Chapter 4 Non-Destructive Testing Flashcards
(86 cards)
1
Q
Briefly describe the basic steps in dye penetrant inspection using the post-emulsified system.
A
2
Q
Suggest 2 suitable magnetic particle inspection techniques to test the longitudinal & transverse cracks on a steel bar. (Give reasons to your choices)
A
3
Q
Compare the liquid dye penetrant test & the magnetic particle test with regard to the advantages and limitations.
A
4
Q
Briefly explain how ultrasound is generated. Give 2 examples of the crystals used to generate ultrasound.
A
5
Q
Compare & contrast the pulse-echo technique & through transmission technique of ultrasonic testing.
A
6
Q
What is “radiography sensitivity”? Name 2 types of IQI that are used to quantify the sensitivity of a radiography.
A
7
Q
Define the term “definition” used in radiography. What are the 2 factors that affect definition.
A
8
Q
How can the radiography definition be improved?
A
9
Q
Name the applications of eddy current testing. What are the types of test coils available? When are they used?
A
10
Q
What is the definition of Non-destructive testing (NDT)?
A
NDT is any method for testing of materials and components that will not
1. cause the destruction of the test piece or component, or
2. impair its designed use.
11
Q
Which area is NDT primarily used in?
A
Detection of faults in materials.
12
Q
What is the purpose of NDT?
A
To determine the existing state or quality of a material, with a view of acceptance / rejection.
13
Q
What are some examples of external faults?
A
- surface finish
- machining marks
- hardened cracks
- blow holes in casting
- surface inclusions.
14
Q
What are some examples of internal faults?
A
- overlaps
- fish-tails
- piping
- micro-segregation
- hydrogen embrittlement
15
Q
How many test methods are covered in this chapter? What are they?
A
7
1. Visual Testing
2. Hydrostatic Testing
3. Dye Penetrant Testing
4. Magnetic Particle Testing
5. X- & Gamma Ray Radiography
6. Ultrasonic Testing
7. Eddy Current Testing
16
Q
What is the most common form of examination?
A
Visual Testing
17
Q
What does Visual Testing consist of?
A
- Viewing the area with the eye,
- aided by magnifying glass,
- borescope
- light source
18
Q
What is a quick and cheap method of detecting various types of cracks?
A
Visual Testing
19
Q
What does the reliability of visual testing depend on?
A
Ability & experience of the inspector
20
Q
What does optical inspection probes permit?
A
Visual inspection of limited access areas such as
1. ducts
2. cavities
3. inside pipes
21
Q
How many types of optical inspection probes are there? What are they?
A
- Rigid and flexible
22
Q
What does a rigid probe consist of?
A
An optical system with a viewing eyepiece at one end.
23
Q
What is the range of sizes of the diameter of rigid probes?
A
2mm to 20mm
24
Q
How would the usage of flexible probes help with inspection?
A
Using flexible probes increase the
1. usefulness &
2. versatility of inspection.
25
What does the flexible probe incorporate?
Flexible probes incorporate a fibre optic image guide & a separate fibre light guide for illumination, both contain within a flexible sheathing.
26
What is the range of sizes of the diameter of the flexible probe?
4mm to 15mm
27
In pressure & leak testing, how are defects revealed?
Defects are revealed by the flow of gas or liquid into or through the defects.
28
What is an example of pressure & leak testing?
Finding leaks in an inner tube of a tyre.
1. Fill the tyre with gas at higher pressure than the surrounding air.
2. Immerse the tyres in water
3. Locate the leaks by the source of bubbles formation.
29
What is hydrostatic testing often used for?
Hydrostatic testing is often used to check for leakage in a
1. welded pressure vessels,
2. piping or
3. valve and
4. fitting section that can be sealed at their own ends.
30
Briefly describe hydrostatic testing.
1. seal the ends of a welded pressure vessel, piping or valve & fitting section
2. apply water pressure of 1.5 to 2 times the working press to these containers
3. leakage is then detected by water / gas seepage OR by changes in pressure of the liquid or gas used.
*** ONLY large defects such as centre-line cracks in welds & pinholes are generally revealed.
31
What gives a sensitive leak test?
Helium, as it is an inert gas with light mass, used with a helium leak detector.
32
What is penetrant inspection used for?
Penetrant inspection is used to detect small cracks / discontinuities which cannot be found by normal visual inspection.
33
What does dye penetrant testing depend on? And how does it affect the process?
Dye penetrant testing depends on the ability of a highly penetrating liquid to seep into any discontinuities in the material to which it is applied.
Therefore, the liquid penetrant
process can only be used to detect defects which are open to the surface of
a homogeneous material.
34
What is the principal advantage of Dye Penetrant Testing?
Dye Penetrant Testing is equally suitable on all homogeneous materials, e.g. metals, plastics,
glass, ceramic. It is not affected by defect orientation and may be applied
without sophisticated or expensive equipment.
35
What are visible penetrants?
Visible penetrants are those which contain a very bright dye, usually red, that is viewed under bright white light.
36
What are fluorescent penetrants?
Fluorescent penetrants contain a dye which fluoresces (glows) brilliantly under filtered
ultra-violet light (often called black light).
37
What are the 3 methods of penetrant removal?
1. Water-washable
2. Post-emulsified
3. Solvent-removable
38
What are water-washable penetrants?
Water-washable penetrants employ a penetrant which is soluble in water. It is the least sensitive of the three methods of penetrant removal.
39
What are post-emulsified penetrants?
Post-emulsified penetrants employ a penetrant which is not soluble in water. It utilizes an oil-base penetrant and as the penetrant does not contain an emulsifier, it has the ability to seep into very fine surface defects. The surface film of penetrant is made water soluble by the application of an emulsifier in a separate operation.
40
What is a solvent-removable penetrant?
The penetrants employed in these processes are neither water soluble nor readily emulsified. They may only be removed satisfactorily by means of a suitable solvent.
41
What are the basic steps in penetrant inspection using the water-washable system?
1. Cleaning the test surface
2. Applying the penetrant
[Fluorescent / dye penetrant seeps into discontinuity]
3. Removing the excess penetrant
[Water spray removes surface penetrant]
4. Applying the developer
[Developer is applied, drawing penetrant to surface]
5. Inspection & Interpretation
[Indication is viewed under visible light / black light (UV rays) if fluorescent penetrant is used]
42
How many precautions are there for dye penetrant testing? What are they?
4.
1. Keep the penetrants & solvents off clothes & skin. These chemical can cause skin irritation.
2. Provide adequate ventilation at all times. Vapours arising from the penetrant / solvent may be toxic.
3. Most materials used in the portable, visible penetrant process are highly flammable.
4. Avoid looking directly at the black-light source. The liquid in the eyeball glows in black light and causes the viewer's sight to become hazy.
43
What are magnetic particle/flaw detection used for?
To detect discontinuities at / near the surface in ferro-magnetic materials.
44
How does magnetic particle/flaw detection detect discontinuities at / near the surface in ferro-magnetic materials?
By applying finely divided ferro-magnetic particles to the surface, the presence of a leakage field, and therefore the presence of discontinuities can be detected.
45
How many methods are there to generate magnetic field? What are they?
2 main ones. with 2 sub methods under each main method.
1. Longitudinal Magnetisation
- Permanent Magnets / Yoke-type Electo-
magnets
- Coil (Solenoid)
2. Circular Magnetisation
- Direct Induction
- Prod Contacts
46
How is magnetic field generated by longitudinal magnetisation using permanent magnets or yoke-type electro-magnets?
When poles of a horse-show magnet / magnetic yoke are placed upon a steel surface, a magnetic flux is set up in the steel from one pole of the magnet to the other. Cracks lying between 90 degrees and 45 degrees to the flux path can be detected if the flux strength is sufficient.
47
How is magnetic field generated by longitudinal magnetisation using coil (Solenoid) ?
It is achieved by fixed current-carrying coils, or portable current-carrying cable coils which encircle the part and induce a longitudinal field.
48
How is magnetic field generated by circular magnetisation using direct induction ?
It is achieved by applying the electrical contacts at each end, or each side
of the area being inspected and allowing the current to flow through
directly.
49
How is magnetic field generated by circular magnetisation using prod contacts ?
Where the part is too large to apply end contacts, the part can be magnetised with prod contacts.
50
What are the factors affecting current requirement?
1. Permeability of the metal
2. Shape of the test piece
3. Thickness of the test piece
4. Type of discontinuity sought
51
What are types of magnet particles for use in the magnetic inspection?
1. Wet Particles
2. Dry Particles
52
What are wet particles?
1. Wet particles consist of a suspension of magnetic particles in a light petroleum
distillate or light oil.
2. Wet particles are coloured, to give good contrast with the surface being inspected, OR have fluorescent coating for viewing with a
black-light source.
3. Wet particles are applied by dipping, immersing or spraying the part.
53
What are dry particles?
1. Dry particles are in powder form and can be obtained in a variety of colours.
2. They are applied using hand shakers, spray bulbs, shaking screens or an air stream.
54
After magnetizing force has been removed, will the magnetism be still present in ferrous materials?
Yes. Ferrous materials usually retain some residual magnetism after the magnetizing force has been removed.
55
What are the effects of residual magnetism?
1. Adverse effects on instruments, eg. aircraft compass.
2. Interference with future machining operations by causing chips to adhere to cutting tools.
3. Adherence of magnetic particles to moving parts thereby accelerating wear.
4. Induction of stray voltages in adjacent circuits.
56
How is demagnetism achieved?
Demagnetism is achieved by subjecting the part to an alternating (reversing), diminishing magnetic field.
57
What is the definition of radiography?
The formation of images on fluorescent screens or photographic material by short wave length radiation such as X-rays and Gamma (γ) rays.
58
What are X-rays?
1. A form of electromagnetic radiation,
2. having very short wave lengths of the order of 10 Å to 0.01 Å, and are
3. produced by an X-ray tube where
4. high energy electrons emitted from a filament on a cathode are
5. accelerated by a high voltage to
6. strike a target on the anode.
*** Only about 1% of the total energy of the electrons is converted to radiation energy of the X-ray beam.
59
What are Gamma (γ) rays emitted by?
Gamma (γ) rays are emitted by the decomposition of radioactive substances such as radium or radioactive isotopes.
60
What does the penetrating power of the gamma rays depend on?
The radioactive source used.
61
What is the wave length of Gamma rays?
Gamma rays usually have shorter wave lengths (0.1 to 0.005 Å) than X-rays (10 Å to 0.01 Å).
62
What are some commonly used radioisotopes for gamma radiography?
1. Cobalt 60
2. Caesium 137
3. Iridium 192
63
What is 'film density'?
It is the degree on film darkness.
64
What does density D determined by?
The amount of light transmitted through the film.
65
What is the formula for film density D?
D = log10 (Io / It)
Io --> incident light intensity
It --> intensity of the transmitted light
e.g. film density of 2.0 transmits 1% of the incident light
66
What does 'definition' mean in radiography?
The sharpness & clearness of the outline of an image.
67
What are the factors affecting definition?
1. Film Unsharpness (Uf)
2. Geometric Unsharpness (Ug)
68
What does film unsharpness depend on?
The film & radiation energy.
69
What is geometric unsharpness affected by?
The relative positions of the specimen & film & by the size of the focal spot.
70
What is the formula for Ug?
Ug = (FF' x t ) / d
71
How can Ug be controlled?
By following these rules:
1. Source size, FF', should be as small as possible.
2. The source-to-specimen distance, d, should be as large as possible.
3. The specimen-to-film distance, t, should be minimum.
4. The radiation rays should be directed perpendicular to the film surface.
5. The film and specimen should lie in parallel planes.
72
What is radiographic sensitivity?
The measure of the smallest discontinuity that can be detected on that particular radiograph.
73
What is the formula for Sensitivity?
Sensitivity % = ( Thinnest diameter observed on radiograph / Thickness of metal under test ) x 100%
74
What are the Image Quality Indicators (IQI) quantified by use of IQI British Standards BS 3971?
1. Wire IQI
2. Step Hole IQI
75
What does Wire IQI consist of?
A set of wires of different diameters of the same material as the specimen being examined.
76
What is the criterion of sensitivity of the Wire IQI?
The diameter of the thinnest detectable wire.
77
How does the Step Hole IQI work?
The hole diameter is equal to the step thickness. The sensitivity is specified
in terms of the smallest hole which can be detected on the radiograph.
78
What does Ultrasonic flaw detection (Ultrasonic Testing) use?
Ultrasonic flaw detection (Ultrasonic Testing) uses ultrasound, that is, sound wave above the audio frequency of 20 kHz.
79
How does ultrasound waves differ from X-rays?
1. The intensity of ultrasound waves is practically undiminished after travelling through several meters of material.
2. Ultrasonic energy cannot pass from metal to air but X-rays pass from metal to air without any loss of energy.
3. Almost all the ultrasonic energy is reflected at a boundary surface. *** what about X-ray?
80
How is ultrasound used in flaw detection normally generated?
By means of piezo-electric materials.
81
What is a piezo-electric material?
It is a material which converts
electrical energy to mechanical energy and vice-versa.
82
What are some common piezo-electric material?
1. Quartz Crystal
2. Barium Titanate
3. Lithium Sulphate.
83
What are the 2 basic construction of ultrasonic probes?
1. Normal Probe
2. Angle Probe
84
What are the techniques of Ultrasonic testing?
1. Pulse-Echo
2. Through Transmission
85
How does Pulse-Echo work?
1. A pulse of ultrasound travels through the specimen until it is reflected
back to the transducer (probe) from an interface, which can be a defect or
the back wall.
2. The reflected sound will produce an amplitude (y-axis) which is displayed on the cathode ray oscilloscope.
3. Since the velocity of the ultrasound is known, the time in terms of distance travelled can be registered along the x-axis.
86
How does "Through Transmission" work?
1. Through transmission ultrasonic flaw detection employs the SAME basic
equipment as pulse-echo
2. BUT uses TWO SEPARATE transducers for the transmitting and receiving functions so that
3. they may be positioned on opposite sides of the specimen.
4. The receiver is used to measure variations in the amplitude of the ultrasound, transmitted through the specimen.
5. Discontinuity of the specimen structure will produce areas of reduced amplitude.
