Ductile fracture of metallic materials can be described by void nucleation, growth and coalescence. The first coalescence of voids defines crack initiation. A quantitative description of material damage by the means of continuum‐mechanics is a very current theme. Using Gurson's model and the yield equation modified by Needleman and Tvergaard a quantitative description of damage condition is possible by connecting stress triaxiality and void volume fraction. In this paper failure curves, in which the effective plastic strain is plotted as a function of stress triaxiality, will be generated by a new yield equation, which is a further development of the Gurson model. The failure curves will be shown for crack initiation and other damage situations. An advantage of the further developed yield equation, compared with the conventional one, can be seen in the fact that not one constant, which is independent of the material, but several material specific values are used. These values can be determined by density measurements at different void volume fractions. The procedure for determination of failure curves will be shown and subsequently test results will be discussed.

In this report a brief overview on the influence of the specimens’ geometry and, therefore, the stress triaxiality upon the elastic plastic fracture mechanics parameters and the stable crack extension will be given. Corresponding references to detailed publications will be given. First, possible variations of testing specimens’ geometries will be discussed and later the influence of stress state upon the plastic strain in case of failure will be quantitatively analysed for axialsymmetric, circumferrentially notched tensile specimens.

In engineering applications, especially for ductile fracture of materials, nucleation, growth and coalescence of voids have often been observed. Currently there is an increase in interest for the effects of voids on the behaviour of engineering materials. In this paper, by the method of combining micro‐ and macro‐parameters, the effects of triaxial stress on the rates of void growth and yield equations are presented for porous materials with power‐hardening. The relations between triaxial stress and the rates of void growth for different n‐values and yield equations with different n‐values and void volume fractions are discussed. Following results have been obtained: For a porous material with power‐hardening, the yield equation can be approximately expressed by an elliptical equation in equivalent stress and triaxial stress. Both the long half‐axis and the short half‐axis of the elliptical equation are functions of the void volume fraction for a given hardening exponent. The triaxial stress has a strong effect on the growth rates of voids. For linear hardening materials, the relation between the growth rate of voids and the triaxial stress is linear. For elastic/perfectly plastic materials with a small void volume fraction, the growth rate of voids can be described in relation to the triaxial stress with an exponential function. The results from this paper are compared with theoretical results from other researchers for elastic/perfectly plastic materials. A good agreement is shown.

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