A wide group of analysis techniques used in science and
industry to evaluate the properties of a
material, component or system without causing
Nondestructive examination (NDE)
Nondestructive inspection (NDI)
Nondestructive evaluation (NDE)
Does not permanently alter the article being inspected
Save both money and time in product
evaluation, troubleshooting and research
Can be used to detect flaws in an in-process machine
NDT methods rely upon use of electromagnetic
radiation, sound, and inherent properties of materials
(such as thermal, chemical, magnetic etc.) to examine
Very short ultrasonic pulse-waves are launched into
materials to detect internal flaws.
Used for steel and other metals and alloys, can also be
used on concrete, wood and composites (with less
Used in many industries including
aerospace, automotive and other transportation
Two methods of receiving the ultrasound waveform:
LEFT: A probe sends a sound wave into a test material. There are two indications, one from
the initial pulse of the probe, and the second due to the back wall echo.
RIGHT: A defect creates a third indication and simultaneously reduces the amplitude of the
back wall indication. The depth of the defect is determined by the ratio D/Ep.
Part is magnetized.
Presence of a surface or subsurface discontinuity in the
material allows the magnetic flux to leak, since air cannot
support as much magnetic field per unit volume as metals.
Ferrous iron particles are then applied to the part.
Particles will build up at the area of leakage and form what
is known as an indication.
Penetrant may be applied to the test component by
dipping, spraying, or brushing
After adequate penetration time, the excess penetrant
is removed, a developer is applied.
Developer helps to draw penetrant out of the flaw
where an invisible indication becomes visible to the
1. Section of material with
a surface-breaking crack
that is not visible to the
2. Penetrant is applied to
3. Excess penetrant is
4. Developer is
applied, rendering the
Short wavelength electromagnetic radiation (high
energy photons) to penetrate various materials.
The amount of radiation emerging from the opposite
side of the material can be detected and measured
Tube exhibiting no cracking Tube exhibiting light cracking
Tube exhibiting moderate cracking Tube exhibiting severe cracking
Uses electromagnetic induction to detect flaws in
Variations in the phase and magnitude of these eddy
currents can be monitored using a second 'receiver' coil, or
by measuring changes to the current flowing in the
primary 'excitation' coil.
Variations in the electrical conductivity or magnetic
permeability of the test object, or the presence of any
flaws, will cause a change in eddy current and a
corresponding change in the phase and amplitude of the
NDT Technique Nature of defect
Ultrasonic Testing Sub-surface, interstitial
Magnetic Particle Inspection Surface and slightly subsurface discontinuities
in ferroelectric materials
Dye Penetrant Inspection Surface-breaking defects in all non-porous
Radiographic Testing Surface, Sub-surface defects
Eddy-Current Testing Surface, Sub-surface defects (depending on
A process plant contained two stainless steel vessels
which had been operating for 21 years. The contents of
the vessels were flammable, mildly toxic and
contained 500 ppm of chlorides. The vessels were
operated from full vacuum up to 15 psi for 20 cycles per
day. They contained an agitator which was used in part
of the process. Both vessels had been hydraulically
tested to 70 psi when new but had not been subjected
to a test since
Company thought that stainless steel would not break.
Hence, no leak detection equipment had been
installed and reliance was placed on plant operators
noticing the smell or observing drips.
A competent Person from a large insurance company
prescribed external visual examination supplemented
by a hammer test every 2 years.
Was this suitable?
The combination of stainless steel and chlorides
immediately raises concerns regarding the possibility
of stress corrosion cracking. While the cracks were
likely to initiate on the inner surface, an external
examination could detect the presence of through wall
cracks only. However, stress corrosion cracks can be
very tight and difficult to see with the naked eye. The
hammer test offers no benefit - who knows what a
good vessel should sound like!
During a thorough examination, it was found that
vessel developed leaks at 40 psi. Further investigation
of the vessel found thousands of through wall cracks.
The vessel had not leaked in service because the
contents were too viscous to pass through the tight
stress corrosion cracks.
Finally it was decided to go for NDT with Dye
Penetrant Inspection using fluorescent dyes, which
give a higher sensitivity and much better results in the
confined, dark space of the vessel.
NDT techniques provide cost-effective and reliable analysis
under realistic conditions.
Each NDT technique has certain capabilities and
limitations and often more than one technique is used to
cover various parts.
Increasing availability of robotic scanners improve the
speed of testing large surfaces, hence minimizing the
THE IMPORTANCE OF WELDING QUALITY CONTROL,K.M. WONG
& Scarlett YEUNG,A.E.S. Destructive & Non-Destructive Testing Ltd
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