ABSTRACT
Conventional ultrasound inspection, a standard non-destructive testing method, uses a coupling medium (e.g. water) because of impedance mismatch. This liquid contact is a drawback because it prevents inspection of many materials. There is a need, then, for air-coupled ultrasound testing, which is now feasible because of low impedance focused narrow band transducers and sensitive electronics, both of which improve the signal-to-noise ratio. We present results obtained on fibre-reinforced plastics, water sensitive materials (e.g. reinforced ceramics), and "shape adaptive" structures to reveal delaminations, impacts, and growth of internal defects. Actuators embedded in "adaptive" structures are used as transmitters while the receiver records the signals. Thus it is possible to image defect areas and non-linear behaviour of potential defects.
ABSTRACT
Non-linear acoustic effects are of increasing interest in close-to-application research in which mostly ultrasonic frequencies above 1 MHz are used. If the excitation frequency is reduced, one can use high power excitation performed locally by a piezoceramic stack actuator that provides much higher vibration amplitudes. The combination of this excitation with a scanning laser interferometer provides a system for rapidly imaging non-linearities and thereby selectively detecting defects in most kinds of materials. This contribution presents various experimental results showing the potential of this new technique.
ABSTRACT
OBJECTIVE: To evaluate the fetal heart rate (FHR) response to vibroacoustic stimulation of fetuses entering the second stage of labor as a predictor of neonatal outcome. METHODS: Three hundred sixteen cases and 316 controls were studied during the second stage of labor. All cases had vibroacoustic stimulation on entering the second stage of labor using an electronic artificial larynx. For control patients, the artificial larynx was not activated. The patients were stratified into groups based on the quality of FHR response: acceleration (n = 124), acceleration followed by deceleration (n = 120), and no response (n = 72). RESULTS: Subsequent FHR accelerations and accelerations followed by decelerations were significantly more frequent in the study group than in controls (77.2 versus 15.2%; P < .05). The frequencies of low 5-minute Apgar scores (below 7) and low umbilical cord arterial pH (below 7.20) did not differ significantly in the group with the acceleration response when compared to those with acceleration followed by deceleration and the no-response groups. The incidence of nuchal cord was significantly higher for the group with a response pattern of acceleration followed by deceleration than for the acceleration and no-response groups (39.2 versus 10.5 versus 11.1%; P < .05). CONCLUSIONS: Vibroacoustic stimulation in the second stage of labor is associated with FHR reactivity, but the quality of FHR response does not predict neonatal outcome and therefore appears to have little value in enhancing the management of the second stage of labor. However, an acceleration followed by deceleration response suggests the presence of nuchal cord.
