The remote bonding configuration allows the application of FBG sensors in extreme environments. But as the length of the fiber increases the attenuation in the fiber becomes significant and the signal-to-noise ratio (SNR) is reduced. The amplitude of wave energy coupled in the fiber depends on the type and quality of the bond, the materials in contact as well as the attenuation of the wave in the fiber. In this paper, FBG sensors with different coatings are used and the amplitude of the wave coupled is studied in each of the cases. Four different material coatings including acrylate, polyimide, aluminium coated and PEEK-coated FBGs are used for the purpose.
Guided waves (GWs) are very popular for the damage detection of thin-walled structures. They propagate large distances and relatively few may be used for damage localization. The problem with their use for complex structures is the signal processing. Due to the presence of multiple modes, and mode conversion and reflections from the structure boundaries and discontinuities the signal processing is indeed challenging. In order to reduce the complexity, lower frequencies are used to limit the excitation only to the fundamental modes. Even then the signal processing may be challenging. So efforts are focussed on the ability of some sensors to detect only a particular wave. This paper aims at investigating the suitability of the polarization maintaining FBG (PM-FBG) for this purpose.
Optical fiber sensors offer several advantages such as small size ability to be multiplexed, immunity to electric and magnetic fields, etc. In addition, another salient feature of fiber optical grating sensors is the ability to be embedded in the structure. The embedding of fiber Bragg grating (FBG) sensors results in a smaller disruption of the planned function of the structural component and allows the placement of the sensor closer to the area most likely to experience deterioration. The FBG sensors have been commonly used for strain and vibration-based structural health monitoring (SHM) but in the last few years, their use for guided waves (GW) based-SHM has been on a rise. This increasing interest is due to the higher sensitivity for GW sensing achieved through the use of FBG sensors in the edge filtering configuration. This study investigates the use of embedded FBG sensors for GW sensing through the thickness of various structures. The effect of different factors such as the depth of the embedding and material properties on the coupling of the waves into the fiber is investigated using a numerical model and experimental results.
Fiber optical Bragg Grating (FBG) sensors are of small size, maybe embedded into structures, and hence are widely used in structural health monitoring (SHM) in various fields. Due to their passive nature, the FBGs have to be used with piezo actuators in a hybrid SHM system. The directional sensitivity makes the signal processing a challenge, also, due to the directionality the damage mapping techniques developed for the omnidirectional PZT sensors cannot be directly applied to FBG sensors. Hence, this paper shows a damage mapping technique that overcomes the directional sensitivity of the FBG sensors. The methodology makes use of quadrant cosine area to obtain damage propagation paths. The damage localization approach works to identify the damage locations by taking advantage of the symmetry of the proposed circular network of PZT actuator-FBG arrangements. It aims at reducing the calculation time by introducing quadrant sector-based calculations of the damage indices.
Composite structures are widely used in many industries. The impact based damages in such structures are the most important disadvantage as they lead to fiber breakage followed by cracks in the structure. The paper analyses the impact damage conditions in the carbon fiber reinforced polymer composite structures. The condition of the structure after the impact was studied. The structure is assessed with the help of guided wave signal processing methods to analyze the structure in both healthy and damage conditions. A higher impact energy-based study was performed to study the growth of the damage. A combined nondestructive based testing was performed on the composite structure to monitor the health of the structure.
Guided waves (GW) allow fast inspection of a large area and hence have received great interest from the structural health monitoring (SHM) community. Fiber Bragg grating (FBG) sensors offer several advantages but their use has been limited for the GW sensing due to their limited sensitivity. FBG sensors in the edge-filtering configuration have overcome the issue with sensitivity and there is a renewed interest in their use. The FBG sensors have directional sensitivity and are passive in nature. This makes it difficult to apply the existing signal processing and damage detection techniques such as tomography, phased array approach etc. As a result, there is a need to develop novel damage detection techniques which take into consideration the specific limitations of the use of FBG sensors. This paper applies the ellipse and hyperbola based techniques for a FBG sensor based network. The paper identifies the specific challenges and applies solutions to overcome those challenges. A comparative assessment of the two methods is presented with the help of experimental data.
Composite structures are widely used in the aviation, automotive industries nowadays. This is because of its corrosion resistance, lightweight, and endurance. Carbon fiber reinforced polymer (CFRP) could be seen even in present-day highend bikes. Even though they are widely used and have a lot of pros, they are prone to various damages and most importantly the impact damage. These impact damages may occur due to accidental drop, low impact drop of tools during testing, etc. This could lead to damage to the material structure, its matrix, fibers, etc. This reduces its entire strength. In our studies, we have focused on the experimental and numerical analysis of the impact damage (ID) detection on multilayered CFRP material using Lamb waves. Lamb waves or guided waves are widely used in various damage detection techniques in the present structural engineering world. The experimental study was performed using a laser Doppler vibrometer (LDV) with piezo zirconate titanate (PZT) based Lamb wave excitation to detect the damage locations. This is followed by signal processing tools to visualize it. The numerical study was conducted using a spectral elemental method (SEM). The results from the experimental, numerical studies revealed the location of damage zones in the CFRP plate.
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