Mechanical Crack Quantitative Detection Methods Of Multi Source Excitation-Fiber Bragg Grating Distributed Sensing

Crack is the most common and most harmful type of damage in mechanical structures.The accurate and effective quantitative detection of mechanical crack is an important guarantee to ensure the normal operation of mechanical equipment and avoid major safety accidents.Nonlinear vibro-acoustic technology(using both low frequency vibration and high frequency ultrasonic excitations,called multi-source excitation)can greatly improve the crack detection sensitivity and solve the shortcomings of traditional ultrasonic detection methods,such as the limitation of ultrasonic wavelength and difficulty in identifying micro cracks and closed cracks.At present,the researches of nonlinear ultrasonic technology are not mature yet;most researches focused on the qualitative detection of damage,damage's quantitative detection are still in its infancy.It is of great significance to study the quantitative detection of crack damage for improving the ability of crack detection and evaluating the performance of mechanical structures.Because of its high sensitivity,strong adaptability,easy to realize distributed measuring,anti-electromagnetic interference,fiber Bragg grating(FBG)has shown a strong potential in the field of mechanical equipment health monitoring.The thesis aims to put forward the method of mechanical structure crack quantitative detection method based on multi-source excitation and FBG distribution sensing to point out the position and range of crack,taking aluminium alloy plates as the objects.The vibro-acoustic technology and FBG distributed sensing technology are combined.The sensing characteristics of FBG for multi-source excitations and modulation mechanism of crack for multi-source excitation are studied.The sensing directivities of FBG for strain are revealed,as well as nonlinear phenomenon of cracks under multi-source excitation.Two quantitative detection algorithms for mechanical cracks are presented and verified by experiments.The main work of the paper is as follows:(1)By studying the influence of strain with different incident angle on FBG sensing sensitivity,it is revealed that the sensing characteristics of FBG for static strain,dynamic strain with low frequency and the strain of Lamb wave are obviously sensing directional.The relation curves of strain's incident angle and FBG sensitivity are drawn.FBG response signals under multi-source excitation and single excitation are compared,and the effects of relative positions of two excitations and different excitation amplitudes on FBG response signals under multi-source excitation are discussed.These researches can provide theoretical and experimental basis for multi-source excitation and FBG distributed sensing crack detection.(2)The mode conversion phenomenon of Lamb wave under crack modulation is researched,the influence of different crack sizes on Lamb scattering intensity is discussed,and the non-linearity(the higher harmonic and sidebands)phenomena caused by crack under multi-source excitation are analyzed.The related factors of sidebands(including excitation amplitude,relative position of excitations)are analyzed.The experiments shows different types of crack can produce sidebands under multi-source excitation.(3)A crack location algorithm utilizing propagating delay of sidebands based on multi-source excitation-FBG distributed sensing is proposed,and a corresponding experimental system is built.The positions of two crack tips are determined,so the range of crack is determined too.The location algorithm can avoid the interference of reflection effect of structural boundary and Lamb dispersion phenomenon.The crack detection problem of small plate can be solved.(4)Another crack quantitative detection algorithm utilizing FBG sensing response angle characteristics is proposed based on the conclusion that FBG has different directional sensing characteristics for S0 and A0 mode,which breaks the limitation that previous crack localization algorithms mostly rely on ultrasonic propagation speed.The topological structure of FBG that two FBGs with different directions pasted on a single measuring point is designed.A crack detection experimental system based on FBG sensor angle direction characteristics is constructed.By studying the layout of FBG detection points and excitation forms on the plate,the limitations of the algorithm were effectively solved.The general location and range of cracks were determined by experiments.