The US Navy pioneered fiber laser sensors for structural health monitoring
Recently, researchers from the Optical Science Department and the Materials Science Department of the US Naval Research Laboratory (NRL) successfully used the distributed feedback fiber laser acoustic emission sensor to detect the acoustic emission signal generated by the crack in the riveted lap joint.
Defect detection of riveted lap joints with fiber laser acoustic emission sensors: This figure shows the initiation and growth of cracks between rivets in the upper left lap joint. The fiber laser sensor (see the illustration at the top right) is mounted on the inspection structure and measures the AE signal generated by the crack defect, which the associated software records as an AE event. A typical data record is shown at the bottom right. As can be seen from the figure, when a defect grows, AE event amplitude will be greatly increased. Source: US Naval Research Laboratory.
"We studied a field structure health monitoring (SHM) automation system that effectively monitors key structural parameters such as temperature, internal stress, impact, and crack defects, and reliably corrects the structural damage before it reaches a critical level It is detected to increase structural safety and speed of information feedback while reducing operational costs for the Navy platform, "said Dr. Geoffrey Cranch, a physicist from the Department of Optometry, who said:" At present, there is no U.S. service that uses in-situ technology to manage Equipment health structure. " In order to achieve this goal, it is essential to have a sensor capable of detecting acoustic emission signals related to the appearance and growth of defects such as cracks in near real time. And, these sensors must be smaller, lighter, easier to manipulate than most existing electronics and have comparable or improved sensitivities. The ultimate goal is to make these components of the system take up little space, and high reliability.
Part of the research funding is provided by the Materials Science Division of the US Naval Research Office (ONR), which is developing a laser sensor with a width about the width of a human hair. During the test, the researchers installed a distributed feedback fiber laser acoustic emission 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 while using An equivalent electrical sensor measures.
This embedded sensor can be used to solve the periodic "fretting" acoustic events of rivets and to detect acoustic emission information from the crack. Time-lapse imaging of the lap joint will allow the observed fracture to be correlated with the measured signal.
In addition to crack detection, this fiber laser sensor is also capable of effectively detecting the impact damage impact and, in addition, has the potential to integrate with existing fiber strain and temperature sensing systems. This provides a multi-parameter sensing capability to meet operational safety requirements of on-site structural health monitoring systems, and it is worth mentioning that this will also significantly reduce overall cost.
"Our research team has demonstrated the ability of this fiber laser sensing technology to detect ultrasonic-induced acoustic emissions from a crack in a simulated fatigue environment," said Cranch. "The novelty of this study is primarily the fiber laser pass Sense of technology and its application, etc. ".
Acoustic signals generated from defects such as cracks can also be measured using piezoelectric sensors and this technique also facilitates existing fault prediction. However, piezoelectric sensing technology is not as practical in many aspects due to its large device size and limited distributed monitoring capabilities.
Cranch emphasized that this technology will likely be applied in many aspects other than the military field. "Our research and application focus is on defense aspects such as navy, such as aircraft, ships and submarines, etc. If some bridges or structures contain critical components that are susceptible to fatigue and failure, then the same technique can be used for these Structure for continuous monitoring. "
At present, no other intrinsic type optical fiber sensor is capable of rivaling the performance achieved by a fiber laser acoustic emission sensor in a laboratory test. Compared with some existing electrical sensing technologies, fiber laser sensors have been shown to have comparable or even higher acoustic emission signal sensitivity. The system is already capable of integrating multiple fiber laser sensors into a bundle of fibers. Currently, the research team is currently working mainly to understand and explain the AE data to calculate some useful parameters (such as failure probability, etc.). Future improvements are mainly focused on the implementation of phased array beamforming technology to effectively identify the specific location of defects such as cracks.
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