Quiz 1
EMA 4714  Materials Selection and Failure Analysis
Wednesday, January 16, 2008
Name ________________________________________
1. [10 points]
With respect to the boat lift Ibeam support failure case, a question was posed  “If
the load on the two cradle support beams was the same [P
total
/2], why did the Ibeam buckle at
only one location  the bearing point for the outboard cradle support beam?”. Use representative
calculations to back up your answer.
Collapse of the beam is predicted to occur when M
f
=
σ
f
*[I/y
m
], where I/y
m
= 1.11 for
bending about the YY axis. For A36 steel, M
f
= 39,960 inlbs.
Since M = P
xx
*a, where a is the distance from the end of the beam to the load bearing
point, a
left
= 5', a
right
= 4'. This means that P
xx
(left) = 666# and P
xx
(right) = 832#.
Since the critical axis rotation angle, = sin
1
[P
xx
/P
appl
], and P
appl
= 0.7*4800/2 = 1680#,
α
= sin
1
[666/1680] = 23.4
o
, or the left
support point of the beam will fail when the axis of
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 Spring '08
 Staff
 Tensile strength, AERMET, cradle support beam, Ibeam support failure, cradle support beams

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