How It Works
The LIZARD® system uses a unique method of electromagnetic testing which can be used on all electrically conducting material. The Patented Field Gradient technology can provide testing modes from Field Gradient Imaging through Alternating Current Field Measurement to Classic Eddy Current including Conductivity measurement in IACS (International Annealed Copper Standard). The following description is intended to provide a basic understanding of the technique.
Using the LIZARD® and Newt patented implementation:
- An alternating current is made to flow at the surface of a conductive test piece.
(The current is induced by the profiled magnetic field of a coil carrying an excitation current).
- The interaction of the electric current with any defect in the test piece is monitored.
(This monitoring measures the flow of current along the flanks of the defect).
- It is necessary that a graduated electric field is created locally to the point of inspection (this is achieved by using a graduated excitation field).
- Sensors for the defect depth must be placed upon graduated regions of the interrogating field.
- Sensors (in the form of planar coils) have their axes at right angles to the inspected surface.
- Electromagnetic array (EMA) signals are resolved into defect and lift-
off components to provide quality control indications and on- line lift- off compensation.
- Alternating Current Field Measurement can be performed with a planar array consisting of excitation and sensing elements having no measurement dead spots, no requirement for on site input of defect length and the ability to provide defect depth at any point along the profile of the defect either in real time or post data analysis.
- Modes of analysis from each sensing element in the array can be selected by interrogating field position with respect to the sensor giving software selectable analysis of defect interaction with the interrogating field (such as phase plane, imaging and so on) enabling the raw data to be processed to provide composite analysis and resolution of difficult defect signatures.
- This is the implementation covered by ASTM E2261-
07 for Alternating Current Field Measurement for the inspection of welds. This standard can be purchased from the following web site - www.astm.org/Standards/E2261.htm
For clarification, Alternating Current Field Measurement is the generic title given
to a group of electrical inspection techniques in which:-
- An alternating current is made to flow at the surface of a conductive test piece.
(The current may be injected by either ohmic contacts or induced by the magnetic field from an adjacent coil or conductor).
- The interaction of the electric current with any defect in the test piece is monitored.
(This monitoring may be by either ohmic contacts to measure voltage profiles or magnetic field sensors at various orientations to measure current profiles).
Using one such implementation by others:
- It is necessary that the current field is uniform (in the case of an induced current, this requires the magnetic field to be uniform too).
- Sensors must be placed at such points that the local interrogating field is uniform.
- Sensors (in the form of coils) have their axes parallel to or at right angles to the inspected surface.
- Separate measurement/ estimation of lift-
off is necessary by the operator as an input to the reverse modelling process in order to compensate for this.
- Defect length must be measured at the time of detection such that this can be input to the reverse modelling process in order to provide defect depth.
It is for these reasons that the LIZARD technology is fast becoming the system of choice for Alternating Current Field Measurement and Electromagnetic Inspection with the advantages detailed below:
The intensity of an alternating current reduces with increase of distance from the surface. Taking the current intensity as 100% at the surface, the current intensity at different depths will be:
Depth Intensity
1 unit 36.8%
2 units 13.5%
3 units 5%
- The value of the unit depth (current intensity 36.8% of surface current intensity) varies with the electrical properties of the material and the frequency of the alternating current.
- Penetration depth is very much less in ferritic (magnetic) material than in other
(non-
magnetic materials).
- The alternating current sheet is distorted by surface-
breaking defects such that the current flow will follow the original surface and the focus of the defect.
- The effects of the distortion of the current sheet may be measured to give a measure of the size of the defect. It is important to note that as the current follows the flanks of the defect it can penetrate more deeply at this point than just taking into account the standard depth of penetration. This means that surface breaking cracks extending much further into the material than the penetration depth can still be detected and measured.
LIZARD® Technology and Advanced Methods of Electro-
Electrical inspection methods rely upon the test piece being an electrical conductor. If the test piece is an insulator, then electrical inspection technique is not possible.
Electrical inspection also relies upon a defect’s interruption and disturbance of an interrogating current (in the case of the LIZARD® this interrogating current is induced by a field gradient) and it is this disturbance which is detected by the system with the LISS software providing the analysis of this data and mapping this to an operator display through various processing algorithms (whilst retaining the raw data for storage or reprocessing).
In the case of contacting methods these rely upon ohmic contacts with the test piece (e.g. ACPD: Alternating Current Potential Drop and Contacting Alternating Current Field Measurement).
Non-
Direct currents would flow throughout the body of the conductive test piece, whereas alternating currents would flow at or near the surface of the conductive test piece and the factors affecting the flow of these are common to all methods.
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Field Gradient Imaging -
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