Materials practice questions

Answer: A) Copper and zinc

Brass is a copper-zinc alloy. Copper-tin alloys are bronzes.

• ACorrect. Brass is a copper-based alloy whose main alloying element is zinc. The zinc content sets its strength, ductility and color.
• BIncorrect. Iron and carbon are the main components of steel and cast iron, not brass.
• CIncorrect. Aluminum and magnesium form aluminum alloys, such as the 5xxx series. Brass is copper-based.
• DIncorrect. Copper and tin are the main components of bronze. Brass uses zinc as its main alloying element, not tin.

Answer: D) Below which the part can survive an unlimited number of load cycles

For steels, the S-N curve flattens at the endurance (fatigue) limit. This is the stress amplitude below which the material can, in theory, survive an unlimited number of cycles without fatigue failure.

• AIncorrect. Even at stresses above the endurance limit, fatigue failure takes many cycles of crack initiation and growth. The endurance limit marks the safe level for unlimited cycles, not a stress that cracks the part on the first cycle.
• BIncorrect. That describes the yield strength, a static property. The endurance limit is a cyclic property and is usually well below the yield strength.
• CIncorrect. That is the ultimate tensile strength. For steels, the endurance limit is much lower, roughly half of it.
• DCorrect. The S-N curve for steels flattens at the endurance limit. Below this stress, cyclic loading does not cause fatigue failure, and testing is typically stopped at about 10^7 cycles.

Answer: B) Cross-link permanently and can't be remelted

• AIncorrect. That's thermoplastics.
• BCorrect. Thermoplastics soften when heated.
• CIncorrect. They're polymers.
• DIncorrect. That isn't the difference.

Answer: B) Composite material

• AIncorrect. It combines fibers and a matrix.
• BCorrect. Strong fibers in a polymer matrix.
• CIncorrect. It's a composite.
• DIncorrect. It's a composite.

Answer: D) Higher yield strength and lower elongation at fracture

Cold working strain-hardens a metal by raising its dislocation density. This increases yield strength and hardness and decreases ductility (elongation); annealing reverses these changes.

• AIncorrect. This describes the result of annealing, which removes the effects of cold work. It is the opposite of what cold drawing does.
• BIncorrect. Strain hardening raises yield strength, but it uses up some of the metal's capacity to deform plastically, so elongation goes down, not up.
• CIncorrect. Cold work raises dislocation density, which makes further dislocation movement harder and therefore raises yield strength.
• DCorrect. Plastic deformation below the recrystallization temperature causes strain (work) hardening. Dislocations multiply and tangle, which raises strength and hardness and lowers ductility.

Answer: A) Room temperature is a large fraction of the polymer's glass transition or melting temperature, so viscoelastic flow occurs

Creep matters when service temperature is a large fraction of a material's melting or softening temperature. Many polymers reach that point at room temperature, so their viscoelastic chain motion lets them deform slowly under a constant load.

• ACorrect. Creep becomes significant once service temperature is a large fraction of a material's absolute melting or softening temperature. For many polymers, room temperature meets that condition, so the molecular chains slowly slide and relax under a sustained load. Steel melts at about 1,800 K, so room temperature is far too low for it to creep.
• BIncorrect. Creep is time-dependent deformation under a sustained, constant load. It is not fatigue, which is damage caused by cyclic loading.
• CIncorrect. Polymers have a much lower elastic modulus than steel. Amorphous polymers also lack the metallic grain boundaries that this option assumes.
• DIncorrect. Dislocation climb is a creep mechanism in crystalline metals at high temperature. Polymer creep comes from chain motion, and it speeds up as temperature rises, not as it falls.

Answer: C) It increases toughness and ductility at the cost of some hardness and strength

Tempering reheats quenched martensite to below the critical temperature. It relieves internal stresses and lets carbides precipitate, trading some hardness and strength for better toughness and ductility.

• AIncorrect. Heating above the upper critical temperature is austenitizing, the step before quenching or normalizing. Tempering is done below the lower critical temperature so that austenite does not re-form.
• BIncorrect. Tempering lets carbon diffuse out of the strained martensite and form carbides. This lowers hardness and strength; it does not raise them.
• CCorrect. As-quenched martensite is very hard but brittle. Tempering relieves internal stresses and lets carbides precipitate, which gives up some hardness and strength in exchange for much better toughness.
• DIncorrect. Diffusing carbon into the surface layer is carburizing, a case-hardening process, not tempering.

Answer: D) A rough surface finish

• AIncorrect. Lower stress improves life.
• BIncorrect. Polishing improves fatigue strength.
• CIncorrect. That improves fatigue life.
• DCorrect. Surface flaws start cracks.

Answer: B) A liquid transforms directly into two solid phases

• AIncorrect. A liquid becomes two solids.
• BCorrect. It's the lowest melting point in the system.
• CIncorrect. A liquid becomes two solids.
• DIncorrect. It's the lowest.

Answer: B) Steel

• AIncorrect. Copper is more noble and acts as the cathode.
• BCorrect. It's more anodic in the galvanic series.
• CIncorrect. It causes galvanic corrosion.
• DIncorrect. The more active metal corrodes.