Manufacturing Processes - Exam 1
Review
1. Explain why the difference between engineering strain and true strain becomes larger as strain
increases. Is this phenomenon true for both tensile and compressive strains? Explain. - Answer-The
difference becomes larger because true strain is based on the actual cross section of the stress material,
while the engineering strain is based on the unloaded cross section. It is true for both tensile and
compressive strains.
2. Which of the two tests, tension or compression, requires a higher capacity testing machine than the
other? Explain. - Answer-Compression requires a higher capacity testing machine because the cross-
sectional area increases in the object being tested. This means a higher load will be needed due to the
increase of the cross-sectional area. This also means larger forces will be needed for the compression
test than of tension.
3. If you pull and break a tensile-test specimen rapidly, where would the temperature be the highest?
Explain why. - Answer-The temperature will be the highest in the necking region because the work input
and strain are in that region. When necking happens the maximum strain is in this region, this is where
maximum dislocation of molecules happens. Dislocation of molecules is where energy is released in the
form of heat, meaning higher temperatures.
4. 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. Explain the reason for this behavior. - Answer-As
discussed in Section 2.2.7 starting on p. 41, with high m values, the material stretches to a greater length
before it fails; this behavior is an indication that necking is delayed with increasing m. When necking is
about to begin, the necking region's strength with respect to the rest of the specimen increases, due to
strain hardening. However, the strain rate in the necking region is also higher than in the rest of the
specimen, because the material is elongating faster there. Since the material in the necked region
becomes stronger as it is strained at a higher rate, the region exhibits a greater resistance to necking.
The increase in resistance to necking thus depends on the magnitude of m. As the tension test
progresses, necking becomes more diffuse, and the specimen becomes longer before fracture; hence,
total elongation increases with increasing values of m (Fig. 2.13 on p. 45). As expected, the elongation
, after necking (postuniform elongation) also increases with increasing m. It has been observed that the
value of m decreases with metals of increasing strength.
5. The note at the bottom of Table 2.4 states that as temperature increases, C decreases and m
increases. Explain - Answer-The value of C in Table 2.5 on p. 43 decreases with temperature because it is
a measure of the strength of the material. The value of m increases with temperature because the
material becomes more strain-rate sensitive, due to the fact that the higher the strain rate, the less time
the material has to recover and recrystallize, hence its strength increases.
6. Explain why materials with high m values, such as hot glass and silly putty, when stretched slowly,
undergo large elongations before failure. - Answer-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. Explain
the reason for this behavior. As discussed in Section 2.2.7 starting on p. 41, with high m values, the
material stretches to a greater length before it fails; this behavior is an indication that necking is delayed
with increasing m. When necking is about to begin, the necking region's strength with respect to the rest
of the specimen increases, due to strain hardening. However, the strain rate in the necking region is also
higher than in the rest of the specimen, because the material is elongating faster there. Since the
material in the necked region becomes stronger as it is strained at a higher rate, the region exhibits a
greater resistance to necking. The increase in resistance to necking thus depends on the magnitude of
m. As the tension test progresses, necking becomes more diffuse, and the specimen becomes longer
before fracture; hence, total elongation increases with increasing values of m (Fig. 2.13 on p. 45). As
expected, the elongation after necking (postuniform elongation) also increases with increasing m. It has
been observed that the value of m decreases with metals of increasing strength.
7. Why have different types of hardness tests been developed? How would you measure the hardness of
a very large object? - Answer-Different hardness tests have been developed because of different
surfaces, the thickness of the material, high range of hardness of a material, and size ratio. It would be
best to use a scleroscope since it can measure large objects and it is portable.
8. What role does friction play in a hardness test? Can high friction between a material and indenter
affect a hardness test? Explain - Answer-Hardness testing deals with either high loads or low loads, and
friction has no effect on high loads. For the low loads surface preparation of the object is needed in
order to reduce the effects of friction.
Review
1. Explain why the difference between engineering strain and true strain becomes larger as strain
increases. Is this phenomenon true for both tensile and compressive strains? Explain. - Answer-The
difference becomes larger because true strain is based on the actual cross section of the stress material,
while the engineering strain is based on the unloaded cross section. It is true for both tensile and
compressive strains.
2. Which of the two tests, tension or compression, requires a higher capacity testing machine than the
other? Explain. - Answer-Compression requires a higher capacity testing machine because the cross-
sectional area increases in the object being tested. This means a higher load will be needed due to the
increase of the cross-sectional area. This also means larger forces will be needed for the compression
test than of tension.
3. If you pull and break a tensile-test specimen rapidly, where would the temperature be the highest?
Explain why. - Answer-The temperature will be the highest in the necking region because the work input
and strain are in that region. When necking happens the maximum strain is in this region, this is where
maximum dislocation of molecules happens. Dislocation of molecules is where energy is released in the
form of heat, meaning higher temperatures.
4. 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. Explain the reason for this behavior. - Answer-As
discussed in Section 2.2.7 starting on p. 41, with high m values, the material stretches to a greater length
before it fails; this behavior is an indication that necking is delayed with increasing m. When necking is
about to begin, the necking region's strength with respect to the rest of the specimen increases, due to
strain hardening. However, the strain rate in the necking region is also higher than in the rest of the
specimen, because the material is elongating faster there. Since the material in the necked region
becomes stronger as it is strained at a higher rate, the region exhibits a greater resistance to necking.
The increase in resistance to necking thus depends on the magnitude of m. As the tension test
progresses, necking becomes more diffuse, and the specimen becomes longer before fracture; hence,
total elongation increases with increasing values of m (Fig. 2.13 on p. 45). As expected, the elongation
, after necking (postuniform elongation) also increases with increasing m. It has been observed that the
value of m decreases with metals of increasing strength.
5. The note at the bottom of Table 2.4 states that as temperature increases, C decreases and m
increases. Explain - Answer-The value of C in Table 2.5 on p. 43 decreases with temperature because it is
a measure of the strength of the material. The value of m increases with temperature because the
material becomes more strain-rate sensitive, due to the fact that the higher the strain rate, the less time
the material has to recover and recrystallize, hence its strength increases.
6. Explain why materials with high m values, such as hot glass and silly putty, when stretched slowly,
undergo large elongations before failure. - Answer-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. Explain
the reason for this behavior. As discussed in Section 2.2.7 starting on p. 41, with high m values, the
material stretches to a greater length before it fails; this behavior is an indication that necking is delayed
with increasing m. When necking is about to begin, the necking region's strength with respect to the rest
of the specimen increases, due to strain hardening. However, the strain rate in the necking region is also
higher than in the rest of the specimen, because the material is elongating faster there. Since the
material in the necked region becomes stronger as it is strained at a higher rate, the region exhibits a
greater resistance to necking. The increase in resistance to necking thus depends on the magnitude of
m. As the tension test progresses, necking becomes more diffuse, and the specimen becomes longer
before fracture; hence, total elongation increases with increasing values of m (Fig. 2.13 on p. 45). As
expected, the elongation after necking (postuniform elongation) also increases with increasing m. It has
been observed that the value of m decreases with metals of increasing strength.
7. Why have different types of hardness tests been developed? How would you measure the hardness of
a very large object? - Answer-Different hardness tests have been developed because of different
surfaces, the thickness of the material, high range of hardness of a material, and size ratio. It would be
best to use a scleroscope since it can measure large objects and it is portable.
8. What role does friction play in a hardness test? Can high friction between a material and indenter
affect a hardness test? Explain - Answer-Hardness testing deals with either high loads or low loads, and
friction has no effect on high loads. For the low loads surface preparation of the object is needed in
order to reduce the effects of friction.
