Siemens Energy in Finspång develop, manufacture and deliver gas turbines to all over the world. To ensure that alloys and designs can withstand the thermo-mechanical loads seen in a gas turbine, especially those in the hotter sections of the turbine stage, creep and crack initiation/propagation testing is performed. Structural joints, such as brazing joints, likely contain defects and have reduced crack propagation resistance compared to the base material bodies which they join. Siemens Energy are interested in spearheading the use of FEM- and crack propagation software such as FRANC3D to model and simulate crack growth in structural joints, to aid in planning and evaluating joint test results.

In this project, a first attempt was made to model crack growth in Ni-base superalloy joints. The joints in question were brazing joints in IN792 with Amdry DF-4B as brazing filler metal. Two geometries were selected for this first attempt: A simple single edge notch specimen (SEN-FSP) and a Reference Case in the form of a creep fracture test specimen with a semi-conical brazing joint. Using the SEN-FSP specimen, joints under pure tensile- and mixed tensile-shear loading were modeled, simulated and compared. ABAQUS was used to perform a shakedown analysis in the SEN-FSP specimen joints, creating a stable and mostly elastic state ready for use with LEFM based FRANC3D. In FRANC3D, a penny shaped crack was introduced for both specimens. For the creep fracture test Reference Case, an approach to modelling concurrent large scale creep and crack propagation was tested for a number of scenarios, involving different degrees of initial stress redistribution. Due to inability to apply separate joint- and base material crack propagation properties in FRANC3D, and a lack of joint property data, the model had to be based on a large number of assumptions.

From results generated by the SEN-FSP- and Reference Case models, the potential is clear, but for such models to leave the pilot stage and transition to being tools capable of generating usable numerical results, improved joint modelling possibilities in FRANC3D combined with ’simulation friendly’ testing for joint properties is necessary.