The Probe

The probe, situated between the field gradient coils in the bore of the magnet, consists of a cylindrical metal tube that transmits the pulses to the 

The significant improvements in sensitivity achieved during the last 5 years have been because of improved probe design and radiofrequency circuits. Since the probe needs to be located very close to the sample, it must be made of a material with a low magnetic susceptibility, for otherwise it would cause distortions of the static magnetic field thereby adversely affecting line shape and resolution. Much research has therefore been undertaken by NMR spectrometer manufacturers to develop materials that 

Recommend the most suitable probe for each of the following laboratories  

What properties should an ideal NMR probe have  

Once the spectrometer purchased, the probe is the most important part which could improve the spectrometer s performance. Upgrading the probe with latest generations can dramatically increase spectrometer s performance and this is the main way of coping with the progress of the technique. 

The other important consideration when selecting a probe is the range of nuclei it is able to observe and for which of these the coil configuration 

The target is also usually considered to have a single titration state and to be unaffected by the position of the probe. However, the GRID program does allow for probe-induced switching between histidine tautomers. 

MIFs are computed for positions of the probe at points on a rectilinear grid superimposed on the target. It is this grid that gives the GRID program its name. 

Grids of target-probe interaction energy values can be read into many molecular graphics programs which can display the MIFs as isoenergy contours or project the energies onto molecular surfaces. 

LC-NMR coupling technology has been recently reviewed [23], and the interested reader is also referred to the detailed text edited by Albert [24]. The miniaturisation of flow probes to the capillary scale has been developed to enhance signal detection sensitivity, as discussed in Section 3.4.2, and more recently the hyphenation of gas chromatography with microflow cells (GC-NMR) has even been demonstrated [25]. 

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The principle physical phenomenon of applying the eddy current method for evaluating the amount of residual austenite in the structure of quenched steel is magnetic induction, involving the influence of the changeable magnetic field on the studied area, found under the probe. 

Fig. 1. A diagram of the principle of the contact probe during the study of the metal matenal structure Ho - field produced by the probe, Hp - field ensuring the measuring signal, Hw - field produced by eddy currents.

Structure defects decrease conductivity of the studied material, and then the intensity of the induced magnetic field is small and the signal received by the probe Hp is big (Fig.2). Low conductivity of austenite is a defects of the structure in case of residual austenite in the martensite structure, which with regard to the magnesite structure is as 1 5. Eddy currents produced in the studied area are subject to excitation in effect of small conductivity of austenite grains in the structure of the studied material. 

The percentage share of the inserts made austenite steel in the martensite structure is refered to the visual field of the probe in the given location. Every probe after performance is given a characteristics, in which the visual field is given, determined using special devices defining the visual field at different distances from the tested object. 

During the inspection of an unknown object its surface is scanned by the probe and ultrasonic spectra are acquired for many discrete points. Disbond detection is performed by the operator looking at some simple features of the acquired spectra, such as center frequency and amplitude of the highest peak in a pre-selected frequency range. This means that the operator has to perform spectrum classification based on primitive features extracted by the instrument. 

The NSC was trained using labeled data acquired during inspection of objects with known defects. Examples of spectra for the object Lower wing skin are shown in Figure 5, the spectra measured for the flawless structures for different number of layers in the upper panel, the spectra corresponding 100% and 50% disbonds in the middle and lower panel, respectively. The size of the disbonds is given as a percent of active surface of the probe used for the test. 

Classifier structures resulting from the training were verified in a blind test. To evaluate the reliability and performance of the NSC it was subjected to a blind test using unknown data containing spectra measured for various sizes and locations of the disbonds (from 50% to over 100% of the probe size). 

Eddy-current non-destructive evaluation is widely used in the aerospace and nuclear power industries for the detection and characterisation of defects in metal components. The ability to predict the probe response to various types of defect is highly valuable since it enables the influence of particular parameters to be studied without recourse to costly and time consuming experiments. The solution of forward problems is also essential in the process of inverting experimental data. 

TRIFOU is a combined Finite Elements/Boundary Integral formulation code. The BIM formulation in vacuum is suitable for NDT simulation where the probe moves in the air around the test block. The FEM formulation needs more calculation time, but tetrahedral elements enable a large variety of specimens and defect geometries to be modelled. TRIFOU uses a formulation of Maxwell Equations using magnetic field vector h, where h is decomposed as h = hs + hr (hj source field, and hr reaction field). 

In this case the probe diameter and the slot length are of similar size. The material chosen has penetration depth of 0.7 ram at the given frequency of 16.9 kHz. The slot depth is 7 times larger than the penetration depth. 

The analysis of the curves obtained in the thin-skin regime ean lead to a simple determination of slot length depending on the dimension of the probe chosen for the inspection. If the size of the probe (outer diameter) is smaller than the defect length we can notice 5 zones relative to the relationship between the position of the probe, the interaction of the induced eddy current and the slot, and the impedance change for the probe. 

Interpretation of the impedance signal for set 2 (imaginary part function of the probe position along the slot)... 

Zone 1 The probe is far away form the slot the interaction with the slot is low. The impedance change is small. This situation is tme until the probe reaches the edge of the slot. The range of the zone is [- x 1 ]... 

Zone 4 The area of coverage interacting with the slot increase. The impedance change increases. This situation is true over the probe depth, that is to say until the all the length of the probe is over the slot. The range of the zone is [x3 x4]... 

The curve is symmetric from the middle of the slot. Hence the length of the defect is determined by the position of its edges at (x2+x3)/2 and -(x2+x3)/2 in the scanning direction of the probe. Of course this result is only true if we can distinguish the 5 zones on the curve. For other relative dimensions, for example a slot smaller than the probe (outer diameter), a curve like in set 1 is obtained, where the zones are confused. 

The case of thin-skin regime appears in various industrial sectors such as aerospace (with aluminium parts) and also nuclear in tubes (with ferromagnetic parts or mild steel components). The detection of deeper defects depends of course on the choice of the frequency and the dimension of the probe. Modelling can evaluate different solutions for a type of testing in order to help to choose the best NDT system. 

Second corner reflection The first corner reflection appears as usual when the transducer is coupled to the probe at a certain distance from the V-butt weld. The second corner reflection appears if the transducer is positioned well above the V-hutt weld. If the weld is made of isotropic material the wavefront will miss (pass) the notch without causing any reflection or diffraction (see Fig. 3(a)) for this particular transducer position. In the anisotropic case, the direction of the phase velocity vector will differ from the 45° direction in the isotropic case. Moreover, the direction of the group velocity vector will no longer be the same as the direction of the phase velocity vector (see Fig. 3(b), 3(c)). This can be explained by comparing the corresponding slowness and group velocity diagrams. 

The probes are assumed to be of contact type but are otherwise quite arbitrary. To model the probe the traction beneath it is prescribed and the resulting boundary value problem is first solved exactly by way of a double Fourier transform. To get managable expressions a far field approximation is then performed using the stationary phase method. As to not be too restrictive the probe is if necessary divided into elements which are each treated separately. Keeping the elements small enough the far field restriction becomes very week so that it is in fact enough if the separation between the probe and defect is one or two wavelengths. As each element can be controlled separately it is possible to have phased arrays and also point or line focussed probes. 

The calibration in UTDefect is perfomed by a side-drilled hole or a flat-bottomed hole. The flat-bottomed hole is approximated by an open circular crack with the probe s beam axis going normally through its centre. This approximation should be sufficiently accurate as long as the crack diameter is larger than about a wavelength. 

In Lakestani (10) modelling work performed within the PISC III project is validated against experiments. Figure 1 shows the pulse echo response from the lower edge of a 10 mm vertical strip-like crack at centre depth 55 mm. The probe has the size 20 mm by 22 ram, is of SV type with angle 45 and has centre frequency 2.2 MHz and an assumed bandwidth of 2 MHz. The calibration is perfomed by a side-drilled hole of diameter 9.5 mm and centre depth 60 mm (the... 

Becker et al. (11) have performed extensive experiments on surface-breaking cracks, tilting both the cracks or the back-side. The cracks are like a half ellipse, but could presumably be reasonably approximated by a strip-like crack. Figure 3 shows a comparison between the experiments and UTDefect for a 2.54 mm crack with varying tilt. The thickness of the plate with the crack is 15.24 ram. The probe is a circular 45 SV probe with frequency 2.25 MHz and diameter 12.7 mm. The experiments are calibrated with a notch but this is presently not... 

It must be admitted that eddy currents are a little unmanageable, they tend to flow where they want as soon as the magnetic field leaves the probe, so we may not know for certain what a flaw is really like. 

Separate driver and receiver and front balancing. The board can be programmed to use absolute probes, differential probes, additive and subtractive fluxes it can be operated in three modes receiver only, driver only, and driver-receiver. All of these configurations only depend on the way the probe is connected, and software configuration. Each board has connections for 2 drivers and 2 receivers. 

The instrument uses a sinusoidal driver. The spectrum is very clean as we use a 14 bits signal generator. The probe signal is modulated in amplitude and phase by a defect signal. The demodulation is intended to extract the cartesian values X and Y of this modulation. 

The operation is quite simple One sets the frequency to the lowest value, adjusts the gain and phase to the desired sensitivity using a special calibration standard discussed below and performs a zero-compensation on a defect free zone of the standard. Now one is ready to test. As one slides the probe across the surface of an aluminum structure, a signal response will be indicative of the presence of corrosion or of the presence of a subsurface edge. 

The sensitivity to defects and other control parameters can be improved by optimizing the choice of the probe. It appears, after study of different types of probes (ferritic, wild steel, insulator) with different geometries (dish, conical,. ..), necessary to underline that the success of a feasibility research, largely depends on a suitable definition of measure collectors, so that they are adapted to the considered problem. 

Material and coating control and evaluation by analyzing the active and reactive parts of the probes impedance. 

The sensibility to defects and other testing parameters of pieces can be modified by the geometry of the piece to be controlled and the conception of the probe. It is sufficient to set the direction of circulation of eddy currents, regulate the magnetic field intensity and choose the coil of the appropriate size. 

Eddy currents and the magnetic flux that is associated to them are proportional to the radial distance of the coil center. The magnetic flux is proportional to the probe induction and consequently to the passing current. The theoretic calculation of this induction is given by the following equation ... 

We improve the penetration of eddy currents by increasing the probe diameter. The useful diameter is generally equal to the coil diameter to which we add four times the standard thickness 8. 

By increasing the probe diameter, we bring down tlie impedance point along the impedance curve with the same way as the electrical frequency or conductivity. We will describe only one type of probes, namely, the probe with ferritic circular section that we could qualify as punctual with an optimal sensibility. In order to satisfy these conditions, tests will be made to confirm these results by ... 

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