Investigation into the Tensile Properties of Stitched and Unstitched Woven Aramid/Vinyl Ester Composites

Abstract

In this study, the influence of stitch bonding on the tensile strength and tensile modulus in plain woven Twaron T-750/vinyl ester composites in the direction of thickness was examined. The effect of stitch density was investigated in longitudinal (warp) and transverse (weft) directions as a parameter. The space (opening) geometry and the deformation caused by the stitching process around stitch points were investigated. The stitching tension, stitch thread type and diameter were kept constant throughout the study. An increase in the tensile strength was observed for low stitching densities while there was no change in the tensile strength for medium stitching densities. A decrease in the tensile strength was observed for high stitching densities. It is especially noted that the tensile strength, which is on the transversal direction perpendicular to stitch direction, decreased with stitching density and tensile deformations initiated much earlier in this direction. It was understood that fiber deviations and resin rich areas were much more effective on these decreases compared with fiber deformation.

Improvement of Mode I Interlaminar Fracture Toughness of Stitched Glass/Epoxy Composites

“…Hence, it may be concluded that the 5 × 5 mm stitch pattern produces the highest mechanical delamination resistance performance of all the tested stitch formations even though creating more fibre damage by increasing the number of stitch penetration points, producing more fibre distortions around the inserted stitching yarns while piercing the stitching needle into the preform panels, thus causing more resin rich regions in the specimens. These factors tend to compromise the tensile properties of the specimens, however, the inserted stitching thread acts to enhance the tensile properties, so research in this area is rendered complex [32,33].…”

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“…The study also found significant differences between the longitudinal and diagonal strength values for unstitched and stitched samples. In another study, Karahan et al [3] also found that the tensile strength in the longitudinal direction increased significantly for lower stitch densities, did not change significantly for medium stitch densities, and decreased significantly for higher stitch densities. In the diagonal direction, there were increment of strength for lower stitch densities and decrement of strength for middle and high stitch densities.…”

Influence of Stitching Parameters on Tensile Strength of Aramid/Vinyl Ester Composites

“…In this subject, Dransfield et al [5] stated that there would be a critical stitch density value in which delamination resistance among layers could be at a maximum level. In the meantime, it was stated by different researchers [12,13] that a very high stitch density would cause misalignment of yarns in the fabric and, more importantly, caused fiber deformation since needle penetration and affected mechanical properties of the structure negatively. Therefore, excessive stitch density negatively affects mechanical properties of composite materials [14 -17].…”

“…In a previous study [12], influence of stitch density on tensile properties of woven aramid/vinyl ester resin composite plates was investigated. The samples used in the previous study [12] were sewn only in uni-direction as parallel. The main distinction of present study the samples used in the study sewn in bi-direction as orthogonal and diagonal.…”

“…The stitch yarn used in the study, yarn thickness, and stitch type parameters were fixed as constant. The samples were produced with the same fabric, resin and production method with the previous study [12].…”