Dr Geoffrey Cranch, a research physicist at the Department of Optics at the US Naval Research Laboratory (NRL), says there is currently no US military service that uses in situ technology to manage the structural health of its equipment. "An automated, in-situ structural health monitoring (SHM) system can monitor critical structural parameters such as temperature, strain, shock and crack, and can reliably detect damage before the damage reaches a critical level," he said. A sensor for detecting the acoustic emission signal associated with the occurrence and growth of the crack in near real time. Such sensors must be smaller than existing electronic products lighter, the sensitivity of a considerable or improved, smaller.

By the US Department of Naval Research Materials Science to provide part of the investment, NRL is developing a 100mw laser pointer sensor, width is about hair width. During the test, the researchers installed a distributed feedback fiber laser acousto-optic sensor in a set of aluminum rivets and measured a 0.5-MHz bandwidth acoustic emission signal generated in a two-hour accelerated fatigue test, using an equivalent Of the electrical sensors.

Embedded sensors are used to solve the acoustic event of periodic "surface abrasion" of the rivet and acoustic emission from the crack information. The time-lapse imaging of the lap will allow the observed fracture to be correlated with the measured signal. In addition to crack detection, the fiber laser sensor also demonstrates the ability to measure the impact of damage and has the potential to integrate with existing fiber strain and temperature sensing systems. This provides a multi-parameter sensing capability to meet the security requirements of the SHM system and significantly reduces total cost of ownership.

"Our research team has demonstrated that the fiber 50mw laser pointer technology is capable of detecting ultrasonic AE emissions from cracks in a simulated fatigue environment. A new part of this work is fiber laser technology and how it can be applied." Sound signals from the crack can also be used Piezoelectric sensors, which have driven the existing failure prediction work. However, Cranch said piezoelectric technology because of its large geometric size and multi-channel capacity of cable, in many applications is not practical.

Cranch believes the technology has great potential in areas beyond the military, where he focuses on naval platforms such as airplanes, ships and submarines, but the technology can also be used in civil aircraft as well as in bridges and buildings. At present, no other fiber-optic sensor can match the fiber laser acoustical emission sensor in the laboratory to achieve performance. Fiber laser sensors have demonstrated comparable acoustic sensitivity to existing electrical sensors, or even higher. The system has been able to integrate multiple fiber blue laser pointer sensors into a bundle of fibers. The present work is explaining useful data for acoustic emission data to calculate failure probabilities.

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