Research On Microcrack Positioning Based On Nonlinear Ultrasound And Probability Scanning Matrix

Early damage,such as microcrack,occupies most of the fatigue life of the materials.Due to the zipper effect and the chain reaction,the microcrack can easily lead to sudden structural failure and system crash in a harsh working environment.The technology of timely discovery and location of microcrack has considerable engineering and economic value.In order to solve the problem that traditional ultrasonic testing technology is difficult to locate the microcrack,the nonlinear ultrasonic testing technology is used to locate the microcrack.In the research,the adjustable angle probe with a plexiglass wedge is used to excite the S0 single mode lamb wave and use it as the detection fundamental wave to suppress the difficulty of signal analysis caused by the dispersion effect.The empirical mode decomposition tool(EMD)is used to decompose and reconstruct the signal to achieve the purpose of filtering.The relative ultrasonic nonlinear coefficient is extracted by the fast fourier amplitude frequency analysis tool from the received signal,and normalized as the damage index.In the research,make the assumption that the microcrack has the highest probability of existence on the center line of the transmission and reception path.The farther away from the center line,the lower the probability of the microcrack existence.The probability distribution conforms to the law of normal distribution.Finally,the product of the damage index and the probability function is taken as the imaging index,and positioning imaging of the microcrack is realized by superimposing the scanning paths in the scanning matrix.In order to study on the problem of positioning of microcrack,three types of simulation models of 6061-T6 aluminum alloys are built by using the ABAQUS finite element simulation software.They are non-destructive model,model with macro damage and model with microcrack.Then,the research on pulse propagation law,the research on the influence of macro damage on the localization algorithm and the research on microcrack positioning are carried out.Then,a nonlinear ultrasonic experimental system was built based on the RAM-5000 SNAP nonlinear ultrasound instrument,and the simulation results are verified experimentally to verify the correctness of the simulation model.Finally,the verified simulation model is used to discuss the factors that affect the positioning accuracy of the algorithm when the microcrack is in different positions,the combination of the transmitting and receiving probes that make up the scanning matrix are different and the sensor arrangement schemes are different.The simulation and experimental results show that the nonlinear probability scanning matrix can effectively locate the microcrack.It can be found that successful positioning can be achieved when at least two scan paths pass through the microcrack.The individual probe damage will not cause the positioning failure.Moderately increasing the scanning path can effectively improve the positioning accuracy.The research results provides a technical reference for the ultrasonic non-destructive testing technology to detect the early damage of structures.