A Novel Fiber Optic Sensor And Its Study On Acoustic Emission Source Localization Of Fiber Reinforced Composites

With the fiber reinforced plastic (FRP) composite laminates are widely used in the field of aerospace, transportation, energy, navigation and military, more and more researchers have drawn attention to the structural health monitoring (SHM) of composites. In order to effectively detect damages and failures of composites due to the static or dynamic load and fatigue, a novel technology instead of traditional nondestructive testing technology is necessary to be developed for achieving the on-line health monitoring of advanced composites materials and structures. Acoustic emission (AE) technique as a kind of modern new non-destructive testing technology, integrating multiple techniques such as sensor technique, measurement technique, signal acquisition and analysis technique, has a broad potential application in the field of nondestructive testing. Traditional AE sensors are usually made of piezoelectric (PZT) ceramic materials, not only susceptible to electromagnetic interference, but also unsuitable for being embedded into FRP composites structure as its bulky structure. In this thesis, a new type of fiber optic acoustic emission sensor (FOAES) is developed, which is able to be embedded in FRP composite laminates. The sensors are applied for the research of AE source localization in anisotropic plate-like composites structures. The main contents in this thesis include:Firstly, a novel FOAES is proposed and developed on the fused-tapered optical fiber coupler. It is packed with a short and thin capillary tube, and easy to be embedded into composite structures. The vibration characteristic of the sensor is established on the perspective of vibration mechanics, which points out that the main influence factor is the pre-tension from the packed tube to the coupled fibers of the sensor. The sensing principle of the FOAES is analyzed with the optical waveguide theory, obtaining the relationship between the fiber core spacing at the sensor coupling part and the sensor sensitivity to the stress wave. According to the mesoscopic structure size of optical fibers in the sensor measured by the electron microscope, optical waveguide transmission properties in the optical fiber sensor are simulated with the method of finite difference beam propagation, and their optical waveguide simulation results provide reference for improving the sensitivity of the sensor during its fabrication. At the same time, a AE source localization monitoring system is built and developed for the novel FOAES. An algebra algorithm of AE source localization is embedded in the system program, which is able to accurately and quickly estimate the AE source location improving the testing efficiency.Secondly, FOAESs are embedded into a FRP plate, constituting of a smart composites plate that has the function of sensing stress waves and identifying the position of the AE source. This technique breaks through the limit of bulky and difficult to be embedded into the composite structures of traditional PZT AE sensor. Through a series of comparison experiments, amplitude frequency response characteristics of the FOAES to stress waves are measured compared with a PZT AE sensor. Their experiment results prove that the new-designed FOAES is completely suitable for the AE source of low-frequency. The sensor can distinguish waveforms feature of the expansion wave (S0wave) and flexural wave (A0wave). Their propagation velocities in different directions are measured in different materials plate by experiments, which support the information for identifying the AE source location.Thirdly, the AE source localization method is researched on the plate of isotropic and anisotropic materials. The FOAES and its monitoring system are used for experimental measurement and validation, including linear and planar localization of AE source. In the study of linear localization method for acoustic emission source, three FOAESs with equally spaced linear array are distributed on the fiber composite plate in any direction. The AE source location is identified using the linear array without the information of propagation velocities of stress waves. A composite wind turbine blade with1.2m length is tested in the method of evenly spaced points. The measurement relative error less than1.3%verifies the linear positioning method has a high accuracy. In the study of planar localization method of AE source, the algebra algorithm is used to calculate the AE source location measured by a square sensor array. Experiment results prove that nonlinear errors and unreal solutions are easy to be obtained using the algebra algorithm with three parameters. An algorithm based on artificial neural network with time differences as inputs is established to solve the above shortcomings, which is suitable for any source localization method of distribution of sensors. The algorithm of source localization using sensors as a diamond array is also proposed for weakening the influence of which is introduced stress wave propagation velocities are different in directions of anisotropic composites materials. Two velocities in the diagonal directions of the diamond are selected for calculating the location of acoustic emission source in the algorithm, which achieve the function of acoustic emission source localization in anisotropic materials plate accurately and fastly.Finally, CFRP composite materials are tested by using drop-weight impact method, which are simulate to natural disasters and harsh environment, such as hails and sands. The drop-weight impact source localization method for CFRP laminates are also researched by using the optical fiber AE source localization system of this article. The impact source location of CFRP laminates is accurately identified by the surface-mounted and embedded sensors. The incidences of AE source localization from impact damage are analyzed, which prove that the measurement results of the FOAES array and the algebra calculate method embedded in the source localization system both have higher precision. In a range of impact energy, the amplitude response of the sensor to stress wave will linearly increase along with the augment of the drop-weight impact energy, which is easy to operate, simple to process data, reliable and so on.Above all, the development of the new FOAES in this thesis expands the research area of nondestructive testing technology. The research results of the FRP composite structures embedded with sensor array provide important reference value for the development of self-sensing smart composite structures. The development of monitoring system for acoustic emission source localization and its application have important research significance for the structural health monitoring (SMH) of advanced composites.