Thermal cyclic loading conditions of nuclear power plant components cause local stress-strain hystereses which are considered to be fatigue relevant events. The contributions of the hysteresis-loops to the fatigue process are evaluated using a damage parameter based on the effective cyclic J-integral which also includes the effects of crack closure. The successful application of such a short crack propagation approach essentially depends on the realistic description of the crack closure. In this context a finite element based algorithm is presented to simulate the opening and closure effects under special consideration of thermal cyclic loading conditions. The concept is based on node release and contact mechanisms. The implications of the crack propagation on the temperature at the crack tip are to be considered. In this context, the consequences of the altered temperature profile as the crack propagates have to be taken into account. It is the aim to formulate Newman-type analytical equations in order to incorporate the influence of crack closure into an engineering approach. Furthermore, the peculiarities of transient thermal loading on the crack propagation behavior are considered. The reduced crack propagation rates due to the temperature gradient in the direction of the wall are investigated numerically in order to describe the reduction of the damage contribution and decelerated crack propagation rates. The effects of changing thermal conditions in the wall on the crack propagation behavior are considered within the numerical algorithm.
The fatigue check of components has to be considered as an essential module of the safety concept as well as the ageing and lifetime management of nuclear power plants. It is based on special safety requirements in the design phase as well as in the framework of the ageing and lifetime management. Considerable efforts of load identification (fatigue monitoring) as well as the strict code conformity are characteristic features. The predominance of thermal cyclic loadings as well as relevant stress/strain amplitudes in the low cycle fatigue regime (LCF) are peculiarities compared to other technical domains. The code based fatigue concept is explained in the first part of the contribution. Furthermore, potentials for the quantification of existing margins are shown. On this, a new approach of mechanistic simulation of the thermal cyclic fatigue process based on short crack fracture mechanics is described in detail in the second part.
The fatigue behavior of the austenitic stainless steel X6CrNiNb18-10 (1.4550, AISI 347) is investigated. Experimental data are generated at room temperature, at elevated temperatures and further under thermo-mechanical conditions. Research is focused on generating data for parameter identification, especially for the improvement of material models. The influence of temperature on the stress-strain behavior and the fatigue life is presented.Finite element simulations are used to describe fatigue crack growth under thermo-cyclic loading conditions. Using the example of a thick-walled tube, the essential parameter for the crack growth rate, the effective cyclic J-integral, is determined for eight temperature transients (defined by fluid temperature, heat transfer coefficient and inner pressure). An approximation of the effective cyclic J-integral allows modeling without consideration of the crack geometry in the model. Keywords: Thermo-mechanical fatigue / biaxial behavior / effective cyclic J-integral / AISI 347 / fatigue crack growth Schlüsselwörter: Thermomechanische Ermüdung / biaxiales Verhalten / effektives zyklisches J-Integral / X6CrNiNb18-10 / Ermüdungsrisswachstum
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