Effect of in-plane fiber tow waviness upon the tensile strength characteristics of fiber reinforced composites of carbon/epoxy AS4/3501-6

Abstract

The purpose of this study was to investigate the strength and effectiveness when induced with 'in plane fiber tow waviness' in a composite ply of carbon/epoxy AS4/3501-6. Fiber waviness is usually induced by infusion processes and inherent in fabric architectures. Composite structural details like ply drops and ply joints can cause serious fiber misalignment. These are usually dependent on parameters such as ply thickness, percentage of plies dropped, and mold geometry and pressure, and pressure of the resin which slides the dry fibers during the resin transfer molding process. Fiber disorientation due to fiber tow waviness in 'in-plane' direction has been the subject of recent studies on wind turbine blade materials and other aerospace laminates with reports of compression strengths and failure strains that are borderline, depending upon the reinforcement architecture, matrix resin and environment. Waviness is expected to reduce compressive strength due to two primary factors. The fibers may be oriented in such a way that the geometry that results because of the orientation may exacerbate the basic fiber, strand, or layer buckling mode of failure. The waviness could also shift the fiber orientation of the axis of the ply longitudinal direction which eventually results in matrix dominated failures for plies normally orientated in the primary load direction (0). Both global and local stress & strain values generated by the finite element model were validated by the traditional mechanical methods using ply/local stiffness matrix and global/reduced stiffness matrix. A precise geometry of waviness on different materials was modeled with different wave severity factor and a parametric study was conducted. Three different defects were modeled where the angle of misalignment ranged from 5 to 15 degrees with wavelength ranging from 1 inch to 1.5 inches and amplitude ranging from 0.03 inches to 0.7 inches. This revealed the effect of 'in-plane fiber tow waviness' on the stress distribution and loss of strength in carbon/epoxy AS4/3501-6. The results clearly show that the effect of 'in-plane fiber tow waviness' leads to resin rich areas which causes high stress concentrations and decrease in the strength ratio, ultimately leading to delamination's.