The quantitative evaluation of damage in woven composites using mode selective excitation of Lamb waves is reported
in this paper. PVDF (polyvinylidene fluoride) comb sensors are used to generate and detect a single plate mode. The top
electrode is a single set of equidistant fingers connected in parallel to the same potential while the bottom electrode is
kept at ground. First, a pair of such sensors is used to generate and detect a single plate mode. Group velocity changes of
a wave packet traveling through the damaged area are used for quantitative damage estimation. Second, a new electrode
configuration is used in order to improve the receiver signal. The proposed configuration referred to as continuous
sensors, is used in structural health monitoring (SHM) for detection of growing cracks. Theoretical and experimental
results are presented. In addition, an analog circuitry to actuate the structure at high frequency (~1MHz) based on energy
tapped from a vibrating cantilever beam (~20Hz) is developed, towards a high-frequency energy-harvested SHM.
Ultrasonic NDI methods have an impressive record of applications on metallic and composite structures. However, limitations arise from the need for a wet couplant between the specimen and the transducer and the rather long inspection times necessitated by point-by-point scanning of large structures. To overcome these constraints, a dry-contact large-area ultrasonic imaging system is being developed for real-time high-resolution NDI applications. This system includes the following: a large ultrasonic source, either piezoelectric or laser-based, a polymer dry-couplant, and a commercially available real-time ultrasonic CCD camera displaying easy-to-interpret images rather than A-scans. Applications of this real-time high-resolution ultrasonic imaging system on metallic and composite structures, using either PZT or laser-based ultrasound generation as the source, are presented. Aluminum and unidirectional and woven composites have been investigated. Images acquired in both through-transmission and pulse-echo modes are presented. Images of artificial defects of different types and shapes in the investigated materials will be demonstrated. The latest developments of the imaging system, with laser-based ultrasound generation as the source, are also reported. The laser-based source provides an efficient solution for some applications of the imaging system. In this configuration, the ultrasound is generated in a 1in. diameter area by an expanded laser beam which heats a constrained absorbing polymer layer. The soft polymer layer is also used as dry couplant to transmit the ultrasound between the test sample and the imaging system.
The paper presents measurements taken with a scanning ultrasonic Doppler vibrometer on a landmine buried separately in sand and in grass-covered soil. The signal obtained with a laser Doppler vibrometer experiences a large variability that is due to loss of spatial coherence upon scattering from moving grass blades. Ultrasonic sensing is not affected by this limitation since the acoustic speckle is much larger than its optical counterpart. Moreover, the slightest hint of air motion enhances the motion of the grass blades, which adds to the optical decoherence and subsequent loss of useful signal. It is shown also that the ultrasonic system has no problem penetrating the layer of grass and detecting the location of the buried target excited by a mechanical shaker.
Representative data pertaining to various critical aspects of air-coupled ultrasonic Doppler sensing of ground vibrations are presented. The behavior of an ultrasonic sensor is systematically compared with that of commercial laser vibrometers. The inherent drawbacks and advantages of both techniques are discussed and evaluated in systematic experiments. The experiments are designed so as to synthesize various scenarios that may be encountered in practice. Thus the vibration sensing capability of ultrasonic vibrometers is investigated in cases including flat and grass-covered surfaces, granular media, with and without ambient air motion. The work is supported by the Office of Naval Research.