Characterization and evaluation of composite laminates by nonlinear ultrasonic transmission measurements

The research work on Nonlinear Elastic Wave Spectroscopy (NEWS) of this contribution is focused on nonlinear ultrasonic transmission through damaged composite laminates for quality assessment. The generation of higher harmonics by a quasi-monochromatic excitation is evaluated. Specially designed and fabricated narrow-band transducers reduced the inherent probe nonlinearity. The transmitted ultrasonic signals were detected by a broad-band transducer, recorded, and Fourier transformed. The resulting amplitude spectra were stored in dependence on the transmitting power. The technique was applied to composite laminate samples with different damage levels. The samples may contain micro-crack fields, cracks, and delaminations. The local ultrasonic transparency, the second and the third-order nonlinearity parameters, and the distortion factor were obtained and are discussed in order to ascertain the significance of the parameters for quality assessment.