Evaluation Of The Mechanical Properties Degradation Of Metal Material And Adhesive Interface Curing Process Using Nonlinear Lamb Waves

As one of the non-destructive testing and evaluation (NDT&E) method, ultrasonic non-destructive testing and evaluation technique has attracted quite many researchers in this field because of its special properties, such as fastness, high efficiency, and wide application. The traditional ultrasonic NDT&E is mainly based on such information as the attenuation, velocity and reflection of ultrasonic waves to evaluate macro damages and defects with in materials. For example, cracks, voids, welding cracking and debonding. However, this traditional ultrasonic technique can’t effectively detect internal micro damage of components and degradation of mechanical property at early stage. Research shows that the nonlinear effect of ultrasonic is sensitive to micro damages. This feature of nonlinear ultrasonic could be used in NDT&E.Because Lamb waves can propagate a long distance with low attenuation, it’s very suitable to detect large area plate and bonding structure. Therefore, this paper mainly studies the response of nonlinear effect of Lamb wave on mechanical performance degradation of metal material and on curing performance of adhesive interface. The experimental materials is a thin plate made from AL6061-T6with thickness of lmm. The adhesive is AB glue and epoxy resin. According to the condition of second harmonic accumulation of Lamb waves, a predetermined angle of plexiglass wedge and ultrasonic transducer were used to excite and receive Lamb waves. The received Lamb wave signal in time domain was processed using Short Time Fourier Transform (STFT) and Fast Fourier Transform (FFT), respectively, to obtain the amplitudes of fundamental and second harmonics. The nonlinear coefficients were therefore obtained. Research on the relationship between nonlinear coefficient and tensile residual strain, and the relationship between nonlinear coefficient and curing time of bonding interface shows that:(1) For the evaluation of material degradation, S1mode of Lamb wave was excited with frequency of3.61MHz. The received Lamb wave signal was processed using STFT; and the nonlinear coefficients were thus obtained. The results show that nonlinear coefficient increases with the tensile residual strain when the residual strain is less than6%. The nonlinear coefficient decreases with the tensile residual strain is in the range of6%～10%. Further more, Al mode of Lamb wave was excited with frequency of3.0MHz, the results show that nonlinear coefficient increases a little from the beginning. However, the nonlinear coefficient nearly keeps unchanged with the increase of tensile residual strain after that. The plastic damage can, therefore, be effectively evaluated using nonlinear coefficient.(2) For the evaluation of the curing of bonding interface, S1mode of Lamb wave was excited with frequency of3.61MHz. The received Lamb wave signal was processed using STFT; and the nonlinear coefficients were thus obtained. The results show nonlinear coefficient decreases with curing time during0-80minutes when the adhesive is AB glue. After that, nonlinear coefficient nearly remained stable during80-360minutes. When the adhesive is epoxy resin of the bonding interface, nonlinear coefficient increased at the beginning and then decreased with the curing time during0-150minutes. Nonlinear coefficient fluctuates during150～300minutes when curing. Nonlinear coefficient nearly remained stable during300-400minutes. Therefore, the different curing processes of the two different adhesives can be characterized using nonlinear coefficient.