Non-destructive Testing & Evaluation of Materials - Tools for Flaw and Damage
assessment of components
MTL7160: Fracture and Fatigue
2nd Lecture: 30-03-2022
Amitava Mitra amitra@iitj.ac.in
Visiting Professor, Department of Physics &
Professor In-charge, Office of Infrastructure Engineering
Indian Institute of Technology Jodhpur, Karwar,
Jodhpur-342037
The Eddy current Technique
Big-5 Non-destructive techniques along with
• Eddy Current (EC)
• Ultrasonics (UT)
• Radiography (RD)
• Magnetic Particle (MPI)
• Dye penetrant (DP)
• But one that causes much apprehension
• People think it to be “too abstract”
• Difficult to visualize
• Fear of the maths
Faraday’s Law and Lenz’s Minus Sign
The electromotive force (EMF) induced in a coil of wire is proportional to the time rate of change
of magnetic field coupling into that coil.
Lenz’s Crucial Extension:
A change in magnetic flux through a circuit induces an electric field in the circuit, which causes a current
to flow in the circuit, which in turn gives rise to a magnetic flux that opposes the original flux.
Eddy Current Technique
When a metallic material is placed close to an alternating magnetic field, current is induced within the
materials which opposes the applied magnetic field. The current induced within the materials due to the
presence of external magnetic field is known as eddy current
Eddy Currents
GENERATION OF EDDY CURRENT
Primary and secondary magnetic field opposing in nature
=> a.c. coil impedance changes
=> any defect in the specimen reduces this change
Eddy Current Technique
When a metallic material is placed close to an alternating magnetic field, current is induced within the
materials which opposes the applied magnetic field. The current induced within the materials due to the
presence of external magnetic field is known as eddy current
Eddy Currents
GENERATION OF EDDY CURRENT
Primary and secondary magnetic field opposing in nature
=> a.c. coil impedance changes
=> any defect in the specimen reduces this change
R
XL ~
Resistance Reactance
Phaser Impedance Z2 = R2 + XL2
Reactance (L)
Diagram
Phase angle = tan-1 (L/R) = R2 + 2 L2
Inductance
= 2 f (frequency)
Resistance (R)
R
Insert a ferrite
core (magnetic)
and only few
number of turns in XL ~
the coil
Normalized Phaser Diagram Resistance Reactance
Normalized Reactance
Ti Impedance Z2 = R2 + XL2
Fe = R2 + 2 L2
Al Inductance
= 2 f (frequency)
(L/L0)
Cu
Initial coil is chosen in such a way that Ro ~ 0, Zo = Lo
Normalized Resistance Z= R +j ω L
(R/L0)
Z/Z0= R/ ω L0+ j ω L/L0
• Absolute Coil: A single-coil used to measure bulk article characteristics like
conductivity, dimension, permeability etc
• Differential Coil: It consists of two coils that electrically oppose each other due to which bulk
characteristics get to cancel out and small defects are observed as a difference in coils
• Surface Probe: It is design to examin the test surface. Contacting , non-contacting
operator held and automated are different types of surface probes used
• ID Probe: These probe consists of coil that is designed to be inserted within a specific cavity
configuration such as interior of tubing drilled holes etc
Impedance Analyser
Factors Affecting Eddy Currents
1. Strength of exciting electromagnetic field
2. Frequency of excitation
3. Electrical conductivity of the material
4. Magnetic permeability of the material
5. Coupling between the exciter and the part under test
Depth of Penetration The Standard Depth of penetration δ is defined as the depth at
which eddy current density is reduced to 1/e times (approximately
37%) the density at the surface.
= depth of penetration
= 1/ (fµ )1/2
f = Frequency of applied signal, = magnetic permeability of
the test object and = conductivity of the test object
Higher frequency of the applied current into the coil (f )
More rate of change of magnetic flux produced into the coil
Larger amplitude of eddy currents High opposing flux
weaker flux at lower levels lower δ.
Higher conductivity ( ) larger amplitude of eddy
currents high opposing flux weaker flux at lower levels
lower δ.
Lower conductivity() lower amplitude of eddy currents
lower opposing flux more flux deeper in the metal
higher δ.
δ at Different Frequencies
At 10 KHz At 240 KHz
Frequency needs to be selected with due consideration to δ
Surface properties need a higher frequency
Sub-surface defects require a lower frequency
For conductivity measurements thickness must be more than 3δ.
Choice of Test Frequency:
Effect of Lift off
Application:
1.6mm hole
5% ID Groove 10% OD Groove 0.25mm dent
2.5mm wide 2.5mm wide
Carbon steel
Y-Channel support
Distance
Nonconductive Coating Measurement
Nonconductive coatings on electrically conductive substrates can be measured very accurately with
eddy current inspection. (Accuracy of less that one mil is not uncommon.) comic
The coating displaces the eddy current probe from the conductive base
material and this weaken the strength of the eddy currents.
This reduction in strength can be measured and related to coating thickness.
Nonconductive
Coating
Conductive
Base Metal
Eddy Currents
Coating Thickness Measurement
Coating is a measure of lift-off
Higher frequencies are used:
Crowded conductivity
Better angular separation
Higher sensitivity to liftoff
Can measure on thinner sheets
Can be calibrated against shims
Material Thickness Measurement
• Thickness measurements are possible with eddy current inspection within certain limitations.
• Only a certain amount of eddy currents can form in a given volume of material
• Therefore, thicker materials will support more eddy currents than thinner materials
• The strength (amount) of eddy currents can be measured and related to the material thickness.
Magnetic Field
From Probe
Test
Material
Eddy Currents
Material Thickness Measurement (cont.)
Eddy current inspection is often used in the aviation
industries to detect material loss due to corrosion and
erosion.
Material Thickness Measurement (cont.)
Eddy current inspection is used
extensively to inspect tubing at
power generation and
petrochemical facilities for
corrosion and erosion.
