Experimental Investigation of Modulation Transfer Phenomenon Due to Shear Horizontal Ultrasonic Wave Interaction with Local Nonlinearity

The Structural Health Monitoring methods based on the nonlinear features of the propagating ultrasonic waves have received a lot of academic attention in the last decade. Numerous works confirm that local nonlinear sources such as fatigue cracks, plastic zones or delaminations, can distort the propagating ultrasonic waves. It manifests itself in a number of nonlinear phenomena e.g. generation of high-order harmonics, side-bands (cross-modulation), frequency shift or modulation transfer. In this work, the nonlinear modulation transfer for shear horizontal guided waves is scrutinized experimentally. Therefore, the amplitude-modulated pumping wave and monoharmonic probing waves are excited simultaneously in a damaged glass plate using PZT wafer transducers. First, the frequencies of generated pumping and probing waves are selected heuristically based on empirical measurements. Next, the influence of the amplitude ratio and the amplitude modulation index of the excited waves on the modulation transfer intensity is addressed. This parametric study allows to identify the excitation parameters for effective monitoring of the state of the tested specimen using methods based on the modulation transfer phenomenon for shear horizontal waves. It also lays the foundation for future research focused on the damage detection and identification. The conclusions resulting from this experimental study can be used to develop monitoring methods of widespread plate-like mechanical structures based on the discussed nonlinear effect.