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A: Multiple Choice (27%) (多選題一題三分，答錯或少答皆扣一分，扣滿三分為止)
1.(
) What materials and structures can you think of (other than metals) that exhibit anisotropic behavior?
(a) wood (b) bone (c) woven material (such as cloth) (d) composite materials
2.(
) Two parts have been made of the same material, but one was formed by cold working and the other by hot working.
Explain the differences you might observe between the two. There are a large number of differences that will be seen between the
two materials, including:
(a) The cold worked material will have a higher strength than the hot worked material, and this will be more pronounced for
materials with high strain hardening exponents.
(b) Since hardness is related to strength, the cold worked material will also have a higher hardness.
(c) The cold worked material will have smaller grains and the grains will be elongated.
(d) The hot worked material will probably have more dislocations, and they will be less evenly distributed.
(e) The cold worked material can have a poor surface finish when in an as-formed condition. Also, it can have better tolerances.
(f) A cold worked material will have a higher recrystallization temperature than a hot worked material.
3.(
) Why the strength of a polycrystalline metal at room temperature decreases as its grain size increases ?
(a) Strength increases as more entanglements of dislocations occur with grain boundaries
(b) Metals with larger grains have less grain-boundary area per unit volume.
(c) Metals will be as able to generate as many entanglements at grain boundaries.
4.(
) It has been noted that the more a metal has been cold worked, the less it strain hardens. Why?
(a) The main effects of cold working are that grains become elongated and that the average grain size becomes larger (as grains
break down) with strain.
(b) Strain hardening occurs when dislocations interfere with each other and with grain boundaries.
(c) When a metal is annealed, the grains are large, and a small strain results in grains moving relatively easily at first, but they
increasingly interfere with each other as strain increases.
(d) For this highly cold-worked material, the stress cannot be increased much more with strain, because the dislocations have
nowhere else to go - they already interfere with each other and are pinned at grain boundaries.
5.(
) By stretching a thin strip of polished metal, as in a tension-testing machine, demonstrate and comment on what happens
to its reflectivity as the strip is being stretched.
(a) The polished surface is initially smooth, which allows light to be reflected uniformly across the surface.
(b) As the metal is stretched, the reflective surface of the polished sheet metal will begin to become dull.
(c) The slip and twin bands developed at the surface cause roughening, which tends to scatter the reflected light.
6.(
) On the same scale for stress, the tensile true stress-true strain curve is higher than the engineering stress-engineering
strain curve. What does the phenomenon occur in a compression test ? (a) During a compression test, the cross-sectional area of
the specimen decreases as the load is increased.
(b) The true stress in compression will be lower than the engineering stress for a given load, providing.
(c) True stress is defined as load divided by the instantaneous cross-sectional area of the specimen.
7.(
) About the effects of rate of deformation and strain rate. It has been stated that the higher the value of m, the more
diffuse the neck is, and likewise, the lower the value of m, the more localized the neck is. What is the reason for this behavior?
(a) With high m values, the material stretches to a shorter length before it fails; this behavior is an indication that necking is
delayed with increasing m.
(b) When necking is about to begin, the necking region's strength with respect to the rest of the specimen increases, due to strain
hardening.
(c) The strain rate in the necking region is also higher than in the rest of the specimen, because
the material is elongating faster there.
(d) Since the material in the necked region becomes stronger as it is strained at a higher rate, the region exhibits a less resistance
to necking.
(e) The increase in resistance to necking thus depends on the magnitude of m.
8.(
) List the factors that you would consider in selecting a hardness test.
(a) type of indenter,
(b) applied load,
(c) method of indentation measurement
9.(
) About selecting a hardness test, what the explanations are correct?
(a) The hardness test selected would depend on the estimated hardness of the workpiece, its size and thickness and if average
hardness or the hardness of individual microstructural components is desired.
(b) The Brinell hardness test leaves a fairly large indentation, thus providing a good measure of average hardness, while the
Knoop test leaves a small indentation that allows for determination of the hardness of the individual phases in a two-phase alloy.
(c) The small indentation of the Knoop test also allows it to be useful in measuring the hardness of very thick layers or plated
layers on parts.
(d) The depth of indentation should be small relative to part thickness, and that any change in the appearance of the bottom
surface the part will make the test results invalid.
B. Quantitative questions (64%)
1.The stress-strain curve is shown as right figure
Arrange (a) the magnitudes of hardness of these four types of materials, A, B, C, D from hard one to soft and the magnitudes of
yield stress of these four types of materials, A, B, C, D from the highest to lowest (b) List at least one possible material for each
type of materials? (c) arrange the magnitudes of Young’s modulus and toughness of A, B and C from the highest to lowest (d)
What materials are possible to be conductive and insulators? and Explain why? (15%)
2. A paper clip is made of wire 0.5 mm in diameter. If the original material from which the wire is made is a rod 25 mm in diameter,
calculate the longitudinal engineering and true strains that the wire has undergone during processing (15%)
3. Calculate the areas under the stress-strain curve (toughness) for the materials shown in Fig. below, (a) plot them as a function of
temperature, and describe your observations. (b) As expected, the elastic modulus of the polymer decreases as temperature increases. Using
the stress-strain curves in the figure, make a plot of the modulus of elasticity versus the temperature. Comment on the shape of the cur ve
(20 %)
4. Cooling curves are shown in Figure below for several Mo–V alloys. Based on these curves, construct the Mo–V phase
diagram.(14%)
5. The true strain, ε at the onset of necking is numerically equal to the strain hardening exponent, n, of the material, prove it
mathematically. (9%)