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MECHANICS OF
MATERIALS
Fifth Edition
Ferdinand P. Beer
E. Russell Johnston, Jr.
John T. DeWolf
David F. Mazurek
Lecture Notes:
J. Walt Oler
Texas Tech University
CHAPTER
2009 The McGraw-Hill Companies, Inc. All rights reserved.
3Torsion
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Contents
Introduction
Torsional Loads on Circular Shafts
Net Torque Due to Internal Stresses
Axial Shear Components
Shaft Deformations
Shearing Strain
Stresses in Elastic Range
Normal Stresses
Torsional Failure ModesSample Problem 3.1
Angle of Twist in Elastic Range
Statically Indeterminate Shafts
Sample Problem 3.4
Design of Transmission Shafts
Stress Concentrations
Plastic Deformations
Elastoplastic Materials
Residual Stresses
Example 3.08/3.09
Torsion of Noncircular MembersThin-Walled Hollow Shafts
Example 3.10
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Torsional Loads on Circular Shafts
Interested in stresses and strains of
circular shafts subjected to twisting
couples ortorques
Generator creates an equal and
opposite torque T
Shaft transmits the torque to thegenerator
Turbine exerts torque Ton the shaft
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Net Torque Due to Internal Stresses
dAdFT
Net of the internal shearing stresses is aninternal torque, equal and opposite to the
applied torque,
Although the net torque due to the shearing
stresses is known, the distribution of the stresses
is not.
Unlike the normal stress due to axial loads, the
distribution of shearing stresses due to torsional
loads can not be assumed uniform.
Distribution of shearing stresses is statically
indeterminatemust consider shaft
deformations.
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Axial Shear Components
Torque applied to shaft produces shearing
stresses on the faces perpendicular to the
axis.
Conditions of equilibrium require the
existence of equal stresses on the faces of the
two planes containing the axis of the shaft.
The slats slide with respect to each other
when equal and opposite torques are applied
to the ends of the shaft.
The existence of the axial shear components is
demonstrated by considering a shaft made up
of axial slats.
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From observation, the angle of twist of theshaft is proportional to the applied torque and
to the shaft length.
L
T
Shaft Deformations
When subjected to torsion, every cross-section
of a circular shaft remains plane and
undistorted.
Cross-sections for hollow and solid circular
shafts remain plain and undistorted because acircular shaft is axisymmetric.
Cross-sections of noncircular (non-
axisymmetric) shafts are distorted when
subjected to torsion.
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Shearing Strain
Consider an interior section of the shaft. As a
torsional load is applied, an element on theinterior cylinder deforms into a rhombus.
Shear strain is proportional to twist and radiusmaxmax and
cL
c
LL
or
It follows that
Since the ends of the element remain planar,
the shear strain is equal to angle of twist.
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Stresses in Elastic Range
Jc
dAc
dAT max2max
Recall that the sum of the moments from
the internal stress distribution is equal to
the torque on the shaft at the section,
andmaxJ
T
J
Tc
The results are known as the elastic torsion
formulas,
Multiplying the previous equation by the
shear modulus,
max Gc
G
max
c
From Hookes Law, G , so
The shearing stress varies linearly with theradial position in the section.
421 cJ
41
422
1 ccJ
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Normal Stresses
Note that all stresses for elements a and c have
the same magnitude
Element c is subjected to a tensile stress ontwo faces and compressive stress on the other
two.
Elements with faces parallel and perpendicular
to the shaft axis are subjected to shear stressesonly. Normal stresses, shearing stresses or a
combination of both may be found for other
orientations.
max0
0max45
0max0max
2
2
245cos2
o
A
A
A
F
AAF
Consider an element at 45o to the shaft axis,
Element a is in pure shear.
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Torsional Failure Modes
Ductile materials generally fail inshear. Brittle materials are weaker in
tension than shear.
When subjected to torsion, a ductilespecimen breaks along a plane of
maximum shear, i.e., a plane
perpendicular to the shaft axis.
When subjected to torsion, a brittle
specimen breaks along planesperpendicular to the direction in
which tension is a maximum, i.e.,
along surfaces at 45o to the shaft
axis.
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ShaftBCis hollow with inner and outer
diameters of 90 mm and 120 mm,
respectively. ShaftsAB and CD are solid
of diameterd. For the loading shown,determine (a) the minimum and maximum
shearing stress in shaftBC, (b) the
required diameterdof shaftsAB and CD
if the allowable shearing stress in these
shafts is 65 MPa.
Sample Problem 3.1
SOLUTION:
Cut sections through shaftsAB
andBCand perform static
equilibrium analyses to find
torque loadings.
Given allowable shearing stress
and applied torque, invert the
elastic torsion formula to find therequired diameter.
Apply elastic torsion formulas to
find minimum and maximum
stress on shaftBC.
MECHANICS OF MATERIALSFE
B J h t D W lf M k
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Sample Problem 3.1SOLUTION:
Cut sections through shaftsAB andBC
and perform static equilibrium analysisto find torque loadings.
CDAB
ABx
TT
TM
mkN6
mkN60
mkN20
mkN14mkN60
BC
BCx
T
TM
MECHANICS OF MATERIALSFE
B J h t D W lf M k
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Sample Problem 3.1 Apply elastic torsion formulas to
find minimum and maximum
stress on shaftBC.
46
4441
42
m1092.13
045.0060.022
ccJ
MPa2.86
m1092.13
m060.0mkN20
462
2max
J
cTBC
MPa7.64
mm60
mm45
MPa2.86
min
min
2
1
max
min
c
c
MPa7.64
MPa2.86
min
max
Given allowable shearing stress and
applied torque, invert the elastic torsion
formula to find the required diameter.
m109.38
mkN665
3
3
2
4
2
max
c
cMPa
c
Tc
J
Tc
mm8.772 cd
MECHANICS OF MATERIALSF
E
B J h t D W lf M k
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3- 14
Angle of Twist in Elastic Range
Recall that the angle of twist and maximum
shearing strain are related,
L
c max
In the elastic range, the shearing strain and shear
are related by Hookes Law,
JG
Tc
G max
max
Equating the expressions for shearing strain and
solving for the angle of twist,
JG
TL
If the torsional loading or shaft cross-sectionchanges along the length, the angle of rotation is
found as the sum of segment rotations
i ii
ii
GJ
LT
MECHANICS OF MATERIALSFi
E
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Given the shaft dimensions and the applied
torque, we would like to find the torque reactionsatA andB.
Statically Indeterminate Shafts
From a free-body analysis of the shaft,
which is not sufficient to find the end torques.The problem is statically indeterminate.
ftlb90 BA TT
ftlb9012
21 AA TJL
JLT
Substitute into the original equilibrium equation,
ABBA T
JL
JLT
GJ
LT
GJ
LT
12
21
2
2
1
1
21
0
Divide the shaft into two components which
must have compatible deformations,
MECHANICS OF MATERIALSFi
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3- 16
Sample Problem 3.4
Two solid steel shafts are connected
by gears. Knowing that for each shaft
G = 11.2 x 106 psi and that the
allowable shearing stress is 8 ksi,determine (a) the largest torque T0
that may be applied to the end of shaft
AB, (b) the corresponding angle
through which endA of shaftAB
rotates.
SOLUTION:
Apply a static equilibrium analysis onthe two shafts to find a relationship
between TCD and T0 .
Find the corresponding angle of twist
for each shaft and the net angular
rotation of endA.
Find the maximum allowable torque
on each shaftchoose the smallest.
Apply a kinematic analysis to relate
the angular rotations of the gears.
MECHANICS OF MATERIALSFi
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Sample Problem 3.4
SOLUTION:
Apply a static equilibrium analysis onthe two shafts to find a relationship
between TCD and T0 .
0
0
8.2
in.45.20
in.875.00
TT
TFM
TFM
CD
CDC
B
Apply a kinematic analysis to relatethe angular rotations of the gears.
CB
CCB
CB
CCBB
r
r
rr
8.2
in.875.0
in.45.2
MECHANICS OF MATERIALSFi
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3- 18
Find the T0 for the maximum
allowable torque on each shaft
choose the smallest.
in.lb561
in.5.0
in.5.08.28000
in.lb663
in.375.0
in.375.08000
0
4
2
0max
0
4
2
0max
T
Tpsi
J
cT
T
Tpsi
J
cT
CD
CD
AB
AB
inlb5610 T
Sample Problem 3.4
Find the corresponding angle of twist for each
shaft and the net angular rotation of endA.
oo
/
oo
o
642
/
o
64
2
/
2.2226.8
26.895.28.28.2
95.2rad514.0
psi102.11in.5.0
.in24in.lb5618.2
2.22rad387.0
psi102.11in.375.0
.in24in.lb561
BABA
CB
CD
CD
DC
AB
ABBA
GJ
LT
GJ
LT
o
48.10A
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3- 19
Design of Transmission Shafts
Principal transmission shaft
performance specifications are:- power
- speed
Determine torque applied to shaft at
specified power and speed,
f
PPT
fTTP
2
2
Find shaft cross-section which will notexceed the maximum allowable
shearing stress,
shaftshollow2
shaftssolid2
max
41
42
22
max
3
max
Tcc
cc
J
TccJ
J
Tc
Designer must select shaft
material and cross-section tomeet performance specifications
without exceeding allowable
shearing stress.
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3- 20
Stress Concentrations
The derivation of the torsion formula,
assumed a circular shaft with uniform
cross-section loaded through rigid end
plates.
JTcmax
J
TcKmax
Experimental or numerically determined
concentration factors are applied as
The use of flange couplings, gears andpulleys attached to shafts by keys in
keyways, and cross-section discontinuities
can cause stress concentrations
Fig. 3.32 Stress-concentration factors
for fillets in circular shafts.
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3- 21
Plastic Deformations
With the assumption of a linearly elastic material,
J
Tc
max
cc
ddT0
2
0
22
The integral of the moments from the internal stress
distribution is equal to the torque on the shaft at the
section,
Shearing strain varies linearly regardless of materialproperties. Application of shearing-stress-strain
curve allows determination of stress distribution.
If the yield strength is exceeded or the material has
a nonlinear shearing-stress-strain curve, this
expression does not hold.
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3- 22
Elastoplastic Materials
At the maximum elastic torque,
YYY cc
JT
3
21 c
L YY
As the torque is increased, a plastic region
( ) develops around an elastic core ( )Y YY
3
3
41
34
3
3
413
32 11
cT
ccT YY
YY
3
3
41
34 1
YYTT
YL
Y
As , the torque approaches a limiting value,0Y
torqueplasticTT YP 34
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3- 23
Residual Stresses
When the torque is removed, the reduction of stress
and strain at each point takes place along a straight line
to a generally non-zero residual stress.
Residual stresses found from principle of superposition
0 dA
J
Tcm
Plastic region develops in a shaft when subjected to a
large enough torque.
On a T-curve, the shaft unloads along a straight line
to an angle greater than zero.
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3- 24
Example 3.08/3.09
A solid circular shaft is subjected to a
torque at each end.
Assuming that the shaft is made of an
elastoplastic material with
and determine (a) the
radius of the elastic core, (b) the
angle of twist of the shaft. When thetorque is removed, determine (c) the
permanent twist, (d) the distribution
of residual stresses.
MPa150Y
GPa77G
mkN6.4 T
SOLUTION:
Solve Eq. (3.32) forY/c and evaluate
the elastic core radius
Find the residual stress distribution bya superposition of the stress due to
twisting and untwisting the shaft
Evaluate Eq. (3.16) for the angle
which the shaft untwists when the
torque is removed. The permanent
twist is the difference between the
angles of twist and untwist
Solve Eq. (3.36) for the angle of twist
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MECHANICS OF MATERIALSthition
3- 25
SOLUTION:
Solve Eq. (3.32) forY/c and
evaluate the elastic core radius3
1
3413
3
41
34
Y
YYY
T
T
ccTT
mkN68.3m1025
m10614Pa10150
m10614
m1025
3
496
49
3
214
21
Y
YY
YY
T
c
JT
J
cT
cJ
630.068.3
6.434
31
c
Y
mm8.15Y
Solve Eq. (3.36) for the angle of twist
o33
3
49-
3
8.50rad103.148630.0
rad104.93
rad104.93
Pa1077m10614
m2.1mN1068.3
Y
YY
Y
YY
Y
JG
LT
cc
o50.8
Example 3.08/3.09
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3- 26
Evaluate Eq. (3.16) for the angle
which the shaft untwists whenthe torque is removed. The
permanent twist is the difference
between the angles of twist and
untwist
o
3
949
3
1.81
69.650.8
6.69rad108.116
Pa1077m1014.6
m2.1mN106.4
p
JG
TL
o81.1p
Example 3.08/3.09
Find the residual stress distribution by
a superposition of the stress due totwisting and untwisting the shaft
MPa3.187
m10614
m1025mN106.449-
33
max
J
Tc
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3- 27
Torsion of Noncircular Members
Planar cross-sections of noncircular
shafts do not remain planar and stress
and strain distribution do not vary
linearly
Previous torsion formulas are valid for
axisymmetric or circular shafts
Gabc
TL
abc
T
32
21
max
For uniform rectangular cross-sections,
At large values ofa/b, the maximum
shear stress and angle of twist for other
open sections are the same as a
rectangular bar.
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3- 28
Thin-Walled Hollow Shafts
Summing forces in the x-direction onAB,
shear stress varies inversely with thickness
flowshear
0
qttt
xtxtF
BBAA
BBAAx
t
ds
GA
TL
24
Angle of twist (from Chapter 11)
tA
T
qAdAqdMT
dAqpdsqdstpdFpdM
2
22
2
0
0
Compute the shaft torque from the integral
of the moments due to shear stress
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MECHANICS OF MATERIALSthtion
3- 29
Example 3.10
Extruded aluminum tubing with a rectangular
cross-section has a torque loading of 24 kip-
in. Determine the shearing stress in each of
the four walls with (a) uniform wall thickness
of 0.160 in. and wall thicknesses of (b) 0.120
in. onAB and CD and 0.200 in. on CD and
BD.
SOLUTION:
Determine the shear flow through the
tubing walls.
Find the corresponding shearing stresswith each wall thickness .
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MECHANICS OF MATERIALShtion
Find the corresponding shearing
stress with each wall thickness.
With a uniform wall thickness,
in.160.0
in.kip335.1t
q
ksi34.8
With a variable wall thickness
in.120.0
in.kip335.1 ACAB
in.200.0
in.kip335.1 CDBD
ksi13.11 BCAB
ksi68.6 CDBC
Example 3.10
SOLUTION:
Determine the shear flow through thetubing walls.
in.kip
335.1in.986.82
in.-kip24
2
in.986.8in.34.2in.84.3
2
2
A
Tq
A