Reflectors

Each reflector contributes a reflection along a raypath to the surface, defined by the normal incidence reflection point (shortest travel time)... 

Secondly, a short pulse duration is required in order to achieve a good axial resolution, i.e. two signals close together should be detected without interference. The task can be, for example, to detect a small reflector close to the surface or back wall of the test object, as the inspection has to cover the total volume as complete as possible, including the near-surface regions. 

The second example shows results obtained with an angle beam probe for transverse waves in coarse grained grey cast iron. Two commercially available probes are compared the composite design SWK 60-2 and the standard design SWB 60-2. The reflector in this example is a side-drilled hole of 5 mm diameter. The A-scans displayed below in Fig. 5 and 6 show that the composite probe has a higher sensitivity by 12 dB and that the signal to noise ratio is improved by more than 6 dB. 

The function h(t) to be restored is the impulse response of the medium x(t) is the transmitted pulse measured by reflection on a perfect plane reflector, for example the interface between air and water and y(t) is the observed signal. 

Real-time reflector location and detection of the probe s scanning trajectory... 

The exact position of reflectors within the weld volume is calculated by means of the known probe position plus weld geometry and transferred to a true-to-scale representation of the weld (top view and side view). Repeated scanning of the same zone only overwrites the stored indications in cases where they reach a higher echo amplitude. The scanning movement of the probe is recorded in the sketch at the top, however, only if the coupling is adequate and the probe is situated within the permissible rotation angle. 

Calibration of the ultrasonic instrument, including plotting of a recording curve (DAC), or a reference reflector for a DOS evaluation, or loading of the existing test dataset... 

Up to now the Reference Block Method and the DGS-Method are world wide the most important techniques for evaluating defect signals in manual Ultrasonic Testing. Even today, individual national standards refer to either one of these two echo evaluation techniques. However, both reflected echo signals from natural defects are compared with an echo from a known reference reflector at the same distance. The result of the evaluation is either... 

The physics of ultrasound is well known and widely described in many publications. Recording amplitudes from model reflectors at different depths by Dr. Josef Krautkramer in 1959 led to the DGS-diagram Echo amplitudes from disk shaped reflectors of different sizes were... 

With the help of the DGS-diagram any defect echo can be evaluated, if the echo of a known reference reflector is available. 

The result of a DGS evaluation is the Equivalent Reflector Size (ERS), the diameter of a disk hit perpendicularly at the same distance of the defect, producing an echo amplitude exactly equal to that of the defect. 

With the reference block method the distance law of a model reflector is established experimentally prior to each ultrasonic test. The reference reflectors, mostly bore holes, are drilled into the reference block at different distances, e.g. ASME block. Prior to the test, the reference reflectors are scanned, and their maximised echo amplitudes are marked on the screen of the flaw detector. Finally all amplitude points are connected by a curve. This Distance Amplitude Curve (DAC) serves as the registration level and exactly shows the amplitude-over-distance behaviour" of the reference reflector for the probe in use. Also the individual characteristics of the material are automatically considered. However, this curve may only be applied for defect evaluation, in case the reference block and the test object are made of the same material and have undergone the same heat treatment. As with the DGS-Method, the value of any defect evaluation does not consider the shape and orientation of the defect. The reference block method is safe and easy to apply, and the operator need not to have a deep understanding about the theory of distance laws. 

Side drilled holes are widely used as reference reflectors, especially when angle beam probes are used (e.g. for weld testing). However, the distance law of side drilled holes is different to that of a flat bottomed hole. In the literature [2] a conversion formula is given which allows to convert the diameter of a side drilled hole into the diameter of a flat bottomed hole and vice versa, valid in the far field only, and for diameters greater than 1.5 times the wave length. In practical application this formula can be used down to approximately one nearfield length, without making big mistakes. Fig. 2 shows curves recorded from real flat bottomed holes, and the uncorrected and corrected DGS curves. 

The echo amplitude Ar of a reference reflector depends on the type, size (diameter) d f, and distance Sr,., of the reference reflector, and additionally on a possible attenuation in the reference block and finally the absolute gain setting of the instrument G f. In a combined DAC/DGS evaluation program we define the following ... 

First, as the element width is very narrow, and though they have a large elevation, the reflector is placed very close to the elements. That is in violation to the standards rules [1], but experience shows no effect on the frequency spectrum. 

In the case the element are focused in the elevation plane (our inner tube probe), it may be more convenient (or easy) to use a flat reflector at the focus plane or at its equivalent focus plane in water, for instance. 

As any conventional probe, acoustic beam pattern of ultrasound array probes can be characterized either in water tank with reflector tip, hydrophone receiver, or using steel blocks with side-drilled holes or spherical holes, etc. Nevertheless, in case of longitudinal waves probes, we prefer acoustic beam evaluation in water tank because of the great versatility of equipment. Also, the use of an hydrophone receiver, when it is possible, yields a great sensitivity and a large signal to noise ratio. 

The encircling probe was characterised with its mirror in water. As we did not own very tiny hydrophone, we used a reflector with hemispherical tip with a radius of curvature of 2 mm (see figure 3c). As a result, it was possible to monitor the beam at the tube entrance and to measure the position of the beam at the desired angle relatively to the angular 0° position. A few acoustic apertures were verified. They were selected on an homogeneous criteria a good one with less than 2 dB of relative sensitivity variations, medium one would be 4 dB and a bad one with more than 6 dB. 

The analysis software rebuild a C-scan of every three reflectors and for every 160 acoustic apertures, and plots (see figure 5) ... 

The acoustical device component is placed in water and is configured like a conventional impulse echo equipment. The ultrasound wave passed the delay path and enters the specimen container through a very thin plastic window. The backside of the container is a steel plate and will also be used as a reference reflector to measure pn. 

Figure B2.1.4 Fluorescence upconversion spectrometer based on the use of off-axis elliptical reflectors for the collection and focusing of fluorescence. Symbols used el, c2, off-axis elliptical reflectors s, sample x, nonlinear crystal. (After Jimenez and Fleming [21].)...

Beryllium is added to copper to produce an alloy with greatly increased wear resistance it is used for current-carrying springs and non-sparking safety tools. It is also used as a neutron moderator and reflector in nuclear reactors. Much magnesium is used to prepare light nieial allo>s. other uses include the extraction of titanium (p. 370) and in the removal of oxygen and sulphur from steels calcium finds a similar use. 

Now encounters between molecules, or between a molecule and the wall are accompanied by momentuin transfer. Thus if the wall acts as a diffuse reflector, molecules colliding wlch it lose all their axial momentum on average, so such encounters directly change the axial momentum of each species. In an intermolecuLar collision there is a lateral transfer of momentum to a different location in the cross-section, but there is also a net change in total momentum for species r if the molecule encountered belongs to a different species. Furthermore, chough the total momentum of a particular species is conserved in collisions between pairs of molecules of this same species, the successive lateral transfers of momentum associated with a sequence of collisions may terminate in momentum transfer to the wall. Thus there are three mechanisms by which a given species may lose momentum in the axial direction ... 

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