ME 401 – Machine Design
Spring 2009, Assignment 7
Unless otherwise stated, assume reliability of 90% and factor of safety of 1. Complete
the remaining problems on the previous assignment (problems 5, 6, 7).
1. For elevated temperatures, fatigue analysis must be conducted differently. Briefly (1
or 2 concise sentences each) describe the following:
a) How is yield strength affected by elevated temperatures? How does that affect
analysis of parts?
b) What is creep, and at what temperature does it become a concern in metals?
c) What effect does elevated temperature have on endurance limit of steel?
d) What effect does elevated temperature have on tensile strength? How does that
affect fatigue analysis? Which modification factors (k
a
, k
b
, etc.) are affected by
tensile strength and hence are affected by temperature?
2. (10 pts) The plate shown below is 0.250 inch thick, 4 inch wide, 20 inch long, with a
0.100 inch diameter hole in the middle. What is the fatigue stress concentration
factor, K
f
, due to the hole if the plate is:
a) ASTM Grade 30 Gray Cast Iron
b) 2024-T6 aluminum alloy
c) AISI 1020 cold drawn steel
d) AISI 1095 normalized steel
3. (20 pts) The plate shown below is made from plain carbon steel. It is 4 inches wide,
20 inches long, and has a 2.00 inch diameter hole in the middle. How thick must the
plate be in order to survive 10
6
stress cycles (infinite life)? Assume localized yielding
is acceptable but gross yielding is not (localized yielding is yielding near the stress
concentration, gross yielding is yielding in the bulk material away from the stress
concentration). Use a values of S
y
=120ksi and S
u
=190ksi. Consider force
fluctuations of:
a) F
max
of 20,000 pounds tension and F
min
of 8000 pounds compression
b) F
max
of 8000 pounds tension and F
min
of 20,000 pounds compression.
F (cyclic)
F
