Research On The Lamb Wave Nondestructive Test For Aluminum Sheet Bending Fatigue

7050 aluminum alloy plant is a commonly material that used as airplane skin. These structures serve in appalling conditions. As external shocks deformation, cyclic loading fatigue and crack propagation, which is caused by rivet hole, are inevitable, there is a great challenge for aircraft’s safety and service life. In order to identify these kinds of damage, we use Lamb wave structure health monitoring system to explore them, which is based on piezoelectric ceramic transducer. The main contents are:Firstly, this paper analyzes the propagation characteristics of Lamb wave. Based on the basic characteristics of Lamb waves, we solve Rayleigh- Lamb equation by the dichotomy iterative method, and then draw the phase velocity and group velocity dispersion curve. Then we use the surface shear stress model to derive the expression of vibration amplitude and draw the amplitude dispersion curve. Considering the specific experimental conditions, we select hanning loaded 5-cycles sine wave as the excitation signal. And we select the diminishing law of characteristic Lamb wave packet’s peak to identify damage qualitatively and quantitatively.Secondly, we set up a detection platform, which is based on active Lamb wave structure health monitoring system. And we optimize the layout of the sensors and improve the preprocessing and bonding process of the sensors. Then we get Lamb wave signals under different frequencies to correct the group velocity dispersion curves and the amplitude dispersion curve under ideal conditions. And we extract the Lamb wave characteristic parameters with the help o f mean filter, the related de- noising method, waveform comparison method, propagation time method and spectral analysis method.Finally, we use active Lamb wave damage detection system to detect 7050 aluminum plant with different kinds of damage, such as impact bending deformation, repeated fatigue accumulation, the hole defects and fatigue crack defects around rivet hole. We can point the stress situation with the help of d irect transmission S0 wave packet peak amplitude’s diminishing law. And there may be a micro-crack in specimen if the amplitude of characteristic wave packets appear sharp decrease. Then hole diameter increases will directly reduce the amplitude of the wave packet transmission and increase the amplitude of the reflected wave packet. So the relative amplitude change in our selected Lamb wave packets can effectively identify damage in the specimen.