Materials Practicals Flashcards
What does the sequence of metallurgical grinding and polishing do?
Removes marks and levels and cleans the specimen surface. Polishing removes the artifacts of grinding.
What is chemical etching and why does it work?
A manufacturing process that removes metal in order to produce parts in a desired shape.
What does etching time depend on?
The etch rate of the acid
What is a grain boundary?
A 2D defects in materials crystal structure. It’s the interface between two grains.
Why do grain boundaries appear dark under a reflected light microscope?
Because the light is scattered and reflected from the sample surface.
Define Hardness
Materials ability to resist plastic deformation
Define Stiffness
The extent to which a materials resists deformation in response to an applied force (rigidity)
Define Toughness
A materials ability to absorb kinetic energy without fracturing
What is Yield Stress?
The stress at which a material starts to deform plastically
What is 0.2% Proof Stress?
The point at which a particular degree of permanent deformation occurs in a test sample
Why do some steels show discontinuous yielding?
Because of higher amounts of interstitial carbon atoms
What is the UTS?
The maximum stress that a material can withstand before fracture.
What is the difference between true stress and engineering stress?
True stress takes into account the change in cross sectional area a specimen will undertake as it is places under stress.
What does an impact test measure?
Toughness
What does a notch do in an impact test?
Controls the location at which the specimen fractures. By creating a structural weakness
Why does modulus measurement during a tensile test require an extensometer?
Because the specimens extension is required when calculating young’s modulous
How would you measure fracture energy from a tensile test?
Find the area under the curve of a stress strain curve to failure
For a given material, what are the effects on YP and UTS of increasing the testing temperature in a tensile test?
Yield point and UTS occurs at lower stress levels
What is the effect on Charpy test data when the test temperature is decreased below room temperature for mild steel and copper?
Steel shows a ductile to brittle transition, copper doesnt
How do you measure hardness and what precautions are necessary to obtain accurate results?
Measuring the depth of an indentation made by a diamond vickers hardness tester.
In a 0.2wt%C plain carbon steel, what are the two microstructures present at room temperature?
Pearlite and Ferrite
What is the name of the microstructure consisting of ferrite and cementite lamella?
Pearlite
How does a dislocation move?
When the atoms from one of the surrounding planes break their bonds and rebond with the atoms at the terminating edge
Define a substitutional solid solution and its effect on hardening
A mixture of two types of atoms in which one atom can replace the other type of atom. This increases hardness
Define an interstitial solid solution and its effect on hardening
A solid solution in which the solute atoms occupy positions between the atoms in the structure of the solvent. This increases hardness
What is work hardening?
Increasing hardness by cold working
What is annealing?
heating and slow cooling in order to remove internal stresses and increase toughness
How is hardness affected by increasing the C content in a plain carbon steel?
Hardness is increased
What is a typical inclusion in mild steel?
oxides and sulfides
Give a size range for a typical non metallic inclusion
1-10µm
What is the mechanism of brittle fracture?
– Little or no plastic deformation
– Catastrophic
– Usually strain is < 5%.
