Commercial aircraft one day might be fitted with networks of sensors that check for defects continuously. Like nerve endings in a human body, in situ sensors offer levels of vigilance and sensitivity to problems that periodic checkups cannot, says Dennis Roach of Infrastructure Assurance and Nondestructive Inspection Dept. 6416.
Such full-time monitoring could supplement, reduce, or even eliminate scheduled structural inspections of aircraft, he says.
“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.
Dennis 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.
Initially the 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.
Aircraft maintenance and repairs represent about a quarter of the US commercial airline fleet’s operating costs, and those costs are rising as aircraft in the fleet age, many well beyond their design lifetimes, says Dennis.
Among the defects commonly encountered are fatigue damage, hidden cracks in hard-to-reach locations, disbonded joints, erosion, impact damage, and corrosion.
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,” he says.
Extension of NDI
Sandia’s SHM work is an extension of its Airworthiness Assurance Program, which for years has focused on development and evaluation of nondestructive inspection (NDI) technologies that aid human inspectors as they go over an aircraft frame or fuselage skin inch by inch looking for the consequences of aging.
Boeing’s recent incorporation of an in situ, or permanently mounted, crack-detection sensor into its NDI standard practices manual for Boeing airframes is the first time a manufacturer has adopted SHM techniques — evidence that the industry is ready to consider new ways of ensuring the safety of aircraft beyond NDI-assisted human inspection, says Dennis.
Several commercial airlines are considering applications, including Delta and Northwest, which have petitioned the Federal Aviation Administration to use embedded crack detection sensors to address specific maintenance requirements.
“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,” Dennis says. “This is a huge step in the evolution of NDI.”
The Sandia team already has developed or evaluated several types of inexpensive, reliable sensors that can be retrofitted into aircraft structures to detect cracks, corrosion, and other flaws (see “In situ monitoring with CVM sensors” at right).
In the future, members of a ground crew might plug a diagnostic system or a 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.
Sandia is part of a group Dennis and other industry partners formed in November 2006 — the Aerospace Industry Steering Committee for Structural Health Monitoring — to address the growing demand for standardized SHM procedures and certification requirements. 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. In a Laboratory Directed Research and Development (LDRD) effort, SHM principles are being applied to monitoring bridges, buildings, and other civil infrastructures. The work has produced a mountable eddy-current sensor that uses electromagnetic waves to detect deep subsurface cracks in metal structures.
Another program with Syncrude Canada Ltd. is studying the application of SHM sensors for real-time health monitoring of mining and oil-recovery equipment.
“In other words, there is recognition that SHM’s time has come, an opinion you would not have heard from many people a few years ago,” says Dennis.
In situ monitoring with CVM sensors
Sandia has developed or evaluated several types of inexpensive, reliable sensors that can be retrofitted into aircraft structures for structural health monitoring (SHM).
One promising sensor, a Comparative Vacuum Monitoring (CVM) sensor, is a self-adhesive rubber patch, ranging from dime- to credit-card-sized. The rubber’s underside is laser-etched with rows of tiny, interconnected channels or galleries, to which 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 system can be set up to alarm or signal a remote site. The sensors — manufactured by Structural Monitoring Systems Inc. (SMS) of Australia — are inexpensive, reliable, durable, and easy to apply, says Dennis. More important, they provide equal or better sensitivity than is achievable with conventional inspection methods, he says.
The Sandia team first conducted laboratory evaluations of CVM sensors on different materials with a variety of thicknesses and structural shapes. Field evaluations of 22 CVM sensors on three commercial aircraft — a Northwest 757 and 767 and a Delta DC-9 — beginning in April 2005 helped validate the lab tests.
As a result of the work, 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.
This recognition of in situ crack detection as an allowable inspection method is an aviation industry first, says Dennis.
The approval is the culmination of a comprehensive, two-year validation program by
Sandia in cooperation with the FAA, Boeing, SMS, a number of US airlines, and the University of Arizona. Work on specific applications for Southwest, Northwest, and Delta Airlines is underway.
Sandia also is developing or evaluating a variety of other sensor systems — or miniaturizing technologies into mountable sensors — that can detect cracks, corrosion, and other flaws in structural elements.
Technologies being considered include flexible eddy-current arrays, capacitive micro-machined ultrasonic transducers, piezoelectric transducers that can interrogate materials over long distances, acoustic emission sensors, embedded fiber optics, nickel strip magnetostrictive sensors, and conducting paint whose resistance changes when cracks form underneath.