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The development of non-destructive evaluation techniques is motivated by the strong demand for the use of lightweight materials in critical components, especially in aeronautical applications, and by the economical drive to keep civil and large mass-produced structures in operation for much longer than originally foreseen. In both situations, it is important to detect damage at the earliest possible stage in order to ensure reliably and safe use. Several linear and nonlinear ultrasonic non-destructive testing techniques are available, nevertheless their practical implementation is still limited. Linear ultrasonic techniques are inadequate when closed cracks need to be detected. The lack in differentiation between sources of nonlinearity makes defects undistinguishable from e.g. nonlinearity induced by mechanical contacts. Additionally, most NDT-techniques are not capable of damage detection under harsh operational conditions. This work presents and validates a technique to detect damage, addressing the aforesaid difficulties in vibrating structures. The nonlinear interaction of a probing Lamb wave with an air-filled delamination in a steel/rubber bilayer undergoing slow, periodical vibrations is studied. The physical properties of the air layer are periodically changing due to compression, and hence the transmission of the probe wave is dynamically modulated. The dispersion characteristics of the Lamb modes in the steel/air/rubber structure are studied for different thermodynamical (isolated/non isolated) models of air confinement. The coupling between the rubber layer and the steel layer strongly increases close to the cut-off frequencies for longitudinal waves. This effect is accompanied by large changes in the physical properties of the probe wave. In the vicinity of these coupling conditions, the cross-modulation of the probe wave is expected to be maximum. This is validated by a simulation of a dynamically changing delamination. In order to elucidate the optimum conditions for cross-modulation, the relation between the cut-off frequency and the delamination properties is analytically studied. The resulting coupling condition is a good approximation of the probe frequency- and compression-range at which the steel-rubber coupling is maximum, and consequently in the vicinity of which the cross-modulation is maximum.Sensing systems to measure transmission amplitudes, using ultrasonic probe waves, are studied. Because the spatial properties of Lamb waves (wavelength, velocity, displacement symmetry) depend on their specific modal nature, transducer arrays can be used to select and amplify a preferred Lamb mode. In this work, three methods of mode selection are validated, using different approaches in how the elements in the transducer array are combined. Wavelength and velocity based mode selection are achieved, with amplification factors close to the experimental maximum values. However, scattering at individual PZT-transducer strips decreases the amplitude of the probe wave propagating through the array, limiting the maximum amplification and the useful number of elements in an array. Symmetry sensitive Lamb mode selection is not feasible, due to the complicated stress patterns of Lamb waves in the PZT transducers. It can be concluded that the choice of transducers has a large influence on the performance of a specific approach.In order to develop diagnostic algorithms based on the modulation of high frequency probe waves by a dynamically changing crack, a laboratory setup is employed in order to acquire data on two type of samples. In an aluminium plate, i.e. the first type of sample, the PZT transducers in the sensing system are subject to strong nonlinear modulation. In the second type of sample, still with a plate-like crack region, the sensing system is placed in a low strain region of the sample. Five new diagnostic algorithms are presented and compared to two linear ultrasonic techniques, i.e. two techniques using the general evolution of the transmission amplitude measured at quasi-static conditions. The new algorithms compare features of the nonlinear relation between the transmission amplitude and the load on the sample with a threshold value, in order to assess the state of the sample. In the development of these algorithms special attention goes to the requirements of a NDT technique in a practical application. The applications are robust to environmental changes, are based on durable components, while being sensitive to vibrating defects. For each specific component to be monitored, a specific choice of a diagnostic algorithm can be made according to the featured components and on the particular circumstances. nrpages: 196 status: published