Measurement of the stiffness coefficients of a viscoelastic composite material with laser-generated and detected ultrasound

A point source-point receiver technique, based on laser generation and laser detection of acoustic waves, is used to measure the stiffness tensor of a composite material. Because of the dispersive nature of these materials, the acoustic signature provided in polymer matrix composites is difficult to interpret. A model for acoustic propagation in composites is developed to analyze the accuracy of the stiffness coefficients determined from the measured acoustic signals. The frequency dependence associated with viscoelastic rheology leads to the numerical implementation of a double inverse transform to calculate the surface displacement generated by a line impact. The model accurately predicts the focusing effects caused by anisotropy and the spreading of the signals caused by dispersion and attenuation. The predicted displacements are in good agreement with the result of experiments performed with a composite plate. The model is used to test a signal processing method based on wavelet transform to measure the group velocities of bulk waves propagating through the plate. Despite dispersion and echoes overlapping, the stiffness coefficients are identified from group velocities with good reliability. The process is then successfully applied to an actual viscoelastic composite material.