Research On Tensile Performance Of The Stitched UACS Laminates Composites

Unidirectionally arrayed chopped strands(UACS)laminates are fabricated by introducing different types of regular slits into the traditional carbon fiber reinforced polymer(CFRP).Compared with traditional CFRPs,newly designed UACS laminates are superior in structural design and safety,such as more uniform distribution of fibers during the process of curing and excellent applicability in fabrication of structural components with complex geometry in some cases.Recent researches have shown that the newly designed Bi-angled UACS laminates perform better tensile properties and fiber fluidity than both the short carbon fiber reinforced composites and other kinds of UACS laminates,which are more suitable for forming complex geometry elements.However,delamination is still the main failure mode in the newly designed UACS laminates,and the carbon fibers with high tensile strength do not play a full role in carrying the load.Based on this,in this study,stitching with Kevlar 29 are introduced into the UACS laminates to enhance the mechanical properties along the thickness direction.The effects of different stitching densities on the tensile properties of the UACS laminates are revealed.The main conclusions are as follows:(1)Based on the static tensile tests,the tensile properties of the Bi-angled UACS laminates with different stitching densities are investigated.In other words,the tensile strength,tensile modulus and failure modes are examined.The experimental results show that the stitched UACS laminates perform better tensile properties than the UACS laminates without stitching,and the tensile strength increase with the decreasing of stitching density.Among them,UACS laminate with stitch scale of 15 mm perform highest tensile strength and increase by nearly 14.0% compared with previous UACS laminates.At the same time,the introduction of stitches improve the delamination resistance of the UACS laminates.However,it has no obvious effects on the tensile modulus and the Poisson rate of the UACS laminates.(2)Commercial finite element software is employed to simulate the tensile properties and progressive damage of the UACS laminates with differ ent stitching densities and the results are compared with the experimental results.Finite element analysis(FEA)results reveal that the delamination of the stitched UACS laminates under loading appears around both the cross area of slits in 0°and adjacent ±45° plies and the slits area near the free edges firstly.Moreover,the stitches have significant inhibitory effects on the propagation of delamination.However,if the stitching density is too high,it may cause more original damage in fabrication and decline of the fiber volume fraction of the laminates.At the same time,the higher stitching density is not conducive to the full play of fiber strength in laminated plates.Finally,laminates failed due to the continuous development of delamination and intensive fiber breakage at the 0° plies.Progressive damage analysis results are in good agreement with the test results,which demonstrates that the introduction of stitches improve the tensile properties of UACS laminates effectively.