Download NON DESTRUCTIVE TESTS

NON DESTRUCTIVE TESTS
By: Imtiaz Ali Shaikh (500410641)
To: Dr. Ravi Ravindran
February 29th 2012
ME8109 Course Project
Contents
Introduction
Purposes of Non-Destructive Testing
Selection of the NDT Method
Eddy Current Testing
Liquid Penetrating Testing
Magnetic Particle Testing
Radiographic Testing
Ultrasonic Testing
Introduction
Non destructive examination is the testing of
materials without destroying the integrity of the
material.
NDT techniques are used for testing materials
mainly for three types of defects
1)Inherent defects which are created during the initial
production of the material.
2)Processing defects which are created during the
processing/manufacturing of the equipment
3)Service defects which are created in the equipment
during service
Purposes of Non-destructive testing
Modern Non-destructive tests are used by
manufacturers
To avoid failures, prevent accidents and save
human life
To make a profit for the user
To ensure customer satisfaction and maintain
the manufacturer's reputation
To aid in better product design
To control manufacturing processes
Selection of the NDT method
The selection of one method over another is
based on the following factors:
Type and origin of discontinuity
Material manufacturing process
Accessibility of the component to perform NDT
Type of equipment available
Time available
Cost
1) Eddy Current Testing
Eddy current testing has its origins with Michael
Faraday’s discovery of electromagnetic induction.
According to Faraday’s Law states when a magnetic
field cuts a conductor or when a conductor cuts a
magnetic field, an electric current will flow through the
conductor.
From Oersted’s discovery, a magnetic flux exists around
a coil carrying current proportional to the number of
turns in the coil and the current.
Principles
The relative motion causes a circulating flow of
electrons, or current, within the conductor
According to French physicist Leon, Eddy current
is caused when a moving (or changing) magnetic
field intersects a conductor, or vice-versa.
According to Lenz’ law, These circulating eddies
of current create electromagnets with magnetic
fields that oppose the effect of the applied
magnetic field .
This opposing magnetic field, coming from the
material, has a weakening effect on the primary
magnetic field and the test can sense this change.
In effect, the impedance of the test coil is
reduced proportionally as eddy currents are
increased in the test piece.
A crack in the test material
obstructs the eddy current
flow, lengthens the eddy
current path, reduces the
secondary magnetic field, and
increases the coil impedance.
If a test coil is moved over a
test piece at constant rate of
speed, a momentary change
will occur in the coil reactance
and coil current. This change
can be detected, amplified,
and displayed by an eddy
current detector.
Important points
ECT is primarily used for testing non
ferromagnetic conducting materials. However,
with modification, it can also be used for
testing ferromagnetic materials.
Cracks lying parallel to the current path will
not cause any significant interruption and may
not be detected crack.
ECT is very important because often the
defects that we are looking for not visible
because paint or some other coating may
cover them. There might also be defects that
are so small they cannot be seen with our eyes
or any other visual method of inspection.
2) Liquid Penetrating Testing
Liquid penetrating testing detects the flaws that are
open to the surface and is a type of visual
inspection.
This method is based upon the principle of capillary
action.
Penetrants are of two types, fluorescent or non
fluorescent (visible).
Several developer types are available, including:
non-aqueous wet developer, dry powder, water
suspendible, and water soluble.
Essential steps
There are four essential steps required
for this test
1) Selection of material with a surfacebreaking crack that is not visible to the
naked eye.
2) Penetrant application
3) Excess penetrant removal
4) Developer application
Inspection
is
performed
under
ultraviolet or white light, depending
upon the type of dye used - fluorescent
or non fluorescent (visible).
•
After excess penetrant has been
removed a white developer is
applied to the sample.
The developer draws penetrant
from defects out onto the surface
to form a visible indication. Any
coloured stains indicate the
positions and types of defects on
the surface under inspection.
Visible penetrants are typically
red in color. Fluorescent penetrants
contain two or more dyes that
fluorescence when excited by
ultraviolet radiation.
3) Magnetic Particle Testing
This method detects flaws that are either surface or
subsurface
This can be considered as a combination of two nondestructive testing methods: magnetic flux leakage
testing and visual testing.
A magnetic flux is send through the material, at the
location of the imperfection a leakage field is
formed. This attracts metal iron dust, which is
sprayed onto the surface.
Principles
A magnetic flux is send through the material by
energising coils. At the location of the
discontinuities a leakage field is formed.
The leakage is a function of the orientation of the
discontinuities to the magnetic field(flex lines).
The leakage is greatest when the discontinuity is
perpendicular to the magnetic field.
When the leakage of the magnetic field is great
enough, a pair of magnetic poles is established at
the discontinuity.
B y applying magnetic particles, the particles
are attracted to the poles and will gather at the
discontinuity, indicating a surface or sub-surface
flaw.
4) Radiographic Testing
Radiography testing detects flaws that are internal or on the inside
surface.
Penetrating radiation can be x-rays or gamma rays.
In the presence of flaws, there will be a differential
absorption of penetrating radiation.
The unabsorbed radiation passes through the test component and
exposes a film.
The basic principle of radiographic inspection of welds is the same as
that for medical radiography.
The most disadvantage of the use of RT is the possible hazards due
to exposure to radiation if proper care is not taken.
Therefore only suitably trained and qualified personnel should
practice this type of testing.
5) Ultrasonic Testing
Ultrasonic testing uses high frequency sound waves
to find hidden internal flaws.
UT is the only practical method having widespread
use in underwater inspection applications for
locating and sizing subsurface defects in
components.
As the test object's sound velocity is known,
therefore it is possible to determine the distance of
the reflector using simple calculation and thus its
exact position in the test object as shown in figure.
The name of the method used in most areas of
application for material testing with Ultrasonic's is
the Pulse Echo Method.
Sound reflections in the audio range are called
echoes
The electrical transmission pulse
triggers the sound pulse at the probe
crystal. At the same time this voltage
pulse is feed to the input of the amplifier
so that the high voltage causes a vertical
deflection of the display sweep, this is
called the initial pulse.
The speed of the pulse is dependent
on the material of the test object (sound
velocity = material constant).
The speed of the display sweep can be
exactly matched to the sound velocity. In
our example the electron beam reaches
scale division 4 while the pulse is at the
opposing side of the test object.
The part of the sound pulse,
which is transmitted through the
couplant and into the probe,
generates a small electrical
reception signal at the crystal
which, via the amplifier, causes
vertical deflection of the beam
spot, this is the back wall echo.
Intermediate echo is caused by
partial reflection of the sound
wave on a discontinuity between
the initial and the back wall echo.
Conclusions
It is very difficult to weld or mould a solid object that has no risk
of breaking in service, so testing at manufacture and during use
is often essential.
During the process of casting a metal object, for example, the
metal may shrink as it cools, and crack or introduce voids inside
the structure. Even the best welders (and welding machines) do
not make 100% perfect welds. Some typical weld defects that
need to be found and repaired are lack of fusion of the weld
to the metal and porous bubbles inside the weld, both of which
could cause a structure to break or a pipeline to rupture.
During their service lives, many industrial components need
regular non-destructive tests to detect damage that may be
difficult or expensive to find by everyday methods.
References
[1] Power Engineering Reference material, Third class, Southern Alberta
Institute of Technology, Calgary.
[2] Baldev Raj T. Jayakumar M. Thavasimuthu, Second Edition Practical Nondestructive Testing
Narosa Publishing House New Delhi
[3] Power Engineering Reference material, Second class, Southern Alberta
Institute of Technology, Calgary.
[4] R Halmshaw, Non-destructive Testing, 2nd ed ., Edward Arnold, Division
of Hodder and Sloughton, London, 1991.
[5] Anonymous. PR Newswire. New York: Feb 7, 2008
[6] joe.buckley.net/papers/eddyc.pdf
[7] PAUL E. MIX Introduction to Non-destructive Testing A Training Guide, 2nd Edition
A JOHN
WILEY & SONS, INC., PUBLICATION
[8] International ATOMIC ENERGY AGENCY, a Non-destructive Testing in Nuclear
Technology
[9] J. H. LAMBLE Principles and Practice of Non-destructive Testing, HEYWOOD &
COMPANY LTD
[10] C.A HOGARTH, Ph.D and j.Blitz, M.Sc, Technique of Non-destructive Testing
[11] Composite.about.com/library/glossary/r/bldef-r4410.htm
[12] PJ Mudge, Practical aspects of NDT reliability in non-destructive testing, Vol. 2,
Pergamon Press, England, 1988
THANK YOU