Countless spot welds and punch rivets are required in order to ensure the structural strength of a vehicle or a vehicle cab. Because this invariably concerns potential crack initiation sites, particularly reliable methods for fatigue analysis are required.
FEMFAT spot offers precisely those methods that are required for correct stiffness and fatigue life simulation of spot-joined components.
Two different approaches are available for this purpose:
For a stress-based assessment, the individual joints are represented by a detailed shell model optimized for stiffness (nugget model). These nuggets can be generated either in FEMFAT with the integrated remesher module or with a different preproccesor. The advantage of using the remesher or the ANSA preprocessor is the fact of the fully automatic definition that is provided.
The next step is the stress analysis of the locally modified model as the basis for the fatigue life prediction.
Another possibility for FEMFAT spot assessment is the usage of Nastran Superelement technology. Here, detailed analysis of notch stresses in the joint is possible even for whole car bodies.
In this case, the spot weld connections are represented by simple connections (beams + connectors, solid + connectors, CWELD, etc.). The fatigue analysis is based on analytical stresses that are calculated from the internal forces and moments in accordance with the JSAE method, which is an improved version of the well-known Rupp method. The overall analysis times are especially short as compared to the stress-based method, not least because the nugget generation can be omitted. However the results are also more mesh-sensitive.
The centerpiece of the fatigue life prediction for both analysis approaches is a database in XML format that has been adjusted in line with test results and which contains comprehensive fatigue-relevant parameters.
This database also allows flexible definition of connections, e.g. the use of different types of connections (spot welds, rivets) and equivalent stresses in one and the same calculation run or separate material assignments for each sheet of a connection, etc. Moreover, specifically for the analysis of bolted connections, nugget elements with a total of three rows can be created (remesher or ANSA) and analyzed.
FEMFAT spot is available as an influence factor for the fatigue analysis in BASIC, MAX and SPECTRAL and can also be combined with WELD.