Arrays of fiber laser sensors can detect and locate cracks in a material

Materials undergoing irreversible structural changes release mechanical energy in the form of acoustic emissions. When these emissions are weak, their detection requires high resolution sensors, such as fiber laser strain sensors, which have sensitivity down to thermodynamic limits. Williams et al. demonstrated both the sensitivity of a single fiber laser sensor and the capability of an array of these sensors to locate the acoustic emission source.

When acoustic emission signals in the form of strain arrive at a sensor, they cause a change in the laser’s cavity length, which alters its frequency. In this way, a single sensor is enough for detecting acoustic emissions generated by a crack. When a number of these sensors are multiplexed, time differences in the arrival of the signal to each sensor can be used to locate the crack, the source of the emission.

“Fiber sensors are small and light and can be multiplexed, so if multiple sensors are required for a particular application, only a single fiber runs from the structure being tested to the measurement system,” said author Caitlin Williams.

In a test of the setup, multiplexed fiber laser sensors were used to locate a crack on an aluminum panel. The arrival time information was found to reliably locate the source of the acoustic emission.

The authors note these arrayed sensors will be useful for crack detection when a visual inspection is not possible or when sensor weight and corrosion resistance are significant factors. These applications include naval and aerospace structures. However, additional work remains to be done to study larger test structures and different materials and to use accelerated fatigue methods with lower background noise in the frequency band of interest.

Source: “Multichannel fiber laser acoustic emission sensor system for crack detection and location in accelerated fatigue testing of aluminum panels,” by Caitlin R. S. Williams, Meredith N. Hutchinson, Joseph D. Hart, Marriner H. Merrill, Peter Finkel, William R. Pogue III, and Geoffrey A. Cranch, APL Photonics (2020). The article can be accessed at https://doi.org/10.1063/1.5133040 .