Eddy Current Inspection
The eddy current method is based on the principle of electromagnetic induction. This involves generating circular electrical currents in a material under test, achieved by the use of a driving alternating magnetic (primary) field.
The current flow induced within the inspected material will produce a magnetic (secondary) field in opposition to the primary field. The magnetic field strength and direction are varying in response to the changing current.
If the electrical currents find an obstruction (defect damage) they have to change their flow direction preferring higher conductivity regions which influences the secondary induced field.
This change of the resultant fields produces a change of electrical impedance measured across a bridge circuit connected to the primary field. This bridge imbalance can be accurately measured. The amplitude of the induced current reflects the volume of material loss and the depth of defect. By this principle, the detection of materials integrity can be assessed.
The eddy current method is a Relative Comparison Inspection Technique. This means that in practice evaluation of the signals received from the specimen under examination must be compared with known signals reproduced from a test sample commonly known as a calibration standard.
Test signals can be generated from known discontinuities (defects) machined into the calibration standards which should be of the same material as that under examination but has no natural abnormalities.
These artificial discontinuities resemble typical failure mechanisms the likes of thinning, pitting, cracking, through holes and vibration damage; however, unfortunately these are of a regular shape unlike those which occur in service.
Partial Saturation Eddy Current (PSEC / MB ECT)
When small defects (diameter pit <5 mm) are expected to be found in ferrous tubes, the sensitivity of RFET is not sufficient. In this case, magnetic biased ECT should be applied. This method (MB ECT) is also very suitable to inspect finned ferrous tubing.
PSEC is a technique whereby a changeable magnetic field is used to partly cancel out the magnetic properties of the ferrous material. The technique is very suitable for detection and quantification of local defects like pitting. PSEC can detect, and distinguish between, internal and external defects. It is also possible to detect overall wall-loss.
To ensure reliability and sensitivity during an examination the maximum possible probe (fill factor) size shall be used. Therefore, the tubes have to be reasonably cleaned especially so of magnetic debris in order to allow the probe to pass through the tube.
The full saturation probe contains conventional eddy current coils and a magnet. The magnetic field of the magnet saturates the material. Once saturated, the relative permeability of the material drops to around one or as low as practical.
Permeability is a material property that describes the ease with which a magnetic flux is established in a component. It is the ratio of the flux density to the magnetizing force and is represented by the following equation:
U = B/H
For non-ferrous metals such as copper, brass, aluminium, etc., and for austenitic stainless steels, the permeability is the same as that of ‘free space’ (i.e. the relative permeability is one).
For ferrous metals however, the value may be several hundred. This has a very significant influence on the eddy current response. In addition, it is not uncommon for the permeability to vary greatly within a metal part due to localised stresses, heating effects etc.
The application of a full saturation eddy current technique depends on the permeability of the material, tube thickness, and diameter. The strength of the magnets used for saturation is very critical in this technique. Weaker magnets will not saturate the material and will produce a high noise to signal ratio.
Full Saturation Eddy Current Technique
Used for the inspection of Ferritic & Ferro Magnetic Materials.
The principle of full saturation eddy current is the same as conventional eddy current. The technique is applicable to partially ferro-magnetic materials such as MONEL, Alloy 2205, nickel and ferritic stainless steel or thin ferro magnetic materials such as ferritic stainless steel.
The full saturation probe contains a conventional eddy current coil and a magnet. The magnet saturates the magnetic field in the material. Once saturated, the permeability of the material drops to one and the principles of conventional eddy current are applicable.
The main challenge with the full saturation technique is to ensure that the material has been fully saturated. This can be confirmed by running a test on a calibration tube. Once the tube is fully saturated, it should produce a normal phase spread on the OD defects in the calibration tube.
It is therefore very important to machine the tube from the same material as that under examination. This is especially important in MONEL whose permeability is not fixed and can vary from batch to batch. In addition, the strength of the magnets in a full saturation probe can vary from vendor to vendor. Weaker magnets will not saturate the material and will produce a noisy signal.
It is imperative to check the quality of the probe before the inspection. The application of full saturation ECT depends on the permeability of the material, thickness and tube diameter. Larger diameter tubes will allow placement of larger magnets, whereby, slightly thicker tubes could be saturated.
Sizing of OD defects is done similar to conventional eddy current. Phase cannot be used for sizing ID defects because the depth of the defect does not influence the phase of the signal. Therefore, the sizing of ID pits is accessed purely on the basis of signal amplitude.