Chapter 8

Question 1

Define “simple fracture.”

Answer 1

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

Question 2

For metals, what two fracture modes are possible?

Answer 2

Brittle and ductile.

Ref: Pg 211

Question 3

What is the general difference between brittle and ductile fracture?

Answer 3

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

Question 4

What are the two steps to any fracture process?

Answer 4

Crack formation and propagation.

Ref: Pg 211

Question 5

With respect to crack propagation, describe ductile fracture.

Answer 5

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

Ref: Pg 211

Question 6

When is a crack said to be “stable?”

Answer 6

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

Ref: Pg 211

Question 7

With respect to crack propagation, describe brittle fracture.

Answer 7

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

Question 8

When is a crack considered to be “unstable?”

Answer 8

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

Ref: Pg 211

Question 9

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

Answer 9

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

Ref: Pg 211

Question 10

What is a microvoid?

Answer 10

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

Ref: Pg 212

Question 11

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

Answer 11

Eleipical crack

perpecdicular

Question 12

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

Answer 12

Ductile

Ref: Pg 212

Question 13

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

Answer 13

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

Ref: Pg 213

Question 14

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

Answer 14

Perpendicular.

Ref: Pg 213

Question 15

True or False

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

Answer 15

True

Ref: Pg 213

Question 16

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

Answer 16

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

Question 17

True or False

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

Answer 17

True

Ref: Pg 214

Question 18

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

Answer 18

Relatively shiny smooth surface.

Ref: Pg 214

Question 19

Define “cleavage” with respect to brittle crystalline materials.

Answer 19

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

Ref: Pg 214

Question 20

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

Answer 20

The fracture cracks pass through the grains.

Ref: Pg 214

Question 21

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

Answer 21

Intergranular.

Ref: Pg 215

Question 22

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

Answer 22

Lower

Ref: Pg 216

Question 23

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

Answer 23

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

Question 24

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

Answer 24

At crack tips, applied stress is significantly amplified.

Ref: Pg 216

Question 25

True or False | The effect of a stress raiser is more significant in brittle than in ductile materials.

Answer 25

True Ref: Pg 217

Question 26

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

Answer 26

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

Question 27

What is the Charpy Test used for?

Answer 27

Used to study the energy absorption of materials at differing temperatures. Ref: Lecture slides

Question 28

Ductile materials have a ____ energy absorption.

Answer 28

High Ref: Lecture Notes

Question 29

Brittle materials have a ____ energy absorption.

Answer 29

Low Ref: Lecture Notes

Question 30

What are the three steps to yield fatigue failure?

Answer 30

1. Crack formation due to stress concentrations and the presence of free surfaces. 2. Crack Growth 3. Fracture Ref: Lecture Slides

Question 31

What is fatigue?

Answer 31

A form of failure that occurs in structures subjected to dynamic and fluctuating stresses. Ref: Pg 229

Question 32

``` 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. ```

Answer 32

True Ref: Pg 229

Question 33

What type of failure is most common?

Answer 33

Fatigue failure Ref: Pg 229

Question 34

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.

Answer 34

True Ref: Pg 231

Question 35

The fatigue limit represents ______.

Answer 35

The largest value of fluctuating stress that will not cause failure for essentially an infinite number of cycles. Ref: Pg 232

Question 36

True or False | Most non-ferrous alloys do not have a fatigue limit.

Answer 36

True Ref: Pg 232

Question 37

Define "Fatigue Strength."

Answer 37

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

Question 38

Define "Fatigue Life."

Answer 38

The number of cycles to cause failure at a specified stress level. Ref: Pg 232

Question 39

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

Answer 39

Surface scratches sharp fillets keyways, threads, and dents Ref: Pg 236

Question 40

When do beachmarks take place?

Answer 40

When the material experiences interruptions in the crack propagation. Ref: Pg 236

Question 41

Each beachmark band represents ____.

Answer 41

A period of time over which crack growth occurred. Ref: Pg 236

Question 42

What is a striation?

Answer 42

The tiny advancing distance of a crack front during a single load cycle. Ref: Pg 236

Question 43

True or False | There may be thousands of striations within a single benchmark.

Answer 43

True Ref: Pg 236

Question 44

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

Answer 44

Rapid failure Ref; Pg 237

Question 45

True or False | Benchmarks and striations do not appear on the region over which the rapid failure occurs.

Answer 45

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

Question 46

What is "creep" with respect to material deformation?

Answer 46

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

Question 47

What is "primary creep?"

Answer 47

The rate of creep decreases with time because the material goes through strain hardening. Ref: Lecture Slides

Question 48

Describe "secondary creep."

Answer 48

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

Question 49

Describe "tertiary creep."

Answer 49

The slope of the creep rate increases with time. Ref: Lecture Slides

Question 50

Which stage of creep has the longest duration?

Answer 50

Secondary Ref: Pg 241

Question 51

What is the most important parameter from a creep test?

Answer 51

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