Principal stresses invariants
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Principal stresses/Invariants. In many real situations, some of the components of the stress tensor (Eqn. 4-1) are zero. E.g., Tensile test

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  • In many real situations, some of the components of the stress tensor (Eqn. 4-1) are zero.

    E.g., Tensile test

  • For most stress states there is one set of coordinate axes (1, 2, 3) along which the shear stresses vanish. The normal stresses, 1, 2, and 3 along these axes are principal stresses.


Stress Acting in a General Direction stress tensor (Eqn. 4-1) are zero.

  • p ds = force acting on a surface element ds

  • Area ds is defined by the unit vector (normal to it)  that passes through R

  • In order to know how p ds changes with the orientation of the area, we have to consider a system of orthogonal axes and the summation of forces

Figure 4.3 Force p ds acting on surface element ds.


Figure 4.4 Same situation as Fig. 4-3 referred to a system of orthogonal axes.

  • The summation of the forces along the axes are:

(4-17)


Where of orthogonal axes.lm1, lm 2, and lm3 are the direction cosines between the normal to the oblique plane and the x1, x2, x3 axes.

  • In indicial notation, Eqn. 4-17 can be written as:

    • This equation defines ij as a tensor, because it relates to vectors p and  according to the relationship for tensor

(4-18)


Determination of principal stresses
Determination of Principal Stresses of orthogonal axes.

  • The shear stresses acting on the faces of a cube referred to its principal axes are zero.

    • This means that the total stress is equal to the normal stress.

  • The total stress is:

  • The normal stress is:

  • If pi and N coincide then

(4-18)

(4-19)

(4-20)


(4-21) of orthogonal axes.

  • Applying equations 4-18 and 4-20 to p1, we have

    OR

  • Similarly, for p2 and p3,

(4-22)

(4-23)

(4-24)


(4-25)

(4-26)


  • The three roots of Eq. 4-26 are the principal stresses given by the following determinant1, 2 and3, of which 1> 2 >3

  • To determine the direction of the principal stresses with respect to the original x1, x2 and x3 axes,

    • we substitute 1, 2 and3 successively back into Eqs. (4-22), (4-23) and (4-24).

    • solve the resulting equations simultaneously for the direction cosines

    • and use:


  • Notes given by the following determinant:

    • The convention (notation) in your text book is different. It uses

      respectively

    • There are three combinations of stress components in Eq. 4-26 that make up the coefficient of the cubic equation, and these are:

(4-27)

(4-28)

(4-29)


  • Notes (cont) given by the following determinant:

    • The coefficients I1,I2 andI3 are independent of the coordinate system, and are therefore calledinvariants.

    • This means that the principal stresses for a given stress state are unique.

      Example:

      The first invariant I1 states that the sum of the normal stresses for any orientation in the coordinate system is equal to the sum of the normal stresses for any other orientation.


  • Notes (cont) given by the following determinant:

    • For any stress state that includes all shear components as in Eq. 4-1, a determination of the three principal stresses can be made only by finding the three roots.

    • The invariants is important in the development of the criteria that predict the onset of yielding.


  • The invariants of the stress tensor may be determined readily from the matrix of its components. Since s12=s21, etc., the stress tensor is a symmetric tensor.

  • The first invariant is the trace of the matrix, i.e. , the sum of the main diagonal terms.

    I1 = s11 + s22+ s33



(4-30)


  • Since the principal normal stresses are roots of an equation involving the stress invariants as coefficients, their values are also invariant, that is, not dependent on the choice of the original coordinate system.

  • It is common practice to assign the subscripts 1, 2, and 3 in order to the maximum, intermediate, and minimum values.


Examples
Examples involving the stress invariants as coefficients, their values are also invariant, that is, not dependent on the choice of the original coordinate system.:

  • (1) Consider a stress state where

    11 = 10, 22 = 5, 12 = 3 (all in ksi)

    and

    33 = 31 = 32 = 0

    Find the principal stresses.


Solution
Solution involving the stress invariants as coefficients, their values are also invariant, that is, not dependent on the choice of the original coordinate system.

Using Eqs. 4-27 to 4-29, we obtain I1 = 15, I2 = 41 and I3 = 0

Substitute values into Eq. 4-30, and we have

The roots of this quadratic give the two principal stresses in the x-y plane. They are:


Examples1
Examples involving the stress invariants as coefficients, their values are also invariant, that is, not dependent on the choice of the original coordinate system.:

  • (2) Repeat example 1, where all the stresses are the same except that 33 = 8 instead of zero.


Solution1
Solution involving the stress invariants as coefficients, their values are also invariant, that is, not dependent on the choice of the original coordinate system.

Using Eqs. 4-27 to 4-29, we obtain I1 = 23,

I2 = 161 and I3 = 328

Substitute values into Eq. 4-30, and we have

The three roots are


Principal shear stresses
Principal Shear Stresses involving the stress invariants as coefficients, their values are also invariant, that is, not dependent on the choice of the original coordinate system.

  • Principal shear stresses, 1, 2 and 3 are define in analogy with the principal stresses.

  • In order to understand how to derive the values, students are advised to see pages 29 and 30 of the text.

  • The principal shear stresses occur along the direction that bisect any two of the three principal axes


Principal shear stresses cont
Principal Shear Stresses (cont.) involving the stress invariants as coefficients, their values are also invariant, that is, not dependent on the choice of the original coordinate system.

  • The numeric values of the Principal shear stresses are:

    Since 1 > 2 > 3, 2 is the maximum shear stress

    i.e.

  • In materials that fail by shear (as most metals do) the orientation of the maximum shear is very important.

(4-31)

(4-32)


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