Strain Wave Acquisition by a Fiber Optic Coherent Sensor for Impact Monitoring

Abstract

A novel fiber optic sensing technology for high frequency dynamics detection is proposed in this paper, specifically tailored for structural health monitoring applications based on strain wave analysis, including both passive impact identification and active Lamb wave monitoring. The sensing solution relies on a fiber optic-based interferometric architecture associated to an innovative coherent detection scheme, which retrieves in a completely passive way the high-frequency phase information of the received optical signal. The optical fiber can be arranged into different configurations in order to meet the requirement of the specific application, the sensor sensitivity being maximized while still ensuring a limited gauge length if a local measure is required. For the active Lamb wave monitoring, this results in a sensing fiber arranged in multiple loops glued on an aluminum thin panel in order to increase the phase signal relative only to the sensing points of interest. Instead, for passive impact identification, the sensitivity is simply increased by exploiting a longer gauge length glued to the structure. The fiber optic coherent (FOC) sensor detects the strain waves emitted by a piezoelectric transducer placed on the aluminum panel or generated by an impulse hammer, respectively. The FOC sensor measurements have been compared with both a numerical model based on Finite Elements and traditional piezoelectric sensors, confirming a very good agreement between experimental and simulated results for both the active and passive impact monitoring scenarios.

Applicability of an interferometric optical fibre sensor for shallow landslide monitoring – Experimental tests

“…Hence, the proposed detection scheme allows one to overcome issues that, until recently, have limited the actual in-situ applicability of this technology that features cost-effectiveness, broadband and high sensitivity. The versatility of the proposed sensing technique has already been widely proven in a variety of engineering contexts, for example, for impact damage diagnostic in aeronautic structures, cavitation onset detection in hydraulic pipes, as well as for predictive maintenance applications in industrial production process (Boffi et al, 2019;Sbarufatti et al, 2017;Ferrario et al, 2016). The sensing scheme can be defined as "integral" as each single measurement provides the sum of the phase difference accumulated by the optical signal during its propagation along the entire length of the sensing cable (ΣΔθ(t)).…”

“…Picarelli et al (2015) report an updated Brillouin frequency shift, that is, a new measurement each 15 min. While for some slow-moving phenomena this sort of temporal resolution would be sufficient, the abrupt nature of shallow failures certainly requires faster monitoring updates that can be easily guaranteed by the proposed interferometric sensor, featuring a wide acquisition bandwidth capability, even up to the MHz, limited only by the adopted ADC (Analog to Digital Converter) sampling board (Sbarufatti et al, 2016). In the presented experiments the sampling frequency of the interferometric sensor, set to 20 kSample/s, was wide enough to yield high precision measurements.…”

Coherent Fiber-Optic Sensor for Ultra-Acoustic Crack Emissions

“…One of the most recent contributions, presented by the authors in a previous work [ 19 ], analyzed a novel fiber-optic based solution for impact detection on an aluminum plate. In this case, the sensor showed promising resulta when compared with commercial PZT sensors.…”

“…In the second configuration instead, the fiber optic is wound like a spiral, and this latter is set at with respect to the metal surface in order to be adhesively bonded directly onto the structure. This solution was investigated by the authors in a previous work [ 19 ], in which this layout showed promising results in impact detection on an aluminum plate. Regarding this configuration, the advantages include the direct detection of waves traveling in the metal and the possibility to increase the sensitivity by adding more loops to the ring, which, however, could compromise the sensor punctuality in the case of a high number of loops.…”

“…As stated in [ 19 ], a further key parameter affecting the FOS response was the ratio between the ultrasonic wavelength and the actual fiber length bonded to the structure. In fact, the sensor performed a spatial integration of the ultrasonic signals over the entire sensing length: if an excessive sensing length was chosen, the overall integration drastically degraded the sensor sensitivity.…”