The in-situ sensors offer levels of vigilance that periodic inspections cannot duplicate.

Structural-health monitoring, or SHM, techniques are gaining the support of airframe manufacturers, airlines, and regulators. The idea is to build in nondestructive inspection (NDI) technologies similar to those in manual inspections so they continuously monitor for unsafe conditions such as cracks in the airframe. A team at Sandia National Laboratories is evaluating some of the first such sensors for aircraft.

Widespread adoption of SHM could significantly cut maintenance and repai r expenses for commercial aircraft, now estimated at about a quarter of fleet operating costs. And the costs are rising as aircraft age, many well beyond their design lifetimes.

To use SHM, ground-crew technicians might plug a laptop into a central port on the aircraft to download structural-health data. Eventually “smart structures” fitted with many sensors could selfdiagnose and signal an operator when repairs are needed.

Ultimately an integrated network of sensors could monitor not only structural elements, but also the health of electronics, hydraulics, avionics, and other systems. The sensors could continually check for the first signs of wear and tear, so technicians can replace equipment when it wears rather than on a periodic maintenance schedule.

The SHM sensors being evaluated at Sandia can detect fatigue damage, hidden cracks in hardto- reach locations, disbonded joints, erosion, impact damage, and corrosion. The hope is to eventually develop sensor designs that can keep tabs on every aspect of an aircraft that affects maintenance.

The Sandia team has developed several types of sensors that mount where flaws often develop. The Comparative Vacuum Monitoring (CVM) sensor is a thin, self-adhesive rubber patch that ranges from dime to credit-card size. It detects cracks in the material beneath it. The patch has laseretched rows of tiny, interconnected channels or galleries, to which a vacuum is applied. Any propagating crack under the sensor breaches the galleries with a corresponding change in pressure.

The CVM sensors provide equal or better sensitivity than conventional inspection methods, according to team leader Dennis Roach.

The sensors were tested in a lab and validated on three commercial aircraft. Boeing recently added the in situ, or permanently-mounted, crack-detection CVM sensor to its Common Methods NDI Manual after a two-year validation program.

Sandia is also working on a variety of other SHM sensors including flexible eddy-current arrays, capacitive micromachined ultrasonic transducers, and piezoelectric transducers. One idea being explored uses a conductive paint that changes resistance as cracks form under the layer.