Mechanical characteristics and damage mechanisms of stitched carbon/epoxy composites under static and fatigue loads

Objective of this thesis is to understand the effect of stitch parameters (stitch density, stitch thread thickness, stitch pattern, stitch orientation) on the in-plane mechanical properties and damage mechanisms of Vectran-stitched and Kevlar-stitched carbon/epoxy composites under static and fatigue loads. Experimental test series comprising static tension, static compression, open hole tension, open hole compression, tension-tension and compression-compression fatigue tests are performed. Characterization of architectural changes induced by stitching process, e.g. fiber breakage, fiber waviness, resin-rich region, stitch debonding, and their correlation with mechanical performance and damage mechanisms is discussed in details. Two analytical works are also described in this thesis: first, a multi-scale modeling scheme to predict 3-D thermo-elastic constants of stitched composites by employing asymptotic expansion homogenization method; second, application of Average Stress Criterion to predict open hole tension strength of stitched composites. Investigation of Vectran-stitched composites shows that moderately stitched composites (stitched 6x6; moderate stitch density) under static tension experience a slight strength reduction, while densely stitched composites (stitched 3x3; high stitch density) exhibit a modest improvement of strength in comparison with unstitched composites. Stitching with either stitch density is found to promote a vast number of cracks, but densely stitched composites are sufficiently effective in impeding the growth of delamination, which translates into higher tensile strength. Accordingly, under fatigue loads, densely stitched composites exhibit better fatigue life due to similar mechanism: delamination impediment provided by the stitches. Tensile modulus of stitched composites is slightly reduced mainly due to fiber waviness. Compression tests on Vectran-stitched composites show that regardless stitch density or thread thickness stitched composites generally exhibit lower compressive strength than unstitched composites. Stitching induces a formation of resin-rich region, which triggers early cracking, fiber splitting and fiber kinking at relatively lower compression load. Analytical work dealing with the predictions of 3-D thermo-elastic constants of Vectran-stitched composites shows that homogenization results are in a good agreement with the experiments. Investigation of Kevlar-stitched composites shows that stitched composites with hole are sensitive to stitch orientation, in which transverse stitching (perpendicular to loading direction) significantly reduces tensile strength due to stitch debonding and ensuing interaction amongst debondings. On the other hand, stitched composites with holes are independent of stitch orientation because the failure is greatly controlled by the stress concentration at the hole rims that surpasses the criticality of stitch debonding. Thus, normal stress distribution in Kevlar-stitched composites with holes can be estimated by Lekhnitskii theory, and open hole tensile strength can be estimated by Average Stress Criterion. Investigation of Kevlar-stitched composites under fatigue loads reveals that stitch pattern of round stitching (stitches encircling the holes) reduces .fatigue life due to damage acceleration around the hole rim, whilst parallel stitching does not pose any significant effect on fatigue life. Finally, recommendation on how to improve the mechanical performance of stitched composites is given. This thesis endorses that stitching is an effective through-thickness reinforcement method for composites in the next generation aircraft.