Evaluation Of Micro Crack In Plate-like Structures Using Fiber Optic-ultrasonic Sensing

The principle of fiber optic-ultrasonic detection is to receive the ultrasonic wave propagating in the material by using optical fiber sensors and then analyze the ultrasonic properties for determining the structural health condition.At present,fiber optic-ultrasonic detection has been widely applied to damage evaluation of the architecture,military,manufacturing,and other important industrial fields.However,it is necessary to further develop suitable and high-precision damage detection techniques for complex aerospace structures in harsh environments.The plate is a basic and important structure in aerospace structures.But it will generate microscopic damage such as micro crack during its long-time service.When the micro crack is subjected to external loading,it will expand and form a macro crack,reducing the strength of the entire structure,and even causing catastrophic failure.Therefore,the detection and evaluation of micro crack is of great engineering significance and values to ensure the reliability and safety of plate-like structures.At present,the linear and nonlinear ultrasonic Lamb wave techniques are the primary ways to evaluate the early damage within plate-like structures.The sensor is the key component to precisely detect Lamb wave.Among various kinds of sensors,the fiber Bragg grating is suitable for the ultrasonic detection of aviation structures due to its advantages of corrosion resistance,anti-electromagnetic interference,and multiplexing.However,its sensitivity and bandwidth need to be further improved to meet the requirements of linear and nonlinear ultrasonic detections.In this study,a high-sensitivity and broad-bandwidth fiber optic-ultrasonic sensing system was designed to detect linear and nonlinear Lamb waves propagating in the metal plate and the composite laminate.The configuration and characteristics of the sensing system,as well as linear and nonlinear Lamb wave were investigated from the aspects of theory,simulation,and experiment and thereby realizing the characterization and evaluation of early micro crack in typical plate-like structures.The main contents of the study are summarized as follows.（1）A new fiber optic-ultrasonic detection system was established,tested,and further developed.Integrating with wavelength shifting and balanced photodetection,a high-sensitivity and broad-bandwidth fiber optic-ultrasonic detection system with proportional-integral-derivative feedback control was designed to accomplish ultrasonic detection with a frequency up to 5 MHz.The minimum detected sensitivity of the system was 9 nε/Hz^1/2.Through the preliminary ultrasonic experiment,it found that the fiber optic-ultrasonic system was sensitive to in-plane motion,had a high sensitivity,and realized the 0-180°sensing angle sensitivity detection of fiber Bragg grating.（2）The principles of fiber Bragg grating,linear and nonlinear ultrasonic detection using Lamb wave,the mechanism of dislocation model and the interaction model of micro crack-ultraconic wave were all studied.It designed a laser-based edge-filter technique for high-sensitivity ultrasonic demodulating from wavelength shift of fiber Bragg grating.Based on the theory of elastic wave,the nonlinear effect and the equation of nonlinear parameter of single-frequency ultrasonic wave were derived using the perturbation method.A dislocation model and an interaction model of micro crack-ultraconic wave were established to study the relationship between the plastic deformation,crack length and ultrasonic nonlinearity.It provides theoretical direction for the development of nonlinear fiber optic-ultrasonic experiments on plate-like structures.（3）Finite element analysis was conducted to simulate the interaction of ultrasonic wave and micro crack in a metal plate or a composite laminate using LS-DYNA.A three-dimensional model of metal plate and composite laminate was established.The plastic zone around the fatigue crack in the aluminum model was considered,and a different length of micro crack was set by using the node replication method.The results show that the nonlinear parameter increased with the growth of the crack length.The property of nonlinearity was affected by the input ultrasonic cycle and frequency,as well as the sensor angle and distance.The interaction between 0-10 number of matrix cracks and the Lamb wave in the composite material was also simulated using the same method.The results show that the increase of matrix cracks will lead to the decrease of ultrasonic amplitude and increase of nonlinear parameters.（4）Linear and nonlinear ultrasonic detection of the micro cracks in metal plate,microscopic matrix cracks,and micro crack caused by impact in composite laminate were experimentally studied.The change of ultrasonic characteristics caused by the micro cracks in the metal plate and the composite laminate was evaluated using a piezoelectric sensor and phase-shifted fiber Bragg grating sensor.All of the experiment results were in good agreement with the simulation.At the same time,this study performed linear and nonlinear ultrasonic experiments of impact damage in the composite laminate by using the two kinds of sensors.The experimental results show that the number of matrix crack and delamination increased with the increase of impact number,leading to the decrease of the ultrasonic amplitude and increase of the nonlinear parameter.In summary,a high-sensitivity and broad-bandwidth fiber optic-ultrasonic detection system was proposed and demonstrated,and finite element simulation that can accurately describe the effect of micro cracks on ultrasonic Lamb wave was discussed in this study.The advanced optical fiber sensing was successfully used to evaluate the propagation of metal fatigue crack,composite matrix crack,and impact damage.The proposed technique is a powerful tool to assess the micro crack damage of aerospace structure,which provides the possibility of structural health monitoring.