The plastic zone instability concept of Vazquez is reviewed and a refined method for determining the conditions for instability is discussed. The plastic zone instability failure criterion is applied to several crack configurations in a thin-walled cylindrical pressure vessel with semi-spherical end caps. The results predict the possibility that, for ductile materials in certain combination of geometry and loading, the failure controlled by the plastic zone instability may precede the failure controlled by the plastic collapse load. Perhaps it may suggests the necessity for consideration of the plastic zone instability along with the plastic limit load.
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