## 2 Materials and Experiment Setups  
The material used in this study to form the composite top-hat and closure plate was carbon fiber prepregs and epoxy MTM44/CF5804A-40%RW-DC. MTM44 is a high performance, 180 \({}^{\circ}\)C (356 \({}^{\circ}\)F) curing, and toughened epoxy resin manufactured to produce primary and secondary aircraft structures. MTM44 is designed for autoclave molding or low-pressure vacuum bag Out-of-autoclave (OoA). The advantages of MTM44 include low density and a high level of damage. The fabric weight is 285 g/m\({}^{2}\) and the prepreg's weight is 475 g/m\({}^{2}\). The volume fraction of the fiber is 60%. The stack sequences used to form the experimental parts (top-hat section) were [\(\pm\)45/0-90/90-0/\(\pm\)45]. To fabricate the top-hat section, a tool is required to serve as a mold (see Figure 1). In this study, the width and the height of the tool were 250 and 60 mm, respectively. The face side was 65 mm long. The tool was made from aluminium series 5754 with a thickness of 2.5 mm. The length of the tool was 800 mm with nine ribs, placed 100 mm apart, welded beneath the tool to prevent any bending or movement during stacking material and manufacturing. Besides welding the ribs to the tool, two metal rods were placed along the length of the tool and attached to the ribs to ensure the stability of the tool during processing. The ribs were made from aluminium series AA5754 with a thickness of 2.5 mm. Aluminium was used to enhance heat transfer when curing and cooling down. In the layup process, layers of the material were carefully stuck in the mold according to the orientation of each. After sticking each ply, a small roller was used with a smooth motion to remove any fold or wrinkling during the process (also to remove any air gaps). The best way to remove folds is to start moving the roller from one edge and working along the plies, with the focus on corners or any concave areas. A Polytetrafluoroethylene (PTFE) release film layer was placed between the tool and the composite to prevent sticking the part with the tool. The PTFE film provides excellent resistance to chemical splashes and is easy to clean (wipe down). It is also very capable of resisting contamination by aqueous and oily fluids. The prepregs were placed on the PTFE film and covered with the release film, then the breathing layer was be placed on top of the release film. The main use of the breathing layer is to absorb resin from the laminate and to ensure complete evacuation of air from the vacuum bag during the process. Bagging film covered the mold and was fixed by the tacky tape to the mold. The vacuum ports were attached over the bagging film. The extra thickness of breathing layers were placed underneath the vacuum port to ensure that the air was still drawn through the port even when a vacuum had compressed the  
Figure 1: (**a**) Overview of fabrication tool and specimen (**b**) tool design. breathing layer and to ensure no resin could be passed through the vacuum port. Figure 2 illustrates the vacuum bagging process used for this study. The composite part was then placed in an oven for curing. The curing temperature was set at 130 \({}^{\circ}\)C for two hours, then the temperature was increased to 180 \({}^{\circ}\)C for another two hours. This was followed by the part cooling down at a rate of 2-3 \({}^{\circ}\)C / min, with the vacuum port still attached to the vacuum during this process. The number of prepreg layers was 2 per layer, resulting in 16 plies for 8 layers. After curing, the contact area for both top-hat and closure plate were roughened using an abrasive material. The purpose of this is to remove any oily area that affects the adhesive material. SikaPower-490 C was used as an adhesive material. SikaPower-490 C is a two-component adhesive which was designed for steel, aluminium and CFRP bonding. The adhesive is suitable for structural and crash resistant application. The adhesive was applied to the roughened area.