HWK 10

White latex paint is an emulsion of acrylate monomers, an iniator, and TiO2 particles that scatter light to give the white color. While drying on the wall, initiators are released and polymerize the monomers into a latex polymer. Excluding the initiators (which effectively become part of the polymer), how many phases are in dried white paint?

2 phases

DOUBLE CHECK?? Titanium dioxide (TiO2) is deposited as an amorphous thin film. When annealed, 75% of the film crystallizes. X-ray diffraction analysis reveals that these crystals are a mix of rutile, anatase, and brookite -- three different crystal structures of titanium dioxide. At this point, how many phases are in the TiO2 thin film?

3 phases -- the two different crystalline forms of TiO2 and the amorphous TiO2.

Possible kinetic barriers that prevent phase transformations from occurring quickly include: (A) Energy costs associated with interface formation. (B) Energy costs associated with the free energy change for the phase transformation. (C) Energy costs associated with the long range diffusion of atomic components to the product phase. (D) Energy costs associated with dislocation motion in a solid.

A and C

Which of the following would increase the time (number of cycles) to fatigue failure in the given material? (A) Flame polishing the surface of a glass component. (B) Cold working a metal alloy. (C) Shot peening a metal gear. (D) Reducing the use temperature of a polymer component to below its glass transformation temperature.

A and C

Fracture is a direct result of... A) ...crack propagation in a material. B) ...dislocation glide in a material. C) ...crack reduction in a material. D) ...elastic deformations in a material. E) ...impeded dislocation motion in a material.

A) ...crack propagation in a material.

Solid state diffusion... A) ...decreases non-linearly with decreasing temperature. B) ...decreases linearly with decreasing temperature. C) ...increases linearly with decreasing temperature. D) ...increases non-linearly with decreasing temperature.

A) ...decreases non-linearly with decreasing temperature.

You will definitely have low confidence in your prediction of the time to product failure if: A) ...the shaper parameter (beta) of the Weibull distribution is equal to 1. B) ...the characteristic lifetime (No) of the Weibull distribution is very small. C) ...the shaper parameter (beta) of the Weibull distribution is much greater than 1. D) ...the characteristic lifetime (No) of the Weibull distribution is equal to 1. E) ...the characteristic lifetime (No) of the Weibull distribution is very large.

A) ...the shaper parameter (beta) of the Weibull distribution is equal to 1. The shape parameter determines how sharp the transition is between parts not failing versus failing. When beta is much larger than 1, this transition is very sharp, and so the time to failure is well defined. If the shape is broader, you have parts that fail at all different times, and so it becomes hard to predict the product's lifetime.

You can be highly confident in your prediction of the time to product failure if: A) ...the shaper parameter (beta) of the Weibull distribution is much greater than 1. B) ...the characteristic lifetime (No) of the Weibull distribution is very large. C) ...the shaper parameter (beta) of the Weibull distribution is equal to 1. D) ...the shaper parameter (beta) of the Weibull distribution is much less than 1. E) ...the characteristic lifetime (No) of the Weibull distribution is very small.

A) ...the shaper parameter (beta) of the Weibull distribution is much greater than 1.

HWK 10.7: Based on these S-N curves, which of these alloys would not fail under cyclic loading of 500 MPa for 105 cycles but would fail under the same loading by 106 cycles? (A) Ti-5Al-2.5Sn titanium alloy (B) 4340 steel (C) 1045 steel

B only

Wooden 4 inch x 4 inch x 12 feet wooden posts hold up the deck that comes off the second floor of your house. On the ground level, you decide to drill 1 inch holes in two of the posts in order to hang a hammock. Compared to the other posts, the posts that now hold the hammock... A) Incorrect answer: ...have a lower load bearing capacity because the notches will act as barriers to dislocation motion. B) ...have a lower load bearing capacity because of the stress concentration near the holes. C) ...have a higher load bearing capacity because of the stress concentration near the holes. D) ...have a higher load bearing capacity because the notches will act as barriers to dislocation motion. E) ...have the same load bearing capacity because both are made of the same material.

B) ...have a lower load bearing capacity because of the stress concentration near the holes.

To measure the fracture toughness of a material, the best technique to use is: A) Toughmetry B) Charpy Impact Testing C) Vickers Hardness Testing D) Dilatometry E) Tensile Testing

B) Charpy Impact Testing

Which of these components is the LEAST likely to suffer from a fatigue failure. A) The axel of a car. B) The bricks in the external walls of a house. C) A diving board. D) The anchor cable for an offshore oil rig. E) The fuselage of an airplane.

B) The bricks in the external walls of a house.

Which of these components when loaded to below its yield strength is likely to suffer a creep failure at room temperature (25 °C) operation? (Choose the BEST answer.) (A) A steel cable holding up a suspension bridge. (B) A nylon cable suspending a yacht from the ceiling in a dry dock. (C) A solder joint under tension from the wire bond on a circuit board. - None of these would be susceptible because creep does not occur at room temperature (25 °C). - None of these would be susceptible because creep does not occur when loading is below the yield strength of the material.

Both B and C

Working as a consultant for an engineering firm, a steam plant manager comes to you for advice. He intends to lower the temperature of the steam running through his steam pipes, but maintain all other operational conditions the same (i.e, steam pressure inside the pipes and the cycling schedules for pressurization and depressurization ). He asks you how this change might affect the serviceable lifetime of the steam pipes. All of the following replies are accurate EXCEPT: A) The lower temperature will decrease the creep rate, decreasing the risk for creep failure. B) By not increasing the pressure in the steam pipes, you will not increase the risk for yielding the material. C) By maintaining the same pressure cycling schedule, you will retain about the same fatigue rate as before, and, thus, your risk for fatigue failure will remain the same. D) All of these replies are accurate. E) None of these

D) All of these replies are accurate.

The filament in an incandescent lightbulb is especially susceptible to failure via: A) Dielectric Breakdown B) Brittle Fracture C) Yielding D) Creep E) Fatigue

D) Creep

HWK 10.17: What is the phase assemblage of this Al-Cu-Sn alloy? A) This material has 3 phases: (1) an Al phase, (2) a Cu phase, (3) a Sn phase. B) This material has 4 phases: (1) an Al-Cu-Sn solid solution, (2) a pure Sn phase, (3) a pure copper phase, and (4) another pure copper phase with a different crystal structure. C) This material has 2 phases: (1) an Al-Cu-Sn solid solution phase and (2) a Cu-Sn phase. D) This material has 3 phases: (1) an Al-Cu-Sn solid solution, (2) a pure Sn phase, and (3) a pure copper phase. E) This material has 2 phases: (1) an Al phase and (2) a Cu-Sn phase.

D) This material has 3 phases: (1) an Al-Cu-Sn solid solution, (2) a pure Sn phase, and (3) a pure copper phase.

The most likely mechanism for creep in a material under high tensile stresses is: A) Cyclic loading. B) Diffusion through the grain boundaries. C) Diffusion through the bulk. D) Crack propagation. E) Dislocation motion (climb).

E) Dislocation motion (climb).

True/False: Systems containing only amorphous solids CANNOT have more than one phase.

False

Assume that the precipitation of pure copper from an Al-Cu alloy is thermodynamically favorable at a given process temperature (i.e., the transformation is spontaneous). How best would you describe the thermodynamics of this phase transformation at this temperature? Formation of the precipitate (INCREASE/DECREASES) the entropy of the system and is (EXO-/EXO- THERMIC)

Formation of the precipitate decreases the entropy of the system and is exothermic.

Assume that Glass A and Glass B are identical in composition and fictive temperature, but Glass A has many cracks of up to 2 mm maximum length while Glass B only has a few cracks with maximum lengths of < 1 mm. Based on this knowledge, which of these statements is most likely true? Glass A will fracture at a (LOWER/HIGHER/SAME) critical stress than Glass B but have the (LOWER/HIGHER/SAME) fracture toughness as Glass B.

Glass A will fracture at a lower critical stress than Glass B but have the same fracture toughness (KIc) as Glass B.

During the phase transformation from liquid to vapor (vaporization): Heat is (RELEASED/ABSORBED) and the entropy of the system (INCREASE/DECREASES)

Heat is absorbed and the entropy of the system increases.

During the phase transformation from solid to liquid (melting): Heat is (RELEASED/ABSORBED) and the entropy of the system (INCREASE/DECREASES)

Heat is absorbed and the entropy of the system increases.

Based on these S-N curves, would you expect ductile cast iron to fail under cyclic loading of 200 MPa for 109 cycles? - Yes - No - No reasonable prediction can be made based upon the available data.

No

You own a brass tuba composed of 90% copper and 10% zinc. Brass is a substitutional solid solution of copper and zinc. How many phases make up your brass tuba?

This is a single phase since it is a solid solution of copper and zinc.

True / False: When you plot failure rate data in the form of ln[ln(1/(1-P))] versus ln(N) where P is the cumulative probability of failure and N is the number of cycles, a linear fit to that data will give a line having a slope that we can use to estimate the shape parameter for the Weibull distribution of that data set.

True

True/False: Adding precipitates to a metal alloy will likely increase its yield strength but decrease its fracture toughness.

True

True/False: Polycrystalline materials have many mis-oriented crystalline grains in their microstructure because the phase transformations used to create these materials (e.g., solidification) occur via a nucleation and growth process.

True

Assuming it is possible for a liquid to transform into a solid at the equilibrium melting temperature, the change in the Gibbs free energy of the substance as it transforms from a liquid to a solid will be: - Zero - Negative - Positive

Zero