Recently, researchers from the Optical Science Department of the US Naval Research Laboratory (NRL) and the Materials Science Department have successfully used distributed feedback fiber green laser pointer acoustic emission sensors to detect acoustic emission signals generated by cracks in riveted lap joints.
Defect detection of riveted lap joints using fiber laser acoustic emission sensor: This picture shows the initiation and growth of cracks between rivets in the upper left lap joint. The fiber laser sensor (see the illustration on the upper right) is installed on the inspection structure to measure the acoustic emission signal generated by the crack defect, and the related software records it as an acoustic event (AE event). A typical data record is shown in the lower right figure. It can be found from the figure that when the defect grows, the AE event amplitude will greatly increase. Image source: US Naval Research Laboratory.
“We have studied an on-site structural health monitoring (SHM) automation system, which can effectively monitor the key parameters of the structure, such as temperature, internal stress, impact, and crack defects; and can reliably monitor the structure before the damage reaches a critical level. It is detected to increase structural safety and information feedback speed, while reducing the operating cost of naval platforms,” said Dr. Geoffrey Cranch, a physicist from the Department of Optical Sciences: “Currently, there is no US service that uses in-situ technology to manage Healthy equipment structure”. In order to achieve this goal, the most critical is the need for a sensor that can detect acoustic emission signals related to the appearance and growth of defects such as cracks in near real time. In addition, this type of sensor must be smaller, lighter, easier to manipulate, and have the same or improved sensitivity than most existing electronic products. The ultimate goal is to make the system of these components small in space and high in reliability.
Part of the research funding provided by the US Office of Naval Research (ONR) Materials Science Department, NRL is developing a laser sensor that is about the width of a human hair. During the test, the researchers installed a distributed feedback fiber laser acoustic emission sensor in a group of aluminum rivets, and measured a 0.5MHz bandwidth acoustic emission signal generated during a two-hour accelerated fatigue test. An equivalent electrical sensor performs the measurement.
This kind of embedded sensor can be used to solve the acoustic events of periodic “fretting wear” of riveted parts and to detect acoustic emission information from cracks. Time-lapse imaging of the lap joint will be able to establish a correlation between the observed fracture and the measured signal.
In addition to crack detection, this fiber laser sensor can also effectively detect the impact of impact damage. In addition, the sensor also has the potential to be integrated with existing fiber optic strain and temperature sensing systems. This provides a multi-parameter sensing capability to meet the operational safety requirements of the on-site structural health monitoring system. It is worth mentioning that this will also significantly reduce the overall cost.
“Our research team has demonstrated the ability of this fiber laser sensing technology to detect the ultrasonic acoustic emission signals generated by cracks in a simulated fatigue environment,” said Cranch: “The novelty of this research lies mainly in fiber laser transmission. Sense technology and its application, etc.”
Acoustic signals generated from defects such as cracks can also be measured by using piezoelectric sensors, and this technology also promotes existing failure prediction work. However, piezoelectric sensing technology does not have much practicability in many aspects due to its large device size and limited distributed monitoring capabilities.
Cranch emphasized that this technology will have the potential to be used in many areas other than the military. “Our research and application focus is on the navy and other defense aspects, such as aircraft, ships, and submarines. If some bridges or building structures contain key components that are susceptible to fatigue and failure, then the technology can also be used for these Continuous monitoring of the structure”.
At present, there is no other intrinsic fiber sensor that can match the performance of fiber laser pointer acoustic emission sensor tested in the laboratory. Compared with some existing electrical sensing technologies, fiber laser sensors have been proven to have a comparable or even higher sensitivity to acoustic emission signals. The system can already integrate multiple fiber laser sensors into a bundle of fibers.