<title>Fiber optic sensor for continuous health monitoring in CFRP composite materials</title>

Abstract

An intensity modulated sensor, based on the microbending concept, has been incorporated in laminates produced from a C/epoxy prepreg. Pencil lead break tests (Hsu-Neilsen sources) and tensile tests have been performed on this material. In this research study, fibre optic sensors will be proven to offer an alternative for the robust piezoelectric transducers used for Acoustic Emission (AE) monitoring. The main emphasis has been put on the use of advanced signal processing techniques based on time-frequency analysis. The signal Short Time Fourier Transform (STFT) has been computed and several robust noise reduction algorithms, such as Wiener adaptive filtering, improved spectral subtraction filtering, and Singular Value Decomposition (SVD) -based filtering, have been applied. An energy and frequency -based detection criterion is put forward to detect transient signals that can be correlated with Modal Acoustic Emission (MAE) results and thus damage in the composite material. There is a strong indication that time-frequency analysis and the Hankel Total Least Squares (HTLS) method can also be used for damage characterisation. This study shows that the signal from a quite simple microbend optical sensor contains information on the elastic energy released whenever damage is being introduced in the host material by mechanical loading. Robust algorithms can be used to retrieve and analyse this information.

Overview of Fiber Optic Sensor Technologies for Strain/Temperature Sensing Applications in Composite Materials

“…In this section we discuss the different type of FOS that can be used with composite materials to measure strain/temperature when embedded inside a composite material. The different types of FOS reported for strain/temperature measurements in composite materials are fiber Bragg grating (FBG) sensors [ 35 , 36 , 37 ], interferometric fiber optic sensors [ 38 ], polarimetric sensors [ 39 ], fiber optic micro bend sensors [ 40 ], distributed sensors (using techniques such as Rayleigh scattering, Raman scattering, and Brillouin scattering) [ 41 , 42 ] and hybrid sensors [ 43 , 44 , 45 ]. The aim of this review is to provide essential information for the smart materials industry in selecting of appropriate types of FOS in accordance with the end-user requirements and applications.…”

“…Fiber optic micro bend sensors are capable of measuring parameters such as pressure, acceleration, displacement, temperature and strain [ 40 , 79 , 80 ]. Microbending is typically caused by defects and small geometrical perturbations in the optical fiber.…”

“…The curvature of radius of microbends is in the order of micrometers. Microbends cause coupling between propagating and radiation modes [ 40 ] which results in the loss of intensity of the light propagating through the fiber. Figure 14 shows the concept underpinning a typical microbending sensor.…”

“…Specifically during delamination of a multilaminated composite structure the stress fields in a damaged layer and an undamaged layer are different. The higher stress level in the damaged layer causes the optical fiber to micro-bend in the material resulting in the attenuation of the output, and therefore this method can be used to measure the strain field due to delamination in a composite material [ 40 ]. As an example the temporal profile of loading in the composite material and the corresponding optical signal attenuation are shown in Figure 15 .…”

Online detection method for transient waves applied to continuous health monitoring of carbon-fiber-reinforced polymer composites with embedded optical fibers

“…[2][3][4][5] where thin plates of CFRP composite materials (with embedded optical fibers) have been damaged during a tensile test up to the complete fracture. A laser light with constant power is put on one side of the embedded optical fiber and the overall light intensity is measured on the other side.…”