In order to make condition-based maintenance a real possibility, there has to be a way to detect damage before it causes failure. That way, parts can be replaced when they are about to wear out rather than because a specific amount of time has elapsed. Now, engineers at Purdue University in West Lafayette, Ind., are working to make this possible by improving structural health monitoring systems for aircraft and other applications.

According to Douglas E. Adams, an assistant professor of mechanical engineering at Purdue, the salient feature of a structural health monitoring system is that it allows you to test a structure while it is in actual operation. "We focus on vibration-based, acoustic emissions," said Adams. "We rely on the simple fact that if you vibrate something, you'll see the damage better." Adams has been collaborating with research assistant Rebecca L. Brown and Mark Schiefer, an engineer with The Modal Shop Inc., a Cincinnati company specializing in test equipment related to vibration and acoustics.

This so-called "black box" serves as the nerve center of the structural diagnostics system.

The system, known as SDNA, for "structural diagnostics using nonlinear analysis," involves a network of actuators and sensors that can be embedded in or attached to a structure such as an aircraft or other vehicle, along with software that Adams wrote to make it possible to interpret the results. The actuators emit high-frequency sound waves that pass through the material to be tested and are detected by the sensors. The multiple sensors make it easier to zero in on the exact location of damage and avoid false alarms.

The system's transmissibility measurements compare the response, or displacement, caused by an input force at different points to see how they are related, so it is not necessary to measure the force that is originally applied. This output-only technique makes it possible to make measurements on, say, an airplane in flight, since wind, engine noise, and other variables are just so much more grist for the mill.

Factors such as temperature change can cause changes in transmissibility, which can result in false reports of damage. But multiple sensors spread over an aircraft in flight can detect the response of several parts of the structure to these changes, with only outlier responses flagged as damage.

A further advantage of the system is its noninvasiveness. "When technicians make inspections, they can inadvertently introduce more damage than was there before," Adams noted.

Composite materials are one application where this technique is particularly useful, he noted, and one much in the news recently. A slight impact can cause a tiny defect inside the composite, invisible from the outside, which may involve a shifting of the material's structure. This can be difficult to see even with standard nondestructive evaluation techniques, he added.

Adams's solution is to build the sensors right into the part when it's manufactured. They can even take the form of bolts, washers, and the like, which help keep the structure together.

Douglas E. Adams demonstrates his system, which tests for damage in structures by sending sound waves through the material.

With less sensitive materials, the system can be retrofitted onto existing structures, but Adams envisions eventually building in structural health monitoring from the beginning. The condition-based maintenance system that this would make possible would save government and industry billions of dollars a year, according to Adams, not to mention saving lives as well.

The ultimate goal is to build smart structures using sensors and computer chips, he said. Smart structures would monitor their own health and alert users to the need for repair when it arises. Should a catastrophic failure impend, the structure would have the capacity to "heal itself"—that is, to make emergency repairs that would hold until the aircraft, for instance, could be returned to the ground for repairs.

While such systems are already showing up in some manufacturing facilities, on machine tools and the like, the aircraft applications that Adams has mainly envisioned will have to wait for FAA certification. Meanwhile, Adams is engaged in expanding his health monitoring approach at the behest of the U.S. Department of Defense. This new system will use a greatly expanded network of sensors and more sophisticated data mining techniques, which should increase reliability in pinpointing damage and predicting maintenance needs.

home | features | weekly news | marketplace | departments | about ME | back issues | ASME | site search

© 2002 by The American Society of Mechanical Engineers