B04 - Crack growth in joined struc­tures

The use of novel manufacturing processes requires the design of components in terms of static strength, fatigue strength, crack sensitivity and crack growth life, which leads to the need for crack propagation investigations of cracked structures. The starting point for these investigations is damage resulting from the joining process as well as initial cracks that occur during operation as a result of fatigue and/or corrosion. Mechanical joints are characterized in particular by contact between the parts to be joined, high stress concentrations, heterogeneous material states, residual stresses and multi-axial, out-of-phase stress states. These factors can also influence the propagation of a crack. At the same time, a wide range of material-geometry or anxillary joining part-tool combinations is present due to the versatility of the process chains. The content of subproject B04 is to investigate crack growth based on technical initial cracks, taking into account the above-mentioned factors and the versatility of the process chains that occur in mechanically joined structures.

The aims of the subproject are the characterization of the fracture-mechanical property profile of mechanically joined structures as well as the consistent description of the crack propagation process and the occurrence of the unstable failure under consideration of the local stress and material conditions. The synergetic use of the methodological competences "experimental crack growth investigations", "theoretical-analytical modeling of crack growth" and "numerical simulation of crack growth" enables a comprehensive and viable consideration of the fracture-mechanical behavior of joined structures in use and a contribution to the holistic description of the joinability under consideration of the versatility process chains. For this purpose, the fracture-mechanical material characteristics of the base materials, the formed joining parts and the joined structures are investigated. In addition, the main material- and process-related influencing factors and their effects on the crack propagation behavior are identified. Fracture mechanical concepts for the prediction of crack growth have to be extended under certain circumstances by these findings for mechanically joined structures. In addition, the simulation program ADAPCRACK3D developed by FAM will be extended so that crack growth can be simulated automatically starting from a defect and the remaining service life can be determined for mechanically joined structures. The investigations of crack growth are intended to provide information on the design of the joined structure and on the significance of the material, geometric and process variable variations that are essential due to the versatility of the joining processes, in order to prevent the propagation of a crack and unstable failure in mechanically joined structures.