Structural Health Monitoring
Structural health monitoring needs technologies of real-time monitoring and nondestructive detection, which are difficult to achieve using conventional methods. Numerous advantages of FFPI sensors, such as immunity to electromagnetic interference, less intrusive size, and greater resistance to corrosion, have made them attractive for structural health monitoring of infrastructures including bridges, buildings, dams, et al.
Figure 6.1 FP pressure sensor.
Engineering applications of FFPI sensors have been demonstrated by Gahan and Donlagic and their coworkers [4,5], respectively. Subsequently, in 2007, Needham invented an FFPI sensor with an air gap . It can use a linear array processor for signal processing in industrial applications, such as gas turbines, engines, pressure vessels, pipelines, buildings, etc., to provide information on pressure, temperature, strain, vibration, or acceleration. An EFFPI strain sensor with sensitivity enhancement was developed for measuring strain of steel plate, as described in [7,8]. The proposed sensor is shown in Figure 6.1. The sensor is composed of a lead-in fiber that also forms the first EFFPI semi-reflective surface, an outer (semi-conical) cladding, a second EFFPI semi-reflective surface, a gutter that surrounds the second EFFPI semi-reflective surface, and a tail fiber that can be of arbitrary length. The strain sensitivity can be improved significantly as the gauge length Lg is much longer than the cavity length Lc.
Bridge’s health monitoring also can be achieved based on EFFPI sensors . Short-length EFFPI strain sensors, which are insensitive to the change of ambient temperature, have been successfully deployed on the concrete-based Hongcaofang Crossroads Bridge in Chongqing, China, as shown in Figure 6.2, which is 210 m long over seven spans. A set of four EFFPI strain sensors were attached to the centers of two spans to measure the static strain of the bridge. The results in Figure 6.3 indicate that this concrete bridge expands with the increase in temperature in daytime and contracts at night due to the temperature drop.
The strain peaks and troughs are just in accordance with the values of the temperature at 3:00 p.m. and 4:00 a.m., respectively. In order to evaluate the accuracy and repeatability of the EFFPI strain sensor,
Figure 6.2 Photograph of the Hongcaofang Crossroads Bridge.
Figure 6.3 Results of the EFFPI strain sensor.
two experiments have been carried out based on a standard cantilever calibration setup with the standard electrical strain gauge as a reference. The results showed that the accuracy and repeatability of the EFFPI strain sensor were around ±1|1?.