Ch 9

Question 1

What are the different types of failure?

Answer 1

simple fracture
ductile fracture: some plastic deformation: slow crack propagation
brittle fracture: no plastic deformation - fast crack propagation

fracture d/t crack propagation
flaws are stress concentrators
fracture toughness

fatigue failure

creep

Question 2

Under what conditions/situations does each type occur?

Question 3

What is the mechanism associated with each failure type?

Question 4

What parameter is used to quantify a material’s resistance to fracture?

Answer 4

fracture toughness: measurement of material’s resistance to brittle fracture

Question 5

What is the role of flaws in promoting brittle fracture?

Answer 5

applied tensile stress amplified at tips of flaws

material eventually fractures when stress at the tip of crack reaches theoretical strength

Question 6

What measures may be taken to reduce the likelihood of each failure type?

Question 7

What are the two types of fracture?

Answer 7

Ductile

Brittle

Question 8

What are the differences between ductile and brittle fracture?

Answer 8

Ductile
-slow crack propagation
-significant plastic deformation
-tearing
-fails with warning

Brittle
-rapid crack propagation
-little or no plastic deformation
-snap deformation
-fails with no warning
-clean break, flat surface

Question 9

What mechanical property does this correspond to? What does this tell us about energy?

Answer 9

Ductility and toughness (energy absorption)

Question 10

What are the stages of moderately ductile failure?

Answer 10

1. necking: a lot of dislocations are being made and repelling one another
2. void nucleation: voids are created at the neck
3. void growth and coalescence: the void grows in size and from many small voids, becomes one larger void
4. crack propagation: a crack grows
5. fracture

Question 11

How do voids originate?

Answer 11

Sometimes as particles, serve as void nucleation sites. The bonding is different/wrong therefore weaker here

Question 12

What are stress concentrators?

Answer 12

Voids

Question 13

What size and shape of cracks/voids are more prone to be stress concentrators

Answer 13

long
sharp

Question 14

Why does the length and shape of a void/crack matter?

Answer 14

length: the force that is applied to the crack is applied to the closest bond point.

shape: if that bond point is pointed, there is really on a few bond which all of that stress is focused on. if the shape is more rounded, there are more bonds over which to distribute the stress

Question 15

Define fracture toughness

Answer 15

how resistant a material is to brittle fracture when a crack is present. how much energy can be absorbed by the material (before failure)

Question 16

Discuss Metals, Ceramics, Polymers and their relative fracture toughness ranges

Answer 16

metals: HIGH fracture toughness
polymers: are ductile but require very little stress to crack
ceramics: LOW fracture toughness

Question 17

Talk about brittle fracture of ductile materials

Answer 17

temperature affects fracture liklihood

titanic displayed brittle fracture even though it was made out of a ductile material because of the very cold atlantic waters

Question 18

How else does temperature affect fracture?

Answer 18

Ductile to brittle transition temperature

BCC metals and polymers at low temperatures display brittle fracture

but at high temperatures display ductile fracture

Question 19

define fatigue

Answer 19

failure under lengthy period of repeated stress or strain cycling
fatigue is responsible for 90 percent of mech. failure

Question 20

Talk about stress v cycle plots and what they show. specifically talk about steel and aluminum

Answer 20

Steel: at high stresses, very few cycles can occur before failure. however, below a certain amount of stress, cycles can occur almost indefinitely

aluminum: more of a curved downward sloping line . eventually, aluminum will break

Question 21

define fatigue limit

Answer 21

the stress at a certain number of cycles where the material fails

Question 22

define fatigue life

Answer 22

total number of stress cycles; to caues fatigue failure at a specified stress amplitude

Question 23

What are three ways to improve fatigue life

Answer 23

1. reduce magnitude of stress
2. surface treatment: put surface into a compressive state to push and cracks that do develop together
3. remove stress concentrators (round corners)

Question 24

define creep

Answer 24

deformation over time at a constant stress

Question 25

How does temperature play a role in creep?

Answer 25

temperature increases the rate of creep

specifically
T>0.4 melting temperture (in K)

Question 26

What are the stages of creep?

Answer 26

1. primary creep: starts of strong the decreases
   2.secondary creep: steady state (constant slope)
2. tertiary creep acceleration of rate
3. rupture/failure

Question 27

In considering the high-temperature creep of a turbine blade, which types of analysis would be the most important?

Answer 27

Larson Miller Parameter

Question 28

Which of the following is NOT TRUE?
Fracture toughness is a design criteria
Fracture toughness is measured in a tensile test.
Fracture toughness predicts when material will fail brittlely.
Fracture toughness is typically higher for metals than ceramics

Answer 28

Fracture toughness is measured in a tensile test.