Basic of eddy current testing part 1by rohit sahu

NON –DESTRUCTIVE TESTING(RME-080)
BASICS OF EDDY CURRENT TESTING
Prepared By : Rohit Sahu
Assistant Professor (ME)
DEPARTMENT OF MECHANICAL ENGINEERING
Disclaimer: The materials provided in this presentation and any comments or information
provided by the presenter are for educational purposes only. Nothing conveyed or provided
should be considered legal, accounting or tax advice.

Introduction
Eddy current inspections has its own origins with the Michael Faraday's discovery of
electromagnetic induction in the year of 1831.
Its development as a non-destructive testing techniques for industrial applications was carried out
during the World War II, in Germany.
Eddy current inspection is one of several methods that use the principal of “electromagnetism” as
the basis for conducting examinations. Several other methods such as Remote Field Testing (RFT),
Flux Leakage etc also use this principle.
Eddy currents are created through a process called electromagnetic induction.
When alternating current is applied to the conductor, such as copper wire, a magnetic field
develops in and around the conductor.
This magnetic field expands as the alternating current rises to maximum and collapses as
the current is reduced to zero.
*Source- NPTEL Lecture 09 Inspection and Quality Control in Manufacturing Prof. Kaushik Pal Department of Mechanical and
Industrial Engineering Indian Institute of Technology – Roorkee and images from web source

Introduction
If another electrical conductor is brought into the proximity of this changing magnetic field, the
reverse effect will occur. Magnetic field cutting through the second conductor will cause an
“induced” current to flow in this second conductor. Eddy currents are a form of induced currents!
(image-1)
Eddy current testing sometimes we are calling it as ECT is widely used in aerospace industry and
in other manufacturing and the service environments that require inspection of thin metal for
potential safety related or quality related problems
What is Eddy current? Eddy currents are electrical currents that induced within a conductor by a
varying magnetic field.
They get either name from eddies that are formed when a liquid or gas flows in a circular path
around obstacles when conditions are right.(image-2)
*Source- NPTEL Lecture 09 Inspection and Quality Control in Manufacturing Prof. Kaushik Pal Department of Mechanical and Industrial Engineering Indian Institute of Technology –
Roorkee and images from web source

Properties of the Eddy Currents: They flow in closed loops within conductor.
They flow in a plane that is parallel to coil winding or maybe the material surface.
They attenuate and lag in phase with depth.
Basic Principles of the Eddy Current Testing:
Eddy current testing works on the principle of electromagnetic induction to detect flaws in conductive
materials.
Electromagnetic induction: When the magnetic flux through a conductor changes, induced currents are
set up in closed paths on the surface of the conductor.
These currents are in direction of perpendicular to the magnetic flux and are called eddy current
Industrial Engineering Indian Institute of Technology – Roorkee and images from web source

How it works?
Step1. Generation of Eddy Currents.
In order to generate the eddy currents for an inspection, a probe is used.
Inside the probe is a length of electrical conductor which is formed into a coil.
Alternating current is passed through the coil which generates an oscillating
magnetic field.
A dynamic expanding and collapsing magnetic field forms in and around the
coil as the alternating current flows through the coil.
The probe and its magnetic field are brought close to a Metal
test piece.
A circular flow of electrons form known as Eddy current will
begin to move through the metal.
Step2. Secondary Magnetic Field is set up in opposite directions.
Eddy currents flowing in the material will generate their own secondary magnetic
field which will oppose the coil's primary magnetic field.
*Source- NPTEL Lecture 09 Inspection and Quality Control in Manufacturing Prof. Kaushik Pal Department of Mechanical and Industrial Engineering Indian Institute of Technology – Roorkee.

How it works?
Step3. Impedance of the coil is changed.
Changes in metal thickness or defects like near surface cracking will
interrupt or alter the amplitude and pattern of the eddy current and thus
resulting the magnetic field.
This in turn affects movement of electrons in coil by varying
the electrical impedance of the coil.
Step4. Change in impedance is analyzed.
Eddy current instrument plots changes in the impedance amplitude and phase angle, which can be used
by a trained operator to identify changes in the test piece.
Factors that affecting the eddy current response?
Electromagnetic factors are: electrical conductivity of the sample (σ), magnetic permeability of the sample
(µ), and excitation frequency (f).
Geometric factors: Coil parameters: loop area, number of turns, diameter of wire.
lift-off: proximity of excitations or pickup coil to the sample(implies coupling efficiency)
sample geometry: thickness compared to skin depth, shape and lateral dimensions;
probe proximity to sample edges.
*Source- NPTEL Lecture 09 Inspection and Quality Control in Manufacturing Prof. Kaushik Pal Department of Mechanical and Industrial Engineering Indian Institute of
Technology – Roorkee.

Eddy Current Density and Depth of Penetration:
 Eddy currents concentrate near the surface adjacent to an excitation coil
and their strength decreases with distance from the coil.
 Eddy current density decrease exponentially with the depth .This
phenomenon is known as the Skin Effect.
What causes the Skin Effect?
 Skin effect arises when the eddy currents flowing in the test object at any
depth produce magnetic fields which oppose the primary field thus
reducing the net magnetic flux and causing a decrease in current flow as
the depth increases.
 Factors affecting the depth of penetration:
 Frequency of the excitation current,
 Electrical conductivity of the specimen,
 Magnetic permeability of the specimen.
*Source- NPTEL Lecture 09 Inspection and Quality Control in Manufacturing Prof. Kaushik Pal Department of Mechanical and Industrial Engineering Indian Institute of
Technology – Roorkee.

Standard Depth of Penetration
The depth that the eddy currents are only 1/e or 37% as strong as
they are on the surface is known as the standard depth of penetration
or skin depth.
It can be calculated by using following equation:
Where
 δ =Standard depth of penetration (m)
 f =Test frequency (Hz)
 μ =Magnetic permeability (H/m)
 σ =Electrical Conductivity (S/m)
Working with this equation:
If relative permeability ( 𝜇r) is given, it must be converted to an absolute permeability (𝜇) as: = 𝜇r 𝜇0
where, 𝜇0= 4𝜋 × 10-7 H/m (permeability of free space).
As per the International Annealed Copper Standard (IACS), electrical conductivity values are often expressed as a
percent of the conductivity of pure annealed copper measured at 25 °C which can be converted to siemens per meter
(S/m) as: 𝜎𝑚𝑎𝑡𝑒𝑟𝑖𝑎𝑙 = % 𝐼𝐴𝐶𝑆 × 𝜎𝑐𝑜𝑝𝑝𝑒𝑟
where, 𝜎𝑐𝑜𝑝𝑝𝑒𝑟 = 5.8 × 107 𝑆/𝑚
Industrial Engineering Indian Institute of Technology – Roorkee. And image from web sources.

Basic of eddy current testing part 1by rohit sahu

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Basic of eddy current testing part 1by rohit sahu

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