Chapter 08 - Non Destructive Test | Basic of NDT and NDT Methods

Question 1

Definition of NDT

Answer 1

The use of non-invasive
techniques to:
– determine the integrity of
a material, component or
structure
– quantitatively measure
some characteristic of an
object.

Inspect or measure
without doing harm

Question 2

What are Some Uses of

NDE Methods?

Answer 2

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

Question 3

Why Nondestructive?

Answer 3

Test piece too precious to be destroyed
• Test piece to be reuse after inspection
• Test piece is in service
• For quality control purpose
• Something you simply cannot do harm to

Question 4

Major types of NDT

Answer 4

Detection of surface flaws
– Visual
– Magnetic Particle Inspection
– Fluorescent Dye Penetrant Inspection
• Detection of internal flaws
– Radiography
– Ultrasonic Testing
– Eddy current Testing

Question 5

What is Visual Inspection and the tools used?

Answer 5

• Most basic and common
inspection method.
• Tools include fiberscopes,
borescopes, magnifying glasses and mirrors.
• Portable video inspection unit with zoom allows inspection of
large tanks and vessels,
railroad tank cars, sewer lines.
• Robots allow inspection of
hazardous or tight areas, such as air ducts, reactors, pipelines.

Question 6

What two main factors of Dye Penetrant Inspection can be attributed to its popularity

Answer 6

– relative ease

– flexibility

Question 7

On what kind of surface can LPI be used and what can of materials are commonly inspected

Answer 7

• LPI can be used to inspect almost any material provided
that its surface is not extremely rough or porous
• Materials that are commonly inspected using LPI include
metals (aluminium, copper, steel, titanium, etc.), glass,
many ceramic materials, rubber, and plastics.

Question 8

Procedure of Dye Penetrant Inspection

Answer 8

• 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.

Question 9

Penetrant Types

Answer 9

Dye penetrants

Fluorescent penetrants

Question 10

Why are Dye penetrants used?

Answer 10

– The liquids are colored so that they provide
good contrast against the developer
– Usually red liquid against white developer
– Observation performed in ordinary daylight
or good indoor illumination

Question 11

Why are Fluorescent penetrants used?

Answer 11

– Liquid contain additives to give
fluorescence under UV
– Object should be shielded from visible
light during inspection
– Fluorescent indications are easy to see in
the dark

Question 12

Dye Penetrant Inspections Primary Advantages

Answer 12

• The method has high sensitive to small surface discontinuities.
• The method has few material limitations, i.e. metallic and nonmetallic,
magnetic
• Large areas and large volumes of parts/materials can be inspected
rapidly and at low cost.
• Parts with complex geometric shapes are routinely inspected.
• Indications are produced directly on the surface of the part and
constitute a visual representation of the flaw.
• Aerosol spray cans make penetrant materials very portable.

Question 13

Dye Penetrant Inspections

Primary Disadvantages

Answer 13

• Only surface breaking defects can be detected.
• Only materials with a relative nonporous surface can be inspected.
• Precleaning is critical as contaminants can mask defects.
• Metal smearing from machining, grinding, and grit or vapor blasting
must be removed prior to LPI.
• The inspector must have direct access to the surface being inspected.
• Surface finish and roughness can affect inspection sensitivity.
• Multiple process operations must be performed and controlled.
• Post cleaning of acceptable parts or materials is required.
• Chemical handling and proper disposal is required.

Question 14

Why is Magnetic Particle Inspection (MPI) used?

Answer 14

• Fast and relatively easy to apply and part surface preparation
is not as critical as for some other NDT methods. – MPI one of
the most widely utilized nondestructive testing methods.

Question 15

How is MPI used and what requirement is needed to it to be used?

Answer 15

• MPI uses magnetic fields and small magnetic particles, such
as iron filings to detect flaws in components.
• The only requirement from an inspectability standpoint is that
the component being inspected must be made of a
ferromagnetic material such as iron, nickel, cobalt, or some of
their alloys.
• The method is used to inspect a variety of product forms such
as castings, forgings, and weldments.

Question 16

(MPI) - Explain the ways you can magnetizing an object

Answer 16

Direct or Indirect

Question 17

Explain Direct magnetizing

Answer 17

Direct magnetization: current is passed directly

through the component.

Question 18

Explain Indirect magnetizing

Answer 18

Indirect magnetization: using a strong external
magnetic field to establish a magnetic field within the
component

Question 19

Advantages of MPI

Answer 19

• One of the most dependable and sensitive methods for
surface defects.
• fast, simple and inexpensive
• direct, visible indication on surface
• unaffected by possible deposits, e.g. oil, grease or other
metals chips, in the cracks
• can be used on painted objects
• surface preparation not required
• results readily documented with photo or tape impression

Question 20

Disadvantages of MPI

Answer 20

• Only good for ferromagnetic materials
• sub-surface defects will not always be indicated
• relative direction between the magnetic field and the
defect line is important
• objects must be demagnetized before and after the
examination
• the current magnetization may cause burn scars on the item examined

Question 21

How is Radiography used?

Answer 21

Radiography involves the use of penetrating
gamma- or X-radiation to examine material’s
and product’s defects and internal features.

Question 22

In Radiography what will the shadowgraph show?

Answer 22

• The resulting shadowgraph shows the internal features and soundness of the part.

-Material thickness and density changes are
indicated as lighter or darker areas on the
film. The darker areas in the radiograph below
represent internal voids in the component.

Question 23

Explain absorption of x-ray in Radiography

Answer 23

• All x-rays are absorbed to some extent in passing through matter
due to electron ejection or scattering.

Question 24

Limitations of Radiography

Answer 24

There is an upper limit of thickness through which the
radiation can penetrate, e.g. γ-ray from Co-60 can
penetrate up to 150mm of steel
• The operator must have access to both sides of an
object
• Highly skilled operator is required because of the
potential health hazard of the energetic radiations
• Relative expensive equipment

Question 25

What is Ultrasonic Testing?

Answer 25

• High-frequency sound waves are transmitted
into a material to detect imperfections or to
locate changes in material properties

Question 26

What is the equation for absorption of x ray

Answer 26

• The absorption follows the equation
I = I0e^(-μx)=I = I0e-^(μ/p)px

Where I is the transmitted intensity;
x is the thickness of the matter;
μ is the linear absorption coefficient (element dependent);
p is the density of the matter;
(μ/p) is the mass absorption coefficient (cm2
/gm)

Question 27

What is the most commonly used ultrasonic (US) testing technique

Answer 27

Echo method
– sound is introduced into a test object
– reflections (echoes) from internal imperfections or
part geometry are returned to receiver
– time interval between transmission and reception of
pulses provide information about the internal
material structure

Question 28

What is Pulse Echo (in US)

Answer 28

Ultrasonic Inspection (Pulse-Echo)
• 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.

Question 29

What is the equation of Pulse Echo

Answer 29

• Measurement of time required for wave to
– travel from transducer through thickness of material
– reflect from back or surface of discontinuity and
return to transducer
• Required time is in microsecond range
𝑑 =𝑣𝑡/2
or
𝑣 =2𝑑/𝑡
d: distance from surface to discontinuity
v: velocity of sound waves in material
t: measured transit time (in and out)

Question 30

What is Longitudinal waves

Answer 30

– Similar to audible sound waves
– the only type of wave which can
travel through liquid

Question 31

What is Shear waves

Answer 31

– generated by passing the ultrasonic
beam through the material at an
angle
– Usually a plastic wedge is used to
couple the transducer to the
material

Question 32

Angle Beam Transducers

Answer 32

Introduction of refracted shear
wave into test material
• angled sound path allows sound
wave to come in from the side
– improvement of detectability of
flaws in and around welded areas

Question 33

What can Angle Beam Transducers be used to analysed

Answer 33

– flat sheets
– plates
– pipes and tubing

Question 34

Eddy currents

Answer 34

• electrical currents which are generated in
conductive materials by inducing an alternating
magnetic field
• current flows in the surface of the material and
its close proximity

Question 35

What can changes in the material can interrupt the flow of eddy currents?

Answer 35

– imperfections
– dimensional changes
– changes of conductivity and permeability

Question 36

What can Eddy Current Testing be used on

Answer 36

Eddy current testing can be used on all
electrically conducting materials with a
reasonably smooth surface.

Question 37

What can Eddy Current Testing be used for

Answer 37

Used for crack detection, material
thickness measurement (corrosion
detection), sorting materials, coating
thickness measurement, metal detection,
etc.

Question 38

Principle of Eddy Current Testing

Answer 38

• The strength of the secondary field
depends on electrical and magnetic
properties, structural integrity, etc.,
of the test object
• If cracks or other inhomogeneities
are present, the eddy current, and
hence the secondary field is
affected.
• The changes in the secondary
field will be a ‘feedback’ to the
primary coil and affect the
primary current.
• The variations of the primary
current can be easily detected by
a simple circuit which is zeroed
properly beforehand

Question 39

Eddy Current Testing; Depth of penetration

Answer 39

Eddy currents concentrate near the
surface adjacent to an excitation coil
The depth at which the eddy current
density has decreased to 1/e, or approx. 37% of the surface density, is
called the standard depth of
penetration 𝛿.

Question 40

Eddy Current Testing; Probe types

Answer 40

• Three major types
– surface probe
– internal bobbin probe
– encircling probe

Question 41

Eddy Current Testing; Applications

Answer 41

• Crack Detection
• Material Thickness Measurements
• Coating Thickness Measurements

Question 42

Eddy Current Testing; Applications - Conductivity measurements for

Answer 42

Material Identification
– Heat Damage Detection
– Case Depth Determination
– Heat Treatment Monitoring

Question 43

Advantages of Eddy Current testing

Answer 43

• Sensitive to small cracks and other defects
• Detects surface and near surface defects
• Inspection gives immediate results
• Equipment is very portable
• Method can be used for much more than flaw detection
• Minimum part preparation is required
• Test probe does not need to contact the part
• Inspects complex shapes and sizes of conductive materials

Question 44

Limitations of Eddy Current testing

Answer 44

• Only conductive materials can be inspected
• Surface must be accessible to the probe
• Skill and training required is more extensive than other techniques
• Surface finish and and roughness may interfere
• Reference standards needed for setup
• Depth of penetration is limited
• Flaws such as delaminations that lie parallel to the probe coil winding and
probe scan direction are undetectable