Chapter 7 - Fracture & Failure

Question 1

What is a fracture?

Answer 1

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

Question 2

What are the 2 steps in fracture?

Answer 2

• Crack formation

- Crack propagation

Question 3

What are the 2 fracture models?

Answer 3

Ductile or Brittle

Question 4

What are the characteristics of a ductile fracture?

Answer 4

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

Question 5

What are the characteristics of a brittle fracture?

Answer 5

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

Question 6

What are the characteristics of a ductile matetrial?

Answer 6

• Again a lot of plastic deformation

- and a lot of energy absorption

Question 7

What are the characteristics of a brittle material?

Answer 7

• Little plastic deformation

- Low energy absorption

Question 8

Describe a very ductile materials fracture?

Answer 8

It looks like two spear tips pointed towards each other

Question 9

Describe a moderately ductile materials fracture?

Answer 9

chubbier with teeth

Question 10

Describe a brittle materials fracture?

Answer 10

straight cut through

Question 11

What type of dislocation occurs in a ductile fracture?

Answer 11

Mediated dislocation

Question 12

Describe the dislocation that occurs in a brittle fracture?

Answer 12

Limited dislocation mobility

Question 13

Describe the process of a brittle fracture?

Answer 13

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

Question 14

What does cleavage mean?

Answer 14

The braking of atomic bonds along specific crystallographic planes

Question 15

What are the 2 types of brittle fracture?

Answer 15

• Transgranular

- Intergranular

Question 16

Describe a transgranular fracture?

Answer 16

• Fracture cracks pass through the grains

- Fracture surface is faceted

Question 17

Describe a intergranular fracture?

Answer 17

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

Question 18

What is a stress riser?

Answer 18

• Can be Macro or Microscopic

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

Question 19

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

Answer 19

The cohesive forces between atoms

Question 20

What is a lower fracture strength explained by?

Answer 20

The effect of stress concentration at microscopic flaws

Question 21

what is the effect of stress concentration at microscopic flaws?

Answer 21

• 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

Question 22

When temperature decreases what can happen to a material?

Answer 22

It can transition from ductile to brittle

Question 23

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

Answer 23

Alloying

Question 24

What temperature do brittle fractures occur at?

Answer 24

Around 4°C

Question 25

What applied stressed will cause fatigue?

Answer 25

- Axial (compression/tension) - Flexural (bending) - Torsional (twisting)

Question 26

What are the fatigue failure steps?

Answer 26

- Crack initiation - Gradual crack propagation - Final catastrophic failure

Question 27

What is fatigue?

Answer 27

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

Question 28

Name the 3 cyclic stresses?

Answer 28

- Reversed stress cycle - Repeated stress cycle - Random stress cycle

Question 29

Describe the characteristics of the reversed stress cycle?

Answer 29

Periodic and symmetric about the zero stress

Question 30

Describe the characteristics of the repeated stress cycle?

Answer 30

Periodic and asymmetrical around the zero stress

Question 31

Describe the characteristics of the random stress cycle?

Answer 31

Random stress fluctuations

Question 32

What is the stress ratio?

Answer 32

R = sigma(min)/sigma(max)

Question 33

Tensile stresses are ........ | Compressive stresses are ......

Answer 33

Positive | Negative

Question 34

How are fatigue properties tested?

Answer 34

In rotating bending tests in fatigue testing apparatus

Question 35

What does a low cyclic fatigue mean?

Answer 35

High loads, plastic and elastic deformation

Question 36

What does a high cyclic fatigue mean?

Answer 36

Low loads, elastic deformation

Question 37

What is the fatigue limit?

Answer 37

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

Question 38

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

Answer 38

Iron (Fe) & Titanium (Ti) alloys

Question 39

What is fatigue strength?

Answer 39

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

Question 40

What is fatigue life?

Answer 40

The number of cycles to fail at a specified stress level

Question 41

What quality determines the crack initiation stage?

Answer 41

The quality of the surface of the material

Question 42

What happens during the first stage of crack propagation?

Answer 42

- Slow propagation along crystal planes - High resolved shear stress - Involves just a few grains - Flat fracture surface

Question 43

What happens during the second stage of crack propagation?

Answer 43

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

Question 44

What are the three stages of fatigue failure?

Answer 44

- Crack initiation in areas of stress concentration - Incremental crack propagation - Final rapid crack propagation

Question 45

What are the factors that affect fatigue life?

Answer 45

- Magnitude of stress (Mean, Amplitude etc) - Quality of the surface (scratches, sharp transition) - Thermal fatigue - Corrosion fatigue

Question 46

What is corrosion fatigue?

Answer 46

- Chemical reactions induce pits which act as stress raisers | - Corrosion also enhances crack propagation

Question 47

What is thermal fatigue?

Answer 47

- When thermal cycling causes expansion and contraction, hence thermal stress, if the component is restrained

Question 48

What is creep?

Answer 48

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

Question 49

What are creep rates used for?

Answer 49

Evaluating materials for anything that involves high temperatures

Question 50

What are the stages of creep?

Answer 50

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

Question 51

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

Answer 51

- instant strain increases - the steady state creep rate increases - the time to rupture increases

Question 52

Creep is generally minimized in materials with two factors?

Answer 52

- High melting point | - High elastic modulus