Numerical simulation of thermal fatigue crack pattern formation by BEM

During the operation of conventional and nuclear power plants, structural components as pipes, nozzles, pressure vessels and steam generators are exposed to repeated thermal transients. The material surfaces affected by such cyclic thermal shocks experience subcritical loading and unloading, which lead to the generation, growth and coalescence of fatigue cracks. Like most technical materials, steel exhibits inhomogeneity and anisotropy of its microstructure. The thermal fatigue damage currently starts from specific weak points in the microstructure (subsequently called flaws), which are usually stochastically distributed. If they fail, a microcrack is formed. The pattern formation proceeds by the growth of those single cracks, whereby repeated bending and branching occurs. During the development of the fatigue crack pattern, the stress field interaction of all cracks has to be taken into consideration. Although such micro crack patterns in general don't endanger the global structural integrity, they can serve as origin for larger fatigue crack growth or for stress corrosion cracking. A general approach to model complex thermal fatigue crack patterns is established, whereas the crack analysis was restricted to approximate solutions. A consistent fracture mechanical analysis to simulate the formation of micro crack patterns in a two-dimensional numerical model for heterogeneous crystalline materials containing an assumed flaw distribution was developed. The simulation is based on the boundary element method (BEM). In the present paper this model is employed to study the formation of thermal fatigue crack patterns. The results show that by means of the BEM the formation of crack pattern with remarkable complexity can be simulated on the basis of an exact fracture mechanical approach. Concerning the application to thermal fatigue cracking it can be concluded, that the simulations take into account all essential influence quantities and produce quite realistic pictures. The quantitative description of microcrack formation and crack branching is important for the evaluation of the residual strength and the residual life time of structural components, which have experienced thermal shocks. Future work is necessary to transfer this two-dimensional analysis to the real three-dimensional situation of surface crack pattern. The developed simulation program can be modified to fit other phenomena of microcracking with regard to materials structure, loading and probabilistic as well