A system level approach is used in modeling fastener fixities as it is impractical to model actual fasteners (the memory and processing penalties would be too large). Therefore, in large system-level models, fastener joints are modeled using approximate fixities.
The clamped area between the nut and bolt is a relatively rigid region due to the pre-tensioning of the bolt, and fastener joints can be approximated as regions of high rigidity held together by NASTRAN CBUSH elements and RBE2s. This analytical approach rigidly attaches the inner faces (in all degrees of freedom) of the fastener holes to corresponding ends of a CBUSH element. The CBUSH elements may be assigned representative stiffnesses to simulate the fasteners. The figure below illustrates fastener joint fixity.
System Level Analysis – Faster Joint. CBUSH element in blue and RBE fixity in red.
This approach assumes:
- Fastener and member interface is infinitely rigid.
- Shear and tensile loads are transmitted through the fastener joint.
- Contact between members is not modeled.
- In the max stress location (cross piece) reacting fastener forces are mainly acting in the fastener tensile direction therefore accounting for member friction will not greatly reduce resulting stress.
It should be noted that stresses at the fixities will be unrealistic due to the nodal fixity and may cause an artificially induced stress concentration (singularity). Unless stress values are below desired material durability limits, the stresses in these areas are ignored and stress values are sampled away from the singularity where stress values are not influenced by the singularity. Extracting detailed joint stresses usually entails a detailed fastener analysis or a detailed sub model.