·Table of Contents
·Methods and Instrumentation
A novel Surface Eddy Current probe with Phase Information on Surface Flaw Depth
H. HOSHIKAWA, K. KOYAMA, and H. KARASAWA
Izumicho Narashino Chiba 275-8575, Japan
Tel: +81-47-474-2396, Fax: +81-47-474-2399
Email : firstname.lastname@example.org
The authors have devised a new surface eddy current probe that provides phase information on surface flaw depth. The flaw signal phase hardly changes by the length and width of flaws, although the signal amplitude changes a great deal. The probe is also lift-off noise free and self-nulling. The authors believe that the probe makes eddy current testing more quantitative and reliable in flaw depth evaluation than the conventional probes.
Knowledge of the depth of surface flaws is significant because material breakdowns are usually directly connected to the depth rather than their length and width. So eddy current testing has always been required to be more quantitative and reliable in evaluating depth of surface flaws.
The signal of eddy current testing by conventional probe often changes much due to the probe lift-off from the test material. Since the large lift-off noise changes signal phase much, the signal phase can hardly be used to evaluate flaws. As a result, most eddy current testing by surface probes usually uses only the amplitude of flaw signal to evaluate flaws because the phase information is eliminated in the process of suppressing lift-off noise. However, the signal amplitude changes not only by the depth of flaws but also by the length and width. Consequently, eddy current testing by surface probe has been considered as an unquantitative and unreliable method for evaluating flaw depth.
The authors have devised a new surface probe for eddy current testing that generates no lift-off noise in principle. Experimental results have indicated that the probe provides the phase information on surface flaw depth and that the signal phase is not influenced much by the length and width of flaws. Thus the probe makes it possible to evaluate flaw depth based on flaw signal phase without much influence from flaw length and width.
The authors hope that the new probe utilizing flaw signal amplitude and phase will make the eddy current testing more quantitative and reliable to evaluate flaws than the conventional probes that usually use only the flaw signal amplitude.
CONVENTIONAL PANCAKE COIL PROBE
Conventional pancake coil probes pick up the variation of the eddy current by flaws to detect them in test materials. Lots of work have been done about coil impedance analysis and have contributed a great deal to the development of eddy current testing [1-2]. However, lift-off noise is unavoidable so long as probes pick up the variation of the eddy current induced by the test coil or the test coil impedance. Since the eddy current induced by test coil changes due to the coil lift-off from the material, the lift-off noise is unavoidable in the eddy current testing by conventional coil probes.
Since the large lift-off noise causes the phase of the probe signal to change a great deal, the signal phase can hardly have been utilized to evaluate flaws using the conventional eddy current probes, except for the tube inspection by inner bobbin probe. The signal phase information is usually eliminated in the signal processing to suppress the lift-off noise. Thus only the flaw signal amplitude has usually been used to evaluate flaws by surface probe. However, since the signal amplitude changes not only by the flaw depth but also by flaw length and width, eddy current testing has not been considered as a quantitative and reliable method of evaluating depth of flaws.
A NEW EDDY CURRENT PROBE
Lift-off noise is unavoidable so long as the probe picks up the eddy current induced by exciting coil. There the authors have thought of two notions in order to design a new probe that suppresses lift-off noise and detects flaws.
With the above two notions in mind, the authors have devised a new eddy current surface probe that is composed of a pancake exciting coil and a tangential detecting coil as shown in Figure 1. The circular exciting coil is adopted because it induces eddy current most efficiently. The exciting coil induces axi-symmetric circular eddy current in the test material with no eddy current circulating across the exciting coil circle when there is no flaw in the test material as shown in Figure 2(a). When there is a flaw crossing the circle, some eddy current circulates along the flaw crossing the circle. Since each part of the detecting coil winding picks up the parallel eddy current component to itself, the tangential detecting coil picks up only the eddy current circulating across the circle as shown in Figure 2(b)-(d). As the new probe scans over a flaw, the detecting coil generates a figure eight signal pattern.
- One of the methods to eliminate lift-off noise in eddy current testing is to develop a
probe picking up the component of eddy current that is generated only by flaws but not
by the probe lift-off.
- Each part of detecting coil windings picks up the parallel component of eddy current to
Fig 1: A new eddy current probe
If the probe has two tangential detecting coils wound perpendicular to each other, it can detect all flaws in every orientation. The impedance of the exciting coil can also be used to monitor the probe lift-off in order to avoid the probe not detecting flaws in the material.
Fig 2: Eddy current induced in the material by a circular exciting coil
The new probe is lift-off noise free because the lift-off of the probe from the material does not cause any eddy current to circulate crossing the exciting coil circle. Thus lift-off noise can be eliminated by detecting only the newly generated eddy current by flaws and by not detecting the eddy current induced by the exciting coil when there is no flaw in the test material. The probe is self-nulling because the detecting coil generates a signal only when a flaw causes some eddy current to circulate across the circle.
Since the probe generates minimal lift-off noise, the authors have also thought that the probe lift-off does not influence much to the flaw signal and that the signal phase can be used for evaluating the depth of surface flaws.
Figure 3 shows the sizes of the new probe and brass plates with an electric discharge machined slit flaw of different depths, lengths, and widths used for the experiments. The test frequency of 32 kHz has been chosen to make the skin depth of the eddy current induced in the material equal to the plate thickness. The exciting coil alone has also been used to conduct experiments of the traditional pancake coil probe.
Fig 3: Experimental setup|
Exciting coil ; outer diameter 7 mm , cross section of windings 1x1mm2
Detecting coil ; length 5mm , height 3mm , cross section of windings 1x1mm2
Brass plate ; 160x160x1.5mm3
Test Frequency ; 32kHz
Figure 4 shows the experimental results of flaw signals and lift-off noises. Figure 4(a) indicates that the conventional pancake coil probe generates far larger lift-off noise than the flaw signals. On the other hand, Figure 4(b) indicates that the new probe generates far larger flaw signals than lift-off noise. Thus it is obvious that the new probe generates flaw signals with far higher signal to noise ratio than conventional pancake probes. The results shown in figure 4(a) indicates that the signal phase of flaws can be used to evaluate flaws.
Figure 5 shows flaw signal patterns for front surface flaws and back surface flaws with different depths obtained by the new probe. The figure indicates that the amplitude and the phase of the flaw signals change according to the depth of front surface flaws and back surface flaws.
Fig 4: Flaw signal and lift-off noise
Figure 6 shows flaw signal patterns obtained by the new probe with different flaw lengths. The flaw length changes the amplitude of signals a lot but keeps the phase almost constant. Figure 7 shows flaw signal patterns obtained by the new probe with different flaw widths. Again, the flaw width changes the amplitude of signals a lot but keeps the phase almost constant. Figure 6 and Figure 7 indicate that the signal amplitude is not proper for evaluating surface flaw depth in the eddy current testing.
Fig 5: Signal flaw patterns for different flaw depths|
Figure 8 shows the flaw signal patterns with the normalized amplitude. The figure indicates that the depth of front surface flaw lags the flaw signal phase and the depth of back surface flaw leads the phase.
Fig 6: Signal patterns of flaws with different lengths
Fig 7: Signal patterns of flaws with different widths
From the signal patterns shown in Figure 8, the authors have derived the flaw depth evaluation curve based on the signal phase as shown in Figure 9. Thus flaw depth can be evaluated by applying flaw signal phase to the curve in Figure 9 without much influence from the variations of flaw length and width. The relation between signal phase and flaw depth is just the same as the one known in the tube inspection by inner bobbin coil probe. The authors believe that the flaw depth evaluation method based on the signal phase improves the evaluation accuracy of eddy current testing.
Fig 8: Flaw signal patterns with normalized amplitude
Fig 9: Flaw depth versus flaw signal phase
The authors have devised a new eddy current probe that has little lift-off noise and provides phase information on surface flaw depth. The signal phase keeps almost constant with the variations of flaw length and width. The authors hope that the probe will enhance the quantitativity and reliability of eddy current testing by a surface probe.
- C. V. Dodd, W. E. Deed, "Analytical Solution to Eddy Current Probe-Coil Problems,"
Journal of Applied Physics, Vol.39, No.6, pp2829-2838 (1968)
- M. Onoe, "An Analysis of a Finite Solenoid Coil Near a Conductor," (in Japanese)
Journal of IEE of Japan, Vol.88-10, No.961, pp1894-1902 (1968)
- H. Hoshikawa, K. Koyama, "A New Eddy Current Probe Using Uniform Rotating Eddy
Current," Materials Evaluation, Vol.56, No.1, pp85-89 (1998)