Study Of Water Absorption Mechanism And Mechanical Properties Of Fully Degradable Composites Based On Microscopic Model

Traditional fiber-reinforced composites are widely used in automotive,aerospace and wind power applications due to their light weight,high strength and stiffness,high temperature resistance and ease of design.However,the main methods of recycling and disposing of these composite waste parts are landfill and incineration,which can cause serious environmental pollution.With the advantages of low density,low cost,high specific strength and high biodegradability,fully degradable composites,represented by plant fiber/polylactic acid(PLA)composites,are beginning to replace synthetic fiber composites in part in the automotive and aerospace industries.The mechanical properties of injection moulded short plant fiber/PLA composites vary in different directions due to the anisotropy of the mechanical properties of the composites caused by the different orientation of the fibers.Composites are subject to the effects of temperature and humidity when used in automotive applications.Composites can absorb water and damage the internal structure,which has a significant impact on the long-term mechanical properties of the composite.In this paper,in order to investigate the mechanical properties of injection moulded short fiber composites in different orientations and the mechanical properties of composite water absorption,the internal microstructure,water absorption mechanism and mechanical properties of injection moulded short jute fiber/PLA composites were investigated.Non-destructive testing of the microstructure of composite materials by X-ray tomography.The fiber length distribution(FLD)and fiber orientation distribution(FOD)were quantified to obtain FLD and FOD probability density functions.The strength and modulus prediction models applied to short jute fiber/PLA composites were developed based on the probability density functions.The water absorption diffusion model and the water absorption kinetic parameters were determined by studying the water absorption behaviour of the composite,and the mechanical properties of composites in aqueous environments were analysed by combining the water absorption behaviour of the composites.Long-term mechanical modelling of short plant fiber/PLA composites in an aqueous environment was developed by studying the theoretical model and the water absorption mechanism of the composites.Specific studies include:1)The microstructure of the composite before and after water absorption was scanned and tested non-destructively by X-ray tomography.The three-dimensional model of the composite was reconstructed by AVIZO software,curves of FLD and FOD probability density functions of short jute fibers and porosity as a function of water absorption time were obtained by quantitative analysis of the porosity of unabsorbed short jute fibers and of the composite after water absorption.2)The mixing rate model and the laminate theory model were modified based on the FLD and FOD probability density functions for short jute fibers.The mixing rate model and the laminate theory model were modified based on the FLD and FOD probability density functions of short jute fibers to predict the tensile strength and modulus of elasticity of short jute fiber/PLA composites at arbitrary load loading angles.Experimental values in 5 directions were compared with theoretical values.3)The short jute fiber/PLA composites were used to carry out water absorption experiments at 23°C and 50°C to obtain water absorption weight gain rate curves.Moisture diffusion model for short jute fiber/PLA composites determined by combining the law of water absorption and diffusion and the saturated water uptake weight gain rate M_mand the moisture diffusion coefficient D of the composites were obtained.The relationship between porosity and time was obtained by quantitatively analyzing the porosity of the composite at each stage of water absorption.4)Tensile experiments were carried out on short jute fiber/PLA composites at various stages of water absorption,and the mechanism of water absorption of the composites was elaborated in relation to the water absorption behaviour,microstructure and changes in mechanical properties at different stages of water absorption.Long-term mechanical modelling of short plant fiber/PLA composites in aqueous environments was developed based on theoretical models of the mechanics of the composites and the water absorption mechanisms of the composites for different time periods of water absorption.To provide a theoretical basis for the study of the water absorption behaviour and long-term mechanical properties of plant fiber/PLA composites,based on the above study of the water absorption behaviour and mechanical properties of short jute fiber/PLA composites.