Axial Yarn Crimping Effects in Braided Composite Materials

Abstract

Braided composite materials develop undulations or crimping in the axial yams when infiltrated with resin, compacted, and cured. This crimping is suspected of producing significant reductions in strength, particularly under compressive loading. In this study, the effects of axial yam crimping were investigated in two-dimensional triaxially braided composite materials under static compressive loading. A methodology was developed to fabricate carbon/epoxy braided composites with minimal axial yam crimping by tensioning the axial yams during the cure process. The compressive strength of braided composites with tensioned axial yams was compared to that of braided composites manufactured in a conventional manner, which produced significant axial yam crimping. Axial yam crimping produced a 30% reduction in axial compressive strength. The axial stiffness, transverse stiffness, and transverse strength were not significantly affected by axial yam crimping. TEXCAD, a mechanics of materials based analysis program, correctly predicted stiffness values but underpredicted the reduction in axial compressive strength due to axial yam crimping. These results suggest that high compaction pressures, which result in greater yam crimping in addition to high fiber volume fractions, may not produce optimal compressive strength performance. These results illustrate the importance of minimizing axial yam crimping for compressive strength critical applications of braided composite materials.