Ductile-Brittle Transition Flashcards
Which type of metals do not exhibit ductile-brittle transition (DBT)?
- FCC metals (aluminum, copper, nickel, austenic stainless
steels) do not exhibit DBT
What types of materials is ductile to brittle transition observed in?
- BCC steels (most widely used steels)
- Some HCP and BCC metals
- Many ceramics
- Many polymers
What is the ductile-to-brittle transition?
The drastic reduction in ductility of a material within a
temperature range
What is the relationship between strain rate and the ductile-brittle transition temperature (DBTT)?
Increasing strain rate increases the DBTT, such that the material exhibits brittle
behaviour over a larger range of temperatures
Are tensile tests sufficiently representative of fracture behaviour? Why or why not?
- Tensile tests are difficult to perform at high strain rates, and therefore can be poorly representative of fracture behaviour
What severe service conditions are impact tests conducted under?
- High strain rate
- Low temperature
- Triaxial stress state (from notch of specimen)
What are commonly used impact tests?
- Charpy and Izod impact tests are commonly used
What is the standard specimen size for the charpy impact test?:
Charpy Impact test uses a standard specimen size:
10 mm x 10 mm x 55 mm bar with a 2 mm notch
What is the fracture surface appearance of brittle fracture and ductile fracture?
- Brittle fracture is shiny (cleavage fracture)
- Ductile region is dull and fibrous (shear fracture)
What does impact testing simulate?
- Impact testing simulates severe failure conditions (triaxial
stress state, as introduced by notch)
What does the Charpy Impact test obtain a measure of?
- Obtain a measure of “Charpy V-notch (CVN) toughness”,
which can be used for comparison of similar materials
What are the test limitations of the Charpy Impact Test?
- does not provide a measure of the
fracture toughness (Kc) - cannot determine “safe stress” for design purposes
Which factor determines whether ductile or brittle failure occurs?
The mode of failure that is
associated with a lower energy
What is ductile fracture a result of?
Ductile failure is a result of dislocation movement
What is brittle fracture a result of?
Crack propagation.
What types of materials have temperature-dependent energy required for dislocation movement?
Most BCC materials
What types of materials require energy for dislocation movement that is independent of temperature?
FCC materials
Describe the relative strength and impact energy of FCC metals?
FCC metals (aluminum and copper alloys) are low-strength, but have a high impact energy
Describe the ductility and impact energy of high-strength materials.
High-strength materials (titanium alloys and high-strength steels) are brittle and with low
impact energies
What happens at position 1 for the Charpy impact test according to Newton’s conservation of energy?
At position 1 (before the hammer is released), the
energy in the system is the
potential energy of the hammer
What happens at position 2 for the Charpy impact test according to Newton’s conservation of energy?
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.
What does the energy absorbed by the specimen during the Charpy impact test represent?
The difference in potential energy of the hammer at
the beginning of its swing (position 1) and end of its
swing (position 3).
Why does a Charpy specimen contain a notch?
It allows us to study brittle fracture, which requires stress
concentration at a crack tip or defect
After impact with the specimen during the Charpy impact test, what can part of the energy absorbed contribute to?
Permanent deformation of the specimen.
Describe the relative Charpy impact energy of brittle materials
Low Charpy impact energy
Describe the relative charpy impact energy of ductile materials
High Charpy impact energy
What is the overall phenomenon that happens to a hammer during a Charpy impact test?
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.
What are chevron markings and fan-shaped ridges indicators of?
Chevron markings and fan-shaped ridges are indicators of brittle fracture, which point to the crack origin
What is an additional method to characterize failure in addition to the charpy v-notch (CVN) energy absorption?
One can
characterize failure according to % shear fracture (or % ductile
fracture region)
What is the relationship between Charpy V-notch impact energy, shear fracture, and temperature?
Increase in temperature = increase in Charpy V-notch impact energy = increase in shear fracture
How does % carbon content impact DBT fracture energy, yield strength, and ductile-brittle transition temperature?
% increase in carbon content = decrease in DBT fracture energy = increase in yield strength = increase in ductile-brittle transition temperature
Explain the relationship between the CVN transition of steels and carbon content
Increasing the
carbon content, while increasing the strength of steels, also raises the CVN transition
of steels
