Mohr's Circle for Plane Stress

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This medium gives Mohr's circle for plane stress. Its the best metjos to calculate normal and shear stress.

Transcript of Mohr's Circle for Plane Stress

P4 Stress and Strain

Dr. A.B. Zavatsky HT08

Lecture 6 Mohrs Circle for Plane StressTransformation equations for plane stress. Procedure for constructing Mohrs circle. Stresses on an inclined element. Principal stresses and maximum shear stresses. Introduction to the stress tensor.

1

Stress Transformation Equationsyy

y1

y y1x1 x1y1 x1 x1

yx xyx

y1

x xy yx

xx1 x1y1

x y1x1 y1

x1 = x1 y1 =

x + y2

+

x y2

( x y ) sin 2 2

cos 2 + xy sin 2

+ xy cos 2

If we vary from 0 to 360, we will get all possible values of x1 and x1y1 for a given stress state. It would be useful to represent x1 and x1y1 as functions of in graphical form.2

To do this, we must re-write the transformation equations.

x1

x + y2

=

x y2

x1 y1 =

( x y ) sin 2 2

cos 2 + xy sin 2 + xy cos 2

Eliminate by squaring both sides of each equation and adding the two equations together.

x1

x + y 2

2

+ x1 y12 =

x y 2

2

+ xy 2

Define avg and R

avg =

x + y2

R=

x y 2

2

+ xy 23

Substitue for avg and R to get

( x1

avg ) 2 + x1 y12 = R 2

which is the equation for a circle with centre (avg,0) and radius R. This circle is usually referred to as Mohrs circle, after the German civil engineer Otto Mohr (1835-1918). He developed the graphical technique for drawing the circle in 1882. The construction of Mohrs circle is one of the few graphical techniques still used in engineering. It provides a simple and clear picture of an otherwise complicated analysis.4

Sign Convention for Mohrs Circley1 y1 y y1x1 x1y1 x1 x1 x1y1 y1x1 y1 x x1

( x1

avg ) 2 + x1 y12 = R 22 avgR

x1

x1y1

Notice that shear stress is plotted as positive downward. The reason for doing this is that 2 is then positive counterclockwise, which agrees with the direction of 2 used in the derivation of the tranformation equations and the direction of on the stress element. Notice that although 2 appears in Mohrs circle, appears on the stress element.5

Procedure for Constructing Mohrs Circle1. 2. 3. Draw a set of coordinate axes with x1 as abscissa (positive to the right) and x1y1 as ordinate (positive downward). Locate the centre of the circle c at the point having coordinates x1 = avg and x1y1 = 0. Locate point A, representing the stress conditions on the x face of the element by plotting its coordinates x1 = x and x1y1 = xy. Note that point A on the circle corresponds to = 0. Locate point B, representing the stress conditions on the y face of the element by plotting its coordinates x1 = y and x1y1 = xy. Note that point B on the circle corresponds to = 90. Draw a line from point A to point B, a diameter of the circle passing through point c. Points A and B (representing stresses on planes at 90 to each other) are at opposite ends of the diameter (and therefore 180 apart on the circle). Using point c as the centre, draw Mohrs circle through points A and B. This circle has radius R. (based on Gere)6

4.

5.

6.

B y B (=90) -xy c xy R

yy

yx xyx

x xy yx

x

A

x1

A (=0) avg x1y1 x7

Stresses on an Inclined Element1. On Mohrs circle, measure an angle 2 counterclockwise from radius cA, because point A corresponds to = 0 and hence is the reference point from which angles are measured. The angle 2 locates the point D on the circle, which has coordinates x1 and x1y1. Point D represents the stresses on the x1 face of the inclined element. Point E, which is diametrically opposite point D on the circle, is located at an angle 2 + 180 from cA (and 180 from cD). Thus point E gives the stress on the y1 face of the inclined element. So, as we rotate the x1y1 axes counterclockwise by an angle , the point on Mohrs circle corresponding to the x1 face moves counterclockwise through an angle 2. (based on Gere)

2.

3.

4.

8

B y1 B (=90) E (+90) -x1y1 x1y1 c D () R 2 A (=0) x1x1 y1 y1x xy yx

yy x

yx xy x

2+180

A

x1y y1x1 x1y1 x1 x1

E

Dy1x1 y19

x

x1y1

x1y1

Principal StressesB (=90) 2p2x xy

B

yy x

yx xy x

Ayx

2

c R

1 2p1 A (=0)

x1y 2

P21

p2

1p1

P1

x

x1y1

210

Maximum Shear StressB (=90) min 2s c max Rsx xy

B

yy x

yx xy x

Ayx

x1

Note carefully the directions of the y shear forces.

A (=0) s x1y1

max max s ss

s max max11

x

Example: The state of plane stress at a point is represented by the stress element below. Draw the Mohrs circle, determine the principal stresses and the maximum shear stresses, and draw the corresponding stress elements.c = avg =R=

x + y2

=

80 + 50 = 15 2

1,2 = c R 1,2 = 15 69.6 1 = 54.6 MPa 2 = 84.6 MPa

(50 ( 15)) 2 + (25)2A (=0)

R = 65 2 + 25 2 = 69.6

2 c B80 MPa 50 MPa y x 80 MPa

1 RB (=90)

A25 MPa 50 MPa

max

max = R = 69.6 MPa s = c = 15 MPa12

50 MPa

y 80 MPa x 80 MPa

25 = 0.3846 80 15 2 2 = 21.0 tan 2 2 = 21 = 21.0 + 180 = 201

25 MPa 50 MPa

1 = 100.5 2 = 10.5

A (=0)

2y54.6 MPa

122 21

c RB (=90)

100.5 84.6 MPa

o

84.6 MPa

10.5o

x

2

54.6 MPa

13

50 MPa

2 2 = 21.0 2 s min = (90 21.0) = 69.0

y 80 MPa x 80 MPa

s min = 34.5

25 MPa 50 MPa

min

taking sign convention into account

A (=0)

2smin

2 RB (=90)

y15 MPa 15 MPa

22 2smax55.5o

c

-34.5o

x15 MPa

max

2 2 = 21.0 2 s max = 21.0 + 90 = 111.0

15 MPa 69.6 MPa

s max = 55.514

Example: The state of plane stress at a point is represented by the stress element below. Find the stresses on an element inclined at 30 clockwise and draw the corresponding stress elements.50 MPa

y 80 MPa x 80 MPa

C ( = -30) -60

A (=0)25 MPa 50 MPa

x1 = c R cos(22+60) y1 = c + R cos(22+60) x1y1= -R sin (22+60) x1 = -26 y1 = -4 x1y1= -69

y 25.8 MPa

y1 4.15 MPa

22 D -60+180

B (=90)

-30

o

x 25.8 MPa

D ( = -30+90)

2 = -30 2 = -6015

4.15 MPa

C68.8 MPa

x1

Principal Stresses 1 = 54.6 MPa, 2 = -84.6 MPaBut we have forgotten about the third principal stress! Since the element is in plane stress (z = 0), the third principal stress is zero. 1 = 54.6 MPa 2 = 0 MPa 3 = -84.6 MPa This means three Mohrs circles can be drawn, each based on two principal stresses: 1 and 3 1 and 2 2 and 3

A (=0)

3

2

1

B (=90)

16

3

1

1

3

3

3

2

1

1 3 1

1

3

1

3

17

The stress element shown is in plane stress. What is the maximum shear stress?x

B

yy x

yx xy x

B

xy yx

A

3

2

1

x1 2 max(1,2) = 1

A

2 3 2 max(2,3) = 2 = 2 2

x1y1

max(1,3) = 1 3 = 1 overall maximum2 218

Introduction to the Stress Tensory

yy yz xx zy

yx xy

zx xz zzz

xx x

xx xy xz yx yy yz zx zy zz

yy

Normal stresses on the diagonal Shear stresses off diagaonal xy = yx, xz = zx, yz = zy

The normal and shear stresses on a stress element in 3D can be assembled into a 3x3 matrix known as the stress tensor.19

From our analyses so far, we know that for a given stress system, it is possible to find a set of three principal stresses. We also know that if the principal stresses are acting, the shear stresses must be zero. In terms of the stress tensor,

xx xy xz yx yy yz zx zy zz

0 1 0 0 2 0 0 0 3

In mathematical terms, this is the process of matrix diagonalization in which the eigenvalues of the original matrix are just the principal stresses.

20

Example: The state of plane stress at a point is represented by the stress element below. Find the principal stresses.50 MPa

y 80 MPa x 80 MPa

x xy 80 25 = M = yx y 25 50 We must find the eigenvalues of this matrix.

25 MPa 50 MPa

Remember the general idea of eigenvalues. We are looking for values of such that: Ar = r where r is a vector, and A is a matrix. Ar r = 0 or (A I) r = 0 where I is the identity matrix. For this equation to be true, either r = 0 or det (A I) = 0. Solving the latter equation (the characteristic equation) gives us the eigenvalues 1 and 2.21

80 25 =0 det 25 50 (80 )(50 ) (25)(25) = 0

2 + 30 4625 = 0 = 84.6, 54.6

So, the principal stresses are 84.6 MPa and 54.6 MPa, as before.

Knowing the eigenvalues, we can find the eigenvectors. These can be used to find the angles at which the principal stresses act. To find the eigenvectors, we substitute the eigenvalues into the equation (A I ) r = 0 one at a time and solve for r.

80 25 x 0 = 25 50 y 0 25 x 0 80 54.6 = 25 50 54.6 y 0

134.6 25 x 0 = 25 4.64 y 0 x = 0.186 y 0.186 1 is one eigenvector. 22