Introduction to non destructive testing (ndt)

Introduction to Non-Destructive Testing (NDT)
Training
Presentation
Presented by : APEXFI for Industrial Inspection Solutions

The use of noninvasive
techniques to determine
the integrity of a material,
component or structure
or
quantitatively measure
some characteristic of
an object.
i.e. Inspect or measure without doing harm.
Definition of NDT

Some Uses of NDT Methods
• Flaw Detection and Evaluation
• Leak Detection
• Location Determination
• Dimensional Measurements
• Structure and Microstructure Characterization
• Estimation of Mechanical and Physical Properties
• Stress (Strain) and Dynamic Response Measurements
• Material Sorting and Chemical Composition
Determination
Fluorescent penetrant indication

Using of NDT Methods
• To assist in product development
• To screen or sort incoming materials
• To monitor, improve or control
manufacturing processes
• To verify proper processing such as
heat treating
• To verify proper assembly
• To inspect for in-service damage
There are NDE application at almost any stage
in the production or life cycle of a component.

Six Most Common NDT Methods
• Visual (VT)
• Penetrant (PT)
• Magnetic (MT)
• Ultrasonic (UT)
• Eddy Current (ET)
• Radiography (RT)

Most basic and common
inspection method.
Tools include
fiberscopes,
borescopes, magnifying
glasses and mirrors.
Robotic crawlers permit
observation in hazardous or
tight areas, such as air
ducts, reactors, pipelines.
Portable video inspection
unit with zoom allows
inspection of large tanks
and vessels, railroad tank
cars, sewer lines.
Visual Inspection (VT)

• A liquid with high surface wetting characteristics is applied to the surface of the part and
allowed time to seep into surface breaking defects.
• The excess liquid is removed from the surface of the part.
• A developer (powder) is applied to pull the trapped penetrant out the defect and spread it on the
surface where it can be seen.
• Visual inspection is the final step in the process. The penetrant used is often
loaded with a fluorescent dye and the inspection is done under UV light to
increase test sensitivity.
Liquid Penetrant Testing (PT)

Liquid Penetrant Testing (PT)
Some of the advantages of liquid penetrant testing are as follows:
(1) Relatively low cost.
(2) Highly portable NDT method.
(3) Highly sensitive to fine, tight discontinuities.
(4) Fairly simple method.
(5) Can be used on a variety of materials.
(6) All surface discontinuities are detected in one operation, regardless of
orientation.
Some of the limitations of liquid penetrant testing are as follows:
(1) Test surface must be free of all contaminants (dirt, oil, grease, paint, rust, etc.).
(2) Detects surface discontinuities only.
(3) Cannot be used on porous specimens and is difficult to use on very rough
surfaces.
(4) Removal of all penetrant materials, following the test, is often required.
(5) There is no easy method to produce permanent record.

Magnetic Particle Testing (MT)
The part is magnetized. Finely milled iron particles coated with a dye pigment
are then applied to the specimen. These particles are attracted to magnetic
flux leakage fields and will cluster to form an indication directly over the
discontinuity. This indication can be visually detected under proper lighting
conditions.

Advantages of magnetic particle testing include the following:
(1) It does not need very stringent pre-cleaning operation.
(2) Best method for the detection of fine, shallow surface cracks in ferromagnetic material.
(3) Fast and relatively simple NDT method.
(4) Generally inexpensive.
(5) Will work through thin coating.
(6) Few limitations regarding the size/shape of test specimens.
(7) Highly portable NDT method.
(8) It is quicker.
limitations of magnetic particle testing include the following:
(1) Material must be ferromagnetic.
(2) Orientation and strength of magnetic field is critical.
(3) Detects surface and near-to-surface discontinuities only.
(4) Large currents sometimes required.
(5) “Burning” of test parts a possibility.
(6) Parts must often be demagnetized, which may be difficult.

Conductive
material
Coil
Coil's
magnetic field
Eddy
currents
Eddy current's
magnetic field
Eddy Current Testing (ET)

Eddy current testing is particularly well suited for detecting surface
cracks but can also be used to make electrical conductivity and
coating thickness measurements. Here a small surface probe is
scanned over the part surface in an attempt to detect a crack.

Some of the advantages of eddy current testing include:
(1) Does not require couplant.
(2) It gives instantaneous response.
(3) Has uncomplicated steps during set-up.
(4) Is extremely sensitive to flaws.
(5) Is very repeatable.
(6) High scanning speeds can be used.
(7) Is very accurate for dimensional analysis of flaws or coating thickness.
Some of the limitations of eddy current testing include the following:
(1) The theory requires a good academic background in electrical principles and in mathematics.
(2) Extremely sensitive to surface variations and therefore requires a good surface.
(3) It is applicable to conductor materials only.
(4) Can be used on non-magnetic and magnetic material but is not reliable on carbon steel for the
detection of subsurface flaws.
(5) Its depth of penetration is limited.
(6) Crack tightness and orientation of eddy current flow to a crack or linear discontinuity will affect
detectability.

Radiographic Testing (RT)
The radiation used in radiography testing is a higher
energy (shorter wavelength) version of the
electromagnetic waves that we
see as visible light. The radiation can come from an X-
ray generator or a radioactive source.
High Electrical Potential
Electrons
-
+
X-ray Generator
or Radioactive
Source Creates
Radiation
Exposure Recording Device
Radiation
Penetrate
the Sample

Top view of developed film
X-ray film
The part is placed between the
radiation source and a piece of film.
The part will stop some of the
radiation. Thicker and more dense
area will stop more of the radiation.
= more exposure
= less exposure
The film darkness
(density) will vary with
the amount of radiation
reaching the film
through the test object.

Some of the advantages of radiographic testing include:
(1) It can be used to inspect large areas at one time.
(2) It is useful on wide variety of materials.
(3) It can be used for checking internal malstructure, misassembly or misalignment.
(4) It provides permanent record.
(5) No calibration needed on the job site.
(6) Devices for checking the quality of radiograph are available.
(7) Interpretation of radiographs can be done in comfortable conditions.
Some of the limitations of this method are:
(1) X-rays and gamma rays are hazardous to human health.
(2) It cannot detect planar defects readily.
(3) Access to both sides of the specimen is required.
(4) Thickness range that can be inspected is limited.
(5) Certain areas in many items cannot be radiographed because of the geometric consideration.
(6) Sensitivity of inspection decreases with thickness of the test specimen.
(7) It is more costly.
(8) It cannot be easily automated.
(9) It requires considerable skill for the interpretation of the radiographs.
(10) Depth of discontinuity not indicated.

High frequency sound waves are introduced into a material and they
are reflected back from surfaces or flaws.
Reflected sound energy is displayed versus time, and inspector can
visualize a cross section of the specimen showing the depth of
features that reflect sound.
f
plate
crack
0 2 4 6 8 10
initial
pulse
crack
echo
back surface
echo
Oscilloscope, or flaw detector screen
Ultrasonic Testing (UT)

Gray scale image produced using
the sound reflected from the front
surface of the coin
Gray scale image produced using the
sound reflected from the back surface
of the coin (inspected from “heads” side)
High resolution images can be produced by plotting signal
strength or time-of-flight using a computer-controlled scanning
system.

Some of the advantages of ultrasonic testing are:
(1) It has high sensitivity which permits detection of minute defects.
(2) It has high penetrating power (of the order of 6 to 7 meters in steel) which allows examination of extremely thick sections.
(3) It has a high accuracy of measurement of flaw position and size.
(4) It has fast response which permits rapid and automatic inspection.
(5) It needs access to only one surface of the specimen.
(6) It is not hazardous to operators, public or equipment.
(7) Portability and operation is electronic.
Some of the limitations of this method are:
(1) Unfavorable geometry of the test specimen causes problems during inspection.
(2) Inspection of materials having undesirable internal structure is difficult.
(3) It requires the use of a couplant.
(4) The probe must be properly coupled during scanning.
(5) Defect orientation affects defect detectability.
(6) Equipment is quite expensive.
(7) Highly skilled manpower is required.
(8) Reference standards and calibration required.
(9) Rough surfaces can be a problem and surface preparation is necessary.

Common Application of NDT
•Inspection of Raw Products
•Inspection Following Secondary
Processing
•In-Services Damage Inspection

Inspection of Raw Products
• Forgings
• Castings
• Extrusions
• etc.

• Machining
• Welding
• Grinding
• Heat treating
• Plating
• etc.
Inspection Following
Secondary Processing

• Cracking
• Corrosion
• Erosion/Wear
• Heat Damage
• etc.
Inspection For
In-Service Damage

Power Plant Inspection
Probe
Signals produced
by various
amounts of
corrosion
thinning.
Periodically, power plants are
shutdown for inspection.
Inspectors feed eddy current
probes into heat exchanger
tubes to check for corrosion
damage.
Pipe with damage

Wire Rope Inspection
Electromagnetic devices and visual
inspections are used to find broken wires and
other damage to the wire rope that is used in
chairlifts, cranes and other lifting devices.

Storage Tank Inspection
Robotic crawlers use
ultrasound to inspect the walls
of large above ground tanks for
signs of thinning due to
corrosion.
Cameras on long
articulating arms are
used to inspect
underground storage
tanks for damage.

Aircraft Inspection
• Nondestructive testing is used extensively
during the manufacturing of aircraft.
• NDT is also used to find cracks and corrosion
damage during operation of the aircraft.
• A fatigue crack that started at the site of a
lightning strike is shown below.

Jet Engine Inspection
• Aircraft engines are overhauled
after being in service for a period
of time.
• They are completely disassembled,
cleaned, inspected and then
reassembled.
• Fluorescent penetrant inspection
is used to check many of the parts
for cracking.

Sioux City, Iowa, July 19, 1989
A defect that went
undetected in an
engine disk was
responsible for
the crash of
United Flight 232.
Crash of United Flight 232

Pressure Vessel Inspection
The failure of a pressure vessel can result in the
rapid release of a large amount of energy. To
protect against this dangerous event, the tanks
are inspected using radiography and ultrasonic
testing.

Rail Inspection
Special cars are used to inspect
thousands of miles of rail to find cracks
that could lead to a derailment.

Bridge Inspection
• The US has 578,000 highway bridges.
• Corrosion, cracking and other damage can
all affect a bridge’s performance.
• The collapse of the Silver Bridge in 1967
resulted in loss of 47 lives.
• Bridges get a visual inspection about
every 2 years.
• Some bridges are fitted with acoustic
emission sensors that “listen” for sounds
of cracks growing.

NDT is used to inspect pipelines to prevent leaks
that could damage the environment. Visual
inspection, radiography and electromagnetic
testing are some of the NDT methods used.
Remote visual inspection using
a robotic crawler.
Radiography of weld joints.
Magnetic flux leakage inspection.
This device, known as a pig, is
placed in the pipeline and collects
data on the condition of the pipe as it
is pushed along by whatever is being
transported.
Pipeline Inspection

Special Measurements
Boeing employees in Philadelphia were given the privilege of
evaluating the Liberty Bell for damage using NDT techniques. Eddy
current methods were used to measure the electrical conductivity
of the Bell's bronze casing at various points to evaluate its
uniformity.

Introduction to non destructive testing (ndt)

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