Structural waves in nondestructive testing

This thesis investigates theoretically and experimentally the application of struc¬ tural waves m nondestructive testing and evaluation The characteristic length of a "structural wave" is large compared to the thickness of the structure in which it propagates If such a wave hits a defect in a mechanical component, waves which propagate in the structure are generated together with local oscillations m the vicinity of the defect The amplitude and the phase of the scattered wave at various frequencies and the arrival time at various locations in the structure contam mformation on the position, size and kind of the defect The number of measurements necessary for the nondestructive testang depends on the geometry of the structure In a "one-dimensional" component such as a beam, rod or a wire, the whole structure can be tested in one single experiment In "twodimensional" components such as plates or shells, several measurements are needed However, a scanning as it is performed in classical ultrasonic testing is not necessary The application of structural waves is therefore very suitable for the fast inspection of large components After an introductory chapter, the behaviour of a structural wave pulse in a cracked rod is investigated m chapter 2 A mathematical model, based on linear-elastic fracture mechanics, is presented It allows the prediction of the reflection and trans¬ mission characteristics of a wave pulse at a defect The theoretical results are com¬ pared to experiments which were performed at cylindrical rods with a lateral crack Chapter 3 describes an effect which has been observed in the context of the experi¬ mental investigations m chapter 2 The direction of the deflection vector of a flexural wave pulse seems to be rotated as the pulse propagates in the rod This effect could be explamed by a small deviation in the rod's properties in the two principal directions of the cross-section This is in analogy to the half-wave plates which are used m laser optics in order to change the direction of polarisation of light The dif¬ ference in the two flexural stiffnesses of the rod is less than 1% It seems to be generated by a variation of the tensile modulus in the longitudinal direction (E^) across the diameter of the rod The experiments to this thesis are described in chapter 4 Transient wave pulses with well-defined frequency spectrum were generated with the help of piezoelectric transducers These are driven far below their first eigenfrequency, usually in the range from 1 to 200 kHz The resulting motion of the structure is measured by means of a heterodyne laser interferometer with phase demodulation It allows the measurement of displacements smaller than 1 nm even at low frequencies (<10kHz) This is not possible if an interferometer with fringe counter or frequency demodulator is used The piezoelectric transducers and the laser interferometer are described m a sepa¬ rate section