Through-the-Thickness Tension Strength of 3-D Braided Composites

Abstract

Low cost of manufacturing textile composites make them a candidate for manufacturing affordable aerospace vehicles. However the material test methodologies and database are needed before they can be confidently applied. The study thus far has been on characterization of mechanical properties of two-dimensional (2-D) textile composites. This paper presents through-the-thickness strength test methodology, analysis, and data for 3-D, triaxially braided composites. The braid preform [012 k/±β12 k] 45% axial was manufactured using a four-step circular braiding machine and BASF G30-500, 12k graphite fiber tows. The preform was consolidated with Tactix 123 matrix. L-beam specimens were tested in tension and through-the-thickness strength was calculated from the measured failure load and the curved beam equation. Strength of braided composites was compared with an equal thickness and ply stacking AS4/3501-6 graphite/epoxy laminated specimen. The average strength of braided, equivalent laminate, and unidirectional composites was 24.2, 28.8, and 40.9 MN, respectively. Through-the-thickness strength of both braided and laminated composites was found to be a matrix dependent property. Failure modes in braided composites were matrix cracks, tow cracks, and tow separation; whereas failures in laminates were delaminations and ply-cracks. Reduction of size of resin pockets in braided composites may improve the through-the-thickness strength.