A 20 inch [508 mm] cubic box modelled with 2D shell located at mid plane of the parts was used to assess the effect of fasteners pretension over a wide range of deformations and stress levels in the components. For this purpose two models representing one quarter of the box were used:
A unique bulk mesh was used for the box in both models with the differences being localised to the zones immediately adjacent to the holes. The image gallery on the right side shows representative pictures for the common and model specific elements. The material properties used and samples of the stiffness values used for the fasteners in the first model are also included.
Excluding the pretension faze, the same loading of 25 psi external pressure was applied to the models in four stages (0-10, 10-15, 15-20 and 20-25) followed by pressure removal for evaluation of the permanent set in the panels and comparative assessment of residual stresses. Just touching contact was enforced at all surfaces prior to loads application in both models.
Contact definitions: In the first representation there are only six sets of contact surfaces between the parts the last one being the upper rigid surface (contact body body 51). In this case a single additional list of contact pairs (defined through a BCTABL1 entry) is sufficient for the entire loading-unloading cycle after the enforcement of the initial contact conditions (just-touching) with BCTABL1=0.
When enlarged, the figure to the right shows at the bottom the above two entry cards. Above there are the cards for the definition of the touching and touched contact bodies (BCONECT) and the physical contact parameters between these bodies (BCONPRP). The geometrical contact parameters used by the solver to determine when contact occurs between two bodies are defined in the card with id 3012.
In the detailed model, the bolts and collars of the 51 fasteners were each separated in two bodies each to enable threads locking after pretension and the gluing of the bolt heads and nut bases to their bases prior to pretension to prevent rigid body singularities. In this arrangement the model has 251 contact bodies, with 313 contact surfaces and over 450 contact configurations that required a more elaborated approach in model building and for the definition of the contact characteristics during the pretension loading which was applied in 27 steps. A colour sample of the tables for definition of contact parameters during the pretension and main loading-unloading is shown in the adjacent image gallery. The supplementary pre-processing work required by the inclusion of 102 additional contact bodies was more than compensated during the solution time and for post-processing of results.
Short brief of results:
The reason for differences in elastic range for the two types panels (un-flanged vs flanged) resides in the restraint role of the fasteners that can be captured only by the second model. Explanations of these restrains together with additional graphs for the stress envelopes and maximum deformations are presented in this video clip.
The evolution of the surface shear stress due to inter surface sliding from zero to 25 psi is included at the end of the video clip mentioned above or by pressing the play button below.
On fasteners pretension, contacts and shell structures… in a fatigue and sizing context