Ductile-Brittle Transition

Question 1

Which type of metals do not exhibit ductile-brittle transition (DBT)?

Answer 1

• FCC metals (aluminum, copper, nickel, austenic stainless
  steels) do not exhibit DBT

Question 2

What types of materials is ductile to brittle transition observed in?

Answer 2

• BCC steels (most widely used steels)
• Some HCP and BCC metals
• Many ceramics
• Many polymers

Question 3

What is the ductile-to-brittle transition?

Answer 3

The drastic reduction in ductility of a material within a
temperature range

Question 4

What is the relationship between strain rate and the ductile-brittle transition temperature (DBTT)?

Answer 4

Increasing strain rate increases the DBTT, such that the material exhibits brittle
behaviour over a larger range of temperatures

Question 5

Are tensile tests sufficiently representative of fracture behaviour? Why or why not?

Answer 5

• Tensile tests are difficult to perform at high strain rates, and therefore can be poorly representative of fracture behaviour

Question 6

What severe service conditions are impact tests conducted under?

Answer 6

1. High strain rate
2. Low temperature
3. Triaxial stress state (from notch of specimen)

Question 7

What are commonly used impact tests?

Answer 7

• Charpy and Izod impact tests are commonly used

Question 8

What is the standard specimen size for the charpy impact test?:

Answer 8

Charpy Impact test uses a standard specimen size:
10 mm x 10 mm x 55 mm bar with a 2 mm notch

Question 9

What is the fracture surface appearance of brittle fracture and ductile fracture?

Answer 9

• Brittle fracture is shiny (cleavage fracture)
• Ductile region is dull and fibrous (shear fracture)

Question 10

What does impact testing simulate?

Answer 10

• Impact testing simulates severe failure conditions (triaxial
  stress state, as introduced by notch)

Question 11

What does the Charpy Impact test obtain a measure of?

Answer 11

• Obtain a measure of “Charpy V-notch (CVN) toughness”,
  which can be used for comparison of similar materials

Question 12

What are the test limitations of the Charpy Impact Test?

Answer 12

• does not provide a measure of the
  fracture toughness (Kc)
• cannot determine “safe stress” for design purposes

Question 13

Which factor determines whether ductile or brittle failure occurs?

Answer 13

The mode of failure that is
associated with a lower energy

Question 14

What is ductile fracture a result of?

Answer 14

Ductile failure is a result of dislocation movement

Question 15

What is brittle fracture a result of?

Answer 15

Crack propagation.

Question 16

What types of materials have temperature-dependent energy required for dislocation movement?

Answer 16

Most BCC materials

Question 17

What types of materials require energy for dislocation movement that is independent of temperature?

Answer 17

FCC materials

Question 18

Describe the relative strength and impact energy of FCC metals?

Answer 18

FCC metals (aluminum and copper alloys) are low-strength, but have a high impact energy

Question 19

Describe the ductility and impact energy of high-strength materials.

Answer 19

High-strength materials (titanium alloys and high-strength steels) are brittle and with low
impact energies

Question 20

What happens at position 1 for the Charpy impact test according to Newton’s conservation of energy?

Answer 20

At position 1 (before the hammer is released), the
energy in the system is the
potential energy of the hammer

Question 21

What happens at position 2 for the Charpy impact test according to Newton’s conservation of energy?

Answer 21

At position 2 (immediately after impact with the
material), potential energy is converted into kinetic
energy of the hammer and energy absorbed by the
specimen.

Question 22

What does the energy absorbed by the specimen during the Charpy impact test represent?

Answer 22

The difference in potential energy of the hammer at
the beginning of its swing (position 1) and end of its
swing (position 3).

Question 23

Why does a Charpy specimen contain a notch?

Answer 23

It allows us to study brittle fracture, which requires stress
concentration at a crack tip or defect

Question 24

After impact with the specimen during the Charpy impact test, what can part of the energy absorbed contribute to?

Answer 24

Permanent deformation of the specimen.

Question 25

Describe the relative Charpy impact energy of brittle materials

Answer 25

Low Charpy impact energy

Question 26

Describe the relative charpy impact energy of ductile materials

Answer 26

High Charpy impact energy

Question 27

What is the overall phenomenon that happens to a hammer during a Charpy impact test?

Answer 27

In a Charpy impact test, the hammer will swing to a lower height (at the end of its swing) after impacting a material specimen that fails in a ductile fashion compared to one that fails in a brittle fashion.

Question 28

What are chevron markings and fan-shaped ridges indicators of?

Answer 28

Chevron markings and fan-shaped ridges are indicators of brittle fracture, which point to the crack origin

Question 29

What is an additional method to characterize failure in addition to the charpy v-notch (CVN) energy absorption?

Answer 29

One can
characterize failure according to % shear fracture (or % ductile
fracture region)

Question 30

What is the relationship between Charpy V-notch impact energy, shear fracture, and temperature?

Answer 30

Increase in temperature = increase in Charpy V-notch impact energy = increase in shear fracture

Question 31

How does % carbon content impact DBT fracture energy, yield strength, and ductile-brittle transition temperature?

Answer 31

% increase in carbon content = decrease in DBT fracture energy = increase in yield strength = increase in ductile-brittle transition temperature

Question 32

Explain the relationship between the CVN transition of steels and carbon content

Answer 32

Increasing the
carbon content, while increasing the strength of steels, also raises the CVN transition
of steels