ANALYSIS OF FORMABILITY OF THICK COMPOSITE COMPONENT UNDER DOUBLE- DIAPHRAGM FORMING PROCESS

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1 ANALYSIS OF FORMABILITY OF THICK COMPOSITE COMPONENT UNDER DOUBLE- DIAPHRAGM FORMING PROCESS Hassan Alshahrani Mehdi Hojjati Concordia Center for Composites, Concordia Center for Composites, 1455 De Maisonneuve Blvd. W, Montreal, 1455 De Maisonneuve Blvd. W, Montreal, Quebec, H3G1M8, Canada Quebec, H3G1M8, Canada Abstract The formability of the out-of-autoclave 5-harness satin woven carbon/epoxy prepreg into a thick single-curved-shaped using double diaphragm forming process was studied. Examining the relationship between deformation mechanisms and the final parts, in terms of observed defects such as thickness variations and void content, was investigated. Optical microscope analysis was performed in order to visualize the yarn deformation and observed defects during the forming process. The results show that thick composite parts can be manufactured by double diaphragm forming which leads to reduce the manufacturing time and cost. Moreover, the success of composite forming is highly dependent on the mechanisms that control the deformation during the process. Keywords Composite forming, Double diaphragm process, Out-of-autoclave prepreg. I. INTRODUCTION Composite parts are manufactured using a range of techniques, both conventional and automated. However, conventional composite manufacturing techniques, such as hand lay-up, tend to be labor intensive, time consuming, and costly. To improve productivity while maintaining the geometrical complexity of components, a three-step process is undertaken: first, flat laminates are laid down by automated machines; next, these laminates are subjected to a forming process; finally, the formed parts are cured. This paper focuses on the forming step, during which heat and pressure are applied to transform the flat laminates into the desired shape prior to curing [1]. The most important sheet-forming processes for composite materials are the diaphragm and stamp forming methods [2]. Both processes exhibit high efficiency and high productivity during formation. However, the stamp forming process is very expensive due to the need for two close-tolerance dies for each shape. Thus, this work will utilize the cheaper and equally efficient diaphragm forming process. A typical double-diaphragm forming process can be described in three steps [3]. First, a flat laminate must first be placed between two deformable sheets known as diaphragms, which are themselves clamped over a forming box. The space between the diaphragms is subjected to a full vacuum seal. Next, the laminate between the diaphragms is heated up to processing temperature. Finally, formation is initiated by applying controlled vacuum pressure to the forming-box cavity below the lower diaphragm. Most previous research [4]-[6] focused on the diaphragm forming that use thermoplastic prepreg. This research has shown that the forming of thermoplastic prepreg material can be influenced by processing parameters, tool geometries, and diaphragm materials. However, the choice of diaphragm materials is limited by the commercial availability of the desired properties. II. MATERIALS AND EXPERIMENTAL A. Materials The material tested in this study was the 5-harness (5HS) satin carbon/epoxy woven-fiber prepreg, toughened with epoxy resin (Cycom 5320) designed for out-of-autoclave manufacturing applications. The fabric s areal weight is 380 g/m2 and the resin content is 36% by weight. The measured thickness of uncured one-ply is approximately 0.55 mm. B. Flat-laminate preparation Flat laminate fabrication, which consist of 32 layers with stacking sequences as shown in Fig. 1, was cut into 300 mm x 70 mm. After stacking of all the layers 185

2 of prepreg, the sample was consolidated using vacuum bagging for 30 minutes at a pressure of 0.1 MPa (see Fig. 2). Fig. 3 Double diaphragm forming setup Fig. 1 Stacking sequences of flat laminate D. Double diaphragm forming procedure Fig. 4 represents all steps that need to be followed during the double diaphragms forming process. Fig. 2 Consolidation process and marked sample C. Double diaphragm forming setup A custom-built set-up was developed which consisting of a vacuum system, a heating system and double-diaphragm tool, as illustrated in Fig. 3. The vacuum system consists of a vacuum box with a control valve and a vacuum pump, for applying vacuum pressure to the cavity between the two diaphragms and within the vacuum box. The vacuum box has a movable plate with interior holes designed to control the height of the tool. To achieve the necessary processing temperatures and to ensure a closed chamber for the curing step, the vacuum box can be adjusted using two electronic jacks. The diaphragm material used in this study was a translucent silicone rubber with 1.6 mm thickness. The double-diaphragm tool has a control valve and vacuum gage to monitor the pressure that is applied between the diaphragms. Thermocouple is used to control the processing temperature. Fig. 4 Double diaphragm forming procedure 186

3 III. RESULTS AND DISCUSSION A. Forming experiment results The aim of this study was to investigate the ability of diaphragm forming for manufacturing of thick composite components. The thick laminate (32 layers) was formed into the desired shape under 70 ᵒC, with a forming rate of 10 kpa/sec. The double diaphragm forming enables to form thick parts even more than 32 layers. However, this requires high applied pressure between double diaphragm films (step 3) to ensure that the laminate remains flat during the preheating process to avoid any compressive state in the sample which leads to wrinkling in most cases. In addition to that, the diaphragm film thickness plays an important role in forming thick parts using such process. No wrinkles were observed in both upper and lower surfaces. However, other defects such as thickness variations and void content were seen. Fig. 5 shows a cross-section of the curved part and marked five thicknesses in the formed sample to analyze thickness variations during the forming process. The original thickness of flat laminate after consolidation process was 14 mm. Table I shows the thickness values after forming process along the curved part. Among these values, the lowest thickness value was found in t3 (top part in the formed sample). This may attributed to the pressure distributions within the vacuum box that need to accommodate the tool shape and the change in yarn geometry. Although all thickness values are relatively close to each other, the difference between the average of these values (11.86 mm) and the original thickness (14 mm) is high (2.14 mm) when the aerospace application is the target. Therefore, selecting an appropriate applied pressure and low forming rate during the double diaphragm forming process help to reduce thickness variation defects in the required parts. During the forming of single-curved parts, inter-ply shear (between layers) and out-of-plane bending are the dominant deformation mechanisms. While, the in-plane shear deformation, which is characterized by rotation of the yarns at their crossovers, is leading during double-curved parts forming. For thick laminates, through the thickness becomes as a significant mechanism. However, it is always related to inter-ply slippage as shown in Fig. 6. Inter-ply slippage mechanism must be occurred during forming process to conform the shape without wrinkles (see Fig. 7(a)). Therefore, processing parameters that facilitate the occurrence of inter-ply slip during forming process must be taken into account. On the other hand, the ply or laminate bending stiffness plays an important role in determining wrinkle formation, especially the size and numbers of wrinkles. High ply-bending stiffness is favorable as long as the desired shape can be achieved. This is in fact due to that compressive loading is occurred in lower surface (bottom layers) to accommodate selected shape. Therefore, low-bending stiffness may lead to out-ofplane deformation (wrinkling) once subjected to compression as shown in Fig. 7(b). Also, the preforming state, such as forming direction to yarn direction, position the sample over the tool, should be taken into consideration during forming process. Fig. 6 Inter-ply slippage during forming of singlecurved part Fig. 5 Cross-section of the curved part TABLE I Thickness values along the curved part Thickness t 1 t 2 t 3 t 4 t 5 Value 12.5 mm 12 mm 11.3 mm 11.6 mm 11.9 mm Fig.7 (a) Inter-ply slippage mechanisms during forming process and (b) effect of bending stiffness on wrinkle formation 187

4 B. Optical microscopic analysis Microscopic analysis was performed in order to visualize the yarn deformation and observed defects during the forming process with the aim to optimize the process conditions. The same sample that cut for thickness variation analysis was used for microscopic images after polishing process. Fig. 8 shows the optical microscopic images in the curved part (marked in color rectangles). Red rectangle represents top eight-layers in the formed sample; a lot of void regions was found within this image whether between layers or between yarns. As we moved from top layers to bottom layers, void content is decreased. Moreover, the yarn shape and its distribution were found to be more realistic in the bottom layers (green rectangle) compared to other layers. Some studies attribute the formation of void defects to the consolidation process of the flat laminate before the forming operations. However, microscopic image for eight-layers that consolidated at the same pressure and period demonstrates small void content as shown in Fig. 9. Out-of-Autoclave prepregs that used in this study are different to most autoclave prepregs in that the upper and lower fiber plies are partially impregnated with resin system. This creates a dry porous medium between the upper and lower portions of the prepreg to permit the evacuation of any entrapped air before the resin wets the dry fibers. During processing, these spaces are progressively infiltrated by resin to produce a uniform and void-free structure. However, the forming step is very short and the yarns need more time to be fully impregnated. Thus, selecting slow forming rate helps to efficiently accommodate the resin flow during forming process. Fig. 9 Microscopic view of 5-HS after consolidation process IV. CONCLUSIONS A double-diaphragm set-up for forming thick flat prepregs into a single-curved shape was developed. Formability of thick OOA prepreg, in terms of processing challenges and observed defects, were discussed. The results show that thick composite parts can be manufactured by double diaphragm forming which leads to reduce the manufacturing time and cost. Selecting an appropriate applied pressure and low forming rate during the double diaphragm forming process help to reduce thickness variation and void defects. However, the yarn deformation, material structure, tool- geometry and resin flow phenomena during forming process have a significate impact on the final yarn shape and resin distributions at the microlevel. The success of forming output is highly dependent on the mechanisms that control the deformation to achieve the final shape. V. ACKNOWLEDGMENTS The authors of this paper would like to acknowledge the financial support of NSERC (Natural Sciences and Engineering Research Council of Canada). Thanks to Bombardier Aerospace for supplying the materials. Supporting provided by Najran University is also gratefully acknowledged. VI. REFERENCES [1] A. Modin, "Hot Drape forming of thermoset prepreg", in Proc. Compos. Manuf. (Pasadena, CA), 1993, pp [2] C. L. Tucker, Forming of advanced composites, (New York: Wiley), p 297, [3] X. Yu, L. Zhang, and Y. Mai, "Modelling and finite element treatment of intra-ply shearing of woven fabric", J. Mater. Process. Technol., vol. 138, pp , Fig. 8 Microscopic images of curved formed part [4] H. Ning, G. Janowski, and U. Vaidya, "Processing and Nonisothermal Crystallization 188

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