Abstract: Based on M. H. Yu unified criterion, the unified solutions to the shapes and sizes of mode Ⅰ,Ⅱ,Ⅲ crack tip plastic zone under small scale yielding (S. S. Y. ) are obtained. The former solutions to this problem by Tresca criterion and Von Mises criterion are only the special cases or linear proximity. The -solutions offered has universal applications to different kinds of materials. From the analysis solutions and diagrams, some new conclusions could be obtained.

Key words: Crack Plastic zone, Twin shear, Yield criterion