Experimental investigation of symmetry properties of second harmonic Lamb waves

Recent studies by multiple authors show contradictory results concerning the symmetry properties of second harmonic Lamb waves. This research experimentally investigates this symmetry issue by examining both symmetric (s1-s2) and anti-symmetric (a1-a2 and a2-a4) mode pairs in aluminum plates under the same experimental conditions. The wedge technique is used to generate and detect ultrasonic Lamb wave signals of a specific mode, and the Morlet wavelet is applied to extract the fundamental and second harmonic amplitudes. The measured normalized second harmonic amplitudes of the three different mode pairs all show a linear increase with propagation distance. However, the slopes of the two anti-symmetric mode pairs are smaller by two to three orders of magnitude than that of the symmetric mode pair considered. Further investigations of these two anti-symmetric mode pairs for plates with different levels of material nonlinearity reveal consistently small slopes that are independent of the level of material nonlinearity. Therefore, this research experimentally demonstrates that the second harmonic generation in anti-symmetric Lamb wave modes is extremely inefficient; this result is consistent with some recent theoretical predictions and thus shows that the use of these anti-symmetric modes is not favorable for the experimental characterization of material nonlinearity. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3691225]