathan White, research scientist Ronald Evans, and program manager Pam Brown from Stryker PMO.
A major challenge with the diagnostic technique is learning how to analyze only one particular part of the wheel that is nestled among other parts. The research paper focused on the technical challenges in developing a method for analyzing a part that is surrounded by others.
"It's one thing to do crack detection on something that's just sitting out for you to test, but it's another thing entirely to find cracks in a spindle when there are a hundred other things surrounding it," Adams said. "This is the novelty associated with what we are doing."
The current test kit requires that the wheels be removed because each wheel supports a different amount of weight, complicating the interpretation of data from the sound waves. This complication is eliminated by jacking up the vehicle and removing each wheel before testing the spindle.
"This is the so-called wheels-up configuration," Adams said. "We are working on a wheels-down configuration, where they won't have to lift the vehicle at all. Just roll it in, test it, roll it back out. We are using devices called actuators to send sound waves through the spindle in the wheels-down test. These actuators let us play higher frequency sounds into the spindle, translating into better results when the wheels are on the ground."
The engineers also are developing new software algorithms that will make the wheels-down system possible and hope to have that work completed within six months.
The research has been funded by the Stryker Program Office.
"The wheels-down method will not only prevent safety hazards by detecting cracks, but it will also reduce the need to do unnecessary maintenance and ensure that the vehicle is ready for full combat operations. The soldier is our number one focus," Brown said.
The Stryker vehicle was named in honor of two Medal of Honor recipients: Pfc. Stuart S. Stryker, who served in World War II, and Spc. Robert F. Stryker, who served in Vietnam.
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