The most accurate results will be obtained using a high signal to noise ratio. A high signal to noise ratio will allow easy identification of a relevant discontinuity with low electronic background noise.
Background noise can be produced from variables that have no interest to the examiner. This would include material configuration, surface roughness, lift-off, permeability, and conductivity.
Abrupt changes in surface curvature result in changes to eddy current signals as probes traverse them. It causes changes in coupling creating a large lift-off signal and the curvature also changes eddy current flow distribution creating an effective resistance change, yielding a signal at an angle to the lift-off direction. The appearance of this type of signal will not change significantly when rescanned at higher and lower test frequency.
Such signals can be difficult to analyze because they depend on how well the probe follows complicated surface curvatures. Basically the direction of the impedance change obeys the following rules when using surface probes:
(a) decreasing radius of curvature on an external surface, e.g., ridge, produce change in the direction of increasing resistivity,
(b) decreasing radius of curvature of an internal surface, e.g., groove, produces a change in the direction of decreasing resistivity.
The most troublesome parameter in eddy current testing is lift-off (probe-to-specimen spacing). A small change in lift-off creates a large output signal.
A particular condition such as ‘wobble’ can be suppressed by making the amplitude of its response at the first frequency equal and its phase 180 ° away from the response at the second frequency and then adding the two signals together. The resulting sum will result in cancellation of the responses and thus a zero signal for that particular condition.
4.3.1 Choice of test frequency
Test frequency is often the only variable over which the inspector has appreciable control.
Material properties and geometry are normally fixed and probe choice is often dictated by test material geometry and probe availability. Choice of a suitable test frequency depends on the type of inspection. Testing for diameter variations normally requires maximum response to fill-factor which occurs at high frequencies. Testing for defects requires penetration to possible defect locations; surface defects can be detected at higher frequencies than subsurface defects. Maximum penetration requires a low frequency which still permits clear discrimination between signals from harmless variations in material properties and serious defects. The above factors show choice of test frequency is usually a compromise.
4.3.2 Phase discrimination
In the majority of cases, no detailed knowledge of the discontinuity types, shapes, depths and orientations exists before the start of the eddy current examinations. Consequently, the majority of the data analysis depends on the phase angle analysis to determine discontinuity parameters.
It is important, however, to detect and to identify discontinuity signals and to separate them from non-relevant background signals before any crack depth analysis can be performed. The phase angle discrimination technique is ideally suited for this separation.
The phase angle discrimination technique depends on the proper choice of test frequencies for providing optimum phase angle separation among different variables. For a given test
The most common practice involving the phase angle discrimination is to rotate the lift-off/fill-factor variations to horizontal and monitor the remaining variables. Based on this concept of maintaining the lift-off/fill-factor as horizontal, a detailed comparison of phase angle separations among variables can be determined.
It should be emphasized that the selected frequency might not necessarily be the ideal frequency for estimating discontinuity depths. The concept of detection first, followed by discontinuity analysis has been an excepted evaluation method.
4.3.3 Filtering
To accentuate desired frequencies and to eliminate undesired frequencies, electronic filtering is employed. Three types of filters can be used; the high pass, the low pass and the band pass.
High pass filtering utilizes a resistance-capacitance circuit, which removes the low frequency components of the eddy current signal from the bridge. This type of filtering can eliminate the effect of gradual variations in conductivity or dimensions on the eddy current inspection response. Low pass filtering employs signal averaging circuits to remove rapid (high frequency) response from electronic noise and from harmonic frequencies related to variations in magnetic permeability. Band pass filters use combinations of both types of circuitry to promote response over a specific range of frequencies and suppress frequencies above and below this range. The effects or each type of filter on the recorded appearance of eddy current signals is illustrated in FIG. 4.3.
Selection of frequencies
For a known discontinuity type, eg: fine cracks, appropriate filters can be calculated by selecting filters either side of the Response Frequency (Fr) refer formula 4.7.
For components with various discontinuities eg: tube inspection, selection of filter frequencies can be optimized by using an appropriate reference sample.
FIG. 4.3. Effects of Filtering.
4.3.4 Magnetic saturation
Eddy current inspection of magnetic materials for defects is difficult or impossible because of random permeability variation. In addition there are skin depth limitations. Without saturation, the initial permeability of steel products can range from 50 to over 500. Since depth of penetration is inversely proportional to the square root of permeability and test frequency, to obtain equal penetration requires a reduction in frequency by the same factor of 50 to over 500. Unfortunately, lowering frequency will move the operating point to where there is poor signal separation between lift-off, permeability and resistivity as well as reduced sensitivity to defects. Therefore magnetic saturation is required to suppress effects of usually harmless permeability variations, which could be mistaken for or obscure, defect signals.
Coupling influence 4.4.1 Vibrations
Vibrations during probe motion can make undesirable signals, or so called ‘probe wobble’.
The multi-frequency technique can suppress this effect can by making the amplitude of its response at the first frequency equal and its phase 180 degrees away from the response at the second frequency and then adding the two signals together. The resulting sum will result in cancellation of the responses and thus a zero signal for that particular condition.
4.4.2 Lift off
When a surface coil is energized and held in air above a conductor the impedance of the coil has a certain value. As the coil is moved closer to the conductor the initial value will change when the field of the coil begins to intercept the conductor. Because the field of the coil is strongest close to the coil, the impedance value will continue to change until the coil is directly on the conductor. Conversely, once the coil is on the conductor any small variation in the separation of coil and conductor will change the impedance of the coil. The lift off effect is so pronounced that small variations in spacing can mask many indications.
The lift off effect is regularly used to measure the thickness of non conductive coatings.
The angle of orientation (tilt) of the probe will also have a significant impact of coupling efficiency. The use of mechanical guide/holders and spring loaded probes can assist in reducing the effect of lift off.
4.4.3 Centring, fill factor
In an encircling coil, or an internal coil, fill factor is a measure of how well the conductor (test specimen) fits the coil. It is necessary to maintain a constant relationship between the diameter of the coil and the diameter of the conductor. Again, small changes in the diameter of the conductor can cause changes in the impedance of the coil. This can be useful in detecting changes in the diameter of the conductor but it can also mask other indications.
For an external coil:
Fill Factor η = (D1/D2)2 (4.5)
For an internal coil:
where
η = fill factor D1 = part diameter D2 = coil diameter
Thus the fill factor must be less than 1 since if ηηηη= 1 the coil is exactly the same size as the material. However, the closer the fill factor is to 1 the more precise the test.
The fill factor can also be expressed as a%. For maximum sensitivity, the fill factor should be as high as possible compatible with easy movement of the probe in the tube. Note that the fill factor can never exceed 1 (100%).
4.4.4 Sensitivity
The distribution of eddy currents on a round bar using an encircling coil is such that the field is maximum at the surface and is zero at the centre of the bar.
The distribution of eddy currents on a flat plate using a surface probe is such that the field is maximum at the surface directly below the coil windings and is zero at the centre of the coil.
4.4.5 Compensation
To optimize probe coupling numerous techniques can be employed, these include;
(a) The use of mechanical guide/holders and spring loaded probes can assist in reducing the effect of lift off.
(b) Appropriate probe diameter to maximize fill factor.
4. . Influence of relative part/probe speed
4.5.1 Instrument frequencies according to speed
Eddy current instruments and recording instrumentation have limited frequency response.
This means they require finite time to respond to an input signal. Frequency response, sometimes called speed of response, is defined as the frequency at which the output signal falls to 0.707 (-3 dB) of the maximum input signal.
A test coil with an effective sensing width W, passing over a localized defect of width w at a speed s, will sense the point defect for a duration of w/s seconds. This signal is approximately equal to one wavelength with a frequency.
The Response Frequency (Fr) is the inverse value of the time taken for the probe to cross the fault and can be shown by the formula:
(4.7) where
S = speed of probe movement (mm Sec -1 ) W = probe width (mm)
w = crack width (mm)
NOTE: For practical purposes crack width can be considered as Zero.
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For example, at a probe speed of 0.5 m/s and probe sensing width of 2 mm, Fr = 250 hertz. If the instrumentation has a frequency response of 250 hertz, the output signal is reduced to 0.707 the input signal and the X-Y signal is distorted. If the instrumentation frequency response is 500 hertz, the output signal decreases only slightly. For this example, the eddy current instrument should have a frequency response equal to or greater than 500 hertz to obtain undistorted signals. Or inversely, if the instrument frequency response is only 350 hertz, the maximum inspection speed should be reduced to 0.25 m/s.
4.5.2 Frequency response of apparatus according to testing speed
Some standards specify maximum permissible scanning speed. For example, according to the Article I-40 of ASME Article 8 Appendix 1, the maximum scanning speed of eddy current probe can be 0.356 m/s for 100 Hz frequency response system. If an eddy current system with a frequency response of 450 Hz is used, it allows and scanning speed of 1.6 m/s.
4. . Reference standards used in eddy current testing
Analysis of eddy current signals is for the most part, a comparative technique. Reference standards are necessary for comparing signal amplitude and phase (shape) of unknown defects to known reference defects. Reference signals are also used for standardizing instrument settings, i.e. sensitivity and phase rotation.
4.6.1 Function of reference samples
Existing national specifications and standards only supply broad guidelines in choice of test parameters. They cannot be used to establish reliable eddy current test procedures for most inspection. The effect of the following can be established:
(a) Varying electrical resistivity (b) Varying thickness
(c) Surface geometry (curvature) (d) Defect length for constant depth (e) Defect depth for constant length
(f) Increasing subsurface defect size for constant defect depth
(g) Increasing distance of subsurface defects from the surface with constant defect size (h) Varying thickness of a non-conducting layer (lift-off)
(i) Varying thickness of conduction layer (j) Ferromagnetic inclusions
1.00mm
0.5mm 0.2mm
Slots of varying depths
Standard Calibration Block
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More than one reference plate would be required to cover a complete range of materials.
FIG. 4.5a illustrates eddy current signals obtained with an absolute surface probe from some of the reference sample defects. FIG. 4.5b illustrates signals from the same defects using differential surface probe.
FIG. 4.5. Eddy current signals with (a) absolute and (b) differential surface probes.
4.6.2 Choice of reference sample
The reference sample shall be a part of and shall be processed in the same manner as the product being examined. It shall be of the similar nominal dimensions and the same nominal composition as the product being examined.
The reference sample shall be long enough to simulate the handling of the product being examined through the inspection equipment. The separation between reference discontinuities placed in the same reference sample shall not be less than the length of the sensing unit of the inspection equipment.
4.6.3 Fabrication and reproducibility of various types of reference samples
Most reference standards consist of drilled holes of various diameters and/or various depth from the external surface. Some reference samples have EDM (electric discharge machining) notches in the circumferential and axial directions and on both internal and external surfaces.
4.6. Inspection method