Processing and characterization of 3D woven and braided thermoplastic composites

This work is aimed at elucidating the processing–microstructure–property relationships of three-dimensional thermoplastic composites. The material processing involves fabric formation and hot-press molding. A powder-impregnated Nylon/carbon yarn was used to form three-dimensional fabrics. Two types of fabrics have been made in the present work, including 3-axis orthogonal woven and two-step braided. The woven fabric is characterized by more evenly distributed fibers along three orthogonal directions, whereas the braided fabric contains most fibers along the axial direction. Compression molding was employed for composite consolidation. Matched molds were designed for making the composites with predetermined thicknesses. The molding thickness and molding temperature were varied to examine their respective effects on the resulting properties. Yarn geometries in the molded composites were studied through microscope observations. The molding significantly distorted the through-thickness yarns of the woven fabric, and the mechanisms of yarn distortion were identified and related to the fabric structure. Material characterization was conducted by means of flexure tests. The loading curves show significant non-linearity with the development of damage. The molding thickness is a critical parameter governing the flexural modulus, flexural strength, and damage modes. A special mounting method was used to permit the examination of the fractured interiors of specimens. How surface loops affect damage modes and how damage grows within these non-uniform materials are discussed in detail.