Ultrasonic pulsed modes

Negishi, K. and Ri, H. U. (1987). Propagation of multi-mode ultrasonic pulses in non-destructive material evaluation. In Ultrasonic spectroscopy and its application to materials Science (Ed. Y. Wada), pp. 70-4. Ministry of Education, Science, and Culture, Japan. [100,102]... 

Sherar, M. D., Noss, M. B., and Foster, F. S. (1987). Ultrasound backscatter microscopy images the internal structure of living tumour spheroids. Nature 330,493-5. [174] Shimada, H. (1987). Propagation of multi-mode ultrasonic pulses in non-destructive material evaluation. In Ultrasonic spectroscopy and its application to Materials science (ed. Y. Wada), pp. 50-6. Ministry of Education, Science and Culture, Japan. [148] Shotton, D. M. (1989). Confocal scanning optical microscopy and its applications for biological specimens. J. Cell. Sci. 94,175-206. [177,200]... 

When ultrasonic energy is applied in a pulsed mode, pulse duration can be an important variable. 

Application of pulses and duration. This variable, which affects ultrasonic probes exclusively, comes into play when operating in a pulsed mode. In USASD, sample dissolution can be favoured by short or long pulses in USASTD, the severity of the treatment requires using long pulses. 

The main variables related to the way in which US energy is applied to the sample-leachant system are the US amplitude, frequency, pulse mode operation, irradiation time, position of the vessel in the ultrasonic field, distance between the probe tip and sample vessel or chamber, and volume and nature of the transmitting liquid. 

The aliquot designated for extraction at neutral pH was extracted three times, with fresh 100-mL acetone/hexane (50/50). An ultrasonic cell disruptor, in pulsed mode at 50 percent duty cycle, was employed to enhance the contact between the extraction solvent and soil. Following each extraction, the soil was allowed to settle, the solvent decanted, and the combined supernatants from the neutral pH extraction were dried through a column of anhydrous sodium sulfate (5-cm bed height 3-cm diameter). Concentration of the extract was performed using a Kuderna-Danish evaporator. Final volume was adjusted to 1 mL using nitrogen blowdown evaporation. 

For many applications the pulse repetition rate / = cfid (which is / = 150 MHz iox d = m) is too high. In such cases the combination of synchronous pumping and cavity dumping (Sect. 6.1.2) is helpful, where only every A th pulse k > 10) is extracted due to Bragg reflection by an ultrasonic pulsed wave in the cavity dumper. The ultrasonic pulse now has to be synchronized with the mode-locked optical pulses in order to assure that the ultrasonic pulse is applied just at the time when the mode-locked pulse passes the cavity dumper (Fig. 6.16b). 

The technical realization of this synchronized system is shown in Fig. 6.16a. The frequency Vs = /27t of the ultrasonic wave is chosen as an integer multiple v =q c I Id of the mode-locking frequency. A fast photodiode, which detects the mode-locked optical pulses, provides the trigger signal for the RF generator for the ultrasonic wave. This allows the adjustment of the phase of the ultrasonic wave in such a way that the arrival time of the mode-locked pulse in the cavity dumper coincides with its maximum extraction efficiency. During the ultrasonic pulse only one mode-locked pulse is extracted. The extraction repetition frequency Vq = (cjld jk can be chosen between 1 Hz to 4 MHz by selecting the repetition rate of the ultrasonic pulses [669]. 

Ultrasonic waves (t5q)ically in the MHz range) can be excited by piezoelectric transducers coupled to the test object. Electromagnetic acoustic transducers (EMAT) (123) cannot be used for ultrasonics of polsrmers and PMC alternatives are discussed in Reference 124. The waves propagate and are scattered and attenuated in the material. The signal can be detected in either reflection or transmission mode (by the transducer emitting the ultrasonic pulse for reflection or by additional transducers at other locations in transmission). Acousto-ultrasonics (AU) (11) records low frequency guided waves (t5q)ically 30-500 kHz) emitted at another location to detect changes in the test objects, eg, in carbon-carbon... 

Another limitation is posed by the shadowing effect in PMC. The delamination closest to the transducer will reflect the ultrasonic pulse to such an extent that delaminations below will be masked, unless their lateral size is larger. Even C-scans taken on both sides do not necessarily guarantee detection of delaminations in all layers. In the reflection mode, repeat echoes from multiple reflections have to be distinguished from real indications, especially in relatively thin PMC shells. 

As ultrasonic sensors work predominantly in pulse mode, the pulse transmission behaviour of the piezo transducer together with the peripheral amplifier and signal conditioning electronic will determine the signal quality. 

Schematic of ultrasonic pulse in through-transmission mode (From Adams et al. 1997)...

In service inspections of French nuclear Pressure Water Reactor (PWR) vessels are carried out automatically in complete immersion from the inside by means of ultrasonic focused probes working in the pulse echo mode. Concern has been expressed about the capabilities of performing non destructive evaluation of the Outer Surface Defects (OSD), i.e. defects located in the vicinity of the outer surface of the inspected components. OSD are insonified by both a "direct" field that passes through the inner surface (water/steel) of the component containing the defect and a "secondary" field reflected from the outer surface. Consequently, the Bscan images, containing the signatures of such defects, are complicated and their interpretation is a difficult task. 

Some of the problems often encountered during ultrasonic inspection of plane specimens are also found on cylindrical specimens. For example, problems associated with the directional characteristic of the ultrasonic transducer. Furthermore, the discontinuity influences the shape and propagation direction of a reflected pulse, causing wave mode transformation. In addition, the specimen influences the shape and amplitude of the reflected pulse by sound absorption. 

Shear moduli C were determined at tiz by the attenuated reflection of pulses (ARP) method described previously (4- ). In this method a 4 psec pulse of ultrasonic energy applied in the shear mode is made to traverse a quartz substrate, impinging on the underside of the film as it reflects repeatedly from the ends of the substrate. 

The pulse-pause mode, which is used to maintain the temperature within a specific range. Ultrasound is applied to the sample while keeping the sample temperature between the limits of this range. This mode allows the sample temperature to be kept within the range while continuing the ultrasonic treatment during the process time. 

General ultrasonic spectrometry relies on direct measurements of the physical changes caused by the US-sample interaction, namely velocity changes and attenuation of the radiation. The different modes of this technique arise from factors such as (1) the way US is applied e.g. as a single frequency, broad-band pulses, scanning frequency), after which modes are named (2) the way US impinges on the sample (normal, parallel, oblique), after which the waves produced in the material (longitudinal, shear, oblique) are named (3) the way the experimental data provided are used viz. amplitude or phase spectra) or processed viz. frequency or time domain). 

The two major techniques in ultrasonic testing arc the pulse-echo mode and the through-transmission mode. In pulse-echo mode, the same transducer is used to emit and receive ultrasonic waves and requires access to only one side of the structure inspected (Fig. 12). The pulse-echo technique is effective on the near side skin laminate only, and the sensitivity decreases as a function of depth [29]. With the through-transmission mode, access to both sides of a structure is required as an emitter probe is placed on one side and a receiver on the other (Fig. 13). This technique measures the signal amplitude of ultrasonic waves transmitted through the material tested and is more sensitive to small defects than the pulse-echo mode. The detection of flaws throughout the whole depth of a... 

Fig. 6.8 Active mode locking (a) sideband generation (b) experimental arrangement with a standing ultrasonic wave inside the laser resonator (c) idealized output pulses...

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