Research On In-situ Ultrasonic Non-destructive Test Used For Common Structures Of Aircraft

As one of the five major nondestructive test (NDT) methods, ultrasonic NDT has been applied in the industry field extensively. Relying on the advanced project of Eleventh Five-Year Plan, we combine the simulation and experimental study on the base of the in-situ ultrasonic NDT of common structures of the aircraft. On the basis of survey on the state of the art of ultrasonic NDT technique, we launch our studies on severel key techniques such as the simulation and realization of ultrasonic NDT system, method of rapid checkout of cracks in bolts, method of quantitative measurement on flaws in rivets and classification of defects in gear in this paper. Our contributions are summarized as follows:Firstly, we carried out the simulation and experiments on the contact pulse-echo ultrasonic NDT system. Aiming at the characteristics of general ultrasonic test system in outfield, we proposed the simulation model of contact pulse-echo ultrasonic NDT system with the combination of conventional circuit model and mathematical model, which was formed on the base of improved electro-acoustic measurement (EAM) model. The couplant was considered as a factor that could not be ignored in this model. Furthermore, we studied the relationship between the intensity of echo and the thickness of the couplant, which could explain the phenomenon in the experiment that this relationship could be expressed definitely. We used the analog ultrasonic tester to measure the parameters of the transducer and the cable, and then introduced the paremeters into the model. The optimal parameters were obtained by debugging the emitting part of the model, which was applied in the design and realization of the ultrasonic NDT system.Secondly, we proposed a parameter estimation method of ultrasonic signal based on particle swarm optimization (PSO) and matching pursuit (MP). Simulations and experiments were studied on the classical model of ultrasonic signal. On the base of the availability of this model, we gave the detailed analysis on some conventional methods of parameter estimation. The difference of accuracy and speed of these parameter estimation methods was compared. Considering the precision and speed of estimation in the test, we introduced a method of parameter estimation of ultrasonic signal based on improved PSO and MP. Particle of smallest adaptive value was obsoleted and new particle was added in the swarm, which retained the number of total particles unchanged. The convergence speed and convergence accuracy of the algorithm was improved effectively. Meanwhile, this method was not so sensitive to the random noise in the ultrasonic signal, and could reduce the calculated amount in parameter estimation. As was shown in the results of experiments, this method can not only deal with the ultrasonic signals of different types, but also with those in different positions. Thirdly, we proposed a new method, entropy of instantaneous frequency (IF), to detect the position of the flaw. Generally speaking, the flaw positon is often confirmed by the observation of different echoes in the screen of ultrasonic tester. However, this method of observation can not distingwish the flaws near the interface. Therefore, from the view of time-frequency analysis (TFA), this paper presented a method based on the entropy of IF in the detection of flaw position. Using the methods for calculating the relative extreme points and determining the window width in the process of solving the entropy curve of IF, we compared the precision of entropies measured by this method and other different time-frequency methods. This method calculated the IF and entropy of IF based on empirical module decomposition (EMD) with the pre-processing of multi-correlation time sequence (MCTS), which overcame the problems of fault intrinsic mode function (IMF) created by EMD. This method reduced the influence of system noise in the test and improved the test accuracy at the same time.Finally, we studied the classification method of ultrasonic defects based on least square support vector machine (LSSVM). In the detection of aircraft parts, different defect leads to different influence. Therefore, it is necessary to distinguish them quickly and accurately. But the signal samples are limited in the outfield test. According to this, we proposed a classification method of least square support vector machine (LSSVM) based on attractive and repulsive particle swarm optimization (ARPSO), which could find the optimal parameters of LSSVM by ARPSO. Under the prerequisite conditions of feature discriminability, the features were reduced by the distance evaluation factor, which could improve the training speed. In the test of flaws in the standard samples and the gear, this method was applied respectively. The selection of kernel parameters and design of multi-class classifier were verified in the experiments. Comparing with other method of LSSVM, this method could combine the merits of classification accuracy of single class, classification accuracy of universal class and the total traing time, which could apply in the in-situ test of aircraft outfield.