Abstract
We demonstrate the influence of the relative humidity (RH) on the wavelength of fiber Bragg grating sensors (FBGS), performing tests with five FBGS at different humidity and temperature conditions. These tests were performed in a climate chamber whose RH changes according to a scheduled profile from 30% to 90%, in steps of 10%. These profiles were repeated for a wide range of temperatures from10∘Cto70∘C, in steps of10∘C. Two different types of instrumentation methods have been tested, spot welding and epoxy bonding, in two different materials, steel and carbon fiber reinforced polymer (CFRP). We discuss the results for each type of sensor and instrumentation method by analyzing the linearity of the Bragg wavelength with RH and temperature.
Highlights
Optical fiber sensors are one of the most suitable options for aircraft structural health monitoring (SHM) systems due to the advantages they offer: immunity to electromagnetic noise, with the consequent increase of safety and a better protection against loss of information; little weight compared with traditional monitoring systems, leading to a reduction of fuel consumption; and wide working temperature ranges [1]
In order to check whether the λFBG 1 Bragg wavelength suffers from hysteresis or not, we have repeated this test for a different sequence of variations of the relative humidity (RH)
Such a reconnection allowed us to obtain measurements from every sensor, since the broken end of fiber Bragg grating (FBG) 1 was placed at the extreme of the array. These results serve us to confirm that the graph of the FBG 1 sensor in Figure 4 shows only the influence of the RH on the coating of the FBG itself, since it is detached from the metallic carrier that holds the FBG in tension, avoiding the λ λ
Summary
Optical fiber sensors are one of the most suitable options for aircraft structural health monitoring (SHM) systems due to the advantages they offer: immunity to electromagnetic noise, with the consequent increase of safety and a better protection against loss of information; little weight compared with traditional monitoring systems, leading to a reduction of fuel consumption; and wide working temperature ranges [1]. When a broad-spectrum light is sent to an FBG core, a certain wavelength of this light is reflected, called Bragg wavelength, allowing the rest of the incoming light to pass through without additional modification [3]. This reflected Bragg wavelength changes according to the applied strain and to temperature conditions. In order to separate the temperature effect from the strain we have used a not bonded FBG as a reference so that we could subtract this temperature effect from the FBGS bonded in the specimen [4] With this procedure the measurement of the real strain on the sample is assured
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