Washington, DC - Navy researchers at the U.S. Naval Research Laboratory (NRL) have demonstrated new fiber optic sensor technology suitable for an in-situ structural health monitoring (SHM) system.

This technology enables autonomous monitoring of key structural parameters such as acoustic emission, temperature, and strain. The information can be used to monitor for damage due to impacts and cracks well before reaching critical levels.

"Primarily focused on monitoring the structural integrity of Navy assets, the technology may also have application on civilian aircraft, ships, and possibly bridges and buildings where continuous monitoring of critical components prone to fatigue and failure would prove beneficial," said Dr. Geoffrey Cranch, research physicist, NRL Optical Sciences Division.

To accomplish this goal, sensors that can detect acoustic emission signatures associated with crack initiation and growth, in near real-time, are required. Such a sensor must be smaller and lighter than existing electrical equivalents, possess comparable or improved sensitivity, be easily multiplexed, and achieve all of these components with a small system footprint and high reliability.

Funded partially by the Office of Naval Research (ONR) Navy Materials Division, the NRL-developed laser sensor is integrated into a shallow groove formed in the lap joint and consists of a single fiber, similar in width to a human hair. In testing the application, optical and material science researchers installed distributed feedback fiber laser acoustic emission sensors into a series of riveted aluminum lap joints and measured acoustic emission over a bandwidth of 0.5 megahertz (MHz) generated during a two-hour accelerated fatigue test. Measurements were also taken with an equivalent electrical sensor.

The embedded sensors were shown to resolve low-level acoustic events generated by periodic "fretting" from the riveted joint in addition to acoustic emissions from crack formation. Timelapse imagery of the lap joint enabled correlation of the observed fracture with the measured signals.

In addition to crack detection, the fiber laser sensor also proved capable of measuring compromising impacts, and the potential to integrate with existing fiber optic strain and temperature sensing systems. Combined, this provides a multi-parameter sensing capability for meeting the full operational safety requirements for a SHM system, as well as significantly lower total ownership costs.

"Our research team has demonstrated the ability of this fiber laser technology to detect acoustic emission at ultrasonic frequencies from cracks generated in a simulated fatigue environment," Cranch said. "The novel part of this work is the fiber laser technology and how it is being applied."

Acoustic signals from cracks can also be measured using piezoelectric sensors, and this technology has driven the existing work on failure prediction. However, the piezoelectric technology is generally not practical for many applications due to its large size and limited multiplexing capability.

Currently there is no other intrinsic optical fiber sensor capable of matching the performance obtained in the laboratory from the fiber laser acoustic emission sensor. The fiber laser sensor has demonstrated acoustic sensitivity comparable to, or greater than that achieved by existing electrical sensors. This system has now been expanded to multiplex many fiber laser sensors onto a single fiber. Efforts are currently underway to interpret the acoustic emission data to calculate useful metrics such as probability of failure. Future enhancements include implementing phased array beam forming techniques to enable crack location.