Due to the highly anisotropic elastic properties of the plies in a fiber-reinforced composite laminate, transverse (shear) waves propagated through the laminate carry rich information about the fiber orientation and ply stacking sequence in the laminate. Such signals can therefore be used in the nondestructive detection of ply lay-up or stacking sequence errors in composite laminates. Several attempts have been reported for exploiting the strong interaction between shear wave polarization and fiber orientation for nondestructive evaluation (NDE) purposes. For example, Hsu et al [2, 5] have used contact shear wave transducers in a "crossed polarizer" configuration in which the polarization directions of the transmitter and receiving transducers on the two faces of the laminate were perpendicular to each other and both were rotated in unison over a full circle. The transmitted signal was found to have good sensitivity for certain ply orientation and stacking sequence anomalies. An experimental problem with making reproducible shear wave measurements is the need for a shear wave couplant. Keeping the coupling condition constant is especially problematic for measurements that require frequent change of the angular orientation of the shear wave transducers. To alleviate this problem, researchers at Iowa State University used EMAT probes for the generation and detection of normal incidence shear waves in a non-contact manner. The composite sample was sandwiched between two aluminum blocks and the EMAT probes were placed on the outside faces of the two blocks. The EMATs can be rotated freely with computer-controlled stepping motors in an angular scan. For uncured laminates, the pressure applied on the blocks was sufficient for shear waves to transmit through. For solid cured laminates, shear couplant was still used between its surfaces and the aluminum; however, the coupling condition was not disturbed by rotation and hence remained constant. In an angular scan, the transmitted shear wave signals at every angular position were acquired. After a full-circle scan, an image as function of angle and time was obtained. Such "angle-time" patterns, especially for the crossed configuration, were found to be very characteristic to the ply lay-up of the laminate and can be used to check errors in ply orientation and stacking sequence in composite laminates effectively. Recently a complete analytical model was also developed by Fei and Hsu for the propagation of shear waves in composite laminates. The analytical model led to a result that is very compact and tractable. The model took into account all the reflected waves at the interfaces in the laminate by including the four partial waves in each layer that are polarized parallel to and perpendicular to the fiber, and propagating in the forward and reversed directions. The incident and transmitted shear waves components were found to be related by four transfer functions, which can be determined experimentally for a given laminate by four measurements, with the transmitter angle and receiver angle at (0, 0), (0, 90), (90, 0), and (90, 90). The model has been experimentally verified using both cured and uncured laminates.
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