Echo mode

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. 

Fig. 6. Test with one segment of 6 sector transducer in pulse-echo mode on aluminium plate, (a) no defect (b) defect simulated with mercury droplet (c) defect position.

The sensors function in a pulse-echo mode that allows the direct measurement of stand-off, from which short and long axes of the borehole diameter are computed. The vertical resolution is 1 in. (25 mm) and accuracy of the diameter measurement is 0.1 in. (2.5 mm). 

As indicated by Fig. 2, there are several testing modes (1) pulse-echo mode, (2) through-transmission mode, (3) reflector-plate mode, and (4) angle-beam mode. 

These methods of analysis are noninvasive and therefore nondestructive and non-hazardous. They can be connected to on-line production processes, thus offering the possibility of automation. They are also economic since the equipment is relatively inexpensive often involving only a single transducer acting as transmitter and receiver in a pulse-echo mode. 

Microwave detection can be performed from the penetrating side (pulse-echo mode) or from the opposite side (through-transmission mode). The through-transmission technique can determine the slate of resin cure in boron fiber laminates. However, it is not suitable for laminates with 0 /60 filament orientation [87]. The propagation of micro-waves (wavelength 10 10 m) is governed by Maxwell s equations of the magnetic field [88]. Maxwell s equations are... 

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... 

Although the pulse-echo mode is possible, this requires a degree of signal processing, and the Lamb wave method is preferred. Indeed, incline insonifieation permits better transitional parameters to be achieved than for normal incidence. 

Figure235 Block diagram of apparatus for transit-time measurements of sound velocity in pulse-echo mode. (Adapted from Schreiber et al., 1973, reproduced courtesy of McGraw-Hill, New York.)...

Pulsed techniques that operate in the so-called spin-echo mode are particularly good for applied NQR, and they naturally lend themselves to remote detection.12... 

We see than that three of the problems likely to be encountered when using echo envelope spectroscopy in place of ENDOR can either be discounted or disposed of. Failure to observe modulation is, assuming adequacy of the electron spin echo equipment, a possible source of information in itself. Lifetime broadening due to phase memory decay, and unwanted complexity of the modulation pattern due to sum and difference terms can both be avoided by performing experiments in the stimulated echo mode. The remaining problem is less tractable and has not yet been solved in a wholly satisfactory manner. This is the problem of transforming the echo envelope to yield a frequency spectrum. 

For example, the relative impedance of some materials are Air/vacuum = 0, water =1.5, glass = 15, copper = 42, and tungsten = 104. Analysis can be done by either using the ultrasound in a transmission mode or in a reflection (pulse echo) mode. For the analysis of the interface between the coating and the substrate, the pulse echo mode has the higher resolution and is capable of detecting interfacial delaminations less than 1 micron in extent. The coating has to have an appreciable thickness, the value of which depends on the material. Acoustic... 

When used in the pulse-echo mode, the returning signals are composed of a series of reflections, some of which have traversed the bond-line and others of which have not. A high-resolution (high frequency) probe is necessary to separate the various components. 

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