Some remarks on the Det.-criterion and caustics

A comprehensive study is made of the Det.(σ_ij)-criterion that involves the weighted difference of the dilatational and distortional component of the strain energy density function. Certain inconsistencies and contradictions when applied to the Mode I crack extension problem are pointed out. Clear distinctions are also made on the meaning of the core region in contrast to the loci of yield initiation and the initial curve of the caustics. Examples are provided to illustrate how overspecification in the initial assumption could lead to obscure results.