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Determination of the stress dependence of the velocity of Lamb waves in aluminum plates

Mode selective excitation and detection together with chirped excitation and digital pulse compression is employed to study the variation of the time-of-flight (TOF) of Lamb waves. The acoustoelastic coefficients for the variation of TOF depend not only on the actual modes but also on the actual frequency or center frequency of the observed propagating wave. The modes are selected from dispersion relations obtained by appropriate modeling. To allow a continuous and monochromatic variation of the load, specially designed apparatus has been designed minimizing the disturbing influence of static and dynamic friction, generally a demerit in the standard force exerting hydraulic equipment. The possibility of delicate maneuverability of force application with minimal deviation with selective excitation and detection of Lamb wave modes lead us to the observation of positive and negative acoustoelastic coefficients and dependencies of these coefficients on applied stress with respect to central frequency are discussed.