## 4 Discussion  
Tensile test and fracture analysis on fiberglass/polyester composites stitched with 0.3 mm nylon thread have been studied. One sample unstitched and four different samples were used, with longitudinal distances between the stitches of 0.5, 1.0, 1.5, and 2.0 cm, for which the tensile test results showed that the stitched samples have, over the unstitch material, slight increases of 3.17%, 1.14%, 0.71%, and 0.56% on UTS. This increase is attributed to the stitching threads that help to carry more energy, but the decrease in tensile strength as the longitudinal distance between stitches is longer is attributed to the fact that it has less thread to absorb energy, and that loosens regions between the stitch's points, which affects resin permeability. The strain of the stitched samples has reductions of 41.66%, 32.47%, 23.85%, and 16.99% over the unstitched samples because of the stitching points, which also contributes to the increases in Young's Modulus of 99.20%, 69.91%, 50.85%, and 31.41% over the unstitched samples, but decreases in toughness of 36.89%, 27.28%, 14.64%, and 8.87%. Fracture analysis shows that although the stitching generates a stress concentration in the matrix and promotes its detachment as the tension increases, it also keeps the glass fibers in place. In Table 4, fracture stages are summarized as follows: In stage 1, tensile starts and no visual indication is shown. In stage 2, both samples start to show minor matrix cracks. Stitched samples cracks are retained and concentrated within the stitched points. Unstitched samples have more strain than stitched samples, and cracks disperse all over the surface. In stage 3, cracks increase, and small zones start to fracture in the matrix. Internal pull-out is shown on both samples, but in stitched samples it is presented near the stitching zone, while in the unstitched sample it is presented all over the surface. In stage 4, external pull-out fibers are shown because of major cracks and fractures within the matrix and the stitching, and again in the stitched samples, the cracks, fracture, and fibers are retained by the now remaining stitching points, while in the unstitches samples the damage is spread all over the specimens and fibers are pulling out from the composite. With these results, it has been shown that the shorter the stitching length distance is, the more displacement is obtained on the tensile test, which is directly proportional to the material toughness, but tensile resistance is compromised. A harder resin is recommended for stitching composites. Stitched composite materials are complex materials to analyze since there are so many variables that can drastically change the properties of the materials. While the stitched materials results show an increase in tensile properties over the unstitched ones, new stress concentrators \begin{table}
\begin{tabular}{c c c} \hline \hline
**Stage** & **Unstitched Sample** & **Stitched Sample** \\ \hline
1 & Tensile test starts. & Tensile test starts. \\  & Separation between the stitching, fibers, and the matrix \\
2 & starts to occur as tensile stress increases. Minor matrix & Separation between the stitching, fibers, and the matrix \\  & cracks start to appear. & starts to occur as tensile stress increases. Minor matrix \\
3 & Cracks increase significantly and small fractures start to & Matrix cracks increase significantly, and small fractures start \\  & appear in the matrix; internal pull-out fibers start. & to appear near the stitching points; internal pull-out fibers \\  & & start. \\
4 & Major matrix cracks and fractures are growing; external & Major matrix cracks and fractures are showed near the \\  & pull-out fibers have appeared. & stitching points; external pull-out fibers have appeared, \\
5 & Fracture of the composite. & retained by stitching points. Some stitches are being \\  & & fractured as well. \\
5 & Fracture of the composite. & Fracture of the composite. \\ \hline \hline \end{tabular}
\end{table}