Fracture I

Question 1

What is the toughness of ductile fracture?

Answer 1

High

Question 2

What is the toughness of brittle fracture?

Answer 2

Low

Question 3

What is the extent of plastic deformation for ductile fracture?

Answer 3

Extensive

Question 4

What is the extent of plastic deformation for brittle fracture?

Answer 4

Little to none

Question 5

What is the appearance of the fracture surface for ductile fracture?

Answer 5

Dull and fibrous

Question 6

What is the appearance of the brittle fracture?

Answer 6

Shiny and crystalline

Question 7

What is the morphology of the ductile fracture?

Answer 7

• Dimples (microscopic)
• Cup and cone
• Evidence of shear
  (failure along paths of
  maximum shear)

Question 8

What is the morphology of the brittle fracture?

Answer 8

• Flat surface
• Evidence of cleavage
  of grains
• Chevron and fanshaped patterns

Question 9

What is the difference between highly ductile fracture, moderately ductile fracture, and brittle fracture?

Answer 9

Highly Ductile Fracture: Specimen necks down to a point

Moderately Ductile Fracture: Some necking

Brittle Fracture: No plastic deformation

Question 10

What is fracture toughness?

Answer 10

• A material property
• A measure of a material’s resistance to brittle fracture when a
  crack is present

Question 11

What is the stress concentration factor (Kt) governed by?

Answer 11

The crack tip radius of curvature.

Question 12

What happens when a crack propagates:

Answer 12

• There is a release of elastic strain energy
• Energy is consumed to extend the crack by creating new fracture
  surfaces

Question 13

What did Griffith propose when a crack will propagate?

Answer 13

• A crack will propagate when the release of
  elastic strain energy exceeds the energy required to form new
  crack surfaces

Question 14

What happens when a crack of length a forms on the surface of an
elastic solid

Answer 14

1. The material within the two triangular volumes is
   unloaded and elastic energy is released
2. Crack surface area per unit thickness created is 2a

Question 15

What is the maximum length for an internal crack?

Answer 15

For an internal crack, maximum length is 2a

Question 16

What are the characteristics of fracture toughness?

Answer 16

• A material property
• A measure of a material’s resistance to brittle fracture when a crack
  is present
• Not equivalent to the stress concentration factor Kt

Question 17

What are the three modes of loading associated with fracture toughness?

Answer 17

Opening mode (Mode I), Sliding mode (Mode II), Tearing mode (Mode III)

Question 18

What does Mode I of loading describe?

Answer 18

Mode I describes plane strain, in which the crack opens and there is no
strain perpendicular to the front and back faces

Question 19

Describe in 5 steps how the brittle fracture process occurs (Written Response).

Answer 19

1. Necking begins when the ultimate tensile strength is exceeded
2. Cavities previously present in material are enlarged
3. Cavities coalescence to form crack
4. Crack propagates
5. When the applied stress can no longer be sustained, fracture occurs

Question 20

What methods can be used to detect cracks?

Answer 20

Visual inspection, non-destructive testing
techniques (NDT), die penetrant, ultrasonic methods

Question 21

What does Y depend on in the plane strain fracture toughness formula?

Answer 21

Y is dependent on the geometry of the specimen, crack, and the
manner of loading

Question 22

Draw a diagram of what a highly ductile fracture looks like.

Answer 22

Check Fracture annotated slides

Question 23

Draw a diagram of what a moderately ductile fracture looks like.

Answer 23

Check Fracture annotated slides

Question 24

Draw a diagram of what brittle fracture looks like.

Answer 24

Check Fracture annotated slides

Question 25

What type of dimples are shown during ductile fracture in a uniaxial tensile test?

Answer 25

Spherical dimples

Question 26

What type of dimples are shown during ductile fracture under shear loading?

Answer 26

parabolic dimples

Question 27

What do mathematicians use the stress concentration factor for?

Answer 27

To study the stress fields at the crack tip

Question 28

What is the reality of the radius of curvature for crack tips

Answer 28

In reality, crack tips have some finite radius of curvature

Question 29

What does the radius of curvature approach with sharp cracks? What does it imply?

Answer 29

0 because it implies that the stress at the crack tip approaches infinity!

Question 30

Why is drilling holes at the end of detectable cracks effective?

Answer 30

It reduces the maximum stress at the crack tip

Question 31

What is the relationship between yield strength, ductility, and plane strain fracture toughness?

Answer 31

Increase in yield strength = decrease in plane strain fracture toughness = decrease in ductility

Question 32

What is a key concept in fracture mechanics about crack tips and stress intensity factors?

Answer 32

Even with two very different geometries and crack dimensions, the same stress field ahead of the crack tip is observed if the same stress intensity factor K exists.

Question 33

What is possible when a crack is detected before failure?

Answer 33

* If detected before failure we can remove the component from service

Question 34

According to Griffith's theory of brittle fracture, what types of energy are associated with crack propagation?

Answer 34

1. Elastic energy released due to the unloading of a volume of material 2. Strain energy absorbed to create new crack surface area

Question 35

What is the stress concentration factor?

Answer 35

Measure of stress amplification at the crack tip

Question 36

What types of defects do all manufactured materials contain?

Answer 36

* Porosities/Cavities * Inclusions (contamination arising from processing) * Surface damage accumulated during manufacturing or during service (nicks and cracks) * Dislocations

Question 37

Write down the equation for the elastic energy released per unit thickness when a crack extends a distance a

Answer 37

Check Fracture annotated slides (slide 20)

Question 38

Write down the equation for the surface energy required per unit thickness to extend a crack from 0 to a

Answer 38

Check Fracture annotated slides (slide 20)

Question 39

What concept can be used to detect cracks in pressure vessels?

Answer 39

“leak before break”

Question 40

What are the macroscopic features of brittle fracture?

Answer 40

Chevron markings and fan-shaped ridges

Question 41

What are the microscopic features of brittle fracture?

Answer 41

Transgranular fracture and Intergranular fracture

Question 42

What is one way to improve the effectiveness of drilling holes at the end of detectable cracks to reduce the maximum stress at the crack tip?

Answer 42

The effect is greatly improved if a compressive residual stress around the hole can be induced.