中文摘要：

考虑到发射和接收换能器对超声兰姆波时域二次谐波信号所带来的不可避免的影响，提出一种基于基频与二倍频兰姆波群速度失配的超声兰姆波二次谐波的时域测量方法。当基频与二倍频超声兰姆波的相速度匹配而群速度失配时，在超声兰姆波传播过程中所发生的二次谐波信号，在时域上可与源于斜劈换能器的二次谐波信号相分离。采用仅源自于基频兰姆波的时域二次谐波的积分振幅，定量描述兰姆波二次谐波的发生效率。以铝板中传播的兰姆波为例，给出了时域二次谐波的具体测量过程。本文提出的测量方法放宽了超声兰姆波二次谐波的测量条件，且扣除了换能器对二次谐波信号所带来的影响，所测得的二次谐波信号完全来自于基频兰姆波时域信号的二次谐波发生效应。

英文摘要：

Taking into account the fact that there is an inevitable influence of the transmitting and receiving transducers on the time-domain second-harmonic signal of the ultrasonic Lamb waves, a time-domain measurement technique of second harmonic has been proposed using mismatch of group velocities between the fundamental and double frequency Lamb waves. When there is a phase velocity matching but group velocity mismatching between the fundamental and double frequency Lamb waves, the second-harmonic signal of the ultrasonic Lamb wave propagation can be separated in the time domain from that generated by the wedge transducers used for generating and receiving the ultrasonic Lamb waves. The present technique uses the integrated amplitude of the time-domain second-harmonic signal that is only produced by the fundamental Lamb wave propagation to quantify the efficiency of the second-harmonic generation. The measurement procedure of the time-domain second harmonic has been demonstrated by an experiment where a fundamental Lamb wave propagates in an aluminum sheet. It is known that, through the measurement technique proposed in this paper, the condition for measurements of the second harmonic of the ultrasonic Lamb waves can be relaxed, and the influences of the wedge transducers on the second harmonic can be deducted completely. The second- harmonic signal measured by the present technique is completely due to the effect of second-harmonic generation of the time-domain signal of the fundamental Lamb waves.