Navy

Fiber laser sensor for structural health monitoring

System is 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

Sensors

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 AE. A typical AE event is shown in the lower right plot. The amplitude of the AE events as a function of time is shown in the lower left plot. Large increases in AE event amplitude are seen when the cracks grow. (U.S. Naval Research Laboratory)

Structural health monitoring (SHM) represents a group of early failure detection processes for a wide variety of structures including bridges, buildings, dams, and runways. Given the aging infrastructure in the United States, SHM is an active field of development.

New SHM systems require large numbers of miniature, lightweight sensors capable of measuring a range of parameters. Fiber optic sensors are a promising technology to provide this measurement capability with large numbers of sensors multiplexed onto a single fiber capable of measuring strain, temperature, and ultrasonics. Fiber optic sensors provide a minimally invasive sensing capability with a greatly reduced number of interconnects and cabling, which is expected to lead to greatly improved system reliability.

There is also interest in measuring high-frequency signals generated either by structural fatigue through crack formation, in the form of acoustic emission (AE), as well as actively generated signals such as Lamb waves generated by piezoelectric sources for damage detection. Piezoelectric sensor performance can be tailored in terms of their size, response and electronic amplification circuitry to provide the required performance for a given application; however, these sensors are bulky and are not practical to be implemented in large numbers.

As maintainers of a large number of engineering structures and being reliant on their operation, the U.S. Navy is actively researching and developing advanced SHM solutions. One such approach utilizes a fiber laser acoustic emission sensor which provides extremely high sensitivity required for passive AE measurement while being capable of integration with strain and temperature sensors. The sensors are integrated into shallow grooves or channels made in panels and perform AE measurements without introducing structural weakness or fatigue into the structure.

The technology measures ultrasonic signals caused by acoustic emission in structures using fiber laser strain sensors. For example, sources of AE include all ultrasonic signals generated from the joints associated with fretting and rubbing of the materials as well as the crack signals generated from material fatigue. In addition, this invention measures Lamb wave signals generated from other sources of acoustic emissions, such as piezoelectrics and laser-induced ultrasound.

In addition to the crack detection, the fiber laser sensor also is capable of measuring compromising impacts and has 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.

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