Research On The Effect Of Different Needle Running Modes On The Mechanical Properties Of Sutured Composites

Compared with traditional composite laminates,stitched composites have high damage tolerance and excellent interlayer performance,and are widely used in military and civilian fields.Stitched composites are prepared by mostly using the needle without rotation to vertically or obliquely pierce the ply fabrics,which causes the stitching difficulty or even the needle to break when the ply thickness is large.The limitations of the stitching equipment make it difficult to prepare large-thickness stitched preforms.It is need to research stitching equipment that can stitch large-thickness preforms.Including a rotary stitching head device,a lateral movement device,a longitudinal movement device and a stitching material fixing device,a variable-speed needle rotary stitching equipment was designed in this article,which was used to puncture 13 layers and 25 layers of fabric at 0 r/min and 2000 r/min.Double-stranded carbon fiber was used to prepare the stitched preform.The stitch pitch and spacing of stitched preform were 5mm.Two thicknesses and two needle operation modes of the stitched composites were prepared with RTM process.Testing the size of the puncture holes in the surface layer of the stitched fabrics,it was found that the area of the rotary puncture holes is larger than that of the straight puncture.By comparing the cross sectional area of the stitching thread and the puncture hole,and exploring the effect of the operation mode of the needle on the fiber bending in the stitched fabrics,it was found that the fiber bending of the stitching composites under the same number of layers are basically the same.Infrared thermal imager was used to observe the heat on the surface of the stitched fabrics,punctured by stitching needle under straight and rotating mode,in order to study the frictional effect of the operation mode of the stitching needle on the fibers in the stitched fabrics,and it was found that the friction effect between the rotary stitching needle and the fibers is greater than that of the straight puncture stitching needle.The four stitching composites were tested for tensile,compression,bending,and inter-laminar shear properties.The non-contact full-field strain measurement system was used to obtain the strain state of the specimen during the tensile experiment.Digital electron microscope and SEM scanning electron microscope were used to observe the failure morphology of the sample and the corresponding failure mode were analyzed.The research results shows that When the number of layers is 13,compared with the straight stitching,the rotary stitching improved the mechanical properties of the composites,and had the most obvious improving effect on the tensile property,increasing the tensile strength and the modulus by 12.57% and 12.24% respectively.When the number of layers is 25,the mechanical properties of straight-punch stitched composites and rotary stitched composites are not much different.The tensile strain cloud map of four stitched composites have regional characteristics.The strain at resin rich area,fiber bending position,fabric layer position without stitching thread and inplane stitching thread position decreased successively.