Experimental method of temperature and strain discrimination with fiber Bragg gratings (FBGs) sensors embedded in carbon fiber-reinforced plastic is proposed. The method is based on two-fiber technique, when two FBGs inscribed in different fibers with different sensitivities to strain and/or temperature are placed close to each other and act as a single sensing element. The nonlinear polynomial approximation of Bragg wavelength shift as a function of temperature and strain is presented for this method. The FBGs were inscribed with femtosecond laser by point-by-point inscription technique through polymer cladding of the fiber. The comparison of linear and nonlinear approximation accuracies for array of embedded sensors is performed. It is shown that the use of nonlinear approximation gives 1.5–2 times better accuracy. The obtained accuracies of temperature and strain measurements are 2.6–3.8°C and 50–83 μεin temperature and strain range of 30–120°C and 0–400 με, respectively.
This paper describes research in the field of fiber optic nondestructive testing and structural health monitoring (SHM) of carbon fiber reinforced polymers (CFRP) by the use of integrated optical fiber sensors (OFS) based on fiber Bragg gratings (FBG). Basic mathematical expressions that represent optical SHM of composites are presented. Some new relationships are derived by considering the non-linear character of the FBG-sensor response and the combined effect of temperature and deformation. Both linear and non-linear coefficients of the sensor elements, as well as combined strain-temperature coefficients before and after the sensors are embedded into the composite panels, have been obtained. Experimental results on the strain state of CFRP under static and dynamic loads demonstrate the effectiveness of the proposed non-linear model for evaluating deformation in composites. It is shown that integrated OFS's allow SHM of composite parts when mechanically loaded to failure, and that they can provide the actual level of strain in the composite parts in real time. SHM improves the operational safety of highly loaded and/or critical aerospace structures by providing real-time stress data, which would permit data-based decisions on overload conditions or imminent failure. Additionally, actual stress data from CFRP samples, or from real parts in use, could show whether the design of the parts should be changed to improve safety margins or to reduce weight.
KeywordsStructural health monitoring • Nondestructive testing • Carbon composite • Optical fiber sensor • Fiber Bragg grating • Deformation • Strain and temperature measurement B Marina Kuimova
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