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A novel structural health monitoring (SHM) methodology, based on nonlinear wave modulation spectroscopy, is presented for the detection of delamination cracks in composites. The basic element is a novel active nonlinear acoustoultrasonic piezoelectric sensor enabling low-cost and wide-frequency operational bandwidth. The active sensor configuration involves two piezoceramic wafer actuators, each one excited with a low-and high-frequency signal respectively, and a piezoceramic sensor, all permanently bonded on the tested structure. Experiments are conducted on two sets of composite strips containing delamination cracks of different sizes. Measured results illustrate first the efficiency of the nonlinear ultrasonics methodology to detect delamination cracks, as well as, the potential and benefits of the new active sensor. The sensitivity of the active sensor response to the crack size and the applied high-frequency carrier signals at the actuators, vary at various frequency and voltage levels indicating the appropriate testing setup. Additionally repeatability of proposed SHM methodology is studied.

Experimental and analytical studies are presented investigating the effect of delamination cracks on the low-frequency dynamic response of composite beam specimens with surface attached piezoelectric actuators and sensors. Frequency response functions of laminated beams with piezoceramic actuators and piezoelectric sensors having single delamination cracks of various sizes are measured. Comparisons with mechanical actuation-accelerometer sensor configurations illustrate the sufficiency and advantages of piezoelectric actuator-sensor pairs in self-monitoring the effects of delamination on the dynamic response. Modal frequencies and damping values of the beams are also measured and their dependence on delamination size is studied. Comparisons with analytically predicted results are shown. Finally, the sensitivity of damage indices based on changes in modal frequency, modal damping, and modal peak density to the crack size is quantified.

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Evaluation of the sensitivity and fatigue performance of embedded piezopolymer sensor systems in sandwich composite laminates AbstractIt has been claimed that embedding piezoceramic devices as structural diagnostic systems in advanced composite structures may introduce mechanical impedance mismatches that favor the formation of intralaminar defects. This and other factors, such as cost and their high strain sensitivity, have motivated the use of thin-film piezopolymer sensors. In this paper, we examine the performance of sandwich composite panels fitted with embedded piezopolymer sensors. Our experiments examine both how such thin-film sensors perform within a structure and how the inclusion of sensor films affects structural performance. Strain-controlled tests on sandwich panels subjected to three-point bending under wide-ranging static and dynamic strains lead us to conclude that embedding thin piezopolymer films has no marked reduction on the tensile strength for a wide range of strain loading paths and magnitudes, and that the resilience of the embedded sensor is itself satisfactory, even up to the point of structural failure. Comparing baseline data obtained from standard surface-mounted sensors and foil gauges, we note that whereas it is possible to match experimental and theoretical strain sensitivities, key propertiesespecially the pronounced orthotropic electromechanical factor of such films-must be duly considered before an effective calibration can take place.