1. 1
INTRODUCTION
The state of stress on any plane in a strained body is said to be ‘Compound Stress’, if, both Normal and Shear stresses are acting on that plane. For, example, the state of stress on any vertical plane of a beam subjected to transverse loads will, in general, be a Compound Stress. In actual practice the state of Compound Stress is of more common occurrence than Simple state of stress.
In a compound state of stress, the normal and shear stress may have a greater magnitude on some planes which are inclined (or, Oblique) to the given stress plane. Hence it is necessary, in a compound state of stresses, to find (i) The normal and shear stress on a plane which is inclined (or, Oblique) to the given stress plane; (ii) The inclination of max. and min. normal stress planes and the values of the normal stress (max. / min.) on them; (iii) The inclination of max. shear stress planes and the values of the shear stress (max.) on them.

2. σx  Normal Stress in x- direction σy  Normal Stress in y- direction 2
Consider an element ABCD subjected to a state of compound stress as shown in the Fig. Let: A E D C B σy τ σx θ
σx  Normal Stress in x- direction
σy  Normal Stress in y- direction
τ Shear Stresses in x & y – directions
θ  Angle made by inclined plane AE wrt vertical
σn Normal Stress on inclined plane
τn Shear Stress on inclined plane
(i) Normal & Shear stress on plane inclined (Oblique) to given stress plane:
Normal Stress, σθ, and Shear stress, τθ, on inclined plane are given by:

3. 2
(ii) The inclination of max. and min. normal stress planes and
the values of the normal stress (max. / min.) on them
Let, θP be the inclination of the plane of max. or min. normal stress and σP be the value of the max. or min. normal stress on that plane, then, from Eqn. (1):
Thus, the condition for max. or min. normal stress to occur on a plane is, shear stress on that plane should be zero. These planes on which shear stress is zero and the normal stress on them being either the max. or the min. are called ‘PRINCIPAL PLANES’.

4. 2
The above Eqn. (3), gives two values for θP, which differ by 900. Thus, there are two mutually perpendicular Principal planes, on which there are only normal stresses, shear stress being zero on them.
On one of them, the value of the normal stress is the max.; it is called the ‘Major Principal plane’, the max. normal stress on it is called the ‘Major Principal Stress’.
On the other principal plane, the value of the normal stress is the min.; it is called the ‘Minor Principal plane’, the min. normal stress on it is called the ‘Minor Principal Stress’.

5. (σx-σy)/ 2
2P τ
±√[(σx - σy/2]2 + τ2
Substituting for sin 2θP and cos 2θP in Eqn. (1), we get the equation for principal stresses as:
The above equation (4) gives two values for principal stresses. The numerically max. of the two values (+ ve or − ve) is the Major Principal Stress, (σMajor or σMax) and the numerically min. (+ ve or − ve) is the Minor Principal Stress (σMinor or σMin) .

6. (iii) Inclination of max. shear stress planes, Max
(iii) Inclination of max. shear stress planes, Max. shear stress Equation.
Let, θS be the inclination of the plane of max. or min. shear stress and τS be the value of the max. or min. shear stress on that plane, then, from Eqn. (2):
Eqn. (5) gives two values for θS, which differ by 900. Thus, there are two mutually perpendicular planes, on which shear stress are max.; numerically equal but opposite in sense.
The planes of Max. Shear stresses are inclined at 450 to the Principal planes.

7. ±√[(σx − σy)/2]2 + τ2
(σx-σy)/ 2
2S τ
Substituting for sin 2θS and cos 2θS in Eqn. (2), we get the equation for Max. shear stresses as:
The above equation (6) gives two values for Max. shear stresses, which are numerically equal but opposite in sense.

8. EQUATINS, NOTATIONS & SIGN CONVENTIONS : 2
Normal Stress, σθ, and Shear stress, τθ, on inclined plane are given by:
Inclination of Principal planes θP . There are two mutually perpendicular Principal planes
The equation for
Principal stresses are:
Inclination of Max. shear stress planes θS = θP Also, given by
Eqn (5). The two mutually perpendicular Max. shear stress planes have
equal & opposite shear stresses.

9. SIGN CONVENTIONS : σx  Normal Stress in x- directionB σy τ σx θ
σx  Normal Stress in x- direction
σy  Normal Stress in y- direction
τ Shear Stresses in x & y – directions
θ  Angle made by inclined plane wrt vertical
σθ Normal Stress on inclined plane AE
τθ Shear Stress on inclined plane AE τθ σθ
All the parameters are shown in their +ve sense in the Fig.
Normal stresses, σ  Tensile stresses +ve.
Shear Stresses, τ, in x – direction & Inclined Plane  Clockwise +ve.
Shear Stresses, τ, in y – direction  Anti-Clockwise +ve.
Angle, θ  measured wrt vertical, Anti-Clockwise +ve.

10. Inclination of Principal planes θP wrt Vertical :
1]. The state of stress at a point in a strained material is shown in Fig. (i) Locate the principal planes and find the principal stresses,
(ii) Locate the Max. shear stress planes and find the max. shear stress. 32
Inclination of Principal planes θP wrt Vertical : 30 C B 80 80 D A
Principal stresses, σP : 30 32
Stresses are in MPa

11. Inclination of Max. shear stress planes θS = θP + 450. D C B 32 30 80
Inclination of Max. shear stress planes θS = θP
θS = = & (wrt Vertical)
Max. shear stress τmax :
2]. The state of stress at a point in an elastic material is shown in Fig.
Find the resultant stress on a plane AE inclined at 550 to horizontal. 40
Shear stress τ = 60 sin 20 = Mpa
Normal stress σx = 60 cos 20 = Mpa
Inclination of AE wrt Vertical θ = 90 – 55 = 350 60
200 C E B 60
200
550 D A 40
Stresses are in MPa

12. Normal Stress, σθ, and Shear stress, τθ, on inclined plane AE:B 40
20.52
56.38
350
Resultant stress, fθ and its inclination α :
Stresses are in MPa

13. Normal Stress, σθ, and Shear stress, τθ,
3]. The principal stresses at a point in a strained material are 80 MPa and 40 MPa both tensile. Find the normal, tangential and resultant stress on a plane inclined at 500 to the major principal plane.
Since σx and σy are principal stresses, the shear stress, i.e, τ, on their planes is zero. 40
Normal Stress, σθ, and Shear stress, τθ,
on plane inclined at θ = 500 to plane of σx : C B 80
500 80 D A 40
Stresses are in MPa

14. Resultant stress, fθ and its inclination α :40 C B 80
500 80 D A 40
Stresses are in MPa
4]. At a certain point in a shaft, the normal stresses across two planes at right angles to each other are 60 MPa (C) and 40 MPa (T). The major principal stress is known to be 150 MPa (C). Find the shear stresses on the two said planes. A D C B 40 60 τ τ
Stresses are in MPa

15. Normal Stress σx = (200×103)/(50 ×100) σx = 40 MPa (T)
5]. A 50mm×100mm tie member of a timber truss has a glued joint (shown in Fig.) at an inclination of 400 to the longitudinal axis. If it is subjected to an axial force of 200 kN, check whether there is a risk of failure. The permissible normal and shear stress for the joint are 25 MPa and 16 MPa respectively. P
Normal Stress σx = (200×103)/(50 ×100)
σx = 40 MPa (T) A E 40
400
Stresses are in MPa
Normal Stress, σθ, and Shear stress, τθ,
on plane AE inclined at θ = 500 to vert. plane :
τθ > 16 MPa,
There is a risk of failure.

16. From (1) and (2), max. safe σx = 28.87 MPa
6]. Determine the max. safe uni-axial force that a strut of C/S area ×103 mm2 can carry safely, if, permissible normal and shear stress are 25 MPa and 12.5 MPa respectively, on a critical plane (shown in Fig.) inclined at 300 to the vertical. P A E σx
300
Let P be the max. safe load and σx be the stress under the load P.
Normal Stress, σθ, and Shear stress, τθ,
on plane AE inclined at θ = 300 to vert. :
From (1) and (2),
max. safe σx = MPa
P = σx × a => × 60 ×103
P = ×103 N = kN

17. (1) Bending stress σ and shear stress τ :
7]. At a transverse section of a beam of rectangular C/S, 100mm × 240mm, the BM is 76.8 kNm (Hogging) and SF is 96 kN. Find at that section, 40mm above the N.A. (i) Principal Stresses and (ii) Max. shear stress.
M.I. → I = 100×2403
= ×106 mm4 80 40
120
100
At 40mm above N.A.
(1) Bending stress σ and shear stress τ :
(2) Principal stress σP and max. shear stress τmax :
C/S A D C B
5.33
26.67
State of stress

18. (2) Principal stress σP and max. shear stress τmax :B
5.33
26.67
State of stress
8]. A propeller shaft of 200mm external diameter and 100mm internal diameter is subjected to a torque of kNm, BM 10 kNm (sagging) and a thrust of 100 kN. Find the Max. and Min. principal stresses and inclinations of their planes.
Stresses at outermost layer
at the top :
(1) Geometrical Propertise of the C/S :

19. (3) Principal stresses and Inclination of their planes :
PROBLEM (8) CONTINUED :
(3) Principal stresses and Inclination of their planes : A D C B
109.43
17.82
Principal stresses, σP :
State of stress
Inclination of Principal planes θP wrt Vertical :

20. MOHR’S CIRCLE :
DEFINITION : Mohr’s circle is a graphical method for solving problems in compound stresses. It is defined as a circle drawn such a way that, the X and Y – coordinates of a point on the circumference of the circle representes, respectively, the normal and shear stresses, on any plane at a point in a material subjected to a Two-Dimensional stress system.
Sign Conventions: Shear stresses on all the planes, (Hz., Vert., Inclined), are taken as +ve, if, anti-clockwise.
All other sign conventions remain the same as for analytical method.
Consider an element ABCD subjected to a state of compound stress as shown in the Fig. Let: A E D C B σy τ σx θ
σx  Normal Stress in x- direction
σy  Normal Stress in y- direction
τ Shear Stresses in x & y – directions
θ  Angle made by inclined plane AE wrt vertical
σθ Normal Stress on inclined plane
τθ Shear Stress on inclined plane

21. MOHR’S CIRCLE : (I) CONSTRUCTION OF MOHR’S CIRCLE :
1]. Select a convenient point as the origine and draw the X and Y – axes.
Choose convenient scale to plot the stresses.
2]. Plot the stresses on the vertical plane, taking σx as x-coordinate and τ as y-coordinate, to get the point ‘V’.
3]. Plot the stresses on the horizontal plane, taking σy as x-coordinate and τ as y-coordinate, to get the point ‘H’.
4]. Join the points ‘V’ and ‘H’ by a straight lne and mark its point of intersection with X-axis as ‘C’, to get the center of the Mohr’s Circle.
5]. With c as center and CV/CH as radius draw a circle to pass through H and V, to get the Mohr’s Circle.

22. MOHR’S CIRCLE :
(II) TO FIND, PRINCIPAL STRESSES, THEIR PLANES AND MAX. SHEAR STRESSES, THEIR PLANES :
1]. Mark the points A and B at which the Mohr’s Circle cuts the X-axis. As y-coordinates of these points are zero, they represent the Principal stresses.
2]. Measure the lengths OA and OB, the max. of the two represents, to scale, the Major Principal stress and the min. represents the Minor Principal stress.
3]. Half of the angle VCA and half of the angle VCB represent the angles of the respective Principal planes wrt to the vertical.
4]. Mark the points D and E at the ends of the vertical diameter of the Mohr’s Circle. As y-coordinates of these points are the max., they represent the Max. shear stresses.
5]. Measure the lengths CD and CE, they represent, to scale, the Max. shear stress.
6]. Half of the angle VCD and half of the angle VCE represent the angles of the respective Max. shear stress planes wrt to the vertical.

23. MOHR’S CIRCLE :
(III) TO FIND NORMAL STRESS, SHEAR STRESS AND RESULTANT STRESS ON ANY GIVEN INCLINED PLANE ‘AE’ :
1]. Draw the radial line CP in the Mohr’s Circle at an angle ‘2θ’ to CV.
2]. The x-coordinate of ‘P’ represents the Normal stress and
the y-coordinate of ‘P’ represents the Shear stress.
3]. The length ‘OP’ represents the resultant stress on given plane ‘AE’.
4]. The angle POA represents the angle of the resultant stress on the given plane ‘AE’ wrt to the normal of the plane.