BME Materials Science 2

Question 1

Which crystal defect involves an extra half-plane of atoms inserted into the crystal lattice?

Accepted Answer

Edge dislocation

Answer

An edge dislocation is characterized by an extra half-plane of atoms, creating a line defect that enables plastic deformation by slip.

Question 2

The Hall-Petch relationship describes how yield strength varies with grain size. As grain size decreases, yield strength:

Accepted Answer

Increases

Answer

The Hall-Petch equation shows yield strength increases as grain size decreases because grain boundaries impede dislocation motion.

Question 3

Which phase transformation occurs when austenite is rapidly quenched to form a body-centered tetragonal structure in steel?

Accepted Answer

Martensite formation

Answer

Rapid quenching of austenite traps carbon in the BCC iron lattice, distorting it into a BCT martensitic structure.

Question 4

A material has a fracture toughness KIc = 50 MPa√m and a yield strength of 500 MPa. What is the critical crack length for plane strain fracture?

Accepted Answer

~3.2 mm

Answer

Using a = (1/π)(KIc/σ)² = (1/π)(50/500)² ≈ 0.0032 m = 3.2 mm for a through crack with Y=1.

Question 5

Which polymer chain configuration results in the highest degree of crystallinity?

Accepted Answer

Isotactic

Answer

Isotactic polymers have all substituents on the same side of the backbone, allowing chains to pack regularly into crystalline regions.

BME - Bachelor of Mechanical Engineering Practice Test