Mechanical Property Degradation Mechanism Of Multiple Impacts And Heat Treatment Of Woven GF/PP Composites

With the rapid development of fiber-reinforced composites in aerospace,transportation and other fields.The limitations of single-fiber reinforced composites in terms of impact resistance are increasingly obvious.The commingle filaments containing thermoplastic filaments can improve the toughness of composite materials,and can also repair matrix cracks after heat treatment.Therefore,hybrid fiber-reinforced composites have a broad promising future.At present,there is a preference for the study of single fiber-reinforced composites,and there is less research on the impact mechanics and heat treatment mechanical property decline mechanism of hybrid filaments reinforced composites.In this paper,plain and three-dimensional orthotropic fabric-reinforced composites were prepared using glass fiber/polypropylene(GF/PP)blended yarns.Single and multiple low-velocity impact and high strain rate impact tests were conducted on composite samples.To investigate the continuous impact mechanical response law and heat treatment mechanical recession mechanism of GF/PP blended filaments composites,this paper proceeds from the following aspects:(1)The design and weaving of two kinds of structural fabric preforms and the preparation of composite specimens are introduced.The weaving principles and process parameters of three-dimensional looms and radial weaving machines are described.The process of preparing composite sheets by vacuum-assisted transfer molding process is introduced,and the fiber volume content of the specimens with the help of muffle combustion method is measured,and find that the fiber volume content of the plain composite is higher than that of the 3D orthogonal composite.The DSC test showed that the PP fibers were in the molten state at 165 °C.The PP melt fills the damage cracks and adheres to the glass fibers separated from the matrix,improving the interfacial contact between the glass fibers and the resin.XRD test proves that after one heat treatment,the crystallinity of PP decreases by 5.56%.The length of connecting chains within the crystal zone varies greatly,when subjected to external force,the shorter connecting chains will break first due to the external force,resulting in the reduction of PP fiber strength and thus the mechanical properties of the composite.Finally,the experimental scheme and data processing methods applied in the subject study are introduced.(2)The influence of local heat treatment on the low velocity impact response of composite plates is analyzed.Morphological characterization of the impacted specimens using an industrial camera and a super depth of field microscope camera.The impact damage modes of composite plates in VH-group,P-group and original control group are analyzed through experiments.The damage mechanism of three-dimensional orthogonal fabric and two-dimensional plain fabric is further explained,and the influence of binding yarn on the impact property of composite is explained.It is found that the pre-impact heat treatment can inhibit the damage propagation and reduce the deformation of the material;The post-impact heat treatment can ensure the integrity of the material structure,but it will seriously degrade the mechanical properties.Three-dimensional orthogonal woven composites are more sensitive to the influence of heat treatment due to the existence of binding yarn.After multiple impacts of 25 J energy,the VH-group 3DOW material is completely perforated and fails.(3)The effect of whole heat treatment on the dynamic response of plain composites under high strain rate impact loading.Based on the experimental results,it was found that the overall pre-impact heat treatment could enhance the stability of the composite against impact performance.Both pre-and post-impact heat treatments bonded the glass fibers in the heated zone into bundles(molten polypropylene action),reducing glass fiber splitting and loosening damage and effectively maintaining the structural integrity of the material after multiple impacts.Using micro-CT 3D scanning technology to characterize the heat-treated specimens,it was found that the heat micro-pores generated by heat treatment absorbed and dispersed the impact energy during the pulse compression process,inhibiting the formation of local macroscopic shear cracks under high strain rate impact loads.