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BMEE209L: Module 4 Flashcards

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1
Q

What is creep?

A

Time dependent permanent deformation under a constant load or constant stress at high temperatures

Creep occurs even if the applied stress is less than the yield strength at that temperature.

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2
Q

What factors contribute to the creep of metallic materials?

A
  • Diffusion
  • Dislocation glide or climb
  • Grain boundary sliding

Polymeric materials also show creep, while ceramics do not.

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3
Q

What are the steps involved in ductile fracture?

A
  • Necking
  • Cavity formation
  • Cavity coalescence to form a crack
  • Crack propagation
  • Fracture
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4
Q

How does ductile fracture differ from brittle fracture?

A
  • Ductile fracture: extensive plastic deformation, stable crack
  • Brittle fracture: little plastic deformation, unstable crack propagation
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5
Q

What conditions can lead to brittle failure in normally ductile materials?

A
  • Low temperatures
  • High strain rates
  • Presence of sharp notches or defects
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6
Q

What common examples illustrate catastrophic brittle fractures?

A
  • Welded ships & tankers during WWII
  • Rails of railways in cold weather
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7
Q

What is the effect of temperature on the stress-strain curve?

A

Temperature affects the tensile properties of materials, influencing their ductility and toughness.

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8
Q

Define tensile toughness.

A

The energy absorbed by a material prior to fracturing, measured as the area under the true stress-strain curve.

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9
Q

What is hardness in materials?

A

A measure of a material’s resistance to localized plastic deformation.

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10
Q

What are the two modes of fracture in metals?

A
  • Ductile fracture
  • Brittle fracture
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11
Q

What is fracture toughness?

A

A measure of a material’s ability to withstand an applied load despite containing a flaw.

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12
Q

What are the three stages of brittle fracture?

A
  • Plastic deformation concentrates dislocations along slip planes
  • Microcracks nucleate due to shear stress
  • Crack propagates to fracture
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13
Q

What is the Ductile to Brittle Transition Temperature (DBTT)?

A

The temperature below which a ductile material behaves in a brittle manner.

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14
Q

How does impact loading affect materials?

A

It makes materials more brittle and decreases toughness.

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15
Q

What is the role of dislocation climb in creep?

A

Atoms move to or from the dislocation line by diffusion, allowing continued slip and deformation even at low applied stresses.

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16
Q

Describe the three stages of creep behavior.

A
  • Primary or transient creep: continuously decreasing creep rate
  • Steady-state creep: constant creep rate
  • Third-stage creep: accelerated deformation leading to failure
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17
Q

What is the relationship between applied stress and rupture time in creep?

A

Higher stress or temperature reduces the rupture time and increases the creep rate.

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18
Q

What is the endurance limit in fatigue testing?

A

The stress below which there is a 50% probability that failure by fatigue will never occur.

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19
Q

What is the role of the geometry factor in the stress intensity factor?

A

It varies based on the specimen and flaw geometry, influencing the calculation of fracture toughness.

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20
Q

What does the term ‘necking’ refer to in ductile fracture?

A

The localized deformation that occurs under tensile load, leading to a triaxial state of stress.

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21
Q

What is the significance of the S-N curve in fatigue testing?

A

It illustrates the relationship between stress and the number of cycles to failure.

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22
Q

Fill in the blank: The stress intensity factor is denoted by _______.

A

K

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23
Q

What are the three stages of creep behavior?

A
  1. Primary (transient) creep
  2. Secondary (steady-state) creep
  3. Tertiary creep
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24
Q

What characterizes primary (transient) creep?

A

Continuously decreasing creep rate

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25
What is the steady-state creep rate?
Slope of the secondary portion of the creep curve
26
What happens during tertiary creep?
Acceleration of the rate and ultimate failure
27
What is typically the main design consideration for short-life creep situations?
Time to rupture or rupture lifetime (tr)
28
What is fatigue in materials?
Lowering of strength or failure due to repetitive stress
29
What are the three stages of fatigue failure?
1. Crack initiation 2. Crack propagation 3. Sudden fracture
30
What is the endurance limit?
The stress below which there is a 50% probability that failure by fatigue will never occur
31
What do ferrous materials exhibit in terms of fatigue?
Profound endurance limit
32
True or False: Non-ferrous metals exhibit well-defined endurance limits.
False
33
What is hydrogen embrittlement?
Severe embrittlement caused by hydrogen in metals
34
What temperature range can lead to temper brittlement in steels?
450 - 590 °C
35
What is notch sensitivity?
The reduction of toughness due to notches caused by poor machining or design
36
What is the Ductile to Brittle Transition Temperature (DBTT)?
The temperature at which a material changes from ductile to brittle fracture
37
What is strain hardening?
Hardening of a ductile metal as it is plastically deformed
38
Fill in the blank: The phenomenon where ductile materials transform to brittle is called _______.
ductile to brittle transition (DBT)
39
What is the modulus of resilience?
The area under the elastic portion of a stress-strain curve
40
What does the S in the S-N curve represent?
Stress
41
What does the N in the S-N curve represent?
Number of cycles to failure
42
What is the theoretical value for stress amplitude below which a material will not fail called?
Fatigue limit or endurance limit
43
What enhances the strength and hardness of some metal alloys through phase transformations?
Precipitation hardening (age hardening)
44
What is the first step in the age-hardening heat treatment process?
Solution treatment
45
What is solid-solution strengthening?
Strengthening by alloying with solute atoms that reduce strain energy
46
What is the relationship between tensile strength and engineering stress-strain curves?
Tensile strength is the maximum stress on the engineering stress-strain curve
47
What is the definition of percent cold work?
The ratio of the original area of the cross-section to the area after deformation
48
True or False: BCC metals have ductile to brittle transition temperatures, while most FCC metals do not.
True
49
What is solid-solution strengthening?
A technique to strengthen and harden metals by alloying with elements that form substitutional or interstitial solid solutions. ## Footnote Alloying elements impose lattice strains on surrounding host atoms, restricting dislocation movement.
50
What is the modulus of elasticity?
The slope of the stress-strain curve in the elastic region, also known as Young’s modulus (E). ## Footnote It is a measure of the stiffness of a component.
51
What does the Hall-Petch equation relate to?
The relationship between yield strength (σy) and average grain diameter (d) in polycrystalline materials. ## Footnote The equation shows that smaller grain sizes lead to higher yield strength.
52
How does grain size affect mechanical properties?
Smaller grain sizes improve strength and toughness due to increased grain boundary area that hinders dislocation motion. ## Footnote High-angle grain boundaries make it difficult for dislocations to traverse, leading to dislocation pile-up.
53
What is the significance of dislocation motion in metals?
Macroscopic plastic deformation corresponds to the motion of large numbers of dislocations. ## Footnote Restricting dislocation motion enhances mechanical strength.
54
What are the units of strain rate?
s⁻¹. ## Footnote Strain rate is the rate at which strain develops in a material.
55
Define plastic deformation.
Deformation that does not return to the original shape after stress is removed. ## Footnote Example: A dent in a car.
56
What is the difference between viscous and viscoelastic materials?
Viscous materials do not return to original shape after stress; viscoelastic materials exhibit behavior between viscous and elastic. ## Footnote Anelastic is typically used for metals, while viscoelastic is associated with polymers.
57
What does tensile testing measure?
The resistance of a material to a static or slowly applied force. ## Footnote It provides information about strength, Young’s modulus, and ductility.
58
What is yield strength?
The stress at which a material begins to deform plastically. ## Footnote It can be determined using the 0.2% offset method.
59
What is ductility?
A measure of the degree of plastic deformation sustained until fracture. ## Footnote Ductility is expressed as percent elongation or percent reduction in area.
60
What is the significance of mechanical properties in aircraft manufacturing?
Materials must be lightweight, strong, and able to withstand cyclic mechanical loading. ## Footnote Aluminum alloys and carbon-reinforced composites are commonly used.
61
What is the definition of stress?
Force per unit area, with units of Pascals (Pa). ## Footnote Types of stress include tensile, compressive, shear, and bending.
62
What does Young's modulus represent?
The slope of the tensile stress-strain curve in the linear regime. ## Footnote It reflects the material's stiffness.
63
What happens to dislocations at high-angle grain boundaries?
Dislocations tend to pile up at grain boundaries during deformation. ## Footnote This makes it more difficult for dislocations to traverse from one grain to another.
64
Fill in the blank: The _______ is a measure of the stiffness of a material.
Young's modulus.
65
True or False: The Hall-Petch equation is valid for both very large and extremely fine grain sizes.
False.

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