Effects of fiber bundle size and weave density on stiffness degradation and final failure of fabric laminates

The mechanical behavior of twill-weave carbon fabric laminates under tensile loading is experimentally studied in this work. The focus is on the effect of the fiber bundle size and weave density of the fabric. The twill-weave carbon fabrics are woven from two different fiber bundles, 3 K and 12 K, with three different weave densities. The carbon fabrics are thus of three types having areal densities between 200 and 600 g/m2, and crimp ratios from 0.049 to 0.12. The laminates are manufactured through vacuum infusion process to minimize the effect of voids and poor resin regions. Tensile tests show serious differences in the behavior of the low and high areal density fabrics although the crimp ratio is about the same in some of the laminates. The low density fabric laminates show a linear stress–strain curve up to a sudden rupture of the laminate. The higher density fabric laminates present a stiffness degradation observed in their stress–strain curve which occurs after a plateau region or just a knee behavior. These high density fabric laminates encounter a severe delamination before the final failure. Digital image correlations (DIC) performed in this work show novel changes of the strain distribution before and after the stiffness degradation in the laminates.