Feature Article Figure 1. Schematic of bagging layup. An aluminum mold was cleaned of surface defects using sand paper, and a non-porous Ethelyne tetrafluoro ethelyne (ETFE) release film was placed. The prepreg layers were cut into required dimensions and laid up according to the required facesheet configuration. Rollers along with hand pressure were used to press the prepregs, starting from the center of the layup and moving progressively towards the edges. This process was repeated for all the prepreg layers to remove entrapped air as well as folds or wrinkles. The tool-side and bag-side facesheet layups were debulked at room temperature under full vacuum for 60 minutes to ensure good compaction. The schematic of the bagging procedure employed for the OOA process is shown in Figure 1. The compacted tool side facesheet layup was laid on the aluminum mold which was covered with the ETFE release film. A layer of film adhesive was placed on the prepreg layup. The honeycomb core was placed on the adhesive layer followed by the bag-side adhesive layer and facesheet stack. Edge breathing using EB1590 edge bleeder was used to evacuate entrapped air from the sandwich layup. A layer of ETFE release film was placed on the sandwich layup followed by an aluminum caul plate. Airweave N10 breather was used to distribute the vacuum pressure evenly. The entire layup was sealed using a double vacuum bagging scheme. The layup was debulked at 49°C (120°F) for two hours. The sample was cured according to the cure cycle shown in Figure 2, and allowed to cool down to room temperature. This was followed by a freestanding post-cure at 232°C (450°F) for two hours. The prepreg cure cycle was used to manufacture the sandwich panel. The samples did not exhibit warping following post cure. 46 Experiments Thermogravimetric Analysis Outgassing of prepreg and adhesive during cure can lead to voids in facesheets and adhesive fillets. These volatiles need to be removed during cure. The evolution of volatiles from the prepreg and adhesive was studied using Thermogravimetric analysis (TGA). The adhesive was removed from storage and allowed to warm up to room temperature. A small sample was removed and the prepreg cure cycle was simulated in a TGA. Air at a flow rate of 40 mL/min. was used as the sample purge gas and the weight loss was recorded. Flatwise Tensile Test Flatwise tensile (FWT) strength primarily serves as a quality control parameter for bonded sandwich panels. This test produces information on the quality and strength of the core-to-facing adhesive bond. The flatwise tensile test for sandwich samples was done according to ASTM C297. Sandwich samples had a facesheet configuration of [0°/90°]s. This test method consists of subjecting a honeycomb sandwich samples to a uniaxial tensile force normal to the plane of the sample. Honeycomb sandwich samples of 50.4 mm x 50.4 mm (2 in. x 2 in.) were cut from the manufactured panel. The sample edges were polished progressively using 80-220 grit sandpaper. The facings of the samples were lightly roughened using 80 grit sand paper. For RT testing, aluminum blocks of 50.4 mm x 50.4 mm x 50.4 mm (2 in. x 2 in. x 2 in.) were bonded to the sample facings using a high strength epoxy, 3M Scotch-Weld DP-460NS. The adhesive was allowed to cure at room temperature for 24 hrs and was post cured at 121°C (250°F) for two hours. Samples for elevated temperature testing at 177°C (350°F) were bonded to aluminum blocks using Metlbond 2550 film adhesive. Test samples were conditioned at 177°C (350°F) in the thermal testing chamber for 2 hours prior to testing. Average relative humidity was SAMPE Journal, Volume 52, No. 3, May/June 2016