Detection of subsurface flaws Introduction

The depth of penetration of the magnetic field produced by eddy current test coils, and therefore of the eddy currents produced by this field is highly dependent of the coil diameter, the larger the coil diameter the greater the depth of penetration. This is the predominant effect in limiting eddy current penetration, and explains why the penetration is often far less than that calculated using the effective depth of penetration formula. Typically, the magnetic field in the axial direction is relatively strong only for a distance of approximately one tenth of the coil diameter, and drops rapidly to only approximately one tenth of the field strength near the coil at a distance of one coil diameter. To penetrate deeply, therefore, large coil diameters are required. However as the coil diameter increases, the sensitivity to small flaws, whether surface or subsurface, decreases. For this reason, eddy current flaw detection is generally limited to depths most commonly of up to approximately 5 mm only, occasionally up to 10 mm.

For materials or components with greater cross-sections, eddy current testing is usually used only for the detection of surface flaws and assessing material properties, and radiography or ultrasonic testing is used to detect flaws which lie below the surface, although eddy current testing can be used to detect flaws near the surface. However, a very common application of eddy current testing is for the detection of flaws in thin material and, for multilayer structures, of flaws in a subsurface layer.

Probe and frequency selection

The essential requirements for the detection of subsurface flaws are, sufficient penetration for sensitivity to the subsurface flaws sought, and sufficient phase separation of the signals for the location or depth of the flaws to be identified. As standard depth of penetration increases, the phase difference between discontinuities of different depth decreases. Therefore, making interpretation of location or depth of the flaws difficult.

Example: If the frequency is set to obtain a standard depth of penetration of 2 mm, the separation between discontinuities at 1 mm and 2 mm would be 57°.

If the frequency is set to obtain a standard depth of penetration of 4 mm, the separation between discontinuities at 1 mm and 2 mm would be 28.5°.

An acceptable compromise which gives both adequate sensitivity to subsurface flaws and adequate phase separation between near side and far side flaw signals is to use a frequency for which the thickness (t) = 0.8δ. At this frequency, the signal from a shallow far side flaw is close to 90⁰ clockwise from the signal from a shallow near side flaw, so this frequency is termed f90. By substituting t = 0.8δ into the standard depth of penetration formula, and changing Hz to kHz, the following formula is obtained:

f90 = 280/ (t²σ) (5.1) where

f₉₀ = the operating frequency (kHz),

t = the thickness or depth of material to be tested (mm), and σ = the conductivity of the test material (% IACS).

FIG. 5.15. Eddy current signals from a thin plate with a shallow near side flaw, a shallow far side flaw, and a through hole, at three different frequencies. At 25 kHz (a), the sensitivity to far side flaws is high, but the phase difference between near side and far side signals is relatively small. At 200 kHz (b), the phase separation between near side and jar side signals is large. but the sensitivity to far side flaws is poor. For this test part, a test frequency of100 kHz (b) shows both good sensitivity to far side flaws and good phase separation between near side and far side signals.

To obtain adequate depth of penetration, not only must the frequency be lower than for the detection of surface flaws, but also the coil diameter must be larger. On flat surfaces, a spot probe, either absolute or reflection, should be used in order to obtain stable signals (see FIG.

5.16). On curved surfaces, a spot probe with a concave face or a pencil probe should be used.

Spring loaded spot probes can be used to minimize lift-off, and shielded spot probes are available for scanning close to edges, fasteners, and sharp changes in configuration.

FIG. 5.16. Spot probes testing a layered structure for corrosion (a) and cracks (b) in the second layer of a two-layer laminate.

Two special-purpose probes should be noted:

Ring probes

Ring probes are doughnut-shaped probes used for the detection of both surface, and subsurface cracks or other flaws at fastener holes when the testing is carried out with the fasteners installed (see FIG. 5.17). These probes are capable of detecting cracks which do not extend far from the hole, for example, those which do not extend beyond the head of the fastener, and therefore may not be detected using a spot probe adjacent to the fastener. Ring probes are commonly absolute reflection probes and may be shielded.

Sliding probes

Sliding probes are also designed to detect both surface, and subsurface cracks or corrosion at fastener holes, but are designed to slide along a row of fasteners (see FIG. 5.18). They are reflection probes, usually with at least two pickup coils. One type has a central driver coil and four small pickup coils equally spaced around it and connected differentially, all housed in a block. Some probes have a recessed slot along the length of the probe for sliding over protruding fastener heads.

Sliding probes give a signal as they pass over a fastener, but if a crack or corrosion is present, a characteristically different and more complex signal is produced. For multilayered structures, characteristically different signals are given for cracks extending along the row of fasteners and for cracks extending normal to the row. In addition, the signals are different for cracks in each layer. Interpretation is therefore done by recognition of the characteristic pattern of the signal from each condition, rather than by analysis of the signal. An appropriate reference sample is essential so the operator can become familiar with the signal patterns. An experienced operator can test a row of fasteners much more rapidly than when using a ring or other type of probe.

Multi-frequency testing can be used to eliminate signals from fasteners, so that the only signals displayed will be flaw signals. The use of multi-frequency inspection for the detection of subsurface cracks and corrosion is becoming more common in use. Two applications of multi-frequency is to, separate the variables due to skin separation and corrosion at similar depths into the material, and to inspect beneath installed fasteners for cracks where the variables caused by the fastener can be eliminated.

FIG. 5.18. A sliding probe intended for detecting surface and subsurface flaws by sliding it along rows of fasteners in the direction of the arrows

Reference samples

Reference samples should be as far as possible identical to the parts to be tested. They should be of the same material (or material of the same conductivity) and thickness as the parts to be tested, and, if a multilayered part is to be tested, each layer should have the same material (or conductivity), thickness, and separation as the test parts. In addition, they should have real or artificial flaws simulating the type sought and at the locations and orientations at which flaws are required to be detected.

Corrosion may be simulated by drilling shallow poles, using either a standard or flat-bottom drill bit. Alternatively, milled slots may be used. Cracks may be simulated by spark-eroded slots, or by narrow saw cuts.

Test procedure

The test procedures are generally similar to those for detecting surface flaws.

5.1.4 Conductivity testing and material sorting