The dimensioning of structural members is usually carried out under the assumption that materials behave either brittle or ductile. The following hypotheses which correspond to one of the two assumptions are often used for strength or yield evaluation (Filonenko-Boroditsch, Festigkeitslehre, Technik, Berlin, 1960, [1]; Gol'denblat, Kopnov, Yield and strength criteria for structural materials (in Russ.: Kriterii prochnosti i plastichnosti konstrukzionnych materialov), Mashinostroenie, Moscow, 1968, [2]; Malmeisters, Tamuzs, Teters Mechanik der Polymerwerkstoffe, Akademie-Verlag, Berlin, 1977, [3]; Sähn et al. Bruch- und Beurteilungskriterien in der Festigkeitslehre, Fachbuchverlag, Leipzig, Köln, 1993, [4]). Because of their simplicity they have established themselves in the engineering practice.
