2. Nondestructive Testing
The use of physical methods for testing materials and
products without harm to those materials and
products.
Nondestructive tests are always indirect. NDT examines material properties
or characteristics
Reliable correlation must be established between the desired property and
the measured property.
- Establishing correlations are often time consuming and expensive
- NDT correlation may require the cooperation of test supervisors,
designers, metallurgists, customer personnel, manufacturing and
testing personnel
3. Requirements for NDT supervisory personnel
•Background knowledge of design, purpose, function etc.
•Ability to communicate, coordinate etc.
Qualification and certification of NDT level III personnel
see page 2 column 2
•SNT-TC-1A and CP-189
•Responsibilities/capabilities of Level III personnel
•Establish techniques & procedures
•Interpret codes standards, specifications & procedures
•Designating a particular NDT method
•Interpreting/evaluating test results to codes, standards and
specifications
•Sufficient knowledge to assist in establishing acceptance criteria
where none is otherwise available
•Familiarity with other NDT methods
•Should be qualified to train and examine level I & II personnel
4. Continuous updating and extension of knowledge is essential
•Broaden knowledge
•Keep up with new developments
•Many text books are available
•Active membership is another source of information
Manufacturing:
Material failures
Determining the source of defects is frequently necessary to eliminate defects
from production parts. Defects are often formed in early stages of production.
Definition: Defect - item of interest becomes unusable.
Two types of material failure:
Fracture – two or more parts, easily recognized
Permanent deformation – less recognizable
5. Causes of material failure :
Mechanical failure is always a result of stresses
1. Static loads
Large discontinuities
Poor dimensional control
Overloading
Poor design
2. Dynamic loads
High number of cycles (millions) i.e. rotating shaft
3. Service at high temperature
Tends to reduce desirable properties
Creep increases with temperature
4. Pressure – creating stress above the materials elastic limit may cause plastic
flow.
6. 5. Corrosive environments
6. Vibration – causes fatigue failure
7. Excess loading
8. Improper use of product
9. Improper maintenance
10. Age – some materials deteriorate with age i.e plastics, glass, paper etc
NDT – for flaw detection
Discontinuity – any variation in material continuity including: geometry, holes,
cavities, cracks, structure, composition or properties.
DISCONTINUITIES ARE NOT ALWAYS BAD
Defect – a discontinuity which has a substantial chance of failure
7. Discontinuities may grow into defects - corrosion, scratch or inherent discontinuity
may grow into a defect under stress from loads, temperature and particularly cyclic
loading.
Purpose of NDT – reliability and serviceability
Specific purposes of NDT:
- Identification or sorting material
- Verification of material properties
- Indication of proper material and suitable quality control during processing
to avoid costly repairs
- Tests to assure completeness, proper relationships, dimensions
- Tests during service to discover possible failure
- Diagnostic tests after failure – useful for design changes.
8. Purpose for use of NDT
Inspection Reliability
- What is the guarantee that all flaws of a certain size will be detected
- What is the largest flaw that can escape detection
Probability of detection & Confidence level
Probability of detection - if 100 flaws of the same size are present and
90 are detected, then the probability of detection is 0.9 or 90%
Confidence Level refers to the probability that 90% of the flaws in the
above study will be detected.
Confidence Level increases with larger sample sizes
i.e. from 1 to 100 to 10,000
Example: A study reveals a 90% probability of detection with a 95% confidence
level.
This means that there is a 5% chance that the 90% probability of
detection is overstated
10. Engineering materials – most metals and those plastics
that are solids and have reasonable strength at room
Temperature.
History of Manufacturing
•Early manufacturing – one-at-a-time
•Interchangeability – standardization and the industrial
revolution
Materials
– picked for their properties i.e. hardness, strength
appearance etc
An intelligent comparison of materials depends on precise meanings
of the terms used and an understanding of how properties are defined
and measured.
11. Some properties are defined by tests and are used directly as design data.
- Modulus may be determined from a tensile test so that a
designer may predict the deflection of a certain size beam
under known loads.
Some tests give relative values or are correlated to material properties
- Hardness tests may give an indication of relative wear
resistance or may correlate to tensile strength but the hardness
values can seldom be used directly in computation for design
loads.
12. Processes:
Converting raw material, which may be in rough, undefined shape, into a
usable product. Processing consists of one or many separate steps
producing changes in shape or properties, or both.
Shape changes – occur when material is a liquid, solid or plastic
- Melting a material and controlling its shape is called casting
- Reshaping in the plastic state is called molding, forging,
press working, rolling or extrusion
- Shaping a solid by metal removal is called machining
Energy Form
Energy is used for shape changes – may be supplied in the form of heat,
mechanical power, chemical reaction, electrical energy or light
Different materials react differently to the same energy system and the
same materials react differently to different energy systems
14. Four classes of material properties:
Chemical - reaction w/other materials especially corrosion
Physical – dependant on atomic structure
•Density, crystalline structure, specific heat, cohesive strength, melting
point
Mechanical – primary importance in design consideration for determining sizes
& shapes for carrying loads.
•Hardness, strength, ductility, toughness etc.
Processing – important for manufacturing
•Castability, weldability, machinability, bending etc.
15. Choice of materials is a compromise between:
•Function
•Strength
•Physical appearance
•Cost – material, processing, inspection
•Safety
16. Loading systems and material failure:
World business is motivated by profit - offering quality products is
critical to the survival of many businesses as society has low
tolerance to product failures.
Designers must consider material stress, strength, and loading.
17. Material stress calculations
Stress – internal forces acting on imaginary planes cutting the body being
loaded. Stress can be calculated by dividing the total force by the area on which
it acts.
Normal stresses – tension and compression where: S = stress
P = force
x-x = A (area)
S = P/A
18. Shear stress calculations: where: S = stress
P = force
z-z = A (area)
Bending - convex side is in tension and the concave side is in compression
S = Mc/I
19. Testing
Test Case: A grinding wheel must be able to withstand rotational speeds of
3500 rpm
•Direct test – the actual product or material is tested for specific
properties or information.
- rotate each grinding wheel to verify strength
•Indirect test – test for strength by means other than rotation,
requires correlation to specific properties
- rap the grinding wheels to create a certain tone
(acceptance criteria)
•Destructive – Destroy the grinding wheel - usually a direct test
- Increase rotational speed to destruction
•Nondestructive – indirect test that requires correlation to
specific properties
– also requires expert evaluation or
interpretation of results.
20. Tension testing (tensile testing) - destructive test which can
determine material properties i.e. strength, ductility, resilience and
toughness.
- Radii in the test specimen to reduce stress risers
- Data is plotted on a stress-strain diagram
- Each type of material has a unique curve or shape on the
stress-strain diagram
21. Stress-Strain diagram (engineers diagram) for steel
A-B elastic range
B - elastic limit
C - yield point
D – work hardening
E – Ultimate strength
F – Breaking/rupturing strength
22. Yield Strength vs Yield
Point
Many materials do not have a well
defined yield point
An artificial point called “yield
strength” may be calculated.
Yield strength is the amount of stress
required to produce a predetermined
amount of strain - USUALLY .002
inch or .2% OFFSET
23. Modulus of elasticity (E)
The ratio of unit stress to unit strain (deformation) - the slope of the
curve within the elastic limit
The relative stiffness or rigidity of materials can be obtained by
comparing their moduli.
E= stress/strain (within the elastic limit)
Ductility:
Tensile testing provides two measures of ductility
Percent elongation = (Lf – Lo) / Lo * 100
Lo = initial length
Lf = final length
Physical measurement – comparing the original area to the smallest area
of the neck at the point of rupture
24. True stress - true strain diagram
Data accounts for cross-sectional area
The greatest difference
is in the plastic flow
region
25. Compression testing:
•Similar set-up as tensile testing: test wood, fiberglass, timber, concrete
•Cast iron has tensile strength one half of its compressive strength
Transverse rupture testing:
•For test brittle materials (low
ductility) - a substitution for
tensile testing
•Tensile testing relies on
localized plastic flow to
correct for equipment/set-up
anomalies
•Materials tested include:
ceramics/glass/reinforced
concrete etc.
26. Shear test:
•Shear strength
test simulates
conditions of
actual loading
of bolts and
rivets
•Load is
applied to
cross sectional
area
27. Fatigue test - Materials subjected to stress cycles
• Fatigue strength – stress that can be applied for arbitrary number for
cycles without failure.
•Endurance limit – highest stress that can be endured with infinite
cycles without failure
•90% of failures of equipment with moving parts include fatigue in some form
Typical S-N curve (Stress and Number of cycles)
28. Creep test:
-Testing of materials for deformation within the elastic limit over long periods
of time.
-Apply constant load to a material at a desired temperature and measure
periodically for deformation.
- Creep tests are carried out for long periods of time – at least 1,000 hours
Notched Bar tests:
-Testing a materials ability to withstand sudden stress or impact from
applied loads (toughness)
-A weighted pendulum/cantilever is lifted to a test height and
released. It swings past the specimen – breaking it – and the
remaining energy is calculated by the height of the follow through
swing.
- Charpy, Izod, Tensile impact
29. Charpy test:
•Impact test which uses a
weighted pendulum
•Measures materials ability to
resist rupture via energy
absorption
Izod test:
•Measures materials ability to
resist rupture via energy
absorption similar to Charpy
test
30. Bend test:
•Free bend test
- specimen is bent slightly then compression applied until
failure or 180 degree bend is obtained
- the angle of bend at the failure is compared with other
tests.
•Guided bend test
- multiple radius guided bends are often used to determine
the smallest radius about which a specimen will bend
180 degrees without fracture – continue to decrease the
radius
Tensile impact testing
•Greater similarity between the test and some conditions can be
provided by tensile impact tests
•Specimens are supported so that impact loads may be applied
•Specimens are NOT notched
31. Hardness testing:
•Indirect test that measures the ability of material to resist near surface
penetration.
•Most frequently used test for determining material properties i.e.
strength, wear resistance and work-hardening qualities.
•Separate raw materials of different composition
Mohs test:
•Scale of ten minerals arranged in order of increasing hardness –
from #1 talc to #10 diamond
•If a material can be scratched by #7 but not scratched #6 by then it
has a hardness of #6
•Used mainly in the field of mineralogy.
File test:
•A file is used to produce metal shavings from a specimen - hardness
is determined by comparing the specimen filings with filings from
standard test blocks
•Not very accurate.
32. Brinell test:
•Typical test: hardened steel 10mm dia ball impressed under a load of 3000kg on material for 10
seconds and the indentation is measured – from the ratio of the force imposed on the indenter to
the size of the impression
•Very consistent, and tensile value can be closely approximated
•Can not be used on very thin materials
Rockwell test:
•Hardness determined by differential depth measurement using a 1/16” steel ball or diamond
penetrator.
•Minor load is applied to reduce effect of dirt scale etc. – then major load is applied.
•A-G Rockwell scales are used
33. Superficial Rockwell test:
•Hardness determined by differential depth measurement using a precision
diamond penetrator – in the same manner as the Rockwell machines.
•Produces shallow impressions – for thin materials or localized surface
measurement.
•N is the superficial Rockwell scale designation
Vickers test:
•Similar to Brinell except that a four-sided diamond pyramid penetrator is
used.
•Vickers and brinell numbers are almost identical i.e. measurements are
calculated from the ratio of the force imposed on the indenter to the size
of the impression
34. Microhardness test:
•Used on very small or very thin materials
•Elongated diamond impression is a few thousandths of an inch long.
•Surface must be highly polished to avoid the effects of surface
imperfections.
35. Safety Factor:
•The ratio between the maximum stress value and working stress value
•Safety factors are used to avoid working too close to maximum values
Calculation:
The working stress of a material with 80,000 psi ultimate tensile
strength is 20,000 psi what is the safety factor in this situation
Safety factor = 80,000/20,000
= 4
38. Space Lattices
Body-centered
•9 atoms
•Metals are hard and strong
•Chromium/iron/molybdenum/tungsten
Face-centered cubic lattice
•14 atoms
•Ductile materials
•Aluminum/copper/gold/lead
Hexagonal close-packed lattice
•17 atoms
•Materials are susceptible to work
hardening
•Cadmium/cobalt/titanium
Iron is body-centered at room temp, face-centered
above 912 deg C and body-centered above 1394 deg C
39. Solidification
Atoms cool and take positions to form unit cells. Cooling is not the same for
every atom and certain ones will take their positions ahead of others and
become a nucleus for crystal formation.
- Atoms give up kinetic energy in the form of HEAT which slows the
cooling process
- Crystal growth continues in all directions - nucleation continues
until the crystal (grain) runs into interference from other grains that
are forming simultaneously about their nuclei.
If two grains with the same orientation meet they will join to form one grain.
Two grains forming about a different axis, the last atoms to solidify will be
attracted to each other however they will assume compromise positions in an
attempt to satisfy their attraction. These “misplaced” atoms about the grain are
known as grain boundaries.
40. Grain size
- misplaced atoms between grains form boundaries
- interruptions in the lattice (boundaries) offer resistance to deformation
- Fine grain with numerous interruptions are stronger and harder than
course grains of the same material compositions.
Grain size exerts an important influence on the mechanical
properties of materials.
Coarse grains
- Coarse grains in harder materials have lower strength than fine grains
- Machine more easily – requiring less power although the surface finish
will not be as good as finer grains
- Easier to harden by heat treatment – but more susceptible to cracking
under the thermal loads
- Will case harden more easily than fine grains
Coarse grains may be more desirable during processing but fine grains are usually
necessary in the final product to provide the best mechanical properties.
41. Work Hardening
Occurs when a load applied to a material exceeds the elastic limit and is
permanently deformed within its crystalline structure
- Elastic properties are not lost – they are enhanced providing
deformation (plastic flow) is produced by cold working.
- The strength of a metal is increased by plastic flow and the elastic limit
is raised
Manufacturers often try to produce improved properties at the same time shaping
is being performed.
Most metals are treated in the solid state to enhance their properties – these
treatments are called:
- Work hardening
- Recrystallization
- Age hardening
- Heat treating of allotropic materials
42. Solid State Changes in Metals
Plastic Deformation (Plastic flow)
- Permanent deformation, fills unoccupied lattice
- Through work hardening; properties are enhanced, elastic limit is raised
Three types of Plastic Deformation: dependant on the type of metal
- Slip: sliding of atomic planes within a grain
- Twinning: occurs when loads are applied suddenly, deforms by twisting
- Rotational: slip on a number of different planes, lattice tends to bend
and rotate to a preferred orientation
43. Cold Work – working material (plastic flow) below the recrystallization
temperature.
Plastic flow fills dislocations (atomic discontinuities) and creates new
dislocations to resist further plastic movement - materials get stronger and
harder. Cold working leaves materials in higher energy, unstable condition.
44. Recrystallization:
Two kinds of change occur when heating cold worked material.
Recovery (stress relieve) – rearrangement/return of some dislocations, some
stresses relieved, no change to crystals
- The objective of stress relieve is to regain electrical, chemical and
corrosion resistance properties without sacrifice to mechanical properties.
- If the temperature is raised too high or for too long hardness and strength
will reduce appreciably.
Recrystallization – nucleation and growth of new smaller strain-free crystals. No
grain-size changes can take place unless cold working is present.
- Achieve maximum ductility
- Allows further metal working especially if deformation stress is close to
ultimate strength (fracture failure)
- Can be a grain refining process
In most cases the last forming process will not be followed by recrystallization in
order that high hardness and strength in the cold worked material may be retained.
45. Grain growth:
If a metals heated at or above its recrystallization temperature after new unstrained grains
have formed the tendency is for some of the new grains to absorb others and grow.
- If a fine structure is desired it is necessary to reduce the temperature quickly after
recrystallization to stop grain growth.
46. Age Hardening – a treatment to develop hardness properties or strength
properties or both. Exact explanation of this phenomena is unknown.
Solution Heat Treat – first step
- Dissolve a maximum amount of precipitant (high energy points) in
solution and freeze it in place by sudden cooling
- Temperature low enough to prevent grain growth and high
enough to ensure maximum diffusion of precipitant
Transition stage by precipitation (aging) – Final step
- Full hardness is developed
- excess metallic component is partially precipitated from the
solid solution (maximum energy state)
Allotropic Changes – metals that change lattice structure upon heating and
cooling to exist in different forms through various temperature ranges.
- Iron changes from BCC to FCC at 912 deg. C, and to BCC at 1394
deg. C
ERROR: Compare the allotropic temperatures given on p. 33 and p. 38
47. Heat Treatment of Steel
Austenization – a grain refinement process. It is a step in a sequence of heat
treatment processes - not a final process.
- Grains are formed by the temperature increase, not the temperature
decrease – formation of new FCC smaller grains.
- Temperature and time critical as large grain growth can occur
Annealing – heat treat process to soften material and increase ductility used in
conjunction with cold working
- decrease hardness and increase machinability
- relieve stress and refine grain size
Normalizing – similar to annealing except metal is not at its softest state, pearlite
is fine instead of course
- high toughness, good machinability
- relieve stress and refine grain size
48. Spheroidizing – iron carbide forms in small spheres in ferrite matrix produces
minimum hardness and maximum ductility
- performed on normalized steels
- improve machinability if high carbon steel
Hardening of Steel – based on the production of high percentages
of martinsite
First step: Austenization –to produce austenite, new FCC grains, smaller
Second step: Fast cooling – steel is quenched using oil, air or water
Third step: Tempering - Softening process to relieve stresses in the steel and
reduce brittleness
- The structural changes caused by tempering are functions of time and
temperature.
49. Corrosion – deterioration of metal by chemical reaction or
electrolysis (transfer of electrons) or both.
Direct chemical action (acid attack) – reactions where coupled anodes
and cathodes existing in the electrolyte are not identifiable. Charge
on the atoms is satisfied.
- Pickling of steel/chemical milling etc
Electrolytic Reaction – flow of electrical current
- Plating process
- Anodes/cathodes to complete a circuit
- Cathodic least corrodible
- Anodic most corrodible
50. Types of Corrosion
General corrosion - appears uniformly i.e. bluish-green copper/dull
silverware
Pitting - is localized and extends deeper into the metal, is more serious and
can be a nuclei for failure
Intercrystalline - is serious, grain boundaries are attacked and crack-like
discontinuities are formed
51. Corrosion Protection
- Metal coatings: electroplating/dipping/metal spray/cladding
- Chemical compounds: anodizing/iron phosphate
- Nonmetallic: paint, varnish, enamel, grease, plastic etc.
- Sacrificial metals, zinc/magnesium/aluminum, metal in high
galvanic series are anodic to metals below them
i.e. zinc is attacked to protect steel
53. Processing raw materials
Ore reduction – starts in blast furnace
- iron ore, coke and limestone are crushed to optimal size, mixed
and fed into the opening
- air is blasted into the bottom and combusts with coke
- operates at 3000 deg. F
- molten iron and slag form at the bottom, tapped off periodically
Pig iron – the product of the blast furnace is tapped and poured into a
crude casting
- Low quality material: brittle and difficult to machine and has
low ductility
- Typically contains 3 - 4% carbon
54.
55. Steel – when carbon content of iron is reduced to less than
2% the new material is called steel.
- Entirely new set of properties
- Trace elements added
- Greater ductility/machineability/weldability
Furnaces:
Crucible – bars re-melted,
carbon and slag float to the
surface and are skimmed
off
- High quality steel
Open-Hearth – flames are
projected on a open
container within the
furnace to keep metal
molten as the carbon is
reduced
56. Bessemer – pig iron is
melted in a container and
oxygen is bubbled through
the melt oxidizing silicon
and carbon
- carbon reduced to 0.05%)
- 5% of the steel made this
way
Electric – heated by electric arc, slag floats to the surface and is skimmed
off. Variation of the crucible method.
- Produces the highest quality steel
- More control of heat and atmosphere
57. Oxygen
- Scrap steel is loaded in the
vessel
- Molten pig iron is poured on top
- A lance blows oxygen on the
mix for about twenty minutes,
lime and fluxes are added
- Most steel is made this way
58. Types of Steel
Plain carbon Steel – 0.05% or less
carbon
- very ductile
- Soft and Weak
Low Carbon Steel - 0.06% to
0.25% carbon
- difficult to harden, low carbon
permits little martensite formation
Medium Carbon Steel – 0.25% to
0.5% carbon
- Can be heat treated
- Working produces tough
materials
High Carbon Steel – 0.5% to
1.25% carbon
- Tool steels, very tough and brittle
materials
59. Alloy steels – Steels that contain quantities of elements greater than impurity
concentration.
- Alloys affect hardenability/weldability/grain size and
toughness/corrosion resistance
Low Alloy steels – addition of small amounts of alloying elements can raise
the yield strength 30% to 40%.
- Structural steel/rolled products with good corrosion resistance
- AISI steels (American Iron and Steel Institute) are alloyed for
improved hardenability and 10% to 20% higher tensile strength
- Improved properties at a higher cost
Stainless Steels – high chromium steels with excellent corrosion resistance
- Frequently referred to as heat and corrosion resistant steels.
Cast steel – relatively small quantities of steel are cast.
- Cast steel is isotropic (lack directional properties)
60. Chapter 6
Nonferrous Metals
and Plastics
Non ferrous metals like aluminum/magnesium/titanium
have densities 1/3 to 1/4 that of steel
High Corrosion resistance
Alloyed with iron and themselves
61. Aluminum
- Expensive to refine (8 Kw hours per pound)
- Excellent ductility and corrosion resistance
- Light material with good strength to weight ratio
- Easy to fabricate
- Aluminum with over 4% magnesium or with cooper added can be
hardened and strengthened
Pure aluminum (electrical grade),
- 68% conductivity of copper 200% on a weight basis
- Excellent corrosion resistance and ductility
- Soft and weak
- Pure aluminum and most alloys are not hardenable by
heat treatment.
62. Copper
- Density is 10% greater than steel
- Excellent ductility/thermal and electrical properties
- 3/4 of cooper is produced in pure form because of its electrical
conductivity
- Principle metal for electrical use
- Excellent corrosion resistance
63. Brass & Bronze
- Brass is cooper (Cu) with 5% to 40% zinc (Zn)
- Bronze is cooper with up to 11% tin (Sn)
- Has better properties than brass, low friction and anti-wear
(good for bearing journals)
- High cost limits the use of tin
Nickel
- 3/4 of all Ni produced is plating material or used an alloy of steel
- Most important property – corrosion resistance
- Good heat resistance
- Good for grain refinement of steel
Magnesium
- Lightest commercial metal
- Good strength and corrosion resistance
- Easily work hardened
- High stress levels at notches – lower impact values
64. Zinc
- Low cost but low strength
- Excellent corrosion resistance
- Plating or coating with zinc is called galvanizing which
accounts for 50% of production
- High formability; pure or slightly alloyed is an excellent roofing
material
Special Groups - usually designed for high stress and elevated
temperature
- Jet turbine engines, high temperature steam piping and boilers,
rocket combustion chambers and nozzles
- High cost to manufacture
65. Cobalt Alloys
- May not contain 50% of any element
- Alloyed with Ni, Cr, tungsten, columbium, manganese
and carbon
- Useful structurally to 1000 deg. C
- Good corrosion resistance and tensile strength
Other Non-Ferrous Metals - Gold/platinum/beryllium
- High chemical inertness
- Relatively rare
- Cost restricts their use
- Beryllium has the highest strength to weight ratio of any
known metal
Titanium
- Ores are abundant, cost of reduction is 100 x that of iron
- Could easily be most important nonferrous metal if low
cost production could be developed
66. Non-Metals
Plastics – a group of large molecule organic compounds, primarily
produced as a chemical product that is susceptible to shaping.
- A monomer is the smallest molecular unit
- All plastics are polymers
- Polymerization is the process of combining monomers into long
chains using heat, light, pressure and agitation.
- Properties depend on degree of polymerization, a wide range of
properties are available – from solids to liquid adhesives.
- Most plastics are synthetic
- Some have origin of natural material; only as a source of elements and
compounds as the chemistry of finished plastic has no direct
connection.
- Excellent insulators
67. Types of plastics
Long Chain Polymers - thermoplastics
- Degree of polymerization is controlled in initial manufacture
of the plastic raw material or resin
- Polymers soften with increase temperature and regain rigidity
as temp is decreased; the process is essentially reversible.
Thermosetting plastics – cross-linking occurs between adjacent chains
- Reaction is chemical in nature and irreversible
- Heating will cause charring and deterioration
- Origin of resin distinguishes different types of plastics
Natural Plastics
- Cellulose may be produced as paper, vulcanized fiber and
cellulose acetate
- Rubber latex
- Wood has some thermoplastic properties that are used in some
manufacturing processes synthetic
- Origin of natural material is only as a source of elements and
compounds.
68. Property Comparisons of thermoplastics and thermosetting plastics
- Thermoplastics lower in strength and hardness but higher in toughness
then thermosetting
- Thermosetting plastics have better moisture and chemical resistance
than thermoplastics
- Ultimate strengths of plastics are lower than metals
- Lower service temperatures than most metals
- Nylon one of few true crystalline plastics, may be hardened working.
- Drawn nylon filaments may have a tensile strength of 50,000 psi
which is actually greater than some low strength steels
70. Modern Manufacturing
Markets
- Products must sell
- Know your competitors
- Product life is limited - market for replacement parts
- Markets are increasing and sales are increasing
- Population is increasing
- The standard of living is increasing
- More leisure time and increased purchasing power
- Market forecasting is difficult but essential
- Electronics over past 50 years
- Technology/Computers/Internet
71. Design
- Quality should be good
- For many consumer goods the appearance may govern the final
choice
- Quality and costs must be balanced
- Availability of facilities affects choice of design
- Proper equipment and skilled personnel required to
produce the product
Processing is usually a complex system
- Manufacturing is usually used to describe processing, starting
with the raw material in a refined bulk form and is concerned
mainly with shape changing.
73. Shape-changing processes
- Shapes changed by addition and subtraction
- Wrought materials are produced by plastic deformation which is
accomplished by hot working (above the re-crystallization
temperature) or cold working.
- Shapes are changed by numerous methods
76. The Process
Casting is the process of causing liquid metal to fill a cavity and solidify into
a useful shape.
The process starts with a pattern
A Mold is a reverse impression constructed from a pattern that
represents the finished product
- Pattern is usually made oversize - to allow for shrinkage
- Sprue is a channel or runner to fill the mold
- Sprue allows for shrinkage
Mold cavity is filled with molten material
Casting is a large industry
Foundries tend to specialize
77. Solidification
Liquid - atoms in high energy state
-Atoms become less mobile as temperature is
lowered
- Finally assume their positions in the space lattice
- Form a crystal
Crystal Growth – starts at the surface
edges to form a skin
-Heat from fusion increases the amount of heat and
must be released, freezing processed is slowed.
-Crystal size limited by interference with other
crystals
-First grains form a skin of fine equiaxed type,
random orientation
Second phase is slower – grain growth is
more orderly
- As crystals form heat of fusion is released
- The mold insulates heat
- Crystals have least interference in direction of heat
- Directional columnar grains form toward heavy
sections
- Dendrites form on sides of columns
78. Third phase
-Cooling rate decreases, temperature tends to
equalize
-Less random nucleation, grains grow more
orderly than rapid cooling
Grain characteristics influenced by
cooling
- Grains on outside are fine equiaxed, form skin
-Columnar and dendrites grow in directions of
heat
-Center is the weakest structure, large equiaxed
Eutectic Alloys
-Similar to pure metal. Solidification takes place
at single temperature, lower than the
components, smaller temp gradient, greater
number of points of nucleation; smaller grains
Noneutectic Alloys
-Freeze through a temperature range (out-side
inward), most products made from noneutectics.
79.
80.
81. Shrinkage – occurs in three stages
First – shrinkage in liquid
- loss of superheat (100 – 500 deg F above melt)
- allows time to pour metal
- Shrinkage can be replaced by adding metal
Second – solidification shrinkage
- Transformation, most materials contract
- Develop minute random voids, microporosity/microshrinkage
- Evolution of gas forms microporosity
- Cavity can be filled with liquid metal
Third – contraction in the solid state
- Primary cause of dimensional change, reason that castings are often made over size
-Cannot be filled with liquid metal
82.
83. Casting design - Direction of freezing extremely important to
allow liquid metal to compensate for contraction
- Feed head, to fill casting and volumetric shrinkage
- Progressive solidification, outer to inner
- Directional solidification, one part of casting to another
- Uniform thickness desirable
84.
85. Pouring
- Most pouring is done with ladles
- Turbulent flow harmful
- Gas and Oxidized can be trapped
- Cold shuts can occur
- Metal can solidify before filling if poured too slowly
- High pouring rate can erode casting walls, cause sand inclusions
Gating System – pouring basin, sprue, runners and ingates
86. Chills
- Absorb heat rapidly
- Help in directional solidification
- Steel/cast iron/copper/cast alloy
- Internal/external
- Faster cooling improves material qualities
87. Sand Molding – is
a reverse pattern
Green Sand
- Sand/clay/moisture
- Used in majority of
castings
- Sand held together by
clay (2% to50%)
- Moisture in the clay
permits flowabilty of
sand around the pattern
88.
89. Cores – inserts that exclude metal flow to form internal surfaces
- usually made of sand green/dry
- Should collapse immediately, not interfere with shrinkage
- Chaplets metal supports of same alloy, become part of the casting
Dry Sand Molds – elimination of moisture reduces defects
- Dried green sand
- Cost of heat/time to dry and handle heavy molds without damage
is expensive
90. Permanent Metal Molds
- Used when quantity justifies added expense
- Made of steel/cast iron
- Used to form copper/Al/Mg/zinc
- High accuracy, good finish, die vs sand
Die casting – same as permanent mold except metal is injected under
pressure into the die – permits uniform cooling of thin castings
Investment Casting
- Master pattern/cast metal pattern/wax pattern
- Wax is dipped in plaster material
- Heated to remove wax - working is pattern destroyed as lost wax
- Plaster preheated and metal is poured
93. Melting equipment
- Cupola – similar to blast furnace
- Crucible furnace – melt small quantities of nonferrous metal
- Pot furnace – quantities of nonferrous, ladled out
- Reverberatory furnace – large brick oven for quantities of
nonferrous metals, uses gas-air and oil-air heating
- Electric Arc furnace – high intensity heat, high purity
- Induction furnace – ac coils generate eddy currents in material,
high purity
95. Permanent union of metallic surfaces by
establishing atom to atom bonds
Bonding – heat and pressure are frequently used
- Cleanliness; exposed atoms are atoms to be joined
- Atomic closeness; melting is most common method
- Elastically or plastically deforming surfaces to establish
closeness
96. Fusion bonding –
most common
- Complete melting of
surfaces
- Strong bonds
- No pressure required
-Metallurgical effects
like casting; grain
size, shrinkage
-Fillers sometimes
added
97. Pressure Bonding
-Heat aids cleanliness
and closeness
-Close union
established by plastic
flow
-Small amounts of
fusion bonding occur;
incidental
-low
shrinkage/distortion
98. Flow bonding
-uses filler metal of different
composition and lower melting
temperature, base metal not
affected. Generally lower
strength than fusion
-Braze welding; fluxes used for
joining, repairing cast iron by
filling cavities
-Brazing; rod, wire, foil heated
by torch/furnace/induction
-Soldering; similar to brazing
except temperature is below
425 deg C, low strength
99. Cold bonding
-Heat not essential
for bonding
-Pressure is
required; causes
plastic flow and
fragmentation of
surface impurities
100. Effects of Welding
High temperature
results in:
-Shrinkage, annealing
and allotropic
transformation; re-
crystallization extends
beyond the melted
material
Low temperature
results in:
-smaller grains and
stronger structure
101. It is often necessary to:
Normalize welds (heat
slightly above the
transformation
temperature) to obtain
uniform grain structure,
typically smaller,
uniform properties and
relieve stress
Stress relieve welds –
typically at 650 deg C,
no grain refinement; less
distortion than
normalizing
108. Shielded Metal Arc Welding
(SMAW)
Welding is generated by an electric arc established
between the flux covered electrode and the base
metal and melts the two together.
109. Benefits of the electrode coating
•Arc stabilization
•Provides gas shielding – decomposition of
cellulose and limestone
•De-oxidation of weld pool
•Strengthens the weld - adds alloying elements
•Increases deposition by adding iron filler
•Slag provides oxide barrier for solidifying metal
•Slag provides thermal insulation of the cooling weld
110. Common defects associated
with SMAW
Porosity
Cracks
Slag
Incomplete fusion
Incomplete penetration
Burn through
Lack of fill
Root concavity
Undercut
Excessive penetration
•The SMAW process is almost totally operator dependant
111. Gas Metal Arc Welding
(GMAW)
Welding is generated by an electric arc
between a continuously fed solid wire
consumable and the base metal
112. Common defects associated
with GMAW
Porosity
Cracks
Incomplete fusion
Undercut
lack of fill
Incomplete penetration
The GMAW process is automated, semi-automated and
machine.
113.
114. Gas Tungsten Arc Welding
(GTAW)
Welding is generated by an electric arc
between a tungsten electrode, a solid wire
consumable and the base metal.
115. Common defects associated
with GTAW
Porosity
Tungsten inclusions
Cracks
Incomplete fusion
Undercut
•The GTAW process is almost totally operator dependant
116.
117. Submerged Arc Welding
(SAW)
Welding is generated by an electric arc between a
continuously fed solid wire consumable and the base metal
while totally submerged in a protective flux.
118. Common defects associated
with SAW
Porosity
Cracks
Slag
Incomplete fusion
•The GMAW process is automated, semi-automated and
machine.
124. Plasma Arc Welding
- Inert gas is
ionized by
passing it
through an
electric arc
- Gas expands
upon striking
the part surface
and gives off
heat forming a
weld pool
126. Electro Slag Welding
Arc starts the
melting process but
is quickly
extinguished
Current is then
passed through
molten slag
Metal is added via
wire feed
Similar to a
continuous casting
process
127. Common causes of defects
•Welder-technique
•Cleaning
•Joint preparation
•Joint design
140. Wrought materials have advantages over their cast counterparts:
•Plastic flow tends to improve strength and ductility
•High duplication accuracy of most deformation processes
•It is difficult to cast very thin sections
10% of steel production is castings the other 90% undergoes deformation of
some sort
Rolling, forging and drawing tend to improve strength
The greatest limitation is the need for a ductile stage. Nearly all metals have
ductility at elevated temperature – the major exception being cast iron - and
may at least be hot worked.
A multitude of manufacturing processes which produce deformation can
produce a multitude of defects. Personnel must be alert of early detection of
defects.
141. Effects of deformation
Loads that exceed the plastic limit redistribute atomic dislocations and change the
grain size and other metallurgical effects:
This is called; Strain hardened, cold worked or work hardened
Ductility is recoverable via recrystallization:
The property changes associated with work hardening are due to strained and
unstable atoms. The changes may be reversed by supplying energy in the form of
heat. Recrystallization is the process of atoms returning to the unrestrained condition
SIMILAR to that which existed before strain hardening.
Hot working:
•When deformation is performed above the recrystallization temperature it is
termed hot working because recrystallization proceeds along with strain hardening.
•The net effect is similar the cold working and then heating above the
recrystallization temperature.
•Hot working permits continuous deformation however if deformation proceeds too
rapidly it is possible, even above the recrystallization temperature, to develop cracks
142. Directional effects
Most metals are polycrystalline (random crystal orientation) – plastic
deformation, crystal fractures, rotations and reorientations lead to loss of
randomness
- Strength is developed in the direction parallel to working
- Drawn wire has strength in drawn direction – where it is
needed most
- Sheet metal losses ductility perpendicular to the rolling
direction which may cause subsequent drawing or
bending operations to be difficult
- Properties are different in different directions
- Directionality from working is never completely eliminated
Direction effects on internal faults
•Cast metal contains discontinuities such as scale, oxides, voids and porosity –
these indications elongate in the direction of flow.
•NDT techniques are developed to interrogate directional discontinuities.
143. Grain size
•For any metal the grain size is determined primarily by the cooling rate.
•Ingots are large casting and they cool slowly resulting in large grains
•Grain size is refined mechanically and thermally
•The grain size immediately after recrystallization will always be small.
All hot and cold working results in crystal fractures which produce smaller grain
size as the material is strain hardened
Re-growth occurs at elevated temperatures – small grain size is not fixed.
Ductility is restored at the cost of strength
144. Ferrous grain size is refined in two ways:
1. Working and recrystallization
146. Hot Working – majority of deformation
- Metal soft/ductile
- Minimum hardness and strength
- Effects of strain hardening continuously relieved by recrystallization at hot-
working temperature
-The effect of faults are minimized by the closing and welding of voids and
the elongation of inclusions
-Major deformation is is done by hot working as cold working is done at the
finishing operations
Cold Working – mainly a finishing operation
- Cold rolled steel with no intermediate heat treatment is hard, strong with
minimum ductility
- Cold rolled with intermediate anneal, intermediate hardness/strength/ductility
- Cold roll with full anneal, low hardness/strength and high ductility
148. There are limitations of castings
- Nearly impossible to cast thin sheet products with
high accuracy across the sheet
- Porosity and associated defects
- Increased brittleness and leakiness
- Poor appearance
Deformation improves properties
- Cold and hot working can often provide the double benefits of
property improvement and shape changing at the same time
Ferrous and nonferrous metals are processed via deformation
- Although working metals adds costs - more 80% of iron based
products are finished as wrought products
Most mill output requires further processing:
- Bar stock
- Cold rolled sheet stock
- Rough rolled billets
149. Hot Rolling – one of the most common mill operations is the rolling of
metals into flat two dimensional forms
- accomplished by passing material between flat or shaped rollers
that squeeze and cause it to flow to an elongated shape.
- For metals with little ductility and for large shape changes the
work is usually done hot to reduce the energy requirements and
possibility of material rupture
Blooms, slabs and billets
- Cast ingot defects removed
- Near surface defects are removed by chipping, grinding
or scarfing (oxygen torch burning)
- Most material starts as CAST ingots which are rolled into
blooms slabs and billets
- Continuous casting eliminates ingots
- Heavy casting slab is introduced directly into the rolling
stands
- Saves; cropping time, material and money
150. Blooms, slabs and billets cont’
- Blooms and billets are square (blooms are larger)
- Slabs are rectangular
- All are destine for further deformation
- Surface oxidation is a problem as working takes place at high
temperatures. Material is cleaned by dipping in acid baths
(pickling) which attacks scale
- Limited accuracy in hot rolling
Cold finishing – properties changed by cold working
- Materials must be ductile – ductility is reduced as the hardness,
yield strength and tensile strength are increased.
- Flat products are called strip/sheet/plate/bar
153. Tube and pipe
- Most pipe and tubing products are produced in mills
- Most pipe made by welding seams – resistance & spiral
- Seamless pipe is rolled and forced into a mandrel -piercing
158. Extrusion – used
extensively with
nonferrous metals
- High energy process, die &
heavy loading
- Cable sheathing
159. Forging – confine metal under sufficient pressure to cause
plastic flow
- Usually performed hot
- Open die - blacksmith
- Closed die - matched dies
- Press forge – slow squeeze
- Drop forged – fast impact
- High strength
- Usually progressive steps
160. Powder Metallurgy – producing metal objects by pressing
or molding powder either with or without fusion of a low
melting constituent only.
- Originally used to sinter materials that were chemically
reduced from a powder or flake i.e. tantalum, osmium &
tungsten
- Can produce porous materials – filters etc.
- Bonding established by heat and or pressure, mechanical
and atomic bonding
- Sintering most often done at elevated temperature
- Higher densities – pressure fills atomic voids/easily
recrystallized
- Property improvements by deformation
- Conventional heat treatments possible
163. Most household products are pressworked
Ductility essential
High cost of special dies
Recrystallization can reduce number of forming steps
Spinning – on a spinning chuck/mandrel
164. Shearing
- A cutting operation – loading to facture with opposed edges
- Straight line is performed on a squaring shear
- Cutoff
- Parting
- Blanking
- Hole making
- Used in finishing operations – trimming and shaving
- Punching, slotting, notching, piercing
165. Bending – localized plastic
flow about one or more
linear axes
- Ductility is required
-inside radius is subjected
to compressive stresses
which may cause an
increase in width
- Outside stresses may cause
an increase in thinning
166. Forming – simple bending and multiple bends made along
more than one axes.
- Surface area is not significantly altered
167. Drawing – involves not only
bending but stretching
and compression of metal over
wide areas
- Automobile fenders,
kitchenware and square and
rectangular box shapes.
- Recrystallization may reduce
the number of steps by
restoring the properties
- Spin forming is versatile, low
cost, but low quality
168. Explosive forming
- Detonated at a predetermined
distance in air or water
- Pressures as high as 4 million
psi are developed
- Shockwave transmits energy
to the work piece
170. Close accuracies/good finish
High tooling costs
Localized energy force – tool is a loading device that
causes plastic deformation and fracture to produce a chip
Cutting tool materials – Carbon tool steel/high speed
steel/carbides/ceramics/diamonds/coated tool materials
171. Machine tools – equipment designed to hold a cutting tool and
work piece and establish a suitable set of motions to remove
material.
- Turning/boring/drilling/milling/grinding
172. Chip types:
- Brittle materials, chips break into short segments
- Ductile materials, chips are continuous long coils
173. Machinability – relative ease with which any material may
be machined
- Three different measurements of machinability – finish, power
consumption and tool life
Surface Finish
- Waviness: variations of conformance relatively widely
spaced
- Roughness: finely spaced surface irregularities
Lay – direction of the predominant tool pattern
174. Numerical control (N/C)
- Programming allows smooth and intricate patterns
- Excellent repeatability
- Some are designed with transducers in the machine elements
which generate feedback
- Disadvantages
- Costly/complex
- Require computers
- Take up more floor space than conventional machines
176. Plastic Processing
Closed die molding
- Similar to die casting
- Time is limited with thermosetting plastics where as
thermoplastic can be re-heated
- Transfer molding (cold chamber die casting) is used with
thermosetting plastics
- Injection molding is similar to hot chamber die casting
Compression molding – closing die provides pressure - forging
178. Casting – acrylic rod or sheet materials include polyesters, epoxies,
and phenolics.
- Produced from thermosetting resins usually in liquid, syrup
form
- Hardening promoted by chemical catalysts
Extrusions – produce sheets, tubes, rods and films.
Reinforced plastic molding – plastic with reinforcing fibers
- Thermosetting resins, glass and wood fibers
- Contact lay up with filler resins
- Compression process for sheet material with curved
surfaces; chairs/table/counter tops/sinks
179. Adhesive Bonding –
adherence to a surface; glue,
cement, adhesive
- Electrostatic and covalent bonds;
sharing electrons by different atoms
- Mechanical interlocking
- Pressure, heat or both required for
some adhesives
- Low distortion as heat input is minimal
- Does not require expensive equipment
or highly trained personnel
180. Composites – mixtures of two or more materials that maintain their
own identities but are attached in such a way that they reinforce each
other
- Metals, nonmetals or combinations of both
- High strength/light weight/high stiffness
181. Laminates – A number of composites put together in the form
of laminates
- Can replace steel in some applications
- Aircraft body/wings
- Fiber glass
- Honeycomb – prefab house doors/walls
182. Mixtures – composites of several materials
- Ceramics
- Concrete: gravel, sand and cement
- Can withstand substantial compressive loads
- Reinforcing wires and rods are inserted when cast
- Rubber: vulcanized and combined with sulfur and other
materials
183. Metal Removal Processes
Electrical Discharge Machining (EDM)
- Removes metal by vaporization caused by the high temperature
of the electrical arc
- Arc at nearest point(s) of contact
- Metal removed from both electrode and work piece
- Useful for special shapes and hard material
184. Metal Removal Processes cont’
Chemical Milling
- A chemical process that uses acid
without electrical action
Electrochemical Machining
- Tool and work piece form
electrodes
- Work piece is positive (anode)
- Same as plating – metal is
removed from the anode
186. Laser – (light amplification by stimulated emission -
- uses light energy
- Cutting
- Drilling holes
- Computerized for numerical control of intricate
patterns
187. Gross Separating Processes
Torch Cutting
- Oxyacetylene flames bring metal to kindling temperature
(exothermic reaction of burning material)
- Assisted by stream of oxygen that causes oxidation
- Can cut steel 5 feet thick
Friction Sawing – metal removal by localized heat in work piece,
generated by rubbing blade or disc
- Edge speeds are 3000 – 7500 meters per minute
- Used for cut off work on bars and structural shapes
189. Surface finishing is often the final stage of production
- Cleaning/polishing
- Deburr
- Corrosion protection
- Chemical change on the part surface
- Mechanical working
- Protective coatings
190. Casehardening – a change of surface properties to
produce a hard, wear resistant shell about a tough
fracture resistant core
- Use low cost low carbon steel alloys to replace higher cost
materials
- Case depth is checked by destructive methods
- Several methods are used to caseharden
Carburizing – diffusing carbon into material by heating the
material to 850 – 930 deg C
Three methods:
- Pack hardening, carbon packed around part
- Liquid method, immersed in molten cyanide
- Gas method, injection of gaseous hydrocarbons
191. Casehardening – cont’
Flame Hardening – surface heated above transformation
temperature
- Gas burners/oxyacetylene torches which heat all or part of
the surface
- Surface is heated quickly so that only a small depth from the
surface goes through the transformation temperature
- Immediately following the torch is water quenching – to
form martinsite
- Only steels that contain sufficient carbon can be flame
hardened
192. Cleaning – in process and finish
- Remove sand from castings, greasy films, coating, coolants,
oils, waxes, scale, oxides, burrs, tool marks, slag etc.
- Liquids, vapors, soaps, and solvents are frequently used
- Mechanical work sometimes added
- Conditioned water is often used to clean
- Pickling baths (sulfuric acid and water) is is often used to
remove scale
- Blasting
- With sand, grit or oxides
- Peening (small round metal pellets)
193. Abrasive Barrel Finishing
- Rolling/tumbling, high polish
- Wire brushing to: deburr, remove rust/spatter/coatings/films
Polishing – surface blending to a glossy finish using felt, rubber, soft
abrasive wheels, etc
Buffing – similar to polishing except fine abrasive is carried in wax or
similar substance
Coatings – used for protection, wear resistance and
increase/reduce coefficient of friction
- cleanliness associated with adhesion
Paint – pigment in a drying oil; tough film
Varnish – usually a clear resin in a solvent without the drying oil;
smoother harder finish than paints
194. Enamel - mixture of pigment in varnish; sometimes with thermosetting
resins which require baking. These baking enamels provide a toughness
and durability which is greater than ordinary paint and enamels.
Lacquers – thermoplastic materials dissolved in fast drying solvents
Vitreous enamels – enamel is a thin layer glass mixed with clay water
and metal oxides fused on to the surface of metal; applied by dip or
spray and fired at 800 deg C
- coatings on washers & dryers, aircraft parts
Metallizing – metal spray methods, thermal coat and plasma
Hot Dip Plating – zinc, tin and lead applied for corrosion protection by
molten metal hot dip process
- Applied molten zinc is called galvanizing, low cost
195. Electroplating – work piece serves as a cathode in an electrical circuit,
dc is applied, current travels through solution
- Thickness usually low .001”
- Rate dependant on the materials, current density, solution
temperature
- Cu, Ni, Cr, Cd, Zinc, tin, Ag and Au
- Corrosion/wear/abrasion resistance
- Results in a dimensional increase
- Usually attractive appearance
196. Chemical Conversions – convert surface metal to chemical
composition in a solution, the work piece is the anode
- Corrosion/wear protection
- Forms an oxide layer
- Little effect on part dimensions
Anodizing – aluminum is the anode
- Usually treated in sulfuric acid
- Can be colored for cosmetics
Chromate Coatings – treatment of zinc and magnesium for corrosion resistance
- Usually treated in chromate acid bath
- Improved adhesion of paint
Phosphate Coatings – treatment of steel in phosphoric acid/salts
- Nonmetallic coating
- Corrosion resistance
- Base for paint coatings
Chemical Oxide – treatment of steel in caustic soda solution heated to 150 deg C
- Cosmetic coating (blacking)
- Poor corrosion resistance
198. Quality Control and Inspection
- Quality control is usually a second step that makes use
of inspection data for process improvement
- Inspection varies with the quality desired
Organization of inspection
- Receiving
- In-process – inspection during manufacture
- First piece – verify a critical step before processing the
remainder of the lot.
- Final
199. Quantity of inspection – is driven by cost, it can be 0 – 100%,
sometimes statistical methods are used to determine sample sizes
- Acceptance sampling plans are essential when inspection cost
is high and the cost of replacing defective parts is low.
- Samples must be random & represent the lot
- Planners must determine the number of defective
parts that would be willingly accepted
- Most economical sample size is a compromise
between improved reliability and inspection costs
- 100% reliability is not always achievable - sample
sizes must take into account inspector fatigue,
monotony, psychological and hypnotic effects
Acceptance Sampling plans (check attributes)
- Used to determine the acceptance of entire lot based on
acceptance of attributes
- Are effective when the cost of inspection is high and the cost
of replacement is low.
200. Producers risk – P1
- If the lot had 1%
defects, there is a 6%
chance that this plan
would reject the lot
Consumers risk – P2
- There is a 10% chance
that a lot with 4%
defects might be
accepted
201. Process control charts – use of
statistical mathematics to control
processes
- Inspection values rarely fall outside of
lines, except when an assignable cause
exists. The variation of points inside the
control limits is from chance alone.
- Data collected is variable, not attributes
- Data collection is more costly but can
reveal more information
- Frequency distribution follows a normal
curve i.e. 99.73% of the measured
values from an entire population are
within +- 3 sigma
- 95.46% is +- 2 sigma
- 68.26% is +- 1 sigma
- Sigma is the Standard Deviation; a
measure of the dispersion of the
measured values
202. Chart construction:
- Process is examined to determine that it is normal i.e. follows a
normal curve
- All assignable causes have been eliminated
- Historical record is made by plotting the mean values of a
number of samples
- If the limits used are +- 3 sigma then not more than .2% of any
plotted points would be expected to fall outside the lines
- When a point does fall out the process is critically examined
for assignable cause
204. Tolerances – can be specified on the print, in notes or block
notes
205. Sources of variation
- Parallax - an illusion
- Temperature
- Equipment
- Human touch, sight and psychological effects
Inspection Equipment:
- Steel rules
- Vernier caliper
- Micrometer
- Sine bar or tables for accurate angle measurement
Indicating Gages And Comparators
- Dial indicators give indirect readings
- Pneumatic gages for pressure & flow i.e. dentist measures
nitrous oxide flow to a patient
- Optical comparators used for dimensions, shapes and
relationships
- Fixed gages - ring, radius, feeler gage, go no-go, plug
206. Surface finish
Surface variation
- Roughness – lines are close together
caused by machining or other process
- Waviness – lines wider than roughness
caused by deflection & warping
- Imperfections, affect fatigue strength
- Roughness often given as root mean
square RMS
Surface Measurements
- Visual comparison
- Electrical measurement, stylus
207. Tips:
•Read the study guide and use the big books as reference material
•Skim through the big books for concepts
•Go through the exam:
- Read each question and answer the easy ones first
- Reading each question will also give clues for answering other questions
- Go back through the exam and answer the questions that are more difficult
- Answer the complex questions last
Read each question carefully, consider the applicability of each answer to the
question – choose the answer that is correct most of the time
Example:
Which of the following is correct regarding liquid penetrant testing?
A. Liquid penetrant testing utilizes red visible dyes
B. Components shall be wire brushed prior to applying penetrant
C. PT a relatively simple method to inspect components for
surface breaking flaws
D. Because of corrosion hazards PT shall precede ultrasonic testing
•Do not leave questions unanswered:
- You will not lose points for wrong answers
- It is better to eliminate one or two of the answers and guess
