The Design Of Single-sided Curved Needle Stitching Head And The Study Of Mechanical Properties Of Laminated Plates For Composites

In this paper,the inter-layer reinforcement technology of composites was taken as the research background,and the research on single-sided curved needle stitching technology of composites was carried out.Through the in-depth analysis of the principle of stitch forming about single-sided stitching,the principle prototype of a single-sided curved needle stitching head was independently designed and combined with the vacuum assisted resin infiltration process to prepare the stitched composite laminate samples successfully.After that,a series of tests and analysis of mechanical properties on samples were carried out,such as tensile strength,flexural strength,Mode I and Mode II inter-laminar fracture toughness.At the same time,the influence mechanism of the stitching parameters on its in-plane mechanical properties and inter-laminar fracture toughness were mainly discussed.All of this can provide a theoretical basis for the structural design and parameter optimization of the stitched composite components.Through the analysis of the principle of stitch forming about single-sided stitching of composite,the single-sided blind stitch was determined as the stitch forming pattern of stitching head designed in this paper.It was cleared that the target design stitching head should include lead mechanism,hook mechanism and feed mechanism.The preliminary planning and analysis of the decomposition motion of each stitching mechanism in the process of continuous stitch were carried out.Through the analysis of the functional requirements of the target design stitching head and the overall planning scheme of the platform,a kind of single-sided curved needle stitching head for composite was independently designed and developed.The displacement-time interaction relationship between the end-executing parts of the stitching head was obtained,and the optimal loop shape,the influence mechanism of thread tension and thread supply-demand relationship on the shape of the loop were established.What’s more,the supply and demand balance conditions of thread in a single stitching cycle was also determined.The stitching/vacuum assisted resin infiltration was used to prepare the sample,and the in-plane mechanical properties of composites were studied,the results indicated that the tensile strength decreases with the increase of stitching density,and the maximum decreasing ratio was 21.14%under the same thread fineness.The flexural strength shows a trend of decreasing first and then increasing,when the stitching density was 10×18,the maximum decreasing ratio was 10.3%,when the stitching density was 10×10,the maximum increasing ratio was 25.33%.Under the same stitching density,the tensile strength and the flexural strength both show a trend of decrease with the increase of thread fineness.Research on the influence mechanism of stitching density and the thread failure modes for the Mode I inter-laminar fracture toughness of the composites were carried out.The results indicated that the stitching technology can hinder the expansion of the inter-layer cracks significantly.Under the same layered crack length,with the increase of stitching density,the Mode I inter-laminar fracture toughness was improved to different extents.When the stitching density was 10×10 the maximum increase ratio is achieved.During the test,when the failure mode of the thread was fracture,the Mode I inter-laminar fracture toughness was improved more than the extraction of the thread.Research on the influence mechanism of stitching density and the thread fineness for the Mode II inter-laminar fracture toughness of the composites were carried out.The results indicated that under the same stitch fineness with the increase of the stitching density,the Mode II inter-laminar fracture toughness G_IIC of composite shows a trend of decreasing first and then increasing.When the stitching density was 10×18,the G_IIC value was lower than the unstitching sample,with a reduction of 11.53%and 14.96%respectively.When the stitching density was 10×10,the G_IIC value reach the highest,increasing 28.76%,42.93%and 100.4%respectively.Under the same stitching density,the Mode II inter-laminar fracture toughness increases with the increase of the fineness of the thread.