What are eddy currents?
Eddy currents are loops of current induced in a conductor due to a changing magnetic field. These currents are created due to a phenomenon called electromagnetic induction. The eddy currents flow in closed loops and look similar to the swirling eddies formed in a stream.
Eddy currents are responsible for power losses in transformers and can cause the temperature of the conductor to increase due to Joules heating. These currents produce their own magnetic fields and these fields oppose the primary magnetic field that created the eddy currents. The magnetic fields created by eddy currents are disrupted due to any change or defect in the conducting material. Due to this property, eddy currents are used in non-destructive testing methods.
What are non-destructive testing methods (NDT)?
Non-destructive testing methods are a variety of non-invasive inspection techniques used to evaluate the component’s material properties or to examine active processing units. NDTs can detect or measure the presence of cracks or corrosion that may not be visible to the eye.
The purpose of the NDT methods is to inspect whether the product is safe in a reliable manner without causing damage. The six most commonly used non-destructive testing methods are:
- Magnetic Particle Testing
- Liquid Penetrant Testing
- Ultrasonic Testing
- Visual Testing
- Radiographic Testing
- Electromagnetic Testing
The methods above are the general test categories that club the testing method with the same core principle together. For example, electromagnetic testing includes remote field testing, alternating current field measurement and eddy current testing. All these tests use the induction of electric current or magnetic field in a conducting part for testing.
What is Eddy Current Testing?
Eddy Current Testing (ECT) is an electromagnetic testing method that uses electromagnetic induction to detect damage or imperfections in a product. This test is commonly used to detect surface or sub-surface flaws in conducting materials. The test is an NDT method and is used in various industries such as aerospace, automotive and electronics.
ECT works on the basic principle of electromagnetic induction in which a conducting coil or probe is excited with alternating current. The sinusoidal current produces a primary magnetic field in the vicinity of the coil. When a conducting material is brought near the excited probe eddy currents are induced in the material. The currents are affected by defects such as cracks, voids, corrosion and inclusions.
How does ECT work?
To detect the defects in the test part an inspection probe is used. The probe is excited with alternating current so as to produce a magnetic field that then induces eddy currents in conducting materials in its vicinity. The nature of material affects the eddy currents and this change can be detected by the use of a second probe or high-precision bridge circuits.
Another method of finding defects is by measuring the coil impedance of the probe. The coil impedance changes with the change in the magnetic field of the eddy current. The magnetic field changes when eddy currents are affected. Defects alter the electrical conductivity and the magnetic permeability of the material, disrupting the eddy currents in turn.
Applications of ECT
Eddy current testing can be used to detect cracks, measure material thickness and coating thickness, detect heat damage, measure conductivity to identify materials, determine case depth and monitor heat treatment.
Eddy current testing is considered an advanced testing method and has various advantages. Apart from being non-invasive, these tests are capable of detecting minute cracks and corrosion that might not be visible on the surface. The tests can also identify, characterize and measure defects. Some computer-based testing machines can even display the results on the computer and store the results in memory. These advantages have led to many industries adopting the eddy current testing method to test sensitive parts where even a small defect can prove catastrophic. At Mikro Innotech, we create innovative and customized ECTs that are crafted to perfectly fit all your industrial requirements.