Investigation of mechanical properties of I beam composites developed with 3D woven fabric

Abstract

The need to provide high rigidity and strength for aircraft structures has created a trend, especially toward fiber-reinforced composites. Load transfer in heterogeneous structures such as composites has been one of the most critical issues needs to be improved due to the poor interphase between fiber and matrix. In addition, the weak interface between the layers also causes delamination behavior, which is a dominant damage mechanism. Laminated composites are mostly damaged by delamination and these effects reduce the hardness, strength, and service life of the structures. By preventing the delamination mechanism, the service life and load-carrying capacity of the composites can be improved. However, delamination prevention or retarding methods cause undesirable effects in the aerospace and automotive fields such as thicker and heavier structures. Delamination resistance has been studied to be increased by methods such as tufting, stitching, and z-pinning, but the interest in 3-Dimensional (3D) weaving has increased due to the formation of resin-rich regions and the damage of the yarns and their orientation in different directions. With an innovative approach, 3D weaving, the out-of-plane yarns placed in the fabric increase the toughness of the structure without the need for a secondary process, and also eliminates delamination, which is an undesirable type of failure. Therefore, 3D weaving has gained importance in the aerospace industry, where structures with both light weight and high mechanical properties are preferred. Within the scope of the thesis, it is aimed to produce preforms in I-beam forms with 3D weaving method to be used as load-bearing structural elements in fields such as aerospace and automotive and to produce high-quality composites from preforms. The main aim is to examine and discuss the effect of both 300, 600, and 1200 TEX E-glass fibers and warp density on the mechanical properties of composite beams. Initially, the optimization of a semi-automated weaving machine capable of 3D weaving was studied. To produce preforms with standard weft and warp density and vertical wall height throughout weaving, the features of controlled warp release with servomotors, automatic tamping system, and determining the weft density over the interface have been added to the semi-automated weaving machine. Then, experiments were run to identify the appropriate warp and weft density for 3D preform production with 300, 600, and 1200 TEX E-glass fibers. As a result, the optimized warp density to achieve a stable height of vertical wall of I-beam is 20 cm-1 and weft density is 4 cm-1 for 300 TEX preforms and warp density is 12 cm-1 and weft density is 4 cm-1 for 1200 TEX preforms. Additionally, 600 TEX preforms were woven with a warp density of both 12 cm-1 and 20 cm-1 weft density of 4 cm-1 for a comparison in both data sets of I-beam fabrics. Moreover, the woven I-beam preforms were then manufactured as 3D composites by vacuum infusion prosess. Custom-designed molds were employed to fabricate the I-beam profiled composites with minimum voids and complete wetting having an efficient composite manufacturing process. The thicknesses of the load-bearing vertical walls of the produced composites were 300 TEX (warp/weft density: 20/4 cm-1), 600 TEX (warp/weft density: 12/4 cm-1), 600 TEX (warp/weft density: 20/4 cm-1), and 1200 TEX (warp/weft density: 12/4 cm-1) measured as 2.21 mm, 2.45 mm, 2.62 mm and 2.78 mm, respectively. Thus, the increase in the linear density (TEX) resulted an enhancement on the thickness of the E-glass fibers. Finally, the effect of TEX on the mechanical properties and failure types of composites was investigated by 3-point bending and compression tests. As a result, the development of a semi-automated weaving machine capable of 3D weaving and the production of 3D weaving preforms in I-beam forms using E-glass fibers with different linear densities have been achieved. More design patterns throughout employing the semi-automated weaving instrument is also possible with the flexible design of the system. By a custom-designed mold system, high-quality 3D composites with low void ratio were manufactured by vacuum infusion method. The correlation of linear density of E-glass fibers through both 3-point bending and compression tests were presented the expected trends including the cover factor discussion when 12 and 20 cm-1 warp density was considered for 600 TEX.