pyNASSIF
(python NASTRAN Stress Intensity Factor)
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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
nonlinearity 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 elasticenergy release rate concept. This method uses
relationships that exist between the stress intensity and the elasticenergy
content of the cracked structure. The elasticenergy 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.

