Kai Bauerbach

Numerical Investigations of Phenomena Caused by the Closure and Growth Behavior of Short Cracks Under Thermal Cyclic Loading

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.Copyright

Fatigue Assessment of Nuclear Power Plant Components Subjected to Thermal Cyclic Loading

Thermal loading conditions of nuclear power plant components cause local stress-strain hystereses. For the fatigue life prediction of nuclear power plant components under thermal cyclic and structural loading a new method based on the local strain approach is to be presented. This method involves finite-element simulations as well as the experience gathered from lifetime assessment methods based on short crack models. The local stresses and strains are obtained from coupled-field FE-analyses. The calculation of the hysteresis-loops relies on appropriate material models and experimentally verified temperature-dependent material parameters in order to describe the elasto-plastic behavior of the material as realistically as necessary. Due to the temperature dependence of the material parameters the resulting hysteresis loops are of non-conventional shapes and similar to those observed under multiaxial nonproportional structural loading. Hence, fatigue methodologies developed for non-proportional loading conditions during the past years bear good prospects for successful application under non-isothermal loading conditions.Copyright

Zur Methodik der Ermüdungsbewertung von Komponenten der nuklearen Kraftwerkstechnik

Kurzfassung Der Nachweis der Betriebsfestigkeit ist wesentlicher Baustein des Sicherheitskonzeptes sowie des Alterungs- und Langzeitbetriebsmanagements kerntechnischer Anlagen. Hoher Aufwand zur Lastidentifikation (Fatigue Monitoring) und starke Regelwerksbindung sind kennzeichnend. Besonderheiten gegenüber anderen Technikbereichen stellen die Dominanz thermozyklischer betrieblicher Belastungen sowie relevante Beanspruchungsamplituden im niederzyklischen Bereich (LCF) dar. Das verfolgte Nachweiskonzept wird in seiner regelwerksseitigen Einbettung im ersten Teil des Beitrages erläutert. Darüber hinausgehend werden Möglichkeiten aufgezeigt, bestehende Margen zu quantifizieren. Hierzu wird ein neuer Ansatz der mechanismenorientierten Simulation des thermozyklischen Ermüdungsvorgangs auf Basis der Kurzrissbruchmechanik im zweiten Teil beschrieben.