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Micromechanics of composites/Average stress in a RVE with finite strain

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Average stress in a RVE

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The average nominal (first Piola-Kirchhoff ) stress is defined as

Recall the relation (see Appendix)

In the above equation, let the volume integral be over and let the surface integral be over . Let the unit outward normal to be . Let the gradient and divergence operations be with respect to the reference configuration. Also, let and let . Then we have

If we assume that there are no inertial forces or body forces, then (from the conservation of linear momentum), and we have

Let be a self equilibrating traction that is applied to the RVE, i.e., it does not lead to any inertial forces. Then, Cauchy's law states that on . Hence we get

Given the above, the average Cauchy stress in the RVE is defined as

Note that, in general, .

The Kirchhoff stress is defined as . The average Kirchhoff stress in the RVE is defined as

In general, .