We report a Brillouin-based fully distributed and dynamic monitoring of the strain induced by a propagating mechanical wave along a 20 m long composite strip, to which surface a single-mode optical fiber was glued. Employing a simplified version of the Slope-Assisted Brillouin Optical Time Domain Analysis (SA-BOTDA) technique, the whole length of the strip was interrogated every 10 ms (strip sampling rate of 100 Hz) with a spatial resolution of the order of 1m. A dynamic spatially and temporally continuous map of the strain was obtained, whose temporal behavior at four discrete locations was verified against co-located fiber Bragg gratings. With a trade-off among sampling rate, range and signal to noise ratio, kHz sampling rates and hundreds of meters of range can be obtained with resolution down to a few centimeters.
This paper presents the design and flight validation of an embedded fiber Bragg gratings (FBG) based structural health monitoring (SHM) system for the Indian unmanned aerial vehicle (UAV), Nishant. The embedding of the sensors was integrated with the manufacturing process, taking into account the trimming of parts and assembly considerations. Reliable flight data were recorded on board the vehicle and analyzed so that deviations from normal structural behaviors could be identified, evaluated and tracked. Based on the data obtained, it was possible to track both the loads and vibration signatures by direct sensors' cross correlation using principal component analysis (PCA) and artificial neural networks (ANNs). Sensor placement combined with proper ground calibration, enabled the distinction between strain and temperature readings. The start of a minor local structural temporary instability was identified during landing, proving the value of such continuous structural airworthy assessment for UAV structures.
The present paper reports the outcomes of activities concerning a real-time SHM system for debonding flaw detection based on ground testing of an aircraft structural component as a basis for condition-based maintenance. In this application, a damage detection method unrelated to structural or load models is investigated. In the reported application, the system is applied for real-time detection of two flaws, kissing bond type, artificially deployed over a full-scale composite spar under the action of external bending loads. The proposed algorithm, local high-edge onset (LHEO), detects damage as an edge onset in both the space and time domains, correlating current strain levels to next strain levels within a sliding inner product proportional to the sensor step and the acquisition time interval, respectively. Real-time implementation can run on a consumer-grade computer. The SHM algorithm was written in Matlab and compiled as a Python module, then called from a multiprocess wrapper code with separate operations for data reception and data elaboration. The proposed SHM system is made of FBG arrays, an interrogator, an in-house SHM code, an original decoding software (SW) for real-time implementation of multiple SHM algorithms and a continuous interface with an external operator.
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