• coalescence
• crack growth
• fracture mechanics
• IBESS
• interaction
• J-integral
• multiple cracks

This thesis is the result of a five-months work at the Fraunhofer LBF research centre.
This thesis explores the recent field of fracture mechanics about the study of interaction and coalescence´of multiple cracks. The work will be focused on propagation of multiple cracks in a butt welded joint specimen intended for heavy industrial application.
The thesis is based on previous research carried forward by Fraunhofer LBF group for Numerical Method and Component Design and all the groups belonging to the IBESS project. The study takes place on short cracks regime in which the use of elastic material hypothesis through the stress intensity factor loses its validity. Instead, an elastic plastic material behaviour is necessary with the introduction of the J-integral parameter, calculated through the use of FEM simulations of interacting and coalescing cracks’ models.
This leads to the definition of two factors necessary for a more accurate study of the propagation of fatigue cracks, the interaction and coalescence factors. A research is conducted to compare the crack growth between experimental results and the IBESS computational algorithm. Also, an improvement of the coalescence factor is required, to solve a singularity problem in the function.
The work showed that the simulations with heat affected zone material behaviour leads to results closer to reality respect the base material behaviour. Furthermore, the new implemented coalescence formula solved the problem of singularity at the beginning of coalescence, maintaining the behaviour of the function close to the experimental results.