Probes and their sensitivity

The following outlines the types of probes commonly used in surface testing, their sensitivity to various conditions, and examples of where each type of probe is used. More information on the types of probes used for particular applications, and probes for special applications, is given in the notes on these applications.

The sensitivity of standard surface testing probes

Standard surface testing probes consist of a coil wound on a core of plastic, ferrite, or other material, and are used with the axis of the coil normal to the test surface. FIG 5.1 shows a typical absolute probe and the eddy current field it induces. Differential probes, with two coils, one on each of two adjacent arms of the bridge circuit, and reflection probes, which contain one coil or set of coils which generates the eddy current field (the driver coil(s)), and a second coil or set of coils which detect the response of the test material (the pickup coil(s)), are also sometimes used. The pickup coils in reflection probes can be either absolute or

differential. Reflection probes show a wider frequency range and higher signal to noise ratio than other types of probe.

All of these coils have the magnetic flux essentially normal to the test surface, and so produce eddy currents in circular paths essentially parallel to the coil windings and the test surface.

FIG. 5.1. Schematic diagram showing a standard eddy current test coil and the eddy current field induced in the test part. The core is not shown.

The eddy current intensity is maximum immediately below the coil windings, and decreases linearly to zero at the centre of the coil. In addition, except for shielded probes (discussed below), the eddy current field extends laterally for some distance away from the coil, the distance increasing as the coil diameter increases and as the depth of penetration increases.

Therefore, large diameter, low frequency coils, which show the greatest depth of penetration, also show the greatest lateral eddy current field.

Flaws can be detected only if they distort the flow of eddy currents. This means that cracks and similar flaws normal to the test surface in any direction can be detected, but flaws parallel to the test surface, like laminations, cannot be detected.

However, if a flaw is shorter than the coil diameter, there is a possibility that it will not be detected because, depending on its location and orientation with respect to the coil, it may not significantly distort the eddy currents (FIG. 5.2). That is flaws can be reliably detected only if their length is approximately equal to or greater than the coil diameter. Because of this, probes for the detection of small surface flaws usually have small diameter coils (approximately lmm to 2 mm diameter), although if only longer flaws are sought, larger diameter coils can be used.

Although flaws which are the same length as the probe diameter or longer can be reliably detected, provided the scanning index (the distance between successive scans) is small enough, the crack signal amplitude increase with increasing crack length up to a maximum obtained when the crack is long enough to distort the entire surface eddy current field.

The depth of surface flaws (the distance the flaw extends below the surface) also affects the signal amplitude - the greater the depth the greater the signal amplitude. The depth of surface flaws also affects the signal phase. As the depth of the flaw increases, it distorts eddy currents deeper below the surface. As the depth of the eddy currents below the surface increases, they show a steadily increasing phase lag with respect to the surface eddy currents, the flaw signal shows a corresponding phase rotation clockwise (see FIG. 5.3). This is true even if the flaw extends beyond the effective depth of penetration. In this case, since the eddy currents cannot flow in their normal paths, they are caused to flow below the flaw and show an increasing phase lag. This may not be true for short deep flaws because the eddy currents may be diverted around the ends of the flaw rather than below the flaw.

FIG. 5.3. Shows the signals obtained from surface discontinuities of varying depths. As the depth of the discontinuity increases, the signal amplitude increases and the signal phase rotates clockwise.

Increasing lift-off always reduces signal amplitude, although it has no effect on the depth of penetration or the phase of the eddy currents. For this reason, probes should be held so that the coil axis is normal to the test surface. However, despite the reduced sensitivity, it is common practice to apply a small piece of Teflon adhesive tape to the tips of small diameter probes to prevent probe wear.

Types of standard surface testing probes (a) Pencil probes

Pencil probes are simply small diameter probes with small diameter coils and are the standard probe for the detection of surface flaws because of their sensitivity to small cracks (see FIG. 5.4). Pencil probes are usually high frequency probes for the detection of surface flaws, but low frequency pencil probes are also available, and are used for the detection of subsurface flaws.

Lang type pencil probes are standard pencil probes where the coil is located a few millimeter up from the probe tip, but an elongated ferrite core channels the magnetic field from the coil to the test surface, so reducing lift-off effects (see FIG. 5.5a). This construction means that probe wear will not affect the coil.

(b) Spot probes

Spot probes are probes with a relatively large flat face for testing flat surfaces for flaws or material properties (conductivity or thickness) (see FIG. 5.5b). The coil diameter may be small or large - the size of the probe does not necessarily give a good indication of the coil size.

Spot probes can also have V notches or curved faces for positioning on curved surfaces to get stable signals.

(c) Spring loaded probes

Spot probes are often spring loaded, as shown in FIG. 5.6, to ensure that lift-off is minimized.

They are often used for measurement of conductivity or the thickness of non conducting coatings. Pencil probes are rarely spring loaded, but spring loaded probe holders are available for pencil probes from some manufacturers.

FIG. 5.4. Various types of pencil probe and a knife probe (lowest). The knife probe is equivalent to a pencil probe with a right angle bend (lower right).

FIG. 5.5. (a) The core and coil of a Lang type pencil probe. (b) A spot probe.

(d) Shielded probes and ferrite cores

Shielding is often applied to pencil probes, and sometimes to spot probes. The eddy current field of standard unshielded probes extends some distance laterally from the coil. The lateral extent of the field can be found experimentally by locating a probe on a surface then moving it towards an edge. At some distance from the edge, an indication (an edge signal) will be obtained. Edge signals are located in the first quadrant clockwise from the lift-off signal, usually at a greater angle from the lift-off signal than for signals from surface cracks. If the probe is scanned near an edge or a hole, the lateral eddy current field will give an edge signal.

Although it is usually evident by its angle that it is an edge signal, a crack signal which may be present simultaneously may not be detected. This is particularly a problem when scanning between two holes or a hole and an edge. Small variations in the distance of the probe from the edge produce a large change in the edge signal, producing a noisy signal, and the possibility of a flaw signal not being observed. Whenever possible the use of a non metallic probe guide should be used to reduce the edge signals while not affecting the crack signal.

Scanning near a hole with a fastener, or near a sharp change in configuration, can give similar problems.

Shielded probes, which usually contain a coil wound on a ferrite core and surrounded by a sleeve ferrite, stainless steel, mu metal or copper (see FIG. 5.7), overcome these problems by restricting the lateral extension of the eddy current field. The use of shielded probes allows scanning close to any of these features without interfering signals. Shielded probes can also be used to measure the length of a surface crack. The probe should be scanned along the crack, monitoring the signal and checking that the probe stays above the crack by moving it slightly laterally to keep the signal at a maximum. As the end of the crack is approached, the signal will decrease in amplitude, and eventually return to the balance position. When the probe is returned to the location where the crack signal just appears, the end of the crack corresponds to the location of the shielding. The other end of the crack can be found similarly.

FIG. 5.7. Effects of cores and shields on field extension.

Other types of surface probe (a) Tangential probes

In a tangential probe, the axis of the test coil is parallel to the test surface, with one side of the coil close to the test surface (see FIG. 5.8). Immediately below the coil, where the eddy currents in the test part are strongest, the magnetic field is essentially parallel to the test surface and in the direction of the axis of the coil, and the eddy currents flow parallel to the

surface at right angles to the field, in the direction of the coil windings. The return paths of the eddy currents are further to the side of the coil and below the surface.

This means that the sensitivity to cracks and similar flaws depends on the direction of the flaw relative to the coil, unlike for standard surface probes. For absolute tangential probes, the maximum sensitivity to surface flaws occurs if the flaw is parallel to the coil axis, and, as the angle between the flaw and the coil axis increases, the sensitivity decreases, reaching zero when the flaw is at 90⁰ to the coil axis (that is, parallel to the windings of the coil).

Tangential probes should therefore only be used when flaws in one direction only are sought.

One of their common applications is for the detection of fatigue cracks in the bead seat radius of aircraft wheels, which always occur in a circumferential direction. Tangential probes allow the whole of the bead seat area to be tested in one scan, unlike standard surface probes, which require a number of scans because a small diameter coil needs to be used to reliably detect short cracks. When a tangential probe is used, a short crack in the bead seat radius will always distort the eddy currents, although since the distortion occurs only at one location in the eddy current path, the relative effect on the eddy currents is less than for a standard surface probe, where, if the coil is immediately above the crack, most of the eddy current field is disrupted.

This means that the crack signal will not be as strong as that for a small pencil probe, however, the technique shows adequate sensitivity, and is used by a number of major airlines with a consequent saving in either operator costs, or the cost of an automated wheel scan system.

FIG. 5.8. A tangential probe showing the direction of the eddy currents at the test surface.

(b) Gap or horseshoe probes

Gap or horseshoe probes use a U-shaped ferromagnetic core to shape the magnetic field of the coil so that it is essentially parallel to the test surface, as shown in FIG. 5.9. This produces eddy currents loops in planes normal to the test surface. At the surface, the surface eddy currents flow at right angles to the line joining the two ends of the core. Consequently, the maximum sensitivity to surface flaws occurs if the flaw is parallel to this line, and, as the angle between the flaw and the line joining the ends of the core increases, the sensitivity decreases, reaching zero when the flaw is at 90⁰ to this line. This type of probe is used for the detection of laminations in sheet material rather than for cracks or other flaws normal to the

FIG. 5.9. A gap (horseshoe) probe.

5.1.3 Testing for surface-breaking flaws