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Material Science > Chapter 8 > Flashcards

Chapter 8 Flashcards

1
Q

Define “simple fracture.”

A

The separation of a body into two or more pieces in response to an imposed stress that is static and at temperatures that are low relative to the melting temperature of the material.

Ref: Pg 211

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2
Q

For metals, what two fracture modes are possible?

A

Brittle and ductile.

Ref: Pg 211

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3
Q

What is the general difference between brittle and ductile fracture?

A

Ductile metals typically exhibit substantial plastic deformation with high energy absorption before fracture. However, there is normally little or no plastic deformation with low energy absorption accompanying a brittle fracture.

Ref: Pg 211

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4
Q

What are the two steps to any fracture process?

A

Crack formation and propagation.

Ref: Pg 211

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5
Q

With respect to crack propagation, describe ductile fracture.

A

Extensive plastic deformation in the vicinity of an advancing crack. The process proceeds slowly as the crack length is extended.

Ref: Pg 211

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6
Q

When is a crack said to be “stable?”

A

When it resists any further extension unless there is an increase in the applied stress.

Ref: Pg 211

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7
Q

With respect to crack propagation, describe brittle fracture.

A

Cracks may spread extremely rapidly, with very little accompanying plastic deformation.

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8
Q

When is a crack considered to be “unstable?”

A

Crack propagation, once started, continues spontaneously without an increase in the magnitude of the applied stress.

Ref: Pg 211

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9
Q

What are the two reasons why ductile fracture is usually preferred to brittle fracture?

A
  1. Brittle fracture can occur suddenly and catastrophically without any warning.
  2. More strain energy is required to induce ductile fracture.

Ref: Pg 211

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10
Q

What is a microvoid?

A

A small cavity that forms in the interior of a structure under tensile plastic deformation.

Ref: Pg 212

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11
Q

Microvoids enlarge, come together, and coalesce to form an _____, which has its long axis _____ to the stress direction.

A

Eleipical crack

perpecdicular

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12
Q

A “cup and cone” fracture is related to which type of fracture?

A

Ductile

Ref: Pg 212

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13
Q

When surveying a “cup and cone” fracture under an electron microscope which characteristic of plastic deformation can be observed?

A

Numerous small spherical dimples. Each dimple is on half of a microvoid that formed and then separated during the fracture process.

Ref: Pg 213

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14
Q

In brittle fracture, the direction of crack motion is very nearly ____ to the direction of the applied tensile stress.

A

Perpendicular.

Ref: Pg 213

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15
Q

True or False

Fracture surfaces of materials that fail in a brittle manner have distinctive patterns that differ from ductile failure.

A

True

Ref: Pg 213

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16
Q

What are some examples of surface features indicating a brittle failure?

A

A series of V-shaped chevron marking may form near the center f the fracture cross-section that points back toward the crack initial site.

Lines or ridges that radiate from the origin of the crack in a fanlike pattern.

Ref: Pg 213

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17
Q

True or False

For very hard and fine-grained metals there is no discernible fracture pattern.

A

True

Ref: Pg 214

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18
Q

Brittle fracture in amorphous materials, such as ceramic glasses, yields a _______.

A

Relatively shiny smooth surface.

Ref: Pg 214

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19
Q

Define “cleavage” with respect to brittle crystalline materials.

A

For most brittle crystalline materials, crack propagation corresponds to the successive and repeating breaking of atomic bonds along specific crystallographic planes.

Ref: Pg 214

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20
Q

Cleavage is said to be a transgranular fracture. Explain what this means.

A

The fracture cracks pass through the grains.

Ref: Pg 214

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21
Q

In some alloys, crack propagation is along grain boundaries; this fracture is termed ____.

A

Intergranular.

Ref: Pg 215

22
Q

The measured fracture strength for most materials are significantly _____ than those predicted by theoretical calculations based on atoms bonding energies.

A

Lower

Ref: Pg 216

23
Q

Explain why the measured fracture strength for most materials is significantly lower than those predicted by theoretical calculations.

A

This discrepancy is explained by the presence of microscopic flaws or cracks that always exist under normal conditions at the surface and within the interior of a body of material.

Ref: Pg 216

24
Q

A crack tip is sometimes called a “stress raiser.” Explain why.

A

At crack tips, applied stress is significantly amplified.

Ref: Pg 216

25
Q

True or False

The effect of a stress raiser is more significant in brittle than in ductile materials.

A

True

Ref: Pg 217

26
Q

Describe in simple terms the ductile-to-brittle transition.

A

For some materials, whether or not they would exhibit a ductile or brittle fracture depends on the temperature of the material.

Ref: Lecture Slides

27
Q

What is the Charpy Test used for?

A

Used to study the energy absorption of materials at differing temperatures.

Ref: Lecture slides

28
Q

Ductile materials have a ____ energy absorption.

A

High

Ref: Lecture Notes

29
Q

Brittle materials have a ____ energy absorption.

A

Low

Ref: Lecture Notes

30
Q

What are the three steps to yield fatigue failure?

A
  1. Crack formation due to stress concentrations and the presence of free surfaces.
  2. Crack Growth
  3. Fracture

Ref: Lecture Slides

31
Q

What is fatigue?

A

A form of failure that occurs in structures subjected to dynamic and fluctuating stresses.

Ref: Pg 229

32
Q 
True or False
Under fatigue (cyclic loading) it is possible for failure to occur at a stress level considerably lower than the tensile or yield strength for a static load.
A

True

Ref: Pg 229

33
Q

What type of failure is most common?

A

Fatigue failure

Ref: Pg 229

34
Q

True or False
For some ferrous, and titanium alloys, the S-N curve becomes horizontal at higher N values. This is a limiting stress level called the fatigue or endurance limit.

A

True

Ref: Pg 231

35
Q

The fatigue limit represents ______.

A

The largest value of fluctuating stress that will not cause failure for essentially an infinite number of cycles.

Ref: Pg 232

36
Q

True or False

Most non-ferrous alloys do not have a fatigue limit.

A

True

Ref: Pg 232

37
Q

Define “Fatigue Strength.”

A

For non-ferrous alloys that do not have a fatigue limit, it is the point at which failure occurs for some specified number of cycles.

Ref: Pg 232

38
Q

Define “Fatigue Life.”

A

The number of cycles to cause failure at a specified stress level.

Ref: Pg 232

39
Q

Give some examples of places where crack nucleation sites would be.

A

Surface scratches
sharp fillets
keyways, threads, and dents

Ref: Pg 236

40
Q

When do beachmarks take place?

A

When the material experiences interruptions in the crack propagation.

Ref: Pg 236

41
Q

Each beachmark band represents ____.

A

A period of time over which crack growth occurred.

Ref: Pg 236

42
Q

What is a striation?

A

The tiny advancing distance of a crack front during a single load cycle.

Ref: Pg 236

43
Q

True or False

There may be thousands of striations within a single benchmark.

A

True

Ref: Pg 236

44
Q

An area having a dull fibrous texture is indicative of ____.

A

Rapid failure

Ref; Pg 237

45
Q

True or False

Benchmarks and striations do not appear on the region over which the rapid failure occurs.

A

True

Ref: Pg 237https://assets.grammarly.com/emoji/v1/1f610.svg

46
Q

What is “creep” with respect to material deformation?

A

The tendency of a solid material to move slowly or deform permanently under the influence of persistent mechanical stresses. It can occur as a result of long-term exposure to high levels of stress that are still below the yield strength of the material. More severe in materials that are subjected to heat for long periods.

Ref: Wikipedia

47
Q

What is “primary creep?”

A

The rate of creep decreases with time because the material goes through strain hardening.

Ref: Lecture Slides

48
Q

Describe “secondary creep.”

A

The rate of creep has a constant slope. It does not speed up or slow down. This is referred to as the work stage o the material.

Ref: Lecture Slides

49
Q

Describe “tertiary creep.”

A

The slope of the creep rate increases with time.

Ref: Lecture Slides

50
Q

Which stage of creep has the longest duration?

A

Secondary

Ref: Pg 241

51
Q

What is the most important parameter from a creep test?

A

The rate of creep with respect to time during secondary creep. This is often called the minimum or steady-state creep rate.

Ref: Pg 241

Material Science (15 decks)

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