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Elements > Chapter 8 > Flashcards

Chapter 8 Flashcards

1
Q

What is a simple fracture?

A

The separation of a body into two or more pieces in response to a static stress

Propagation of cracks accompanies fracture

Two general types of fracture

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2
Q

What are the two general types of fracture?

A

Ductile and brittle

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3
Q

What are three properties of ductile fracture?

A

1) slow crack propagation
2) accompanied by significant plastic deformation
3) fails with warning

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4
Q

What are three properties of brittle fracture?

A

1) rapid crack propagation
2) little or no plastic deformation
2) fails without warning

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5
Q

Are ductile or brittle cracks more desirable?

A

ductile fracture

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6
Q

What kinds of fracture have large necking? No necking?

A

Very ductile: large necking
Moderately ductile: slight necking
Brittle: no necking

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7
Q

What’s the name for the shape a ductile metal makes at fracture?

A

cup-and-cone fracture

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8
Q

What does a brittle fracture look like?

A

flat surface

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9
Q

What could a pipe’s ductile fracture look like? Brittle fracture?

A

ductile: one piece, large deformation

brittle: many pieces, small deformations

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10
Q

What are the five stages of moderately ductile failure?

A

1) necking
2) void nucleation
3) void growth and coalescence
4) crack propagation
5) failure

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11
Q

What do the surface photographs of a brittle fracture look like?

A

They display V-shaped, chevron markings

The V features point to the crack initiation site

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12
Q

What are the two types of crack propagation?

A

Intergranular crack propagation (between grains)

Transgranular crack propagation (through grains)

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13
Q

Why does fracture occur?

A

As a result of crack propagation

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14
Q

Why are measured fracture strengths of most materials much lower than predicted?

A

1) microscopic flaws (cracks) always exist in materials
2) magnitude of applied tensile stress amplified at the tips of these cracks

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15
Q

How do you calculate the stress at the crack tip?

A
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16
Q

How do you calculate the stress concentration at the crack tip?

A
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17
Q

Is stress concentration higher for sharp cracks or blunt tips?

A

sharp cracks because they cracks spread with lower stresses

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18
Q

What are the tip shapes for cracks of brittle materials? Ductile materials?

A

brittle: sharp

ductile: blunt (because of a deformed region)

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19
Q

Which cracks grow first?

A

The largest, most highly stressed cracks

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20
Q

When does crack propagation & fracture occur?

A

When the stress at the crack tip (σm) > (σc) critical stress for crack propagation

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21
Q

What is fracture toughness?

A

Measure of material’s resistance to brittle fracture when a crack is present

qualitatively scales with impact resistance

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22
Q

What is the variable for fracture toughness? How do you calculate it?

A

K sub c

Y=1

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22
Q

When is Kc (fracture toughness) independent of thickness?

A

When a specimen thickness is much greater than crack dimension

23
Q

What is plane strain fracture toughness?

A

characterizes the ability of a material to resist the propagation of a crack or fracture under conditions of plane strain. Plane strain conditions occur when the stress state is such that the deformation in one direction is constrained, meaning that the material cannot expand in that direction. This often happens in thick structures where the deformation is constrained in one dimension

Thickness is greater than the crack dimension so Kc is independent of thickness

K sub (ic)

24
Q

Is plane strain fracture toughness high or low for ductile materials? brittle?

A

ductile: high

brittle: low

25
Q

What is the formula for crack growth condition?

A
26
Q

What do the plots of
1) critical stress for crack propagation (σc) v. crack length
2) inverse
look like?

A
27
Q

What will the parameter Y be consistent with?

A

The same component and alloy

28
Q

What test do you use to test ductile materials for brittle failure? How do you compute the impact energy?

A

Impact Test

Compute the impact energy from the difference between initial height h and final height h’

  • charpy and izod
29
Q

What are three test conditions promoting brittle fracture in impact tests?

A

1) high strain rate
2) deformation at low temperatures
3) presence of a notch

30
Q

What is the influence of T on impact energy?

A

there are three kinds of behavior observed for the different crystal structures/strength materials

31
Q

What kind of metals have a ductile-to-brittle transition temperature? At what temperature should they be used?

A

BCC metals

Only used at temperatures where ductile

32
Q

What is fatigue?

A

Failure under lengthy period of repeated stress or strain cycling

33
Q

What are the key parameters to fatigue?

A

S (stress amplitude), σm (stress at crack tip), and cycling frequency

34
Q

What are the two key points about fatigue?

A

1) It can cause part failure even if they applied stress < critical stress

2) It’s responsible for 90% of mechanical engineering failures

35
Q

How is fatigue data plotted?

A

Stress amplitude (S) vs. log of number N of cycles to failure

36
Q

What are the two types of fatigue behavior observed?

A

1) Fatigue limit: Sfat:

2) no fatigue limit if S<Sfat
- some materials there is no fatigue limit

37
Q

What is the fatigue life Nf?

A

Toal number of stress cycles to cause fatigue failure at specified stress amplitude

38
Q

What are the three general techniques to improve fatigue life?

A

1) Reducing magnitude of mean stress: think of the SN graph
2) surface treatments: shot peening and carburizing
3) design changes: removing stress concentrations

39
Q

How does decreasing mean stress affect fatigue life? What does the graph look like?

A

It increases fatigue life

40
Q

How do compressive surface stresses affect surface hardness?

A

Compressive surface stresses increase surface hardness by suppressing surface cracks from growing

41
Q

What are the two surface treatments for imposing compressive surface stresses & improving fatigue life?

A

1) Shot peening: surface compressive stress due to plastic deformation of outer surface layer
2) Carburizing: carbon atoms diffuse into outer surface layers

42
Q

What is a design change to improve fatigue life?

A

replace sharp corners with rounded corners to reduce stress concentration

43
Q

What is creep?

A

ΔE/Δt

A measure of deformation (strain) vs. time at constant stress

44
Q

What temperature does creep occur at in most metals?

A

elevated temperatures

T> 0.4(Tm)

45
Q

What are the three phases of creep? What are on the x and y axis?

A

1) primary creep: slope (creep rate) decreases with time
2) secondary creep: steady-state (constant slope)
3) tertiary creep: slope (creep rate) increases with time, so an acceleration of rate

46
Q

How does creep behave with increasing temperature and stress?

A

The steady-state creep rate increases with increasing T and stress

The slope of the curve increases and gets shorter

Rupture lifetime (tr) decreases

47
Q

What is steady-state creep rate?

A

The graph is linear because strain hardening is balanced by recovery. The slope has a dependence on T and stress (increases)

48
Q

What does a steady-state creep graph look like?

A
49
Q

How do material scientists predict creep rupture lifetime?

A

It’s impractical to wait years, so they increase the temperature and Larson-Miller parameter (m). Then solve it using the graph (psi v. m) and relevant equation.

50
Q

What is fatigue failure? What are two important things to remember?

A

Stress fluctuations with time

1) occurs at applied stress < TS
2) Important parameters: fatigue limit, fatigue strength/lifetime

51
Q

what is tr?

A

Creep Rupture lifetime

52
Q

What are the two categories of fatigue?

A

Stress-controlled or strain-controlled

53
Q

What are the most heavily used structural materials?

A

Copper alloys

54
Q

What are three reasons for fracture?

A

static stress, fatigue, and creep

Elements (15 decks)

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