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, for 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 sensing fiber can be arranged into different layouts, depending on the requirement of the specific application, in order to enhance the sensor sensitivity while still ensuring a limited gauge length if punctual measures are required. For 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 only in correspondence to the sensing points of interest. Instead, for passive impact identification, the required sensitivity is guaranteed by simply exploiting a longer gauge length glued to the structure. The fiber optic coherent (FOC) sensor is exploited to detect 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 good agreement between experimental and simulated results for both active and passive impact monitoring scenarios.

Keywords: Lamb wave; coherent detection; finite element model; impact force reconstruction; interferometric fiber optic sensors; modelling.

Figure 1

Polarization and phase diversity coherent detection scheme exploiting a 90° optical hybrid.

Figure 2

Phase-diversity coherent detection scheme exploiting a 3 × 3 optical coupler. The third 3 × 3 output is properly terminated to avoid light reflections.

Figure 3

Test layout for (a) active and (b) passive impact monitoring cases. FOCS: fiber optic coherent sensor. PZT: piezoelectric.

Figure 4

(a) Schematic of the actuator and sensor layout for active monitoring tests; (b) detail of the sensing optical fiber arranged in 20 loops and glued to the aluminium plate for active monitoring tests.

Figure 5

(a) Detail of the sensing optical fiber glued to the aluminium plate for passive impact monitoring; (b) Power spectral density of the impact response of the PZT sensor located in the vicinity of the FOC sensor.

Figure 6

Example of tone burst displacements as a result of the axis-symmetric model.

Figure 7

Schematic of the axis-symmetric Finite Element model (FEM) of the plate, with indication of the three different load configurations selected to model A0 and S0 Lamb wave propagation modes and the impact.

Figure 8

Toneburst signals acquired by the FOC sensor as a function of different number of fiber loops.

Figure 9

Gauge length effect on the Lamb wave signal acquired by the FOC sensor: (a) experimental results and (b) numerical simulation. Results are presented as a function of $L_g/\lambda$.

Figure 10

Comparison of FOC sensor vs. FEM simulated signals at increasing Lamb wave frequencies. The analytical time of arrival (ToA) for A0 and S0 modes is also provided as reference.

Figure 11

Comparison of FOC and PZT sensors vs. FEM simulated signals at 175 kHz Lamb wave frequency. The analytical ToA for A0 and S0 modes is also reported as reference.

Figure 12

Comparison of FOC (a,b) and PZT (c,d) sensors vs. FEM simulated signals after an impact event. On the left (a,c), no strain wave reflection is included in the FEM approximation. On the right (b,d), FEM is designed to account for first boundary reflections.

Figure 13

Schematic of the strain wave reflections at boundary edges after impact.