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Arlon Online Engineering Library - TechRef!

Stress & Strain

Principal Stresses
For the special case of a two-dimensional stress state, the equations for principal stress reduce to

     principal stress

The two nonzero values calculated from this equation are temporarily labeled sigmaa and sigmab and the third value sigmac is always zero in this case. Depending on their values, the three roots are then labeled according to the convention: algebraically largest = sigma1, algebraically smallest = sigma3, other = sigma2.

Mohr's Circle - 2D Stress
To construct a Mohr's circle, the following sign conventions are used.
sign convention

1. Tensile normal stress components are plotted on the horizontal axis and are considered positive. Compressive normal stress components are negative.
2. For constructing Mohr's circle only, shearing stresses are plotted above the normal stress axis when the pair of shearing stresses, acting on opposite and parallel faces of an element, form a clockwise couple. Shearing stresses are plotted below the normal axis when the shear stresses form a counterclockwise couple.

The circle drawn with the center on the normal stress (horizontal) axis with center, C, and radius, R, where
cr
The two nonzero principal stresses are then:
     sigmaa = C + R
     sigmab = C - R
mohrs circle

The maximum inplane shear stress is taoin = R. However, the maximum shear stress considering three dimensions is always
     maximum stress

Hooke's Law
Three-dimensional case
     epsilonx = (1/E)[sigmax - nu(sigmay + sigmaz)]     gammaxy = taoxy/G
     epsilony = (1/E)[sigmay - nu(sigmaz + sigmax)]     gammayz = taoyz/G
     epsilonz = (1/E)[sigmaz - nu(sigmax + sigmay)]     gammazx = taozx/G

Plane stress case (sigmaz = 0):
plane stress case

Uniaxial case (sigmay = sigmaz = 0):     sigmax = Eepsilonx or sigma = Eepsilon, where
sigmax, sigmay, sigmaz = normal stress
epsilonx, epsilony, epsilonz = normal strain
gammaxy, gammayz, gammazx = shear stress
E = modulus of elasticity
G = shear modulus
nu = Poisson's ratio

Elastic Strain Energy
If the strain remains within the elastic limit, the work done during deflection (extension) of a member will be transformed into potential energy and can be recovered.

If the final load is P and the corresponding elongation of a tension member is delta, then the total energy U stored is equal to the work W done during loading.
     U = W = Pdelta/2
energy curve

The strain energy per unit volume is
     u = U/AL = sigma2/2E    (for tension)

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