Performance Evaluation of Unstitched, Stitched and Z-Pinned Textile Composites Under Static Loading

Abstract

Advances in conventional tape laminates and textile composites provide aircraft manufacturers important technology, but the industry lacks the confidence to use these composites to manufacture wing and fuselage structures due to high cost and low damage tolerance. In order to overcome the high cost and to improve the damage tolerance of composites, researchers have developed new through-the-thickness reinforcement techniques, such as stitching through the thickness. This reinforcement technique can be used to join the skin, stiffeners, ribs and spars to form an integral structure. The structures are typically more damage tolerant, contain fewer fasteners and are less expensive to manufacture than conventional composite or metallic structures. Furthermore, stitching reduces the manufacturing time and labor compared to drilling holes for fasteners, and may eliminate the problems of fatigue and/or corrosion from galvanic reactions with metal fasteners. Woven composites with through the thickness reinforcements such as stitching have good properties not only in mutually orthogonal directions but also in the transverse direction and more balanced properties than traditional tape laminates. They are also expected to have better fatigue and impact resistance due to the interlacing. Another benefit is reduced manufacturing cost by reducing part count. Because of these potential benefits, these composites are being considered for various applications including primary/secondary components for aerospace structures. The objective of this effort is to develop experimental tools for comparing the performance of these composites reinforced by stitching to unstitched composites. Identification of damage mechanisms and forces available to grow damage is essential for identifying the primary parameters that determine performance. Accurate determination of the driving forces will require extensive manufacturing and experimentation. However, once the reinforcement techniques are well understood, it is anticipated that simplified experiments can be developed that could be used routinely by designers to evaluate the effects of the reinforcements on damage tolerance. This paper specifically addresses the performance evaluation of stitched low cost manufactured composites subjected to static loading. Static tension and compression testing was conducted to determine the Ultimate Tensile and Compressive Strengths, Young’s Moduli and Poisson’s Ratio. Two different stitch patterns or stitch densities were used for comparison. The first density was five rows of stitching per inch of width, with eight stitches per inch over the entire length. The second density was three rows of stitching per inch of width, with four stitches per inch over the entire length.