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Crack detection in riveted lap joints with fiber laser acoustic emission sensors. The graphic shows the initiation and growth of cracks between rivets in a lap joint in the top left. A fiber laser sensor (illustrated in the top right inset), adhered to the structures, measures the acoustic emission signals generated by the cracks and software records them as acoustic events. A typical event is shown in the lower right plot. The amplitude of the events as a function of time is shown in the lower left plot. Large increases in an acoustic event amplitude are seen when the cracks grow. (U.S. Naval Research Laboratory)

Fiber laser sensor technology for structural health monitoring

According to Dr. Geoffrey Cranch, a research physicist from the Optical Sciences Division of the U.S. Naval Research Laboratory (NRL), none of the U.S. military services are currently using in-situ technologies to manage the structural health of their assets.

“An automated, in-situ structural health monitoring (SHM) system, capable of monitoring key structural parameters such as temperature, strain, impacts, and cracks, and capable of reliably detecting damage well before reaching a critical level is needed to increase safety and readiness while lowering operational cost of Navy platforms,” he said.

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

Along those lines and with funding partially provided by the Office of Naval Research Materials Science Division, an NRL-developed laser sensor, about the width of a human hair, was designed to be integrated into a shallow groove. In testing the application, 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 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. Time-lapse 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 an SHM system as well as a 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,” said Cranch. “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, Cranch says that piezoelectric technology is generally not practical for many applications due to its large size and limited multiplexing capability.

Cranch believes that the technology has possible applications beyond the military. “Our focus is on Navy platforms, such as aircraft, ships and submarines, but the technology could also be used on civilian aircraft,” he said. “Applications to bridges and buildings are also possible if there are critical parts prone to fatigue and failure that would benefit from continuous monitoring.”

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 lasers 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.

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