Eddy Current Testing
State of the art
How powerful is Eddy Current (ET) Testing?
Eddy current testing is used to detect defects on the surface and near-surface defects in the material.
From 0.05 mm to 0.15 mm crack depth, very dependent on usable frequency and materia
The material to be tested must be electrically conductive
Non destructive testing with Eddy Current Testing (ET)
VOGT Ultrasonics your service provider for eddy current testing
VOGT Ultrasonics is your reliable partner
how it works
Eddy Current test sequence
Planning the inspection and defining the inspection requirements
Depending on the targeted penetration depth and resolution, it is necessary to select how the eddy current inspection shall be performed.
Step no. 1
Preparation of the workpiece
For eddy current testing, a simple cleaning of the workpiece is sufficient.
Step no. 2
An inspector uses a probe that generates a magnetic field and scans the workpiece with this magnetic field. Induction creates a magnetic field in conductive parts of the workpiece. The interactions between the magnetic field of the probe and the workpiece are recorded and evaluated. Any anomalies can be examined in more detail.
Step no. 3
Evaluation and reporting
Of course, the data is finally evaluated and comprehensively documented, both to prove a successful inspection and to record possible defects for reworking.
Step no. 4
How does Eddy Current Testing work?
Eddy current testing is based on the physical phenomenon of electromagnetic induction. In an eddy current probe, an alternating current flows through a wire coil and generates an oscillating magnetic field. When the probe and its magnetic field are placed near a conductive material, such as a metal test specimen, a circular stream of electrons called an eddy current begins to move through the metal – like swirling water in a stream. This eddy current passing through the metal creates its own magnetic field, which interacts with the coil and its field through mutual inductance. Changes in metal thickness or defects such as cracks near the surface interrupt or change the amplitude and pattern of the eddy current and the resulting magnetic field. This affects in turn the movement of electrons in the coil by changing the electrical impedance of the coil. The eddy current inspection device records changes in impedance amplitude and phase angle that can be used by a trained inspector to detect changes in the test piece.
The eddy current density is highest near the surface of the component, so this is the area with the highest inspection resolution. The standard penetration depth is defined as the depth at which the eddy current density is 37% of its surface value, which in turn can be calculated from the test frequency and the magnetic permeability and conductivity of the test material. Thus, variations in the conductivity of the test material, its magnetic permeability, the frequency of the alternating current pulses driving the coil, and the coil geometry affect the test sensitivity, resolution and penetration depth.
There are many factors that influence the potential of eddy current testing. Eddy currents moving into materials with higher conductivity values are more sensitive to surface defects, but penetrate less deeply into the material. Additionally the depth of penetration also depends on the test frequency. Higher test frequencies increase resolution near the surface but limit penetration depth, while lower test frequencies increase penetration depth. Larger coils test a larger volume of material from each position as the magnetic field penetrates deeper into the workpiece, while smaller coils are more sensitive to small defects. Variations in the permeability of a material create noise that can limit defect resolution due to larger background variations.
While conductivity and permeability are properties of the test material that are beyond the control of the inspector, the test frequency, coil type, and coil size can be selected according to the test requirements. For a given inspection, the resolution is determined by the probe type, while the detection capability is controlled by the material and device properties. For some inspections, multiple frequencies are run to optimize results. Several smaller probes, or probes, can also be used to achieve the best resolution and penetration depth required to detect all possible defects. It is always important to choose the right probe for each application to optimize inspection performance.
Pro & Contra
Advantages of Eddy Current Testing (ET)
- Detection of surface cracks and near-surface cracks from a size of 0.05 mm
- Detection of defects due to insulating surface coatings
- Non-contact method, therefore suitable for testing high-temperature surfaces and underwater surface
- Effective for test objects with physically complex geometries
- Provides immediate feedback
- Portable and lightweight equipment
- Short preparation time – surfaces require only minor cleaning
- No coupling agent required
- Can measure the electrical conductivity of test objects
- Can only be used on conductive materials
- Penetration depth depends on many factors
- Very susceptible to changes in magnetic permeability, making it difficult to test welds of ferromagnetic materials
- Defects that are parallel to the surface of the component cannot be detected.
- Careful signal interpretation is required
ET with VOGT Ultrasonics
Eddy current testing uses electronic probes that pass through various types of tubes or along the surfaces of materials to locate possible defects.
An eddy current is a current that runs in the opposite direction to the current induced by a probe into a conductive material.
Ein Wirbelstrom ist ein Strom, der entgegengesetzt zu dem von einer Sonde in ein leitendes Material eingeleiteten Strom verläuft.
The cost of an eddy current inspection varies significantly depending on the geometry of a workpiece and the desired penetration depth and resolution. Please contact us for more detailed information.
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