No abstract

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.

The constraint parameter triaxial stress which results from loading types and component geometries plays an important role in the failure behaviour of materials. Especially the local failure initiation of materials depends very strongly on triaxial stress. The failure behaviour of a material under different stress states (i.e. different constraint) differs. In this paper effects of triaxial stress on the failure behaviour of steels are presented by means of theoretical analysis, experiments and finite‐element‐calculations. A theoretical model describing the effects of triaxial stress on failure initiation of materials has been developed based on theoretical analysis. Corresponding experiments were conducted to determine the material constants in the failure model of materials. In order to obtain local damage parameters finite‐element calculations have been conducted. According to the results from theoretical analysis, experiments and finite‐element calculations the damage curves of materials, in which the critical equivalent plastic strain is plotted as a function of triaxial stress, have been obtained. Furthermore, the failure behaviour of materials under different triaxial stresses has been discussed.

During the design stage of the today operating German PWRs, one major design target was to manage the materials ageing behavior of the ferritic reactor pressure vessel (RPV) steel in oder to prevent a brittle fracture of the RPV under all possible loads. Very conservative thermal hydraulic input data were used in this context for the assessment of loss of coolant accidents. Today more realistic thermal hydraulic analyses based on large scale experimental results are available as well as more sophisticated fracture mechanics methods to assess postulated flaws. Using these tools an additional joint analysis of six German PWR RPVs is performed. A fully representative spectrum of all possible large and small break loss of coolant accidents is investigated. Locations of interest are the irradiated RPV core beltline region as well as those regions with negligible irradiation but higher loading, like the RPV MCL nozzles and the flange to cylinder transition area. The thermal hydraulic work comprises global system analyses with subsequent local analysis considering mixing phenomena like plume forming below the injecting nozzles. Dependent on the location of the postulated flaw (core or nozzle region), the postulated leak size is differing. In the core weld region medium leak sizes (100 to 200 cm2) with subsequent injection into the cold leg lead to dominating loading, whereas in the nozzle the small leak sizes (3 to 20 cm2) with injection into the hot leg are more important. This is due to the higher pressure at smaller leak sizes and the larger thermal stresses caused by the high temperature of the nozzle flange and the low temperature of the injected ECCS-water. But independent of the number of transients it turned out that all load paths of the postulated flaws and the investigated leak sizes show a similar behavior. As the result of this additional joint safety analysis of six German PWR RPVs with different injection modes, a brittle fracture of all RPV regions can be excluded with sufficient safety margin.