Finite Element Analysis (FEA) Evaluation
Using finite element analysis (FEA) the authors modeled the thermal movements occurring in the edge seal of a large IGU as a result of temperature variations for three commercially available spacer bars of different material and design.
An IGU window size of 1.49 m (58.8 in.) by 2.13 m (84 in.) was chosen for the FEA modeling. This size simulates a large sliding glass door, with the same aspect ratio (1.42857) as the 0.35 m (14 in.) by 0.50 m (20 in.) test panes. Taking advantage of symmetry, the model needs only to take one quarter of the full size into account. In the model, the pane is supported along the bottom perimeter edge as shown by the arrows in Fig. 1. Element nodes on symmetry planes are constrained to remain on the symmetry planes.
Galvanized steel and aluminum spacers are modeled with dimensions 12.3 mm (0.485 in.) wide and 8 mm (0.315 in.) deep and a wall thickness of 0.4 mm (0.016 in.) for aluminum and 0.5 mm (0.020 in.) for galvanized steel. Comer keys for galvanized steel and aluminum spacers are modeled as solid polyamide (Nylon*® 6) keys that are “bonded” to the spacers (the model does not allow for any slippage between the spacer and the corner keys). Figure 2 shows the spacer and corner key designs and dimensions chosen.
The stainless steel spacer is modeled with dimensions of 11.5 mm (0.454 in.) wide and 6 mm (0.235 in.) deep with a wall thickness of 0.2 mm (0.01 in.). The corner key is modeled as “bonded” to the spacers and as a solid. The actual corner key is hollow and the metal is split at the inner radius, therefore the stress predicted by the model is much higher than the actual stress expected in service. Figure 3 shows the stainless steel spacer and comer key designs and dimensions chosen.
The edge seal of the galvanized steel and aluminum spacer is modeled with the secondary seal constrained by “bonding” to the spacer and the glass panes, but not to the nylon corner key, reflecting the fact that secondary sealants generally have poor adhesion to plastic corner keys. For the stainless steel spacer, the edge seal is modeled with the secondary seal constrained by “bonding” to the glass panes and the spacer, as was the case for the galvanized steel and aluminum spacers. However, the corner keys in this instance are bonded to the secondary seal as well as the edges in contrast to that described for the other two spacer types.
The secondary sealant thickness—measured as coverage above the spacer—is assumed as 6 mm. The glass thickness is modeled as 6 mm (0.236 in.) and a 0.3 mm (0.019 in.) gap is assumed for the primary
seal. The polyisobutylene (PIB) material itself was not modeled, due to the low strength of the material. Figure 4 shows the edge-seal configurations for the aluminum and galvanized steel spacers and the stainless steel spacer.
FIG. 3—Stainless steel spacer and comer key designs and dimensions.
FIG. 4—Edge-seal configurations for aluminum and galvanized steel and stainless steel spacers.
FIG. 5—Corner deflection of aluminum spacer frame (exaggerated by factor of 100).