Ultrasonic Evaluation For Carbon Nanotubes Reinforced Aluminum Matrix Composites

As a new type of aluminum matrix composites, carbon nanotubes reinforced aluminum matrix(CNTs/Al) composite materials have important potential in the aerospace field, instrumentation field and some high-technology fields. However, it is difficult to make carbon nanotubes distributed in the aluminum substrate evenly in the process of materials preparation, which makes carbon nanotubes reunion defects formed easily in composites. The existence of reunion defects severely reduces the mechanical properties of composite materials. In addition, the size of carbon nanotube is from nano-order to micro-order, so there will exist tiny carbon nanotubes reunion defects in the CNTs/Al composite materials easily, which brings more difficulties to the testing and evaluation of composites. In this paper, the nonlinear ultrasonic testing technology was proposed for detecting the carbon nanotubes reunion defects in CNTs/Al composite materials, and compared with the results detected by traditional linear ultrasonic testing technology, realizing the preliminary ultrasonic evaluation of CNTs/Al composite materials.First of all, prepare the blocks by friction stir processing method. By changing the times of friction stir processing, the different CNTs/Al composites in which the severity of carbon nanotubes reunite is different were obtained. And the carbon nanotubes distribution uniformity and the tensile properties of all blocks were analyzed by destructive testing, through which the carbon nanotubes reunite severity of each block was verified.Secondly, the macro carbon nanotubes reunion defects in composites were detected by ultrasonic feature scan imaging testing technology. Besides that, the acoustic velocities and the attenuation coefficients of each block were measured. And the carbon nanotubes reunion defects were associated with velocities and attenuation coefficients respectively.Thirdly, according to the nonlinear acoustic principle and the finite amplitude nonlinear testing principle, the nonlinear ultrasonic testing experiment system was designed and built. Then the nonlinear influence factors including signal stimulus,transducers and acoustic couplant were analyzed, and the linear calibration methods of experiment system were studied, and the results show that the system can be appliedto nonlinear ultrasonic testing experiment.Finally, the relative second order nonlinear coefficients of carbon nanotubes reunion defects in each block were measured by this experiment system, and the relationship between carbon nanotubes reunion defects and the relative second order nonlinear coefficients was established, and compared with the results of attenuation coefficients.The results show that: for the macro carbon nanotubes reunion defects in CNTs/Al composites,the nonlinear response is not very strong, the traditional linear ultrasonic testing technology can detect the defects more effectively; for the tiny carbon nanotubes reunion defects in CNTs/Al composites, linear ultrasonic testing technology and the nonlinear ultrasonic testing technology both can detect the defects effectively,but the the nonlinear ultrasonic testing technology is better.