Peter Dumstorff

Variational Extended Finite Element Model for Cohesive Cracks: Influence of Integration and Interface Law

According to a recently proposed variational formulation of the Extended Finite Element Method for cohesive crack propagation analyses [1] the length and the direction of new crack segments are determined on a global level from minimizing the total energy of the system. The focus of this paper is laid on the influence of the numerical integration and of the cohesive interface law on the energy distribution and, consequently, on the predicted crack trajectory. These influences are investigated by means of crack propagation analyses in plane structures made of quasi-brittle materials.

Setup, Validation and Probabilistic Robustness Estimation of a Model for Prediction of LCF in Steam Turbine Rotors

Flexibility and availability together with fast startup times become more and more important for steam turbine operation. Exact knowledge about the turbine components stresses and lifetime consumption during transient operation is a prerequisite in order to meet these requirements. A transient FE model of an intermediate pressure steam turbine rotor was generated, allowing the prediction of temperature and elastic stress field during turbine startup, load changes and shutdown. Operating data of the steam parameters and of a thermocouple inside the wall of the turbine inner casing were used to indirectly validate the thermal FE model in order to reproduce the measured metal temperatures in a proper accuracy. Subsequently a probabilistic sensitivity study was performed in order to identify the influence of scattering or not well known boundary conditions on the calculated lifetime consumption of the steam turbine rotor during a cold start. This in fact provides information about the accuracy of the prediction. The results of the sensitivity study also help to improve the model accuracy by identifying the boundary conditions with the largest impact on lifetime prediction uncertainty, i.e. the boundary conditions that need further investigation.

Aspects of crack propagation and hygro-mechanical coupling using X-FEM

Numerical prognoses of durability of structures made of cementitious materials such as concrete requires the consideration of environmental loads in addition to external loading [1]. For problems such as cracking of concrete structures, joints in rocks or shear bands in soft soils, the moisture transport in the opening discontinuities has to be taken into account in durability oriented analyses. The paper is concerned with a concept for coupled hygro-mechanical analyses considering discontinuities in the context of the extended finite element method [2, 3]. The spatial discretization of the displacement field as well as the moisture field, represented by the fluid pressure, are additively decomposed into a regular part and an enhanced part representing possible jumps in the primary variable. The extension to hygromechanical couplings involves the consideration of jump conditions across discontinuities as well as fluid transport in the intact part and the discontinuity, respectively. The present formulation is restricted to fully saturated conditions. The coupled model is investigated by means of an academic benchmark example, characterized by an artificially introduced crack, is analyzed numerically considering a hygromechanical loading scenario.