The use of guided waves in the ultrasonic nondestructive evaluation of structural components, e.g., bonded plates and composite laminates, has received considerable attention in recent years. Highly accurate and efficient experimental techniques have been developed to generate, record and analyze these waves in laboratory specimens, leading to an improved capability in flaw detection and material characterization in a variety of materials [1-4). A convenient method to generate guided waves in a plate or laminate is the so-called leaky Lamb wave (LLW) technique. It has been demonstrated in several recent papers [5-7) that phase velocity and amplitude of guided waves composite laminates can be determined very accurately in a broad range of frequencies and velocities by the LLW technique.
The time histories and spectral characteristics of acoustic pulses reflected from fiberreinforced composite laminates immersed in water are recorded in the laboratory and analyzed through a generalized ray theory and an exact theory. Calculated results for an unidirectional laminate and two angle-ply laminates are compared with measured data. It is shown that for a unidirectional laminate a finite number of mode-converted waves contribute to the overall signal reflected from a thick specimen resulting in certain irregular behavior of the reflected pulses. A phenomenological model of wave attenuation is introduced in the theoretical simulation and the damping parameters are shown to have a strong influence on the amplitude of the reflected pulses. The phase velocity of the guided waves in the laminates are shown to be nearly independent of water loading and material dissipation in a broad frequency range. Agreement between measured and calculated results is found to be excellent to very good in all cases.
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