Dispersion relations of elastic shear waves in crossply fiber reinforced composites

Results of theoretical and experimental studies of the dispersion effects for the propagation of elastic shear waves in crossply fiber-reinforced composites are presented. These crossply composites are formed from elementary layers and stacked with a given sequence in predetermined directions defining a superperiod. Two laminated and periodic materials exhibiting a fourfold and an eightfold axis of rotation, respectively, have been investigated. When the elastic wavelength becomes comparable to the composite period, spatial dispersion effects are expected: departure from the linearity of the dispersion curves, stop bands, and in the eightfold-axis material, an acoustical activity induced by the helical stacking of the elementary layers. A theoretical analysis of these various effects is presented, and experimental results obtained through an ultrasonic spectroscopy method are given