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Engineering Materials > Chapter 7 - Fracture & Failure > Flashcards

Chapter 7 - Fracture & Failure Flashcards

1
Q

What is a fracture?

A

The separation of a body into pieces due to stress at temperatures below melting point

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2
Q

What are the 2 steps in fracture?

A
  • Crack formation

- Crack propagation

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3
Q

What are the 2 fracture models?

A

Ductile or Brittle

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4
Q

What are the characteristics of a ductile fracture?

A
  • occurs in most metals
  • extensive plastic deformation ahead of crack
  • Crack is ‘stable’
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5
Q

What are the characteristics of a brittle fracture?

A
  • Occurs in ceramics, ice & cold materials
  • Little plastic deformation
  • Crack is unstable
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6
Q

What are the characteristics of a ductile matetrial?

A
  • Again a lot of plastic deformation

- and a lot of energy absorption

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7
Q

What are the characteristics of a brittle material?

A
  • Little plastic deformation

- Low energy absorption

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8
Q

Describe a very ductile materials fracture?

A

It looks like two spear tips pointed towards each other

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9
Q

Describe a moderately ductile materials fracture?

A

chubbier with teeth

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10
Q

Describe a brittle materials fracture?

A

straight cut through

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11
Q

What type of dislocation occurs in a ductile fracture?

A

Mediated dislocation

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12
Q

Describe the dislocation that occurs in a brittle fracture?

A

Limited dislocation mobility

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13
Q

Describe the process of a brittle fracture?

A
  • No plastic noticeable plastic deformation
  • Crack propagation is very fast
  • Crack propagates perpendicular to the direction of applied stress
  • Crack propagates by cleavage
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14
Q

What does cleavage mean?

A

The braking of atomic bonds along specific crystallographic planes

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15
Q

What are the 2 types of brittle fracture?

A
  • Transgranular

- Intergranular

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16
Q

Describe a transgranular fracture?

A
  • Fracture cracks pass through the grains

- Fracture surface is faceted

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17
Q

Describe a intergranular fracture?

A
  • Fracture crack propagation is along grain boundaries which are weakened by impurities
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18
Q

What is a stress riser?

A
  • Can be Macro or Microscopic

- these are flaws in th material caused by a reduction of the cross sectional area

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19
Q

What is the fracture strength of a brittle solid related to?

A

The cohesive forces between atoms

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20
Q

What is a lower fracture strength explained by?

A

The effect of stress concentration at microscopic flaws

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21
Q

what is the effect of stress concentration at microscopic flaws?

A
  • applied stress is amplified at the tips of micro-cracks, voids, notches etc that are called stress raisers
  • The magnitude of the amplification depends on the orientation of the micro-cracks
22
Q

When temperature decreases what can happen to a material?

A

It can transition from ductile to brittle

23
Q

What process can help raise the temperature that the ductile to brittle transition occurs at?

A

Alloying

24
Q

What temperature do brittle fractures occur at?

A

Around 4°C

25
Q

What applied stressed will cause fatigue?

A
  • Axial (compression/tension)
  • Flexural (bending)
  • Torsional (twisting)
26
Q

What are the fatigue failure steps?

A
  • Crack initiation
  • Gradual crack propagation
  • Final catastrophic failure
27
Q

What is fatigue?

A

When a material will fail at a considerably lower stress than the tensile or yield strength due to cyclic stresses

28
Q

Name the 3 cyclic stresses?

A
  • Reversed stress cycle
  • Repeated stress cycle
  • Random stress cycle
29
Q

Describe the characteristics of the reversed stress cycle?

A

Periodic and symmetric about the zero stress

30
Q

Describe the characteristics of the repeated stress cycle?

A

Periodic and asymmetrical around the zero stress

31
Q

Describe the characteristics of the random stress cycle?

A

Random stress fluctuations

32
Q

What is the stress ratio?

A

R = sigma(min)/sigma(max)

33
Q

Tensile stresses are ……..

Compressive stresses are ……

A

Positive

Negative

34
Q

How are fatigue properties tested?

A

In rotating bending tests in fatigue testing apparatus

35
Q

What does a low cyclic fatigue mean?

A

High loads, plastic and elastic deformation

36
Q

What does a high cyclic fatigue mean?

A

Low loads, elastic deformation

37
Q

What is the fatigue limit?

A

A maximum stress amplitude below which the material never fails no matter the number of cycles

38
Q

The fatigue limit only occurs in some materials, name two?

A

Iron (Fe) & Titanium (Ti) alloys

39
Q

What is fatigue strength?

A

The stress at which fracture occurs after a specified number of cycles

40
Q

What is fatigue life?

A

The number of cycles to fail at a specified stress level

41
Q

What quality determines the crack initiation stage?

A

The quality of the surface of the material

42
Q

What happens during the first stage of crack propagation?

A
  • Slow propagation along crystal planes
  • High resolved shear stress
  • Involves just a few grains
  • Flat fracture surface
43
Q

What happens during the second stage of crack propagation?

A
  • Faster propagation perpendicular to the applied stress
  • Crack grows (blunting and sharpening process at tip)
  • Rough fracture surface
44
Q

What are the three stages of fatigue failure?

A
  • Crack initiation in areas of stress concentration
  • Incremental crack propagation
  • Final rapid crack propagation
45
Q

What are the factors that affect fatigue life?

A
  • Magnitude of stress (Mean, Amplitude etc)
  • Quality of the surface (scratches, sharp transition)
  • Thermal fatigue
  • Corrosion fatigue
46
Q

What is corrosion fatigue?

A
  • Chemical reactions induce pits which act as stress raisers

- Corrosion also enhances crack propagation

47
Q

What is thermal fatigue?

A
  • When thermal cycling causes expansion and contraction, hence thermal stress, if the component is restrained
48
Q

What is creep?

A

Is a time dependent and permanent deformation when a material is subjected to high temperatures
E.G. Turbine blades

49
Q

What are creep rates used for?

A

Evaluating materials for anything that involves high temperatures

50
Q

What are the stages of creep?

A

1) instant deformation
2) Primary/transient creep
3) Secondary/steady-state creep
4) Tertiary

51
Q

How does an increase in stress or temperature effect the creep?

A
  • instant strain increases
  • the steady state creep rate increases
  • the time to rupture increases
52
Q

Creep is generally minimized in materials with two factors?

A
  • High melting point

- High elastic modulus

Engineering Materials (31 decks)

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