Testing of Materials

1. OML751 TESTING OF MATERIALS
UNIT III – NON DESTRUCTIVE TESTING
Mr. S. Muthu Natarajan M.E., (Ph.D.),
Assistant Professor/Mechanical
Kamaraj College of Engineering and Technology (Autonomous)
Madurai District
Mobile: +91-9566470389

2. Visual inspection, Liquid penetrant test,
Magnetic particle test, Thermography test –
Principles, Techniques, Advantages and
Limitations, Applications. Radiographic test,
Eddy current test, Ultrasonic test, Acoustic
emission- Principles, Techniques, Methods,
Advantages and Limitations, Applications.
Syllabus

3.  Visual inspection is the simplest, fastest and most widely used non-
destructive testing method.
 Visual inspection is carried out with naked eye(unaided) or using
some optical aids (aided) such as mirrors, magnifying glasses and
microscopes etc.
 Visual inspection is defined as the examination of material or
component for conditions of nonconformance using light and the
eyes alone or in conjunction with various aids.
 Visual inspection often also involves shaking, listening, feeling and
sometimes even smelling the component being inspected.
Visual Inspection

4.  Visual inspection is commonly employed to support other NDT
methods
 Other NDT methods require visual intervention to interpret images
obtained while carrying out examinations. At some point, all NDT
methods fall back on visual testing.
 For Example, LPT uses dyes that rely on the inspectors ability to
visually identify surface indications.
 Digital detectors and computer technology have made it possible to
automate visual inspections. This is known as machine vision
Visual Inspection (Contd…)

5. Visual testing is commonly used
 To detect surface characteristics such as surface finish, scratches,
cracks, colour, wear and corrosion
 To check alignment of mating surfaces
 To check the evidence of leaking
 To check internal side defects
Characteristics

6. Advantages
 Simple and easy to use
 Relatively inexpensive
 Testing speed is high
 Testing can be performed on components which are in-service
 Permanent records are available when latest equipments are used
 Almost all materials can be inspected
Limitations
 Limited to detection of surface flaws
 The test results depend on skill and knowledge of tester
 Eye resolution is week
 Eye fatigue
Advantages & Disadvantages

7. Classified on the basis of use of aids used
 (i) Unaided or direct visual testing
 (ii) Aided visual testing
Unaided or direct visual testing
 As the name suggests, the unaided visual testing is carried out with
naked eye (without using any optical aids)
 The most important instrument in visual inspection is human eye
Classification of Visual Inspection

8.  Human eye is the most fascinating and valuable tool in NDT
 It has greater precision and accuracy than many of the most
sophisticated cameras. It has unique focusing capabilities and has the
ability to work in conjunction with the human brain so that it can be
trained to find specific details or characteristics in a test specimen.
 It has the ability to differentiate and distinguish between colors and
their tones/shades characteristics as well
 Human eye is capable of assessing many visual characteristics and
identifying various types of discontinuities
 The eye can perform accurate inspections to detect size, shape,
colour, depth, brightness, contrast and texture
Eye

9.  As the name suggests, the aided inspection is carried out with the
help of optical aids (such as magnifying glasses, microscopes,
borescopes, fiberscopes) and a variety of other optical imaging and
image enhancement tools.
 The optical aids are mainly used for
(i) Magnification of defects which cannot be detected by
unaided visual inspection
(ii) Assisting in the inspection of defects
(iii) areas where not easily accessible to human eye
Aided Visual Inspection

10.  The optical aids used for visual inspection are
(i) Magnifying Mirrors (small, angled mirrors)
(ii) Magnifying glasses, eye loupes, multi-lens magnifiers,
measuring magnifiers
(iii) Microscopes(optical and electron)
(iv) Boroscopes
(v) Fiberscopes and videoscopes
(vi) Telescopes
(vii) Periscopes
(viii) Optical comparators
Equipments Used

11.  When inspecting areas not easily accessible, a magnifying mirror can
be used
 Depending on the test specimen, the mirror can be of any size.
Magnifying Mirrors
Magnifying Glass
 A magnifying glass can be used for closer inspection of suspicious
looking areas
 It generally consists of a simple lens for lower power magnification
and double or multiple lenses for higher magnification

12.  Microscope is a multiple element magnifier for providing very high
magnified image of small object
 The simple microscopes consists of a convex lens. The object is
placed between lens and focus length of lens, so that an erect, virtual
and magnified image is formed. The size of the image of an object
depends upon the angle subtended at the eye by the object.(known as
visual or angle)
Microscopes

13.  Borescope are optical instrument designed for remote viewing of
objects. They are used to inspect the inside of a narrow tube, bore or
chamber.
 Borescopes is a precision optical instrument with built-in
illumination
 Borescopes, also called endoscopes or endoprobes consist of superior
optical systems and high intensity light sources.
 Some Borescopes provide magnification option, zoom controls.
 Because of the variety of applications, Borescopes are manufactured
in rigid, extended, flexible and micro designs.
Boroscopes

14.  Modern fiberscope and videoscopes, due to their small size and
flexibility, can provide access to internal areas inaccessible to rigid
borescopes.
 Using these, digital images can be captured and processed in real
time. With the aid of laser lights, the area and depth of many surface
defects can also be determined
Fiberscopes and Videoscopes
Telescopes
 Telescope is an instrument that collects radiation from a distant
object in order to produce an image of it.
 An optical telescope uses visual radiations
 The telescopes are used for providing visual examination of the
inaccessible surfaces

15.  Optical comparators are the magnifying devices for visual
examination and measurement.
 A comparator produces 2D enlarged image of an object on a large
ground-glass screen.
 Optical comparators project the image of small parts onto a large
projection screen. The magnified image is then compared against an
optical comparator chart, which is a magnified outline drawing of the
workpiece being gauged
Optical Comparator

16.  Inspection of cleaning in machines
 Checking for corrosion, erosion and deformities of machine
components
 Checking for ruptures, cracks and wear of parts in the equipment.
 Monitoring of manometers, pressure and temperatures
 Monitoring of oil level, greasing and greasing apparatus.
 Monitoring of the operational conditions of systems or machines.
 Visual Testing of welds, Pumps, Hydraulic systems, Belt Pulley,
Forging Discontinuities
Applications of Visual Inspection

17. Magnifying Mirrors

18. Magnifying Glass

19. Microscopes

20. Boroscopes

21. Fiberscopes

22. Telescopes

23. Optical Comparator

24.  This method is used to reveal surface discontinuities by bleed-out of
a colored or fluorescent dye from the flaw.
 The technique is based on the ability of a liquid to be drawn into a
"clean" surface discontinuity by capillary action
 After a period of time called the "dwell time", excess surface
penetrant is removed and a developer applied. This acts as a blotter
that draws the penetrant from the discontinuity to reveal its
presence.
Liquid Penetrant Testing

25.  Liquid penetrant testing is one of the most widely used NDT
methods. Its popularity can be attributed to two main factors: its
relative ease of use and its flexibility.
 It can be used to inspect almost any material provided that its surface
is not extremely rough or porous.
 Materials that are commonly inspected using this method include;
metals, glass, many ceramic materials, rubber and plastics
 However, liquid penetrant testing can only be used to inspect for
flaws that break the surface of the sample (such as surface cracks,
porosity, laps, seams, lack of fusion, etc.).

26. Steps of Liquid Penetrant Testing
 The exact procedure for liquid penetrant testing can vary from case
to case depending on several factors such as,
 the penetrant system being used,
 the size and material of the component being inspected,
 the type of discontinuities being expected in the component
 the condition and environment under which the inspection is
performed.
General Steps Followed in Liquid Penetrant Testing Method,
1. Surface Preparation 2. Penetrant Application
3. Penetrant Dwell 4. Excess Penetrant Removal
5. Developer Application 6. Indication Development
7. Inspection 8. Clean Surface

27. Surface Preparation
 One of the most critical steps of a liquid penetrant testing is the
surface preparation.
 The surface must be free of oil, grease, water, or other contaminants
that may prevent penetrant from entering flaws.
 The sample may also require etching if mechanical operations such
as machining, sanding, or grit blasting have been performed.
 These and other mechanical operations can smear metal over the
flaw opening and prevent the penetrant from entering.
Penetrant Application
 Once the surface has been thoroughly cleaned and dried, the
penetrant material is applied by spraying, brushing, or immersing
the part in a penetrant bath.

28. Penetrant Dwell
 The penetrant is left on the surface for a sufficient time to allow as
much penetrant as possible to be drawn or to seep into a defect.
 Penetrant dwell time is the total time that the penetrant is in contact
with the part surface. Dwell times are usually recommended by the
penetrant producers or required by the specification being followed.
 The times vary depending on the application, penetrant materials
used, the material, the form of the material being inspected, and the
type of discontinuity being inspected for.
 Minimum dwell times typically range from 5 to 60 minutes.
Generally, there is no harm in using a longer penetrant dwell time as
long as the penetrant is not allowed to dry.

29. Excess Penetrant Removal
 This is the most delicate step of the inspection procedure because
the excess penetrant must be removed from the surface of the
sample while removing as little penetrant as possible from defects.
 Depending on the penetrant system used, this step may involve
cleaning with a solvent, direct rinsing with water, or first treating the
part with an emulsifier and then rinsing with water.

30. Developer Application
 A thin layer of developer is then applied to the sample to draw
penetrant trapped in flaws back to the surface where it will be
visible. Developers come in a variety of forms that may be applied
by dusting (dry powders), dipping, or spraying (wet developers).
Indication Development
 The developer is allowed to stand on the part surface for a period of
time sufficient to permit the extraction of the trapped penetrant out
of any surface flaws.
 This development time is usually a minimum of 10 minutes.
Significantly longer times may be necessary for tight cracks.

31. Inspection
 Inspection is then performed under appropriate lighting to detect
indications from any flaws which may be present.
Clean Surface
 The final step in the process is to thoroughly clean the part surface
to remove the developer from the parts that were found to be
acceptable.

32. Advantages & Disadvantages
PROS CONS
High sensitivity (small discontinuities can
be detected).
Only surface breaking defects can be
detected.
Few material limitations (metallic and
nonmetallic, magnetic and nonmagnetic,
and conductive and nonconductive
materials may be inspected)
Only materials with a relatively non-
porous surface can be inspected
Rapid inspection of large areas and
volumes
Pre-cleaning is critical since contaminants
can mask defects
Suitable for parts with complex shapes
Metal smearing from machining,
grinding, and grit or vapor blasting must
be removed
Indications are produced directly on the
surface of the part and constitute a visual
representation of the flaw
The inspector must have direct access to
the surface being inspected
Portable (materials are available in
aerosol spray cans)
Surface finish and roughness can affect
inspection sensitivity
Low cost (materials and associated
equipment are relatively inexpensive)
Multiple process operations must be
performed and controlled

33. Penetrants are carefully formulated to produce the level of sensitivity
desired by the inspector. The penetrant must possess a number of
important characteristics,
Spread easily over the surface of the material being inspected to
provide complete and even coverage
Be drawn into surface breaking defects by capillary action.
Remain in the defect but remove easily from the surface of the part.
Remain fluid so it can be drawn back to the surface of the part
through the drying and developing steps.
Be highly visible or fluoresce brightly to produce easy to see
indications.
Not be harmful to the material being tested or the inspector.
Penetrants

34. Types of Penetrants
Penetrant materials come in two basic types:
Type 1 - Fluorescent Penetrants
They contain a dye or several dyes that fluoresce when exposed
to ultraviolet radiation
Type 2 - Visible Penetrants
They contain a red dye that provides high contrast against the
white developer background
Fluorescent penetrant systems are more sensitive than visible
penetrant systems because the eye is drawn to the glow of the
fluorescing indication. However, visible penetrants do not require a
darkened area and an ultraviolet light in order to make an inspection.

35. Penetrants are then classified by the method used to remove the excess
penetrant from the part. The four methods are:
Method A - Water Washable:
Penetrants can be removed from the part by rinsing with water
alone. These penetrants contain an emulsifying agent (detergent) that
makes it possible to wash the penetrant from the part surface with water
alone. Water washable penetrants are sometimes referred to as self-
emulsifying systems.
Method B – Post - Emulsifiable, Lipophilic:
The penetrant is oil soluble and interacts with the oil-based
emulsifier to make removal possible.
Types of Penetrants (Contd…)

36. Method C - Solvent Removable:
They require the use of a solvent to remove the penetrant from
the part.
Method D - Post-Emulsifiable, Hydrophilic:
They use an emulsifier that is a water soluble detergent which
lifts the excess penetrant from the surface of the part with a water
wash.

37. Penetrants are then classified based on the strength or detectability of
the indication that is produced for a number of very small and tight
fatigue cracks. The five sensitivity levels are:
Level ½ - Ultra Low Sensitivity
Level 1 - Low Sensitivity
Level 2 - Medium Sensitivity
Level 3 - High Sensitivity
Level 4 - Ultra-High Sensitivity
The procedure for classifying penetrants into one of the five
sensitivity levels uses specimens with small surface fatigue cracks. The
brightness of the indication produced is measured using a photometer.
Types of Penetrants (Contd…)

38.  The role of the developer is to pull the trapped penetrant material
out of defects and spread it out on the surface of the part so it can
be seen by an inspector.
 Developers used with visible penetrants create a white background
so there is a greater degree of contrast between the indication and
the surrounding background.
 On the other hand, developers used with fluorescent penetrants both
reflect and refract the incident ultraviolet light, allowing more of it
to interact with the penetrant, causing more efficient fluorescence.
Developers

39.  This test is used to detect the hot crack which can happen during
solidification process of deposited weld metal, and it might happen
in weld metal or in weld heat affected zone. The surface lack of
fusion also can be identified by this test.
 The surface porosity is a common surface defect that can be found
visually and more accurately by dye penetration test. The
acceptance criteria for the liquid penetrant test for welding have
been addressed on the ASME Code Section VIII Div 1 Mandatory
appendix 8.
LPT Application on Welding

40.  The casting surface porosity, surface shrinkage, hot tear and cold
shut can be detected by liquid penetrant inspection. The acceptance
criteria have been addressed in ASME Section VIII Div 1.
Mandatory Appendix 7.
LPT Application on Casting
LPT Application on Forging
 The forging surface detects are Laps and Bursts which both can
easily be identified by performing a liquid penetrant test.

41. Magnetic Particle Testing - Introduction
1 • Ferromagnetic Materials
2 • Diamagnetic Materials
3 • Paramagnetic Materials
Types of Magnetic Materials
 Those materials which can be strongly magnetized and are suitable
for MPT (Magnetic Particle Testing).
 Example: Iron, Nickel, Cobalt alloys, etc.
 These materials are not magnetized in direct proportion to the
applied magnetizing force.
 Beyond the saturation point, part cannot be further magnetized.
Ferromagnetic Materials

42. Diamagnetic Materials
Paramagnetic Materials
 Those materials which are feebly repelled by strong magnet.
 In diamagnetic materials all the electrons are paired so there is no
permanent net magnetic moment per atom.
 Example: Most elements in the periodic table, including copper,
silver, and gold, are diamagnetic.
 Those materials which can be magnetized but weakly.
 Example: Oxygen, magnesium, molybdenum, and lithium.

43. Methods of Generating Magnetic Fields
1
• Direct Magnetization
2
• Indirect Magnetization
Direct Magnetization
 With direct magnetization, current is passed directly through the
component. The flow of current causes a circular magnetic field to
form in and around the conductor.
 When using the direct magnetization method, care must be taken to
ensure that good electrical contact is established and maintained
between the test equipment and the test component to avoid damage
of the component.

44. Indirect Magnetization
 Indirect magnetization is accomplished by using a strong external
magnetic field to establish a magnetic field within the component.
Demagnetization
 After conducting a magnetic particle inspection, it is usually
necessary to demagnetize the component.
Removal of a field may be accomplished in several ways
 By heating the material above its curie temperature (for instance, the
curie temperature for a low carbon steel is 770°C).
 When steel is heated above its curie temperature then it is cooled
back down, the orientation of the magnetic domains of the individual
grains will become randomized again and thus the component will
contain no residual magnetic field.

45. Magnetic Particle Testing
 Magnetic particle testing is one of the most widely utilized NDT
methods since it is fast and relatively easy to apply and part surface
preparation is not as critical as it is for some other methods.
 This method uses magnetic fields and small magnetic particles (i.e.
iron) to detect flaws in components.
 The component being inspected must be made of a ferromagnetic
material (a materials that can be magnetized) such as iron, nickel,
cobalt, or some of their alloys.
 Underwater inspection is another area where magnetic particle
inspection may be used to test items such as offshore structures and
underwater pipelines

46. Basic Principle
 It can be considered as a combination of two nondestructive testing
methods: magnetic flux leakage testing and visual testing.
 For the case of a bar magnet, the magnetic field is in and around the
magnet. Any place that a magnetic line of force exits or enters the
magnet is called a “pole” (magnetic lines of force exit the magnet
from north pole and enter from the south pole).
 When a bar magnet is broken in the center of its length, two
complete bar magnets with magnetic poles on each end of each piece
will result.
 If the magnet is just cracked but not broken completely in two, a
north and south pole will form at each edge of the crack

47.  The magnetic field exits the north pole and reenters at the south pole.
The magnetic field spreads out when it encounters the small air gap
created by the crack because the air cannot support as much magnetic
field per unit volume as the magnet can.
 When the field spreads out, it appears to leak out of the material and,
thus is called a flux leakage field.
 If iron particles are sprinkled on a cracked magnet, the particles will
be attracted to and cluster not only at the poles at the ends of the
magnet, but also at the poles at the edges of the crack. This cluster of
particles is much easier to see than the actual crack and this is the
basis for magnetic particle inspection

49. General Steps Followed in Magnetic Particle Testing Method,
Surface Preparation
Initial Demagnetization
Magnetization
Application of Magnetic Particles
Viewing
Marking of Defects
Demagnetization
Removal of Ink from Components

50. Surface Preparation
 Loose rust & scale removed from part in order to prevent the
contamination of ink.
 The paint should be removed locally to provide adequate contact
areas for current flow.
Initial Demagnetization
 Components which have been machined on magnetic chucks or
handled near the magnetic field , has some residual magnetisms
 So for avoiding false indication always demagnetize the component
before testing.
Magnetization
 Direct or indirect method of magnetization can be used

51. Application of Magnetic Particles
 The metal used for the particles has high magnetic permeability and
low retentivity.
 High magnetic permeability is important because it makes the
particles attract easily to small magnetic leakage fields from
discontinuities, such as flaws.
 Low retentivity is important because the particles themselves never
become strongly magnetized so they do not stick to each other or the
surface of the part
On the basis of carrying agent, it is of two types,
1
• Wet Method
2
• Dry Method

52. Wet Method
 The particles used are suspended in oil or liquid and obtained in
form of thick paste or powder.
 Generally more sensitive than the dry because the suspension
provides the particles with more mobility and makes it possible for
smaller particles to be used (the particles are typically 10 μm and
 smaller) since dust and adherence to surface contamination is
reduced or eliminated.

53. Dry Method
 Dry magnetic particle products are produced to include a range of
particle sizes.
 The fine particles have a diameter of about 50 μm while the course
particles have a diameter of 150 μm (fine particles are more than 20
times lighter than the coarse particles).
 Dry particle inspection is well suited for the inspections conducted
on rough surfaces.

54. Viewing
Magnetic Particles Source of Illumination
Black or Red Paste or Powder Day Light
Fluroscent Paste or Powders Black Light
Marking of Defects
 All indication should be marked after allowing the magnetic
particles (ink) to drain.
Demagnetization
 Demagnetization can be done by heating the component above its
curie temperature
Removal of Ink from the components
 Manual Removal by cleaning the surface of the component