For the first time ever, a team of researchers successfully developed and tested networked acoustic emission sensors that can detect airframe damage on conceptual composite UH-60 Black Hawk rotorcraft.
Researchers with the U.S. Army Research Laboratory and the U.S Army Aviation and Missile Research, Development and Engineering Center said their discovery opens up possibilities for new onboard features that could immediately alert the flight crew to the state of structural damage like matrix cracking and delamination as they occur, giving the crew greater opportunity to take corrective actions before catastrophic failure.
ARL has been studying several possible alternatives to rotorcraft airframe health monitoring. This effort, which began almost two years ago, makes a strong case for integrated real-time damage sensing methodologies on future airframe structures.
The sensing method can be used to reliably detect and locate the initiation and growth of damage that may occur during service.
“Future Army airframe structures are required to be lighter, safer and ultra-reliable,” said Dr. Mulugeta Haile, research aerospace engineer. “To achieve these the Army must adopt a combined strategy of implementing advanced structural design methods, improved structural materials and integrated damage sensing and risk prediction capabilities.”
He said the team turned to acoustic emission tests because other methods such as ultrasonic and radiography require an external energy source in the form of a directed wave.
“The external energy has the undesirable effect of interfering with other systems of the aircraft. In addition, other methods are not as good as AE in detecting early damage,” he said.
Acoustic emission sensing is a passive non-destructive technique for detection of damage in the very early stage, and long before the structure experiences catastrophic failure. Unlike other methods, Acoustic emission detects damage in real-time (or at the instant the damage is happening).
The fact that AE is passive means that it does not require an external energy to detect damage. It relies on the energy that is initiated within the structure, Haile explained.
“The novelty of the current work is that we introduced several new concepts on wave acquisition control and signal processing to recover damage-related information in networked acoustic emission sensors,” Haile said.
“The Eureka moment was when the sensing network consistently identified and located the initiation and progression of damage during a prolonged fatigue test that lasted over 200,000 cycles — a feat that has never been achieved before.”
The ARL sensing network is composed of several lightweight transducers encapsulated in 3-D printed non-intrusive sensor mounts. Sensors of the network are optimally distributed in multiple zones to maximize coverage as well as the probability of damage detection.