Growing damage in various materials and constructions often causes high-energy high-frequency acoustic waves. By monitoring these so called 'acoustic emission' events, growing damage can be detected in an early stage before the damage results in catastrophic failure or loss of material functionality. For monitoring large composite structures, an optical acoustic emission monitoring system is presented. The sensing part of the sensor consists of a single mode fiber that is used in a polarimetric configuration. This sensor is combined with transient signal detection techniques (filtering, frame-to-frame analysis, recursive noise estimation) to detect perturbations of the light in the sensor. In this paper the acoustic monitoring capabilities of the system are demonstrated by carrying out bending tests on a short test model for an Intelligent Power and Data Transmitting Composite Coiled Tubing (PDT-coil).
Permanent damage in various materials and constructions often causes high-energy high-frequency acoustic waves. To detect those so called ‘acoustic emission (AE) events’, in most cases ultrasonic transducers are embedded in the structure or attached to its surface. However, for many applications where event localization is less important, an embedded low-cost multimode optical fiber sensor configured for event counting may be a better alternative due to its corrosion resistance, immunity to electromagnetic interference and light-weight. The sensing part of this intensity-modulated sensor consists of a multimode optical fiber. The sensing principle now relies on refractive index variations, microbending and mode-mode interferences by the action of the acoustic pressure wave. A photodiode is used to monitor the intensity of the optical signal and transient signal detection techniques (filtering, frame-to-frame analysis, recursive noise estimation, power detector estimator) on the photodiode output are applied to detect the events. In this work, the acoustic emission monitoring capabilities of the multimode optical fiber sensor are demonstrated with the fiber sensor embedded in the liner of a Power Data Transmission (PDT) coil to detect damage (delamination, matrix cracking and fiber breaking) while bending the coil. With the Hankel Total Least Square (HTLS) technique, it is shown that both the acoustic emission signal and optical signal can be modeled with a sum of exponentially damped complex sinusoids with common poles.