1 JJp 3D2DDo

This expression reduces to Eq 35 when '/= 1. When '/= 0, it becomes:

The yield surfaces are plotted in Fig. 8 for U= 0 and 1, where they are compared with the surfaces obtained by Fleck (Ref 17). It should be noted that these surfaces have vertices where they meet the Em axis. The direction of the strain increment vector is nonunique for pure hydrostatic stress states, which can lead to computational difficulties when implementing this model. In practice, this nonuniqueness has been bypassed by inserting a circular arc at the vertex to ensure a smooth continuous yield surface (Ref 19).

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Eq 41 i /

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