Beer F.P., Johnston E.R., DeWolf J.T., Mazurek D.F. Mechanics of Materials

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SAMPLE PROBLEM 7.1

A single horizontal force P of magnitude 150 lb is applied to end D of lever

ABD. Knowing that portion AB of the lever has a diameter of 1.2 in., deter-

mine (a) the normal and shearing stresses on an element located at point H

and having sides parallel to the x and y axes, (b) the principal planes and the

principal stresses at point H.

SOLUTION

Force-Couple System. We replace the force P by an equivalent force-

couple system at the center C of the transverse section containing point H:

P 5 150 lb

T 5

150 lb

18 in.

5 2.7 kip ? in.

150 lb

10 in.

5 1.5 kip ? in.

a. Stresses S

, S

, T

at Point H. Using the sign convention shown

in Fig. 7.2, we determine the sense and the sign of each stress component

by carefully examining the sketch of the force-couple system at point C:

5 0 s

1.5 kip ? in.

0.6 in.

518.84 ksib

2.7 kip ? in.

0.6 in.

517.96 ksib

We note that the shearing force P does not cause any shearing stress at

point H.

b. Principal Planes and Principal Stresses. Substituting the values of

the stress components into Eq. (7.12), we determine the orientation of the

principal planes:

tan 2u

2 s

217.96

0 2 8.84

521.80

2 u

5261.0°and180° 2 61.0° 51119°

5230.5°and159.5°b

Substituting into Eq. (7.14), we determine the magnitudes of the principal

stresses:

max, min

1 s

2 s

1 t

0 1 8.84

0 2 8.84

1 17.962

514.42 6 9.10

max

5113.52 ksib

524.68 ksib

Considering face ab of the element shown, we make u

5 230.58 in Eq. (7.5)

and find s

5 24.68 ksi. We conclude that the principal stresses are as

shown.

 1.5 kip · in.

T  2.7 kip · in.

P  150 lb





 7.96 ksi



 8.84 ksi



 0



 30.5



max

 13.52 ksi



min

 4.68 ksi

18 in.

1.2 in.

10 in.

4 in.

447

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PROBLEMS

448

7.5 through 7.8 For the given state of stress, determine (a) the

principal planes, (b) the principal stresses.

7.9 through 7.12 For the given state of stress, determine (a) the

orientation of the planes of maximum in-plane shearing stress,

(b) the maximum in-plane shearing stress, (c) the corresponding

normal stress.

7.1 through 7.4 For the given state of stress, determine the normal

and shearing stresses exerted on the oblique face of the shaded tri-

angular element shown. Use a method of analysis based on the equi-

librium of that element, as was done in the derivations of Sec. 7.2.

10 ksi

15 ksi

60

Fig. P7.1

80 MPa

40 MP

55

Fig. P7.2

6 ksi

60

5 ksi

Fig. P7.3

45 MPa

27 MP

75

18 MPa

Fig. P7.4

40 MPa

35 MPa

60 MP

Fig. P7.5 and P7.9

12 ksi

15 ksi

4 ksi

Fig. P7.7 and P7.11

12 ksi

5 ksi

8 ksi

Fig. P7.8 and P7.12

50 MPa

10 MP

15 MPa

Fig. P7.6 and P7.10

7.13 through 7.16 For the given state of stress, determine the nor-

mal and shearing stresses after the element shown has been rotated

through (a) 258 clockwise, (b) 108 counterclockwise.

90 MPa

30 MPa

60 MP

Fig. P7.14

80 MPa

50 MP

Fig. P7.16

12 ksi

6 ksi

8 ksi

Fig. P7.15

8 ksi

5 ksi

Fig. P7.13

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449

Problems

7.19 A steel pipe of 12-in. outer diameter is fabricated from

-in.-thick

plate by welding along a helix that forms an angle of 22.58 with a

plane perpendicular to the axis of the pipe. Knowing that a 40-kip

axial force P and an 80-kip ? in. torque T, each directed as shown,

are applied to the pipe, determine s and t in directions, respec-

tively, normal and tangential to the weld.

7.20 Two members of uniform cross section 50 3 80 mm are glued

together along plane a-a that forms an angle of 258 with the hori-

zontal. Knowing that the allowable stresses for the glued joint are

s 5 800 kPa and t 5 600 kPa, determine the largest centric load

P that can be applied.

7.17 and 7.18 The grain of a wooden member forms an angle of 158

with the vertical. For the state of stress shown, determine (a) the

in-plane shearing stress parallel to the grain, (b) the normal stress

perpendicular to the grain.

400 psi

15⬚

Fig. P7.18

1.6 MPa

4 MP

15⬚

Fig. P7.17

22.5°

in.

Weld

Fig. P7.19

25⬚

50 mm

Fig. P7.20

100 kN

␤

100 kN

80 mm

Fig. P7.21 and P7.22

7.21 Two steel plates of uniform cross section 10 3 80 mm are welded

together as shown. Knowing that centric 100-kN forces are applied

to the welded plates and that b 5 258, determine (a) the in-plane

shearing stress parallel to the weld, (b) the normal stress perpen-

dicular to the weld.

7.22 Two steel plates of uniform cross section 10 3 80 mm are welded

together as shown. Knowing that centric 100-kN forces are applied

to the welded plates and that the in-plane shearing stress parallel

to the weld is 30 MPa, determine (a) the angle b, (b) the corre-

sponding normal stress perpendicular to the weld.

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450

Transformations of Stress and Strain

7.25 The steel pipe AB has a 102-mm outer diameter and a 6-mm wall

thickness. Knowing that arm CD is rigidly attached to the pipe,

determine the principal stresses and the maximum shearing stress

at point K.

7.23 A 400-lb vertical force is applied at D to a gear attached to the

solid 1-in. diameter shaft AB. Determine the principal stresses and

the maximum shearing stress at point H located as shown on top

of the shaft.

7.2 4 A mechanic uses a crowfoot wrench to loosen a bolt at E. Knowing

that the mechanic applies a vertical 24-lb force at A, determine the

principal stresses and the maximum shearing stress at point H

located as shown on top of the

-in. diameter shaft.

6 in.

2 in.

400 lb

Fig. P7.23

24 lb

10 in.

6 in.

Fig. P7.24

200 mm

6 mm

150 mm

51 mm

10 kN

Fig. P7.25

7.26 The axle of an automobile is acted upon by the forces and couple

shown. Knowing that the diameter of the solid axle is 32 mm,

determine (a) the principal planes and principal stresses at point

H located on top of the axle, (b) the maximum shearing stress at

the same point.

3 kN

350 N · m

0.15 m

0.2 m

Fig. P7.26

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451

Problems

7.30 For the state of plane stress shown, determine (a) the value of t

for which the in-plane shearing stress parallel to the weld is zero,

(b) the corresponding principal stresses.

7.27 For the state of plane stress shown, determine (a) the largest value

of t

for which the maximum in-plane shearing stress is equal to

or less than 12 ksi, (b) the corresponding principal stresses.

8 ksi

10 ksi



Fig. P7.27

60 MP

20 MPa



Fig. P7.28

15 ksi

8 ksi



Fig. P7.29



12 MP

2 MPa

75

Fig. P7.30

7.28 For the state of plane stress shown, determine the largest value of

for which the maximum in-plane shearing stress is equal to or

less than 75 MPa.

7.29 Determine the range of values of s

for which the maximum in-

plane shearing stress is equal to or less than 10 ksi.

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452

Transformations of Stress and Strain

7.4 MOHR’S CIRCLE FOR PLANE STRESS

The circle used in the preceding section to derive some of the basic

formulas relating to the transformation of plane stress was first

introduced by the German engineer Otto Mohr (1835–1918) and is

known as Mohr’s circle for plane stress. As you will see presently,

this circle can be used to obtain an alternative method for the solu-

tion of the various problems considered in Secs. 7.2 and 7.3. This

method is based on simple geometric considerations and does not

require the use of specialized formulas. While originally designed

for graphical solutions, it lends itself well to the use of a

calculator.

Consider a square element of a material subjected to plane

stress (Fig. 7.15a), and let s

, s

, and t

be the components of the

stress exerted on the element. We plot a point X of coordinates s

and 2t

, and a point Y of coordinates s

and 1t

(Fig. 7.15b). If

is positive, as assumed in Fig. 7.15a, point X is located below the

s axis and point Y above, as shown in Fig. 7.15b. If t

is negative,

X is located above the s axis and Y below. Joining X and Y by a

straight line, we define the point C of intersection of line XY with

the s axis and draw the circle of center C and diameter XY. Noting

that the abscissa of C and the radius of the circle are respectively

equal to the quantities s

ave

and R defined by Eqs. (7.10), we con-

clude that the circle obtained is Mohr’s circle for plane stress. Thus

the abscissas of points A and B where the circle intersects the s axis

represent respectively the principal stresses s

max

and s

min

at the

point considered.

We also note that, since tan (XCA) 5 2t

y(s

2 s

), the angle

XCA is equal in magnitude to one of the angles 2u

that satisfy Eq.

(7.12). Thus, the angle u

that defines in Fig. 7.15a the orientation

of the principal plane corresponding to point A in Fig. 7.15b can be

obtained by dividing in half the angle XCA measured on Mohr’s cir-

cle. We further observe that if s

. s

and t

. 0, as in the case

considered here, the rotation that brings CX into CA is counterclock-

wise. But, in that case, the angle u

obtained from Eq. (7.12) and

defining the direction of the normal Oa to the principal plane is

positive; thus, the rotation bringing Ox into Oa is also counterclock-

wise. We conclude that the senses of rotation in both parts of Fig. 7.15

are the same; if a counterclockwise rotation through 2u

is required

to bring CX into CA on Mohr’s circle, a counterclockwise rotation

through u

will bring Ox into Oa in Fig. 7.15a.†

Since Mohr’s circle is uniquely defined, the same circle can be

obtained by considering the stress components s

, s

, and t

x9y9

, cor-

responding to the x9 and y9 axes shown in Fig. 7.16a. The point X9 of

coordinates s

and 2t

x9y9

, and the point Y9 of coordinates s

and

x9y9

, are therefore located on Mohr’s circle, and the angle X9CA in

Fig. 7.16b must be equal to twice the angle x9Oa in Fig. 7.16a. Since,

as noted before, the angle XCA is twice the angle xOa, it follows that

†This is due to the fact that we are using the circle of Fig 7.8 rather than the circle of

Fig. 7.7 as Mohr’s circle.





max



max



min



min





(a)

Fig. 7.15 Mohr’s circle.



max



min



(b)

Y ,

(

)





(



)

X ,





(

)

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453

the angle XCX9 in Fig. 7.16b is twice the angle xOx9 in Fig. 7.16a. Thus

the diameter X9Y9 defining the normal and shearing stresses s

, s

and t

x9y9

can be obtained by rotating the diameter XY through an angle

equal to twice the angle u formed by the x9and x axes in Fig. 7.16a.

We note that the rotation that brings the diameter XY into the diameter

X9Y9 in Fig. 7.16b has the same sense as the rotation that brings the

xy axes into the x9y9 axes in Fig. 7.16a.

7.4 Mohr’s Circle for Plane Stress









max



min



x'y'

(a)(b)

ABOC





x'y

(

)

X' ,





x'y

(

)

Fig. 7.16

The property we have just indicated can be used to verify the

fact that the planes of maximum shearing stress are at 458 to the

principal planes. Indeed, we recall that points D and E on Mohr’s

circle correspond to the planes of maximum shearing stress, while A

and B correspond to the principal planes (Fig. 7.17b). Since the

diameters AB and DE of Mohr’s circle are at 908 to each other, it

follows that the faces of the corresponding elements are at 458 to

each other (Fig. 7.17a).



ave



(a)(b)

BC A



max



min



max



max

45

90



Fig. 7.17

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454

Transformations of Stress and Strain

The construction of Mohr’s circle for plane stress is greatly simpli-

fied if we consider separately each face of the element used to define

the stress components. From Figs. 7.15 and 7.16 we observe that, when

the shearing stress exerted on a given face tends to rotate the element

clockwise, the point on Mohr’s circle corresponding to that face is

located above the s axis. When the shearing stress on a given face tends

to rotate the element counterclockwise, the point corresponding to that

face is located below the s axis (Fig. 7.18).† As far as the normal

stresses are concerned, the usual convention holds, i.e., a tensile stress

is considered as positive and is plotted to the right, while a compressive

stress is considered as negative and is plotted to the left.





(a) Clockwise Above





(b) Counterclockwise Below





Fig. 7.18 Convention for plotting shearing stress on Mohr’s circle.

†The following jingle is helpful in remembering this convention. “In the kitchen, the clock

is above, and the counter is below.”

CF A

(MPa)

(b)







EXAMPLE 7.02

For the state of plane stress already considered in Example 7.01,

(a) construct Mohr’s circle, (b) determine the principal stresses,

stress.

(a) Construction of Mohr’s Circle. We note from Fig. 7.19a that

the normal stress exerted on the face oriented toward the x axis is

tensile (positive) and that the shearing stress exerted on that face tends

to rotate the element counterclockwise. Point X of Mohr’s circle, there-

fore, will be plotted to the right of the vertical axis and below the hori-

zontal axis (Fig. 7.19b). A similar inspection of the normal stress and

shearing stress exerted on the upper face of the element shows that

point Y should be plotted to the left of the vertical axis and above the

horizontal axis. Drawing the line XY, we obtain the center C of Mohr’s

circle; its abscissa is

ave

1 s

50 1 1210

5 20 MPa

Since the sides of the shaded triangle are

CF 5 50 2 20 5 30 MPa

andFX 5 40 MPa

the radius of the circle is

R 5 CX 5 21302

1 1402

5 50 MPa

10 MPa

40 MPa

50 MPa

(a)

Fig. 7.19

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(b) Principal Planes and Principal Stresses. The principal

stresses are

max

5 OA 5 OC 1 CA 5 20 1

0 5

0 MPa

5 OB 5 OC 2 BC 5 20 2

0 5230 MPa

Recalling that the angle ACX represents 2u

(Fig. 7.19b), we write

tan 2

2 u

5 53.1°u

5 26.6°

Since the rotation which brings CX into CA in Fig. 7.20b is counterclock-

wise, the rotation that brings Ox into the axis Oa corresponding to s

max

in Fig. 7.20a is also counterclockwise.

counterclockwise brings CA into CD in Fig. 7.20b, a further rotation of

458 counterclockwise will bring the axis Oa into the axis Od corresponding

to the maximum shearing stress in Fig. 7.20a. We note from Fig. 7.20b

that t

max

5 R 5 50 MPa and that the corresponding normal stress is s9 5

ave

5 20 MPa. Since point D is located above the s axis in Fig. 7.20b,

the shearing stresses exerted on the faces perpendicular to Od in Fig. 7.20a

must be directed so that they will tend to rotate the element clockwise.

(MPa)

(b)

max













53.1°2

90

R  50



max





 30

min





ave



20'

 



45



70 MPa

max





50 MPa

max





30 MPa

min





20 MPa'





20 MPa'



(a)

Fig. 7.20

455

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456

Transformations of Stress and Strain

Mohr’s circle provides a convenient way of checking the results

obtained earlier for stresses under a centric axial loading (Sec. 1.12)

and under a torsional loading (Sec. 3.4). In the first case (Fig. 7.21a),

we have s

5 PyA, s

5 0, and t

5 0. The corresponding points

X and Y define a circle of radius R 5 Py2A that passes through the

origin of coordinates (Fig. 7.21b). Points D and E yield the orienta-

tion of the planes of maximum shearing stress (Fig. 7.21c), as well

as the values of t

max

and of the corresponding normal stresses s9:

max

5 s¿ 5 R 5

(7.18)

In the case of torsion (Fig. 7.22a), we have s

5 s

5 0 and

5 t

max

5 TcyJ. Points X and Y, therefore, are located on the t axis,

and Mohr’s circle is a circle of radius R 5 TcyJ centered at the origin

(Fig. 7.22b). Points A and B define the principal planes (Fig. 7.22c)

and the principal stresses:

max, min

56R 56

(7.19)



P/A



(b)(a)(c)



PP'



max



Fig. 7.21 Mohr’s circle for centric axial loading.

CBA



max



max





min



(a)(b)(c)



Fig. 7.22 Mohr’s circle for torsional loading.

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