Annular Pocketed Flange Modeling and Simulation

To verify the effectiveness of the annular pocketing of a flange to improve joint stiffness, a solid model of a typical space application flange was created and finite element analyses were performed, simulating both unpocketed and pocketed flange types for comparison.

Solid Model

A solid model of a pocketed flange was created using I-DEAS (version 8) solid modeling and simulation software from Structural Dynamics Research Corporation (SDRC). This model is shown below in Figure 1:

Figure 1: Solid model of a circular pocketed flange.

The outer diameter of the flange is nine inches, the bolt circle (through holes for eight number 10 screws) is eight inches in diameter, and the inside diameter is seven inches. The length of the cylindrical shell section is two inches and its thickness is 1/8 inch. The flange thickness is 1/4 inch, the pocket depth is 1/16 inch, and the inner and outer lands are 1/8 inch wide.

Finite Element Models

A solid 1/8 inch mesh was applied to the solid model and two load cases were run simulating a tensile load applied to the top of the cylindrical section.

Simulations

A pressure load of -100 psi was applied to the top surface of the cylindrical section to simulate a tensile load in the system. For the unpocketed flange, the outer land was constrained as well as the inner surfaces of the holes to simulate the constraints of the fasteners. The results of this case are shown below in Figure 2 and Figure 3:

Figure 2: Exagerated deformation of simulated unpocketed flange in tension.
Model file PocketFlange3ModelE.mf1

There is significant lifting of the inner flange area.

Figure 3: Exagerated deformation of simulated unpocketed flange in tension, side view.
Model file PocketFlange3ModelE.mf1

Using the same load, a constraint was added at the surface of the inner land of the pocket to simulate the preload induced by the fasteners. The results of this case are shown below in Figure 4 and Figure 5:

Figure 4: Exagerated deformation of simulated pocketed flange in tension.
Model file PocketFlange3ModelD.mf1

The total vertical deflection of the cylindrical section is much less than for the simulated unpocketed flange.

Figure 5: Exagerated deformation of simulated pocketed flange in tension, side view.
Model file PocketFlange3ModelD.mf1

A third case, suggested by Don Jones, was analyzed (not shown here) with a force load on the screw holes to replace the constraint. This case was intended to demonstrate that gapping of the inner land between the screws would not occur with moderate tensile loads. This was demonstrated by observing that the inner land in the regions between the screw holes remained in compression with the tensile load applied.

Conclusion

The finite element analysis, while not exact, shows potentially significant stiffness increase for the pocketed flange design. Only pure tension was analyzed, but its relevance to the bending case is apparent.

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