Non Destructive Testing
Non Destructive Testing
Non Destructive Testing
(MML 469)
Index
1. Introduction
2. Visual Inspection
3. Eddy Current Testing
4. Magnetic Particle Inspection
5. Ultrasonic Testing
6. Radiography
7. Acoustic Emission Testing
8. Liquid Penetrant Test
Introduction
Factor of Safety
𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀 𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆
• Factor of Safety =
𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷 𝐿𝐿𝐿𝐿𝐿𝐿𝐿𝐿
• Factor of Safety for Bridges = 2 to 4
• Factor of Safety for Motor Yatches = 6 to 10
• Factor of Safety for Space Shuttles = 1.25 to 1.4
Non Destructive Testing (NDT)
• Those testing methods in which material under test is not destroyed.
• Purpose = Suitability of a particular part/system to satisfactorily
perform its intended function.
• NDT = Non Destructive Evaluation (NDE)/Non Destructive Inspection
(NDI)
Need for NDT
No material is perfect.
• Lower defects
• Locate and assess severity
To develop confidence in newer advanced materials.
Better Understanding of Materials.
• Lower Factor of Ignorance
• Lower Factor of Safety
• Lower c/s area
• Lighter components/material saving
Need for NDT
Quality and Reliability
• Incoming material/components
• During manufacturing
• Service life
Categorically state the rejection criteria – harmless / harmful /
beneficial defects.
Need for NDT
Development of instruments/software
• Quickly detect the defects
• Identify nature/shape/size critically
• Lower factor of ignorance
Continuous monitoring of high risk structures (e.g. storage tanks)
Online monitoring (e.g. welding defects)
Thickness Measurement
Need for NDT
Evaluation of material behavior
• Stages leading to failure
• Fiber reinforced composites
Predicting Fatigue Behavior
For evaluation of
• Grain size/mechanical properties
• Quantify composition of alloys
• Identification/differentiate materials
• Measurement of fiber volume fraction
Need for NDT
Characterization of newer materials
Increase safety and cost savings
Surface characterization/finish
Engineering “Postmortem”
Locate area of stress concentrations
Designs and NDT
OBJECTIVES OF DESIGN ENGINEER IS TO:
• Imagine and design the shape.
• Visualize the functional environment.
• Estimate the required physical and mechanical properties.
• Select suitable material and a commensurate manufacturing process
in a cost effective manner.
• Appreciate the variability of manufacturing processes that lead to
statistical variation in physical and mechanical properties and
dimensions.
Designs and NDT
• Stipulate the tolerance margins with respect to mechanical properties
and dimensions.
• Too close the tolerance margins leads to rejection and increases the
cost of production.
• Realize the fact that structural components undergo degradation or
damage during service.
Fail Safe Approach
• Component is replaced after its design life is reached, no defects
during service life due to applied safety factors.
Damage Tolerant Approach
• It is stipulated that in the presence of defects up to a certain limit,
failure of components is a very probability 10−6 𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝 . After
this time interval, the possibility of continued usage, repair or
rejection of the structure is decided by periodic inspections.
Different types of NDT techniques
A. VISUAL:
• Visual test
• Optical test
B. BROAD/CONVENTIONAL:
• Liquid Penetrant Inspection (LPI)
• Magnetic Particles Flow Detection (MPI/MPT)
• Radiography (RT)
• Ultrasonic (UT)
• Eddy Current (ET)
• Acoustic Emissions and Acousto – Ultrasonic Testing (AT/AET)
• Vibration Tech, Leak Detection, Holographic Interferometry
• Dielectric Method
Comparison of NDT and DT
Flaws
• Based on Stage
• Based on Position
• Based on Defects
• Casting Defects:
1. Blow Holes
2. Porosity
3. Gas Porosity
4. Misrun
5. Mismatch
6. Short Pour
7. Shrinkage/Flash/Fins
8. Inclusions
Various steps carried out to perform NDT
1. Preparation of Test Surface.
2. Application of Testing Medium/Signal.
3. Modification of Testing Medium/Signal.
4. Conversion of Signal into a Conventional Form.
5. Interpretation of Signal.
6. Verification of Test Results.
Visual Inspection
Visual Inspection
• Used for detecting and examining a variety of surface flaws, such as
corrosion, contamination, surface finish, and surface discontinuities
on joints (for example, welds, seals, solder connections and adhesive
bonds).
• Used for detecting and examining surface cracks, which are
particularly important because of their relationship to structural
failure mechanisms.
• Visual inspection often provides a useful supplement.
• The methods of visual inspection involve a wide variety of equipment.
• Examination with naked eye.
Visual Inspection
• Examination with interference microscopes for measuring the depth of
scratches in the finish of finely polished or lapped surfaces.
• Some of the equipment used to aid visual inspection includes:
1. Flexible or Rigid Borescopes for illuminating and observing internal,
closed or otherwise inaccessible areas.
2. Image sensors for remote sensing or for the development of permanent
visual records in the form of photographs, videotapes, or computer –
enhanced images.
3. Magnifying systems for evaluating surface finish, surface shapes (profile
and contour imaging), and surface microstructures.
4. Dye and fluorescent penetrants and magnetic particles for enhancing the
observation of surface cracks (and sometimes near – surface conditions
in the case of magnetic particle inspection).
Types of Designs of Borescopes
1. A rigid borescope with a lamp at the distal end.
2. A flexible fiberscope with light source.
3. A rigid borescope with a light guide bundle in the shaft.
Rigid Borescopes
• Rigid borescopes are generally limited to applications with a straight –
line path between the observer and the area to be observed.
• Lengths from 0.15 m to 30 m.
• Diameters from 0.9 mm to 70 mm.
• Magnification is usually 3 to 4× but powers up to 50× are available.
Flexible Borescopes
• Flexible borescopes are used primarily in the applications that do not
have a straight passageway to the point of observation.
• The two types of flexible borescopes are:
1. Flexible Borescopes
2. Videoscopes with a CCD image sensor at the digital tip
• Diameters from 1.7 mm to 13 mm.
• Lengths up to 12 m.
• Special quartz fiberscopes are available in lengths up to 90 m.
Magnetic Particle Inspection
Magnetic Particle Inspection Test
• MPT = Magnetic Particle Test
• MPI = Magnetic Particle Inspection
• The word magnet is derived from a place called Magnesia because
magnetic rocks are common there.
MPI
• A method of locating surface and subsurface discontinuities in
ferromagnetic materials.
• Ferromagnetic materials = Fe, Ni, Co and their alloys.
• Surface and subsurface discontinuities can be inspected using MPI.
• Cracks transverse to the applied magnetic field will have more
distortion i.e. more leakage field formed at and above the surface of
the part.
MPI
• This magnetically held collection of particles forms an outline of the
discontinuities and indicates its location, size, shape and extent.
• Magnetic particles are applied over a surface as dry particles, or as
wet particles in a liquid carrier such as water or oil.
• Depth of crack cannot be determined.
• The presence of leakage field, and hence the presence of the
discontinuity, is detected by the use of finely divided ferromagnetic
particles applied over the surface, with some of the particles being
gathered and held by the leakage field.
Principles of MPT
1. Materials in magnetic field
2. Magnetized and form continuous circuit
3. If surface or subsurface defect magnetic field is deflected
4. Leakage field
5. Attract magnetic particles/ink
6. Outline of discontinuity
7. Location/Size/Shape
• Cu, Al, Ti, Mg, Pb, SS, Ceramic, Brass, Bronze … can not be
magnetized.
Basic Procedure of MPT
Precleaning
Interpretation of Indications
Basic Procedure of MPT
Forged/Rough/Scaled
Machined Surface
Surface
Pickling/Shot Blasting
Degrease
Demagnetize
Demagnetize
Magnetize (Longitudinal)
Inspect
Demagnetize
Application of magnetic particle/inspect under UV
Evaluate
Advantages of MPT
• Sensitive of locating small and shallow surface cracks in ferromagnetic
materials.
• Indications are produced directly on the surface and constitute
magnetic pictures of actual discontinuities.
• Quick and easy to operate.
• Little or no limitation on the size or shape of the part being inspected.
• No elaborate precleaning is necessary.
• Cracks filled with foreign materials can be detected.
Limitations of MPT
• Used only on ferromagnetic materials.
• Thin coatings of paint and other nanomagnetic coverings, such as
plating, adversely affect the sensitivity of MPI.
• For best results, the magnetic field must be in a direction that will
intercept the principal plane of the discontinuity, this sometimes
requires two or more sequential inspections with different
magnetizations.
• Demagnetization following inspection is often necessary, if not then
wear resistance will decrease.
Permanent Magnet Yoke
Description of Magnetic Field
• Alnico = 12% Al, 26% Ni, 24% Co, balance Fe.
• Alnico permanent magnet is used.
• No open poles.
• Magnetic Fields are used to reveal discontinuities.
• A horseshoe magnet will attract magnetic particles to its ends, or
poles.
• Magnetic flux flows from the south pole to the north pole.
• The magnetic lines of force forms the ringlike assembly because no
external poles exist.
Description of Magnetic Field
• Magnetic particles dusted over the assembly are not attracted to the
magnet even though there are lines of magnetic force flowing
through it.
• A ringlike part magnetized in this manner is said to contain a circular
magnetic field that is wholly within the part.
• Horseshoe magnet forming a closed, ringlike assembly, which will not
attract magnetic particles.
Description of Magnetic Field
• If an air gap exists between the end of the magnet and the magnetic
material, the poles will attract magnetic materials.
• Magnetic particles will cling to the poles and bridge the gap between
them.
• Any radial crack in a circularly magnetized piece will create a north
and south pole at the edges of a crack.
• Magnetic particles will be attracted to the poles created by such a
crack, forming an indication of the discontinuity in the piece.
What is Leakage Field
• The field is set up at cracks or other physical or magnetic
discontinuities in the surface are called leakage fields.
• The strength of a leakage field determines the number of magnetic
particles that will gather to form indications.
• Strong indications are formed at strong fields and weak indications at
weak fields.
• The density of the magnetic field determines its strength and is partly
governed by the shape, size and material of the part being inspected.
Description of Magnetic Fields Magnetized
Bar
• A straight piece of magnetized material (bar magnet) has a pole at
each end.
• Magnetic lines of force flow through the bar from the south to the
north pole.
• Because the magnetic lines of force within the bar run the length of
the bar, it is said to be longitudinally magnetized or to contain a
longitudinal field.
Description of Magnetic Fields
• The direction of the magnetic field in an electromagnetic circuit.
• Is controlled by the direction of the flow of magnetizing current
through the part to be magnetized.
• The magnetic lines of force are always at right angles to the direction
of current flow.
• To remember the direction taken by the magnetic lines of force
around a conductor.
• Consider that the conductor is grasped with the right hand so that the
thumb points in the direction of current flow.
• The fingers then point in the direction taken by the magnetic lines.
Description of Magnetic Fields (Circular
Magnetization)
• Electric current passing through any straight conductors such as a
wire or bar creates a circular magnetic field around the conductor.
• When the conductor of electrical current is a ferromagnetic material,
the passage of current induces a magnetic field in the conductor as
well as in the surrounding space.
• A part magnetized in this manner is said to have a circular field or to
be circularly magnetized.
Description of Magnetic Fields (Longitudinal
Magnetization)
• When electric current is passed through a coil of one or more turns, a
magnetic field is established lengthwise or longitudinally, within the
coil.
• The nature and direction of the field around the conductor that forms
the turns of the coil produce longitudinal magnetization.
Description of Magnetic Fields Effect of Flux
Direction
Description of Magnetic Fields Effect of Flux
Direction
Types of Materials
Easily magnetized
Ferromagnetic 𝜇𝜇 ≫ 1
𝑋𝑋𝑚𝑚 is very high
Difficult to magnetize
Magnetic behavior in
Paramagnetic 𝜇𝜇 > 1
magnetic field
𝑋𝑋𝑚𝑚 is in between
Residual Method
Continuous Method (Material should have
high Retentivity)
Magnetization Method
• Usually the harder the material, the higher the retentivity.
• The continuous method can be used for most parts.
Magnetizing Current
Single Phase and 3 Phase AC Current
Single Phase and 3 Phase AC Current
• Half wave rectified single – phase AC is used with dry powder as
magnetic particles.
• DC = bulky unit
• AC = handy unit
• Magnetic Yolk of single – phase AC = Handy
• DC Yolk = Bulky
Rectified Single Phase AC Current
• The pulsation of the current is useful because it imparts some slight
vibration to the magnetic particles, assisting them in arranging
themselves to form indications.
• Half wave current used in magnetization with prods and dry magnetic
particles, provides the highest sensitivity for discontinuities that are
wholly below the surface, such as those in castings and weldments.
• Magnetization employing surges of direct current are used to increase
the strength of magnetic field.
• Therefore by suitable current – control and switching devices, pass a
very high current for a short period (< a second) and then reduces the
current, without interrupting it, to a much lower value.
Alternating Current
• Single phase AC, used directly for magnetization purposes, is taken
from commercial power lines (𝑓𝑓 = 50 𝑜𝑜𝑜𝑜 60 𝐻𝐻𝐻𝐻).
• When used for magnetization, the line voltage is stepped down, by
means of transformer, to the low voltages required.
• At these low voltages, magnetizing currents of several thousand
amperes are often used.
• Portable, mobile and stationary units are available.
Alternating Current
• A longitudinal magnetic field is usually established by placing the part
near or inside of a coil.
• Magnetic lines of force are parallel to the long axis of the test part.
Power Sources
• Portable, mobile and stationary equipment is currently available, and
selection among these types depends on nature and location of
testing.
Portable Unit
• Light weight (16 to 40 kg) power source units.
• Can be taken to inspection site.
• Are designed to use 115 −, 230 −, 𝑜𝑜𝑜𝑜 460 − 𝑉𝑉 AC.
• Can supply magnetizing current outputs of 750 to 1500 A in half –
wave or AC.
• Machines capable of supplying half – wave current and AC.
• Have continuously variable (infinite) current control.
• Can be used for magnetic particle inspection in wide range of
applications.
Portable Unit
• Primary applications of this equipment is hand – held prod inspection
utilizing half – wave output in conjugation with dry powder.
• In general, portable equipment is designed to operate with relatively
short power supply cables, and the output is very limited when it is
necessary to use longer cables.
• Major disadvantages of portable equipment are:
1. Limited amount of current available.
2. For the detection of deep – lying discontinuities and for coverage of
a large area with one prod contact, a machine with high – amperage
output is required.
Portable Unit
3. Also, portable equipment cannot supply the full – wave DC
necessary for some inspections and does not have the accessories
found on large mobile equipment.
Mobile Unit
• Mounted on wheels to facilitate transportation.
• Mobile equipment usually supplies half – wave or alternating
magnetizing – current outputs.
• Inspection of parts is accomplished with flexible cables, yokes, prod
contact clamps and coils.
• Instruments and controls are mounted on the front of the unit.
• Magnetizing current is usually controlled by a remote – control switch
connected to the unit by an electric cord.
• Quick coupling connectors for connecting magnetizing cables are on
the front of the unit.
The procedure used for MPI
Magnetization of Component:
• The component is magnetized creating a magnetic field. A suitable
method is used to achieve optimum magnetization.
Application of Magnetic Particles:
• Depending upon carrying agent magnetic particles can be classified
into two types:
1. Wet Method –
• In this method the particles are suspended in liquid media (oil or
water). The concentration of particles is 2% by volume and is made
sure that particles are dispersed with proper consistency in the bath.
The procedure used for MPI
2. Dry Method –
• In this method the particles are carried to the surface by air. It is
made sure that the coating of dry particles is light and evenly
distributed, since heavy coatings will impede particle movement.
Viewing and Identification of Defects:
• The particle movement is viewed under proper illumination,
preferably daylight. The fluorescent powder is viewed under the
backlight inside darkened booth.
• The area under inspection is covered with transparent adhesive film.
When this film is peeled off, it comes out with magnetic particles
adhering to corresponding indications
The procedure used for MPI
Demagnetization:
• The residual magnetization is removed after particle inspection. This
is usually done by heating material to approximately 1033 𝐾𝐾.
Removal of Ink:
• The ink is removed from the component by paraffin oil wash by hand
brush.
Magnetic Field
DC
Penetrate
HWDC or FWDC
To Magnetize
Confined to the
AC surface
(Skin Effect)
HWDC or FWDC
HWDC FWDC
1) HWDC or Pulsating DC most often used to Depth of the subsurface magnetic field is more.
power electromagnetic yokes.
2) Good particle mobility. Particle mobility is less.
Three Phase Full Wave Form
• Mostly in industries.
• Favourable power transmission.
• For stationary magnetic particle equipment.
• Advantages of both power forms.
Electromagnetic Yokes
Coils
• The flux density passing through the interior of the coil is proportional
to the current,𝐼𝐼, in amperes, and the number of turns in the coil, N.
Central Conductor
Inspection Method for Hollow Components
Circular Magnetic Field Strength (F),
Distribution (D) and Intensity
• When current passed through conductor, F = 0, at the center and
maximum at the surface.
• F at the surface of the conductor decreases as the radius of conductor
increases.
• F outside the conductor is 𝛼𝛼 to the current strength.
• F inside the conductor depends on the current strength, magnetic
permeability of the material.
• F outside the conductor decreases with distance from the conductor.
Magnetic Particle Test Method
• Dry Method
1. Finely divided dry ferromagnetic particles/powders.
2. Gray, Black or Red.
3. Suitable for field work, rough castings.
• Wet Method
1. Fine Magnetic Particles suspended in kerosene.
2. Particle size 10 − 50 𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚.
3. Suitable for fine defects.
Dry Method
Florescent Method
• Ferromagnetic particles are coated with florescent dye.
• Examined under UV light (𝜆𝜆 = 3650𝐴𝐴).̇
• Suitable for deep holes, corners, keyways.
High Carbon and Low Carbon Steels
• High Carbon Steels:
1. Hysteresis Loop area is high.
2. Retentivity is high.
3. Residual magnetization is high.
4. Residual magnetization method for MPI is used.
• Low Carbon Steels:
1. Retentivity is low.
2. Continuous Method is used.
3. Magnetic Field is on while testing.
Standardization and Calibration in MPT
KETOS RING/STANDARD TEST BLOCK:
• Purpose: Equipment should work under conditions of acceptable and
reproducible sensitivity.
• High degree of confidence.
• Artificial discontinuities.
• 12 holes of same diameter (0.07 in) at different depths.
• Maximum current that gives a satisfactory indication at each hole is
established and recorded for the specific magnetic unit.
Standardization and Calibration in MPT
KETOS RING/STANDARD TEST BLOCK
• Failure to obtain a satisfactory indication:
1. Concentration of magnetic particles below the optimum level.
2. Ammeter reading is incorrect.
3. Other malfunctioning.
Standardization and Calibration in MPT
PIPE GUAGE
• Disk of highly permeable material divided into triangular 4/6/8
sections by non – ferromagnetic material.
• Divisions act as artificial discontinuities that radiate out in different
directions from center.
• Can be used to measure:
1. Adequate field strength.
2. Orientation of magnetic field.
Application of Magnetic Media (Wet vs. Dry)
Quality and Process Control in MPI
DRY MAGNETIC PARTICLES:
• Magnetic particles come in a variety of colors. A color that produces a
high level of contrast against the background should be used.
Quality and Process Control in MPI
WET MAGNETIC PARTICLES:
• Wet particles are typically supplied as visible or fluorescent. Visible
particles are viewed under normal white light and fluorescent
particles are viewed under UV light.
Quality and Process Control in MPI
WET MAGNETIC PARTICLES:
• To detect concentration of particles in the suspension:
1. A sample is taken in a pear – shaped 100 mL centrifuge tube having
a stem graduated.
2. Leave it undisturbed for 30 – 60 min.
• Acceptable Ranges:
1. 0.1 to 0.4 mL for fluorescent particles.
2. 1.2 to 2.4 mL for visible particles.
Interpretation of Indications
• After applying the magnetic field, indications that form must be
interpreted. This process requires that the inspector distinguish
between relevant and non – relevant indications.
Crane Hook with Service Induced Crack
Gear with Service Induced Crack
Drive Shaft with Heat Treatment Induced
Cracks
Eddy Current Testing
Eddy Current Inspection
• Based on the principles of electromagnetic induction.
• Application:
1. Measure or identify such conditions and properties as electrical
conductivity, magnetic permeability, grain size, heat treatment
condition, hardness, and physical dimensions.
2. Detect seams, laps, cracks, voids, and inclusions.
3. Sort dissimilar metals and detect differences in their composition,
microstructure, and other properties.
Eddy Current Inspection
4. Measure the thickness of a non – conductive coating on a
conductive metal, or the thickness of a non – magnetic metal
coating on a magnetic material.
5. Applicable to ferromagnetic and non – ferromagnetic metals.
• Electromagnetic Induction was discovered by Faraday in 1831.
Functions of a Basic System
• Part is placed within or adjacent to an electric coil.
• Alternating current (exciting current) is passed.
• It will cause eddy current to flow in the part as a result of
electromagnetic induction.
• These currents flow within closed loops in the part, and their
magnitude and timing (or phase) depends upon:
1. The primary field established by the exciting current.
2. The electrical properties of the part.
3. The presence or absence of flaws in the part.
4. The electromagnetic fields established by currents flowing within the part.
Eddy Current Inspection
• At A – A, no crack is present, and the eddy current flow is symmetrical.
• At B – B, where a cack is present, the eddy current flow is impeded and
changed in direction, causing significant changes in the associated
electromagnetic field.
• The condition of the part can be monitored by observing the effect of the
resulting field on the electrical characteristics of the exciting coil, such as its
1. Electrical Impedance
2. Induced Voltage, or Induced Current
• The effect of the electromagnetic field can be monitored by observing the
induced voltage in one or more other coils placed within the field near the
part being monitored.
Basic Principal of Eddy Current Testing (ET)
• When AC is applied to the conductor, such as copper wire, a magnetic
field develops in and around the conductor.
• A dynamic expanding and collapsing magnetic field forms in and
around the coil as the alternating current flows through the coil.
• When an electrically conductive material is placed in the coil’s
dynamic magnetic field, electromagnetic induction will occur, and
eddy currents will be induced in the material.
• Eddy currents flowing in the material will generate their own
“secondary” magnetic field which will oppose the coil’s “primary”
magnetic field.
Eddy Current Testing (ET) – Surface NDT
Method
Eddy Current Testing
• Electromagnetic Induction.
• Faraday’s Law.
• If a conductor carries AC current.
𝑑𝑑𝜙𝜙
• It will have change in magnetic field
𝑑𝑑𝑡𝑡
• It will induce emf in same or nearby conductor.
𝑑𝑑𝜙𝜙
• Induced EMF = 𝑉𝑉𝐿𝐿 =
𝑑𝑑𝑑𝑑
𝑑𝑑𝑑𝑑
• 𝑉𝑉𝐿𝐿 = 𝑁𝑁 ×
𝑑𝑑𝑑𝑑
• 𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤, 𝑁𝑁 = 𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛 𝑜𝑜𝑜𝑜 𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡
Eddy Current Inspection
• Electrical impedance of the coil or induced voltage of either the
excitation coil or other adjacent coil or coils is measured.
Eddy Current Inspection
• Elements of a typical inspection system:
The output of the inspection coil is fed to the amplifier and detected or
demodulated by the detector.
Eddy Current Inspection
• Excitation of the inspection coil.
• Modulation of the inspection coil output signal by the part.
• Processing of the inspection coil signal prior to amplification.
• Amplification of the inspection coil signals.
• Detection or demodulation of the inspection coil signal.
• Display of signals on a meter, an oscilloscope, an oscillograph, or a
strip chart recorder; or recording on magnetic tape.
Operating Variables
1. Coil Impedance
2. Electrical Conductivity
3. Magnetic Permeability
4. Lift – Off and Fill Factors
5. Edge Effect
6. Skin Effect
Coil Impedance
• When DC is flowing in a coil, the magnetic field reaches a constant
level, and the electrical resistance (R) of the wire is the only limitation
to current flow.
• In AC current in coil, two limitations are imposed :
1. The AC resistance of the wire, R
2. Inductance resistance, 𝑋𝑋𝐿𝐿
• The AC resistance of an isolated or empty coil operating at low
frequencies or having a small diameter is very nearly the same as the
DC resistance of the wire of the coil.
Coil Impedance
• The ratio of AC resistance to DC resistance increases as either the
frequency increases or the wire diameter increases.
• In the discussion of eddy current principles, the resistance of the coil
wire is often ignored, because it is nearly constant. It varies mainly
with wire temperature and the frequency and spatial distribution of
the magnetic field threading the coil.
• Inductive reactance, 𝑋𝑋𝐿𝐿 , is the combined effect of :
1. Coil Inductance
2. Test Frequency
Coil Impedance
• Total resistance to the flow of AC in a coil is called impedance, Z, and
comprises of both:
1. AC Resistance, R
2. Inductive Impedance, 𝑋𝑋𝐿𝐿
• 𝑍𝑍 = 𝑅𝑅 2 + 𝑋𝑋𝐿𝐿2
• 𝑋𝑋𝐿𝐿 = 2𝜋𝜋𝜋𝜋𝐿𝐿0
• 𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤, 𝑓𝑓 = 𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡 𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓 (𝑖𝑖𝑖𝑖 𝐻𝐻𝐻𝐻)
• 𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤, 𝐿𝐿0 = 𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶 𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼 (𝑖𝑖𝑖𝑖 𝐻𝐻𝐻𝐻𝐻𝐻𝐻𝐻𝐻𝐻)
Coil Impedance
• When a metal part is placed adjacent to or within a test coil, the
electromagnetic field threading the coil is changed as a result of eddy
current flow in the test object.
• Both the AC resistance and the inductive reactance of the coil are
affected.
• The resistance of the loaded coil consists of two components, namely,
1. The AC Resistance of the Coil Wire
2. The Apparent, or Coupled Resistance caused by the presence of the
test object.
Coil Impedance
• Changes in these components reflect conditions within the test
object.
• Typical impedance – plane diagram derived by placing an inspection
coil sequentially on a series of thick pieces of metal, each with a
different IACS electrical resistance or conductivity rating. The
inspection frequency was 100 𝑘𝑘𝑘𝑘𝑘𝑘.
Impedance Components
• The coil is assumed to have inductance, 𝐿𝐿0 , and negligible resistance.
• The part being inspected consists of a very thin tube having shunt
conductance, G, closely coupled to the coil.
• When an AC is applied, some energy is stored in the system and
returned to the generator each cycle and some energy is dissipated or
lost as heat per cycle.
• The inductive reactance component, 𝑋𝑋𝐿𝐿 , is directly proportional to the
energy stored per cycle.
Impedance Components
• The resistance component, R, of the impedance is directly
proportional to the energy dissipated per cycle.
• The impedance, Z, is equal to the complex ratio of the applied
voltage, E, to the current, 𝐼𝐼.
• The term complex is used to indicate that, in general, the AC and
voltage do not have the same phase angle.
Impedance Components
• Figure (b) to (d) show three impedance diagrams for three conditions.
• When only the coil is present, the circuit impedance is purely
reactive, that is, Z = 𝑋𝑋𝐿𝐿 = 𝜔𝜔𝜔𝜔 = 2𝜋𝜋𝜋𝜋𝜋𝜋 (Figure(b)).
• When only the conductance of this equivalent circuit is present ( a
hypothetical condition for an actual combination of inspection coil
and part being inspected), the impedance is purely resistive, that is,
1
𝑍𝑍 = = 𝑅𝑅 𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹 𝑐𝑐 .
𝐺𝐺
Impedance Components
• When both coil and conductance are connected (Figure (d)), the
impedance has both reactive and resistive components in the general
instance, and the impedance.
• 𝑍𝑍 = 𝑅𝑅 2 + 𝑋𝑋𝐿𝐿2
• An angle, 𝜃𝜃, is associated with the impedance, Z. This angle is a
function of the ratio of the two components of the impedance, R and
XL.
Impedance Components
• Points and loci on impedance – plane diagrams can be displayed using
phasor representation because of the close relationship between the
impedance diagrams and the phasor diagrams.
• In a given circuit with input impedance, Z, applying an impressed
fixed circuit, 𝐼𝐼, will produce a signal voltage, E in accordance with
Ohm’s Law (𝐸𝐸 = 𝐼𝐼𝐼𝐼). This signal voltage can be displayed as a phasor.
• With 𝐼𝐼 fixed, the signal voltage E is directly proportional to the
impedance, Z. Thus, the impedance plane can be readily displayed
using the phasor technique.
Phasor Representation of Sinusoids
• In Figure, three vectors, A, B, and C which are rotating
counterclockwise with radian velocity, 𝜔𝜔𝐿𝐿 = 2𝜋𝜋𝜋𝜋𝜋𝜋.
• The equations that describe these vectors are of the form:
• 𝐾𝐾 sin 𝜔𝜔𝜔𝜔 + 𝜙𝜙
• 𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤, 𝐾𝐾 = 𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒 𝑡𝑡𝑡𝑡 𝐴𝐴, 𝐵𝐵, 𝑎𝑎𝑎𝑎𝑎𝑎 𝐶𝐶.
• 𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤, 𝜙𝜙 = 𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒 𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝 𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎
• These equations are plotted in Figure (b). The length of these vectors
A, B, and C determine the amplitude of the sinusoids generated in
Figure (b).
Phasor Representation of Sinusoids
• The physical angle between, the vectors A and B, or between A and C,
determines the electrical phase angle, between sinusoids.
• In Figure (b), these angles are +90° 𝑎𝑎𝑎𝑎𝑎𝑎 − 45°, respectively.
• The three vectors, A, B, and C are considered to be rotating at
frequency, 𝑓𝑓, generating three rather monotonous sinusoids.
Phasor Representation of Sinusoids
• This system of three vectors rotating synchronously with the
frequency of the sinusoids is not very useful, because of its high rate
of rotation. However, if rotation is stopped, the amplitudes and phase
angles of the three sine waves can be easily seen in a representation
called a phasor diagram.
• In eddy current inspection equipment, the sine wave signals are often
expanded in quadrature components and displayed as phasors on an
x-y oscilloscope, shown in Fig. (c). Usually, only the tips of the phasors
are shown
Phasor Representation of Sinusoids
• Thus, A and B in Fig. (c) show the cathode ray beam position
representing the two sinusoids of Fig. (b).
• Point C represents a sinusoid 𝐴𝐴 sin 𝜔𝜔𝜔𝜔 having the same amplitude as
Asinωt, but which lags or follows it in phase by an electrical angle
equal to 45°.
• The points indicated as C' represent sinusoids having the same phase
angle as C sin t, but with different amplitudes.
• The concept of a phasor locus is introduced by varying the amplitude
gradually from the maximum at C to zero at the origin O.
Phasor Representation of Sinusoids
• This results in the beam spot moving from C to O, producing a locus.
• In contrast, a shift of the phase angle of a sinusoid causes a
movement of the phasor tip around the origin O as shown by the arc
DE. Here, D represents a sinusoid having the same amplitude as the
sinusoid represented by A but leading it by 30°. Increasing this phase
angle from 30 to 60° results in the phasor locus DE.
• When both amplitude and phase changes occur, more complicated
loci can be formed as shown at F and G.
Eddy Current Testing
• Resistance Depends on:
1. Type of Material (Resistivity) = 𝜌𝜌
2. Length and cross section area
3. Temperature
1
• 𝜎𝜎 = 𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸 𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶 =
𝜌𝜌
𝑙𝑙 1
• 𝑅𝑅 = 𝜌𝜌 → 𝑅𝑅 ∝ 𝑙𝑙 𝑎𝑎𝑎𝑎𝑎𝑎 𝑅𝑅 ∝
𝐴𝐴 𝐴𝐴
The International Annealed Copper Standard
• Resistivity is a unique value for a specific material and is temperature
dependent.
• International Annealed Copper Standard (IACS) indicates relative
conductivity of a material compared to that of Cu.
• Conductivity value in Siemens/meter can be converted to %IACS by
multiplying the conductivity by 1.7241 × 10−6 .
1.7241×10−6
• = %𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼
𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅
• 1.7241 × 10−6 × 𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶 = %𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼
Eddy Currents
• Closed loops of induced current circulating in planes perpendicular to
the magnetic field.
• Travel parallel to the coil’s windings and flow is limited to the area of
the inducing magnetic field.
• Concentrate near the surface adjacent to an excitation coil.
• Their strength decreases with distance.
• Eddy current density decreases exponentially with depth. This
phenomenon is called skin effect.
Depth of Penetration and Current Density
• The depth that eddy current penetrates into a magnetic field is
affected by:
1. The frequency (f) of the excitation current.
2. Electrical Conductivity 𝜎𝜎 .
3. Magnetic permeability of the material 𝜇𝜇 .
• There is a decrease in the depth of penetration with increase in
1. frequency
2. electrical conductivity
3. magnetic permeability
Depth of Penetration and Current Density
1
• 𝛿𝛿 = , 𝛿𝛿 = 𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆 𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷 𝑜𝑜𝑜𝑜 𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃 = 𝑆𝑆𝑆𝑆𝑆𝑆 (𝑖𝑖𝑖𝑖 𝑚𝑚𝑚𝑚)
𝜋𝜋𝜋𝜋𝜋𝜋𝜋𝜋
𝐻𝐻
• 𝜎𝜎 𝑖𝑖𝑖𝑖 %𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼 𝑎𝑎𝑎𝑎𝑎𝑎 𝜇𝜇 (𝑖𝑖𝑖𝑖 )
𝑚𝑚𝑚𝑚
X-ray film
= less exposure
= more exposure
Top view of developed film
General Principles of Radiography
• The energy of the radiation affects its penetrating power.
Higher energy radiation can penetrate thicker and more
dense materials.
• The radiation energy and/or exposure time must be
controlled to properly image the region of interest.
Thin-Walled Area
Flaw Orientation
Optimum
Radiography has
sensitivity limitations
Angle = easy to
when detecting cracks. detect
= not easy
to detect
X-rays “see” a crack as a thickness variation and the larger the variation, the
easier the crack is to detect.
When the path of the x-rays is not parallel to a crack, the thickness variation is less,
and the crack may not be visible.
IDL 2001
0o 10o 20o
Radiation Sources
Two of the most commonly used sources of radiation in
industrial radiography are x-ray generators and gamma ray
sources. Industrial radiography is often subdivided into “X-
ray Radiography” or “Gamma Radiography”, depending on
the source of radiation used.
Gamma Radiography
• Gamma rays are produced by
a radioisotope.
• A radioisotope has an
unstable nuclei that does not
have enough binding energy
to hold the nucleus together.
• The spontaneous breakdown
of an atomic nucleus resulting
in the release of energy and
matter is known as
radioactive decay.
Gamma Radiography (cont.)
• Most of the radioactive
material used in industrial
radiography is artificially
produced.
• This is done by subjecting
stable material to a source
of neutrons in a special
nuclear reactor.
• This process is called
activation.
Gamma Radiography (cont.)
Unlike X-rays, which are produced by
a machine, gamma rays cannot be
turned off. Radioisotopes used for
gamma radiography are
encapsulated to prevent leakage of
the material.
Protective Layer
Phosphor Layer
After exposure:
Laser Beam
A/D
Converter
Imaging
Plate 110010010010110
Motor
Computed Radiography (cont.)
Digital images are typically sent to a computer
workstation where specialized software allows
manipulation and enhancement.
Computed Radiography (cont.)
Examples of computed radiographs:
Real-Time Radiography
• Real-Time Radiography (RTR) is a term used to describe a form
of radiography that allows electronic images to be captured and
viewed in real time.
• Because image acquisition is almost instantaneous, X-ray
images can be viewed as the part is moved and rotated.
• Manipulating the part can be advantageous for several reasons:
It may be possible to image the entire component with one
exposure.
Viewing the internal structure of the part from different
angular prospective can provide additional data for analysis.
Time of inspection can often be reduced.
Real-Time Radiography (cont.)
• The equipment needed for an RTR includes:
Technicians who work with radiation must wear monitoring devices that
keep track of their total absorption, and alert them when they are in a
high radiation area.