MACHINE DESIGN  An Integrated Approach
6101
W
980.7
N
=
(a)
3.
Figure D3
a
in Appendix D shows that the maximum bending moment occurs at the righthand support and is
M
max
WL b
−
()
⋅
:=
M
max
1.275
10
6
×
Nmm
⋅
=
This is repeated bending so
M
a
M
max
2
:=
and
M
m
M
a
:=
4.
The stress in the board at the point of maximum bending moment is
Area moment of inertia
I
wt
3
⋅
12
:=
I
8.329
10
5
×
mm
4
=
(b)
Alternating stress
σ
a
M
a
t
⋅
2
I
⋅
:=
a
12.2
MPa
=
(c)
Mean stress
m
M
m
t
⋅
2
I
⋅
:=
m
12.2
MPa
=
(d)
5.
For repeated (fluctuating) bending, the factor of safety for Case 3 loading is
N
f
S
f5E8
S
ut
⋅
a
S
ut
⋅
m
S
f5E8
⋅
+
:=
N
f
2.4
=
(e)
PROBLEM 610
Statement:
An overhung diving board is shown in Figure P64a.
A 100kg person is standing on the free
end.
Assume crosssectional dimensions of 305 mm x 32 mm.
What is the fatigue safety factor for infinite life if
the material is brittle fiberglass with Sf = 39 MPa @ N = 5E8 cycles and Sut = 130 MPa in the longitudinal
direction?
700 = a
2000
= L
R
R
1
2
P
Units:
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This note was uploaded on 05/19/2011 for the course EML 2005 taught by Professor Arakere during the Spring '05 term at University of Florida.
 Spring '05
 Arakere
 Machine Design

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