As is known, ductile strength is characterized by the shear resistance of a material Sshf, and brittle strength is characterized by the breakaway resistance of a material Sref. Under a complex stress state, the ductile and brittle strength criteria of a material are functions of all three true stresses S1, S2 and S3. If the functions from S1, S2, S3, expressing shear and tear resistance, are chosen correctly, then the mentioned strength criteria for a given material and its physical state before loading will be constant, independent of the type of stress state ...