(python NASTRAN Stress Intensity Factor)
Stehlin Engineering developed a tool, which aim is to compute the stress intensity factor (K) along a given crack path in function of the crack length in structures idealized by PLATE elements in FEM models based on the NASTRAN solver. The tool is able to deal with two crack tips at the same time (1 thru crack or 2 edge cracks). pyNASSF is a script written in python, which interacts with the NASTRAN solver and the NASTRAN input/output files.
The method permits to calculate the SIF of up to 2 thru cracks fronts in virtually any thin structures. The cracks are implemented in a NASTRAN FEM model by unzipping the nodes along the crack path. This method permits to compute stress intensity for geometries, which are difficult to handle using standard solutions available in standard CG software like AFGROW and NASGRO or by compounding of solutions of other sources or when conservatism in the analysis has to be reduced. This also permits to assess the impact of parameters like fasteners flexibility, stiffener effect, secondary bending effects, etc. on the crack growth behaviour. When required, more complex structural behaviour such as non-linearity due to large displacements or contacts can also be assessed. In case of 2 edge cracks, it automatically accounts for the load redistribution between the parts.
pyNASSIF is based on the elastic-energy release rate concept. This method uses relationships that exist between the stress intensity and the elastic-energy content of the cracked structure. The elastic-energy content is calculated using FEM model of the cracked structure and an interface with NASTRAN is provided to automatize all analyses steps.
The method is straightforward and powerful and can be used to solve very complex cases.