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materials engineering lab > Test 2: creep, fatigue, cold work, micro hardness > Flashcards

Test 2: creep, fatigue, cold work, micro hardness Flashcards

1
Q

Creep (def)

A

permanent deformation of materials when subjected to a constand load/stress for a long period of time

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2
Q

What is often the limiting factr in a part’s lifetime?

A

Creep phenomenon

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3
Q

How is temperature related to Creep Fx’s?

A

A higher temp gives more severe creep fx’s. The temperature a material starts to creep depends on it’s melting point In metals if temp is >40% of it’s melting temp then creep is significantly affected

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4
Q

How is temperature related to Creep Fx’s?

A

A higher temp gives more severe creep fx’s. The temperature a material starts to creep depends on it’s melting point In metals if temp is >40% of it’s melting temp then creep is significantly affected

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5
Q

In what applications does Creep need to be watched closely?

A

Displacement-Limited applications , Rupture-limited, stress-relaxation limited

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6
Q

Describe Displacement limited applications

A

those that must maintain precise dimensions. Ex) turbine rotos in jet engines.

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7
Q

Describe Rupture limited applications

A

those in which precise dimensions are not essential but fx must be avoided Ex) high pressure steam tubes and pipes

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8
Q

Describe Stress-relaxation limited applications

A

those in which an initial tension relaxes with time Ex) in suspended cables and tightened bolts.

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9
Q

What is the generalized creep behavior?

A

Primary or transient creep occurs first then the creep rate continuosly decreases. On a creep strain v. time graph, this is shown as slope of curve diminishing with time.

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10
Q

What are 4 variable that affect creep rate?

A
  1. Material properties 2. Exposure time 3. Exposure temperature 4. Applied load
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11
Q

For creep: If either stress increases or temperature increases what happens?

A
  1. instantaneous strain at the time of stress application increases
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12
Q

For creep: If either stress increases or temperature increases what happens?

A
  1. instantaneous strain at the time of stress application increases 2. the steady-state creep rate is increased 3. The rupture lifetime is diminished
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13
Q

Describe the tensile creep testing method

A

a constant tensile load is applied on specimen at a specific temperature. the heating element allows for creep testing at different temperatures.

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14
Q

Describe the compressive creep testing method

A

A compressive load is applied on the specimen at a specific temperature

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15
Q

Describe the Flexural Creep testing method

A

A load is applied to a simply supported specimen at mid-span

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16
Q

How is a creep test generally conducted

A

Conducted until a given amount of deflection is reached, then the rate of deformation v. time is plotted.

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17
Q

What is a creep-rupture test?

A

if creep test is continued till fx occurs

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18
Q

What is “minimum/ steady state creep rate”?

A

An engineering design parameter considered for long-life applications. Graph: slope of secondary portion of creep curve

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19
Q

What is “time to rupture”

A

Time required till rupture at a constant load.

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20
Q

What is “time to rupture”

A

Time required till rupture at a constant load. Used for relatively short-life creep situation.

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21
Q

What is the “creep modulus”?

A

ratio of initial applied stress to creep strain at a specific time

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22
Q

What is “creep strength/limit”?

A

The highest stress that a material can withstand for a specified length of time without exceeding the specified deformation at a given temperature.

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23
Q

What is the Larson-miller parameter procedure used for?

A

It is a data extrapolation technique. A creep/rupture test is performed at temperatures in excess of those required for a shorter amount of time with a comparable stress level. The data is extrapolated in order to estimate how a material will act outside of the lab setting

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24
Q

Draw the larson-miller parameter plot and describe the equation

A
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25
Q

What is fatigue?

A

The Progressive, localized & permanent structural damage that occurs when a material is subjected to cyclic or fluctuating load.

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26
Q

Describe mechanical value relationships for a material under fatigue.

A

Maximum stress values are less than the ultimate tensile stress limit and may be below the yield stress limit of the material

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27
Q

List 5 stages of fatigue failure

A
  1. Cyclic plastic deformation prior to fatigue crack initiation
  2. Nucleation (initiation of fatigue cracks at flaws on surface)
  3. Short crack or small crack phase
  4. Crack propagation or growth rates,once initiated cracks grow at accelerating rate
  5. final instability or failure as crack propogates through remaining cross-sectional area.
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28
Q

What factors are necessary to cause fatigue fracture?

A
  1. applied stress of sufficient magnitude
  2. large variation of fluctuation in applied stress
  3. sufficiently large number of cycles of the applied stress
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29
Q

What are three common ways in which stresses may be applied?

A

Axial, torsional, and flexural.

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30
Q

Elastic Deformation

A

Materials return to their original shape after a small load/stress is applied.

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31
Q

Plastic Deformation

A

Deformation that remains after the load causing it is removed.

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32
Q

Anisotropic

A

A material not possessing the same properties in all directions

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33
Q

Isotropic

A

A material possessing the same properties in all directions

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34
Q

Grain Growth

A

The increase in average grain size of polycrystalline materials, proceeds by grain boundary motion

35
Q

Recrystallization

A

the formation of the new set of grains that are strain free. material becomes softer and more ductile

36
Q

How can a plastically deformed metal specimen be restored to it’s predeformed state?

A

Microstructural and mechanical characteristics can be restored to their predeformed states by appropriate heat treatment where recovery, recrystallization, and grain growth occur

37
Q

What occurs during recovery?

A

There is a reduction in dislocation density and alterations in dislocation configurations

38
Q

Cold working

A

AKA: work hardening, strain hardening.

An increase in hardness and strength caused by plastic deformation at temperatures below the recrystallization range.

39
Q

Hot working

A

Plastically deforming metal at such a temperature and strain rate that recrystallization takes place simultaneously with the deformation, thus avoiding strain hardening.

40
Q

What are 4 hot working processes?

A
  1. Hot rolling
  2. Forging
  3. Extrusion
  4. Hot drawing
41
Q

Describe the Hot rolling process

A

A material is passed through two rolls that rotate in opposite directions and the gap is smaller than the material. The deformation produces elongated grains.

42
Q

Describe the Hot working process, Forging

A

Forming a hot or cold metal into a fixed shape by hammering, upsetting, pressing, or rolling

43
Q

What kind of process is Extrusion? Describe it

A

Bulk deformation process.

A bar of metal is forced through a die oriface by a compressive force that is applied to a ram. the extruded piece that emerges has the desired shape and a reduced cross sectional area.

44
Q

Describe the Hot working Process, Drawing

A

Pulling a metal piece through a die which has a tapered bore. Tensile force is applied on the exit side.

Results in reduction in cross section with corresponding increase in length

45
Q

What are 4 cold working processes?

A
  1. Squeezing/cold rolling
  2. Bending
  3. Shearing/ Blanking
  4. Drawing (into sheet metal or wire/rod/tubing)
46
Q

What are advantages of Cold working over hot working? (7)

A
  1. No heat
  2. possibility of different textures on top and bottom of a sample
  3. Better surface finish
  4. superioir dimensional control
  5. better reproducability and interchangeability
  6. Improved strength, fatigue, and wear properties
  7. minimization of contamination
47
Q

What are disadvantages of cold working? (6)

A
  1. Higher forces required to initiate deformation
  2. heavier and more powerful equipment required
  3. less ductility avaliable
  4. Metal surfaces must be clean and scale free
  5. intermediate anneals may be required to compensate for the loss of ductility
  6. Undesirable residual stresses may be produced
48
Q

How does cold working affect mechanical properties?

A
  1. increase yield strength
  2. increase ultimate tensile strength
  3. Reduce ductility
49
Q

What happens to the bulk of a material as cold working is carried out?

A
  • A material becomes more brittle as it is cold worked.
  • Dislocations are increased making it more difficult for plastic deformation to occur
50
Q

Describe why roll configurations are important

A
  • Rolls will experience elastic deformation when milling materials so the rolls must be combined to reduce the deformation.
  • Small rolls are advantageous because less force is needed to deform a material.
  • If rolls are on different sides of the main rolls it is called a cluster.
  • Named as rolls stacks from bottom to top.
51
Q

How are grains changed due to deformation?

A
  • Since volume of the material must stay the same, the area is reduced while length is increased.
  • Grains are elongated in the direction of rolling.
  • For forging, grains are “smashed” alond the width, length and height
52
Q

Describe Anisotropic grain behavior due to cold rolling.

A
  • Hall Petch Equation
    • as grains get smaller, the strength increases
  • Cold rolling results in
    • SL: short longitudinal
      • along major long axis, long grains but less strength
    • ST: Short transverse
      • along width, small round grains, greatest strength
    • LT: long transverse
53
Q

When cutting a anisotropic material what should be considered?

A

When blanking/cutting a piece out of a cold rolled material, the orientation of the cold rolled direction should be noted since anisotropic has different properties in different directions.

54
Q

What is metallography?

A

The analysis of structral characteristics of a metal/ alloy in relation to its physical and mechanical properties. Done by preparing a metal surface for analysis by grinding, polishing, and etching to reveal microstructual constituents. The sample can then be analyzed using macro/microscopy

55
Q

What is the difference between what macroscopy and microscopy examine?

A
  • Macroscopy
    • reveals details such as uniformity or structure, presence of defects
  • Microscopy
    • reveal structural details such as grain size, shape and distribution of secondary phases, and non-metallic inclusions
    • can illustrate history of mechanical and heat treatments given to the metal or alloy
56
Q

What are typical applications of Microscopy?

A
  • Microhardness testing
  • grain size measurement
  • study of the distribution of secondary phases
  • observation of non-metallic inclusions
  • locating cracks and defects in the specimen
57
Q

What are the stages of preperation for microhardness testing?

A
  1. Sectioning
  2. mounting
  3. coarse grinding
  4. fine grinding
  5. rough polishing
  6. fine polishing
  7. etching
58
Q

What does mounting a sample involve?

A

Mounting a sample gives a safe way to hold the sample during griding and polishing operations.

Samples are mounted in phenolic, bakelite, or epoxy resin

59
Q

Describe grinding preperation for microhardness testing

A
  • Grinding removes epoxy layer and prepares specimen for further polishing.
  • removes sharp/jagged edges
  • surface must be kept cool to control heat generated
60
Q

Describe Rough polishing for microhardness testing

A
  • ROugh polishing removes surface deposits/levels irregularities
  • prepares surface for further polishing
  • top &/or bottom surfaces are made parallel and flat by polishing with emery paper. (size range decrease: 250-800 grit by abrasive paper)
61
Q

Describe fine polishing for microhardness testing

A
  • Systematic abrading of the specimen with a series of grits of decreasing coarseness to produce a smooth surface required for microscopic examination
  • used to make sample scratch free
  • uses abrasive suspension on a rotating wheel
62
Q

What are some commonly used abrasives for microhardness testing?

A

Alumina, chromium oxide, diamond dust, magnesium oxide

63
Q

Describe Etching for microhardness testing

A

Used when it is necessary to observe the microstructure & grain size in specimen.

After polishing the microstructural constituents are revealed by using a suitable chemical or electrolytic etchant

64
Q

What occurs during a microhardness test (after preperation)?

A
  • Static indentation made with loads NOT EXCEEDING 1 kgf. (15-1000gf)
  • One Indenter which is either
    • Vickers Diamond pyramid
    • Knoop elongated diamond pyramid
  • On microscopic scale with high precision instruments
65
Q

What are typical evaluations that microhardness testing is used for?

A
  1. Bulk hardness of small samples
  2. hardness of welds
  3. hardness of thin coatings
  4. testing very thin materials like foils
66
Q

Vickers Hardness number

A

Applied load (kgf) divided by surface area of indentation. where d of SA is the average of d1 and d2

67
Q

Describe how the Vickers Hardness test works

A
  • Diamond indenter wuth square-bases pyramid shape with angle of 136 degrees between opposite faces
  • load: 10gf-1000gf
  • force maintained for 10-15 sec
  • diagnals of indentation are measure and applied in formula to get HV (vickers hardness number)
68
Q

What are disadvantages of Vickers test?

A
  • Need to optically measure the indent size
  • time consuming
69
Q

Describe the process of bending

A

Cold working. Plastic deformation of materials about a linear axis with little or no change in surface area

70
Q

Describe Shearing/blanking as a working process

A

Cold working.

The mechanical cutting of materials without the formation of chips or the use of burning or melting.

71
Q

Describe Drawing of sheet metal as a process

A

Cold working.

Forming of parts where plastic flow occurs over a curved axis

72
Q

Describe drawing of wire/rod/tubing as a process

A

reducing the cross section of the material by pulling it through a die

73
Q

Reversed stress cycle

A
  • One of the 3 stress cycles with which loads may be applied to the sample.
  • simplist stress cycle
  • sine wave
  • max stress and min stress differ by neg. sign
74
Q

Repeated stress cycle

A
  • one of three stress cycles with which loads may be applied to sample
  • most common
  • max stress and min stress are asymmetric
  • sine wave
75
Q

Random variation of stress and frequency

A
  • Once of 3 stress cycles with which loads may be applied to the sample
  • random variation
76
Q

Describe Low-cycle fatigue

A

something can be bent past its yield point (it will stay bent) without breaking, but repeated loading on the same section will cause material failure.

77
Q

List a few factors which affect fatigue life (10)

A
  1. geometry
  2. stress concentration
  3. residual stresses
  4. size and distribution of internal defects
  5. direction of loading
  6. grain size
  7. environment
  8. temperature
  9. surface quality
  10. material type
78
Q

How do engineers design against fatigue?

A
  1. infinite lifetime concept: design to keep stress below threshold of fatigue limit
  2. design, conservatively, for a fixed life after a certain amount of time
  3. damage tolerant design: instuct user to inspect part periodically and replace part if crack is of a critical length
79
Q

What is an S-N curve?

A

Stress v. Cycles to failure. Uses “stress amplitude” plotted on y axis and logarithm of # of cycles to failure on x axis.

80
Q

Fatigue life

A

The number of cycles that will cause failure at a certain stress level . on S-N plot

81
Q

Fatigue limit

A

A characteristic of the material and its geometry. If a material is loaded below the fatigue limit, the material will not fail, regardless of number of cycles it is subject to. On S-N plot

82
Q

Fatigue strength

A

Stress at which failure occurs for a given number of cycles, On S-N plot

83
Q

Draw an S-N plot

A
84
Q

What are 4 types of fatigue testing?

A
  1. Reversed stress: tension-compression
  2. Repeated Stress: tension-tension
  3. Irregular or random
  4. Flexural fatigue: sheet and plate bending

materials engineering lab (2 decks)

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