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.
It has been proven that the embedment of optical fibers into a composite material could offer an alternative to robust piezoelectric transducers used for Acoustic Emission (AE) monitoring. In this configuration optical fibers are used as intensity-modulated sensors. A set of propagating elastic waves is generated whenever damage occurs in the composite material. These waves locally modify the optical and geometrical properties of the optical fiber and hence can be detected by them as a transient signal that modulates the light intensity. In this paper a method for detecting the transients by on-line signal processing is presented. It is then applied to optical signals resulting from tensile tests performed on CFRP composites material with embedded optical fibers. By means of the Short-Time Fourier Transform (STFT), the level of the noise added to the signal is estimated by filtering the time trajectories. This filter is continuously adapted according to the principle of minimization
of the mean squared error. Finally the detection is achieved by a constant false alarm rate power-law detector. This technique is fast and doesn't take into account neither the statistical distribution of the noise nor the frequency content of the transients as long as the frequency component distribution can be approximated by an exponential law. The detected transient features can be correlated with the AE results but an off-line analysis and classification is still needed.
An intensity modulated sensor, based on the microbending concept, has been incorporated in laminates produced from a C/epoxy prepreg. Pencil lead break tests (Hsu-Neilsen sources) and tensile tests have been performed on this material. In this research study, fibre optic sensors will be proven to offer an alternative for the robust piezoelectric transducers used for Acoustic Emission (AE) monitoring. The main emphasis has been put on the use of advanced signal processing techniques based on time-frequency analysis. The signal Short Time Fourier Transform (STFT) has been computed and several robust noise reduction algorithms, such as Wiener adaptive filtering, improved spectral subtraction filtering, and Singular Value Decomposition (SVD) -based filtering, have been applied. An energy and frequency -based detection criterion is put forward to detect transient signals that can be correlated with Modal Acoustic Emission (MAE) results and thus damage in the composite material. There is a strong indication that time-frequency analysis and the Hankel Total Least Squares (HTLS) method can also be used for damage characterization. This study shows that the signal from a quite simple microbend optical sensor contains information on the elastic energy released whenever damage is being introduced in the host material by mechanical loading. Robust algorithms can be used to retrieve and analyze this information.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.