Abstract: The shape and size of plastic zone at crack tip is discussed by Mises, Tresca, Twin shear stress yield criterions respectively in this paper. It is pointed out that along the boundary line of plastic zone at craok tip, proper mechaniccal parameters and their amounts are selected and compared to predict the critical extension of the complex crack. Here, corresponding crack critical extension criterions are developed by using the three yield criterions and choosing three parameters, Tomax, Eomax, rpmin (size of plastic ,zone).Predicted results of crack extensimn are compared with each
critereion,

Key words: Complex cracks, Crack growth, Plastic zone