Structures with smarts

A technician inspects the belly of a Boeing 737.

Takeoffs and landings, cabin pressurizations, rapid temperature changes, moisture, and turbulence all take a toll on the skin and structure of a commercial aircraft. Over time these repeated stresses can cause tiny cracks and other flaws to form that can, if not remedied, grow into more serious defects.

To find and repair flaws in their earliest formative stages, the airlines schedule inspections at maintenance facilities, where human inspectors aided by high-tech scanners go over each aircraft inch by inch looking for the consequences of aging.

Future commercial jets might be fitted with networks of sensors that watch continuously for the initial signs that defects are forming. Like nerve endings in a human body, these in situ, or permanently installed, sensors offer levels of vigilance and sensitivity to problems that periodic checkups cannot.

Such full-time monitoring could supplement, reduce, or even eliminate scheduled structural inspections, says Dennis Roach, who leads a Sandia team that is evaluating some of the first sensors for structural health monitoring, or SHM, for aircraft and other safety-critical equipment.

“With sensors continually checking for the first signs of wear and tear, you can restrict your maintenance efforts to when you need human intervention,” he says.

Rising costs

In a retired jet at Sandia, Dennis Roach and Ciji Nelson prepare SHM sensors.
(Photo by Randy Montoya)

Cracks are just one form of airframe defect. Using nondestructive inspection (NDI) tools that help them locate defects in hard-to-reach places without dismantling the aircraft, inspectors also encounter metal-to-metal adhesive failures, material erosion, impact damage, and corrosion, to name a few.

Sandia’s Airworthiness Assurance Program for years has focused on development and evaluation of NDI technologies for defect detection in aging aircraft.

Every day a commercial aircraft isn’t flying costs the company tens of thousands of dollars, costs that are eventually passed on to the air traveler.

Aircraft maintenance and repairs now represent about a quarter of the U.S. commercial airline fleet’s operating costs, and those costs are rising as aircraft in the fleet age, many well beyond their design lifetimes, he says.

“Airline passengers want safe and reliable transportation at a reasonable price,” says Roach. “The aviation industry is continually seeking technologies and processes that enhance aircraft safety and reduce costs.”

First acceptance

Dennis Roach with a Comparative Vacuum Monitoring sensor showing galleries etched into the sensor’s underside.
(Photo by Randy Montoya)

Initially, full-time monitoring sensors are envisioned for hot spots where flaws are expected to form. Eventually the work could lead to “smart structures” with many sensors that would self-diagnose and signal an operator when repairs are needed.

The Sandia team already has developed or evaluated several types of inexpensive, reliable sensors that can be retro-fitted into aircraft structures for SHM.

“The ultimate dream — and it’s only a dream right now — would be to take a poplar tree, put it into a tank, let it sit for three days, then come back and watch as the ethanol comes pouring out of the spigot,” says Simmons. “Though we’re probably decades away from that, this project aims to consolidate the pretreatment steps and get us one step closer to realizing that vision.”

One promising candidate, called a Comparative Vacuum Monitoring (CVM) sensor, is a self-adhesive rubber patch, ranging from dimeto credit-card-sized. The rubber’s underside is laser-etched with rows of tiny, interconnected channels or galleries to which an air pressure is applied. Any propagating crack in the material under the sensor breaches the galleries and the resulting change in pressure is monitored.

The sensors, made by Structural Monitoring Systems, Inc. (SMS) of Australia, are inexpensive, reliable, durable, and easy to apply, says Roach. They also provide equal or better sensitivity than is achievable with conventional inspection methods, he says.

Boeing recently included CVM technology in the Boeing Common Methods NDI Manual, which allows airlines to work with Boeing and the FAA to seek certification of the sensors for specific applications on specific aircraft.

Growing demand

Boeing’s acceptance was the culmination of a comprehensive, two-year validation program by Sandia in cooperation with the FAA, SMS, a number of U.S. airlines, and the University of Arizona. Work on additional applications for Southwest, Northwest, and Delta Airlines is under way.

To help address the growing demand for standardized SHM procedures and certification requirements, Sandia helped form the Aerospace Industry Steering Committee for Structural Health Monitoring in November 2006. The international group includes manufacturers, regulators, government agencies, the military, and universities.

The Sandia team also continues to seek acceptance for SHM outside the aerospace industry. Besides aircraft, SHM techniques could monitor the structural well-being of spacecraft, weapons, rail cars, bridges, oil recovery equipment, buildings, armored vehicles, ships, wind turbines, nuclear power plants, and fuel tanks in hydrogen vehicles.

“Any structure that operates in a fatigue environment with cyclical stresses or other structurally- degrading environment could benefit from frequent sensor monitoring rather than relying only on scheduled inspections,” Roach says.

Smart structures

In the future, members of a ground crew might plug a diagnostic system or laptop into a port on the aircraft and download structural health data collected during flight. Ultimately an integrated network of sensors could monitor not only structural materials but also the health of electronics, hydraulics, avionics, and other systems.

The Sandia researchers already are using computerized prognostic and health management software to help them recognize the first signs of fatigue in large metal structures, says Roger Hartman, manager of Sandia’s Infrastructure Assurance and NDI Department.

“You begin to evolve a systems approach to making important infrastructure elements safer and more reliable,” Hartman says.

“When we set out to do NDI, in the back of our minds we knew that eventually we wanted to create smart structures that ‘phone home’ when repairs are needed or when the remaining fatigue life drops below acceptable levels,” adds Roach. “This is a huge step in the evolution of NDI.”