Study of Voids Effect on Tensile Strength of Carbon Fiber Reinforced Composites for Structural Applications

Abstract

Advanced carbon fiber reinforced polymer composites (CFRP) are being particularly used for aerospace structural applications because of their high strength, stiffness and light weight. To design a structural component thorough characterization of composite material is required. In the present study the high strength carbon T700 12K fiber with bi-functional epoxy resin LY556 and hardener HY5200 was selected for fabrication of NOL ring specimens on a cylindrical mandrel by filament winding technique. Voids are inevitable during the fabrication in the fiber reinforced composites. The matrix-dominated properties such as: inter laminar shear strength, bending properties, tensile strength and modulus, fatigue and fracture toughness are influenced by voids, while the fiber-dominated mechanical properties are not significantly influenced by voids. Different thicknesses of NOL rings of 1.3, 1.4, 1.6 and 1.7mm are made and defects like voids were identified through non-destructive testing methods. Ultrasonic test is used to identify the voids in the NOL ring specimens. Ultrasonic wave transmission was observed at a particular zone which gave the resemblance of induced voids in the specimens. Composite properties are directly proportional to the fiber volume fraction. Physical properties of different thickness of laminates were evaluated. It is observed from the test results composite with less void content gave better compaction and strength compared with more void content. The results also showed that resin mix viscosity influences the void formation and strength of the filament wound composite. I. INTRODUCTION The mechanical characterization of fiber reinforced composites are determined by the interfacial adhesion properties between fiber and matrix. The transfer of the stress is through the combination of physical adhesion, chemical bonding and mechanical inter locking. However effective fiber matrix interface is required to transfer the stress from the matrix to the load bearing fibers. If any void/porosity are present in fiber matrix interface the bonding strength will decrease which results in strength decrease 1-6. To avoid the voids during fabrication of specimens thorough mixing of resin and hardener is required and no bubble should be formed during impregnating the fiber in the matrix. Composite materials are heterogeneous materials they are prone to have certain defects inside the composite laminates. Many factors are associated with strength characteristics of composites such as resin content, fiber volume fraction, void content, loading direction and loading rate etc. Other factors contribute to the formation of void including the curing pressure, resin system, environmental conditions and so on, some of which are almost unavoidable. The mechanical properties of composite directly related to the reinforcing fiber content. It is therefore important that the fiber ratio should be monitor during the manufacturing of structural component. The role of fiber in a composite plays a vital role. The main factors that govern the fiber contribution in composite are the mechanical properties of fiber, the surface interaction of fiber and resin (Interface), the amount of fiber in the composite (Fiber volume fraction) and orientation of fibers in the composites. The strength and stiffness of a composite laminates will increase in proportion to the amount of fiber content. Although literature indicates that mechanical properties decrease as the void content increases 7-8 , the magnitude of the effect of void content on the cylindrical shape filament wound CFRP is not evident. Hence in this study particularly carbon fiber reinforced filament wound composite specimens fabricated with varying process parameters. Another important factor which effects the strength of the composite is viscosity of resin mix 9. To derive the full potential of the fiber in composite matrix viscosity play an important role. In this work, effect of physical properties on filament wound composite specimens were studied.