A characterization of the properties of a stitched hybrid composite laminate

The goal of this research was to obtain a fundamental understanding of the material properties of a hybrid stitched composite laminate. To that end, over 160 samples have been tested in seven different test configurations. These test configurations were four point flexural bending, unnotched tension and compression, open hole tension and compression, shear, and mixed mode interlaminar fracture.; The material tested was a hybrid AS4/IM7/3501-6 carbon-fiber/epoxy-matrix stitched laminate with through-thickness stitching of various thread types and stitch densities. The laminate was made of carbon fiber plies in a [±45/0 ₂/90/0₂/±45]_n orientation. Two thicknesses were tested, 54 plies (n = 6), and 36 plies (n = 4). Dry carbon fiber plies were stitched together to provide a preform before epoxy was applied using the Resin Film Infusion (RFI) process. The Boeing Co. manufactured the materials in support of the NASA Advanced Composites Technology program.; Unique research contributions include the development of a new in-plane shear sample configuration, evaluation of open hole tension and compression characteristics under finite width conditions and the development of a new mixed mode test method.; The following conclusions were made. The flexural tests established that stitching had a negative impact upon flexural strength. The in-plane shear strength tests used a modification of an existing test configuration to find shear strength.; The open hole and unnotched tension and compression tests demonstrated the material was relatively notch insensitive. More importantly, the open hole tests ascertained that existing strength prediction methods can be applied to composite laminates with hole diameter/sample width ratios as great as 1/2, provided finite width effects and material orthotropy are compensated for.; The unnotched compression tests demonstrated that stitch density and type had both positive and negative effects upon composite laminate compression strength. Stitching weakened the in-plane fibers, but increased the through-thickness strength of the material. These two factors have opposite effects upon compression strength.; Perhaps the most important contribution of this research is the establishment of a new mixed mode test method to experimentally determine the fracture toughness of composite laminates. The Single Leg Four Point Bend test configuration combines the advantages of several existing test arrangements and eliminates many of their disadvantages.