| Bamboo is one of the main forest resources in China.It grows fast and has good properties.Even when bent to touch the ground under the pressure of snow,it will not break,known as "plant steel" reputation.Natural bamboo fiber with high specific strength and specific modulus is known as "natural glass fiber".With epoxy resin as matrix of bamboo fiber/epoxy composite materials,with light weight,high strength,good fatigue resistance and buffer performance,and with excellent features such as low cost,low energy consumption and environmental protection,can replace the glass fiber composites,play a bamboo fiber renewable and recyclable advantage,applied to automotive components,shell,decorative and packaging materials,etc.In this paper,the bamboo fibers was rolled from Cizhu bamboo(Neosinocalamus affinis)by using rolled-separated technology for processing into different specifications.According to the threads twisted of fiber yarn,the bamboo fiber twisting machine was developed,and the twisting bamboo fiber,or TBF with certain tensile strength was prepared by twisting technique.The structure of TBF was analyzed,and its resin impregnation and hygroscopic properties were tested.A new type of TBF hoop filament-wound composites,or TBF-NOL was prepared by fiber winding technology.The properties of the composites with different fiber types and different forming methods were tested by means of ESEM,X-CT,DMA,mechanical testing machine.The properties of the TBF-NOL were analyzed and the forming mechanism of TBF-NOL was revealed.According to the moisture absorption characteristics of bamboo fiber,the moisture adsorption,hygroscopic and aging properties of bamboo fiber,TBF and TBF-NOL were studied by means of DVS and moisture absorption environment treatment.The hygroscopic mechanism of TBF-NOL were analyzed and its service life was predicted.The detailed research content and relevant conclusions are as follows:1)The processing technology of TBF was determined and its tensile strength reach 70.56 N.Based on the size and structure of bamboo fiber which is rolled and separated,and combined with the threads twisted of fiber yarn,the bamboo fiber twister was developed independently.The twist technology of bamboo fiber and the working principle of the equipment were described.The structural characteristics and mechanical properties of TBF were analyzed.The diameter and tensile strength of TBF were 1.48 mm and 70.56 N,respectively,by using rolled-separated technology and twisting technique,which reached the application requirements of the fiber winding technology.By combining theoretical analysis and experimental comparison,TBF with smaller diameter and better strength can be prepared by optimizing twist process parameters and improving fiber bonding mode.2)The impregnation state of TBF resin was obtained,and an improved impregnation method of fiber resin was proposed.With the process of wet winding molding,the epoxy resin adhered to the surface of TBF,but failed to penetrate into the pore structure of the fiber.The twist structure of TBF increased the difficulty of resin impregnation.By means of vacuum impregnation,the porosity between fiber bundles was avoided and the impregnation voids decreased from 15.8% to 6.5%.The impregnation properties of TBF resin and the distribution of impregnation voids were understood,which provided the direction for the improvement of the impregnation process of composite resin with TBF hoop filament-wound composites.3)The evaluation system of plant fiber NOL was established,and the mechanical properties of TBF-NOL were obtained.NOL ring is the most commonly used means to evaluate the properties of filament-wound composites.The results show that it is reasonable to compare the properties of filament-wound composites of plant fiber yarns with different sizes and specifications by using NOL rings with 5 layers winding structure.The tensile strength,shear strength,bending strength and cyclic tensile strength of TBF-NOL were 45.62 MPa,18.31 MPa,105.41 MPa and 32.31 MPa,respectively,and showing good interfacial bonding performance.However,due to the porous structure inside TBF,the mechanical properties of the composites are lower than those of glass fiber composites.4)The structural characteristics and forming mechanism of TBF-NOL were revealed.The TBF-NOL are composed of bamboo fiber reinforced phase,epoxy resin matrix phase and multistage characteristic void damage phase.The TBF in the NOL composites were arranged in parallel and surrounded by a resin matrix.The void damage phase can be divided into fiber void,resin curing void and impregnation void.The forming mechanism of TBF-NOL is essentially TBF resin impregnation and positioning arrangement,that is,TBF impregnate with resin,and coat on the surface of TBF,fail to enter the vascular tissue and parenchyma within TBF,the TBF are arranged into a set of positions.After the resin is heated and cured,a large number of bubble pores are generated on both sides of TBF,and the position of the fiber is fixed.5)The change of water adsorption properties of TBF-NOL was described,and the moisture content prediction model of TBF-NOL in the process of absorption-desorption was established.DVS moisture adsorption method was used to study and the results showed that the water adsorption process of bamboo fiber and TBF conforms to the characteristics of type II isothermal adsorption curve,and the moisture content change in the absorption-desorption process could be predicted by GAB model.The water adsorption process of TBF reinforced epoxy resin composites conforms to the characteristics of type visothermal adsorption curve,and the change of water content in the desorption process could be predicted by Dose Resp model.PEK model was used to better explain the water adsorption mode of monomolecular and multi-molecular layers of TBF,while the water adsorption mode of TBF was changed to monomolecular layer mode when it was combined with epoxy resin.Hygroscopic aging has a significant effect on the moisture adsorption properties of composites.6)The hygroscopic mechanism of TBF cyclic composites was defined,the bending strength attenuation model of TBF-NOL was established.The moisture absorption process of TBF-NOL conforms to Fick's second diffusion law.After 70? water immersion treatment,the moisture content and mechanical properties of TBF-NOL changed greatly,its maximum moisture content was 21.17%,its shear strength and bending strength decreased by 36.77% and 46.84%,respectively.For TBF-NOL with multi-porosity structure,in the hygroscopic process,the first is the hygroscopicity of the resin matrix,the second is the moisture transfer through cracks and micropores,and the last is the hygroscopicity of TBF.During the aging of composite properties,the dry shrinkage and wet expansion of TBF led to the appearance of dry shrinkage cracks,which were separated from the resin matrix and layered failure occurred in the test of bending properties.According to the formula of residual strength model,establishes the prediction TBF ring to thermosetting resin matrix composite materials under low temperature and humidity environment,the service life of the model under the condition of 20 ? water immersion,TBF composite material has good service ability for a long time,when its performance can keep for a long time in between 40% and 50%,need to reduce the pore content in the composites,improve its safe use. |
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