Interfacial, Structure And Mechanical Properties Of Ramie Fiber Reinforced Poly(Lactic Acid)(Pla) Composites

Glass fiber reinforced composites have been widely used in the industrial field with their advantages of moderate strength and low cost, playing an important role in the economy development. However, their large energy consumption and difficult recyclability have brought a heavy burden for the ecological environment. All countries in the world have made the industrial and environmental development more coordinated, making composites more environmental friendly. Natural fiber reinforced biopolymers have been rapidly developed as a new kind of renewable, recyclable and carbon storage composites. But they have lower mechanical properties than glass fiber ones because of the low tensile strength of short natural fiber and its random orientation in the composite. In order to develop their industrial applications with high load bearing capacity, it is very essential to produce natural fiber reinforced biodegradable composites with good mechanical performance. In this study, much effort has been made on how to improve the mechanical properties of ramie fiber reinforced poly (lactic acid)(PLA) composites. There are four parts in our work:(1) Analyses and characterization of the composite interfacial properties;(2) Effect of the cyclic load pretreated fabrics on the composite mechanical properties and its mechanism;(3) Fabrication, mechanical and sound absorption properties of3D cellular matrix composites;(4) Micro-structural modeling and simulation of3D cellular matrix composites.(1) The composite interfacial properties were respectively analyzed and characterized. Most of the published studies have measured the composite mechanical properties to investigate how fiber surface modification affected the composite interfacial properties. In this part, the composite interfacial adhesion was investigated in terms of the fiber/matrix interfacial shear strength (IFSS) using the micro-bond test and its shear lag model. The alkali, silane and alkali-silane combined treatments could greatly improve the IFSS respectively by about35%,20%and40%. And the combined treatment was thought as the most efficient method for enhancing the interfacial adhesion. For fiber surface morphology, the alkali or silane treated fibers had cleaner surface than the untreated ones. For fiber surface wettability, the dynamic contact angle of the alkali treated fibers was increased to46.7°and that of the silane treated ones was decreased to56.7°, compared with that of the untreated ones (38.5°). For fiber crystalline structure, the crystallinity of the silane treated fibers was decreased to36.3%while that of the untreated ones was67.1%.(2) Effect of the cyclic load pretreated fabrics on the composite mechanical properties and its relative mechanism were investigated. It was reported that the cyclic load treatment can help natural fibers and their staple yarns achieve the improved tensile properties which could be reversed over time. But the enhanced tensile performance may bring more difficulties in the weaving process. So in this part, woven fabrics were directly pretreated with the cyclic loading along the warp direction and then reinforce thermoplastic matrix by the hot-press approach. For fabric surface morphology, the warp counts became7-12%bigger and the weft counts were7-9%smaller after the treatment. For fiber crystallinity and crystalline orientation, no significant differences between untreated and treated fibers were found. And effect of the pre-treatment on the fabric tensile properties and its composite mechanical properties was investigated. The fabrics treated by70%of the mean fracture load of the untreated ones with10cycles had the greatest improvement of about38%on the tensile strength because of the decreasd yarn crimp in the fabric and enhanced fiber orientation along the yarn axis. Compared with the control group, the cyclically loaded ramie fabric reinforced PLA composites had35%higher tensile strength,32%higher Young’s modulus,20%higher flexural strength and17%higher flexural modulus, mainly due to the increased fiber volume fraction and improved tensile properties of the fabrics.(3) Fabrication, mechanical and sound absorption properties of3D cellular matrix composites were studied. In order to achieve good resin infiltration, three dimensional fabrics woven with the co-wrapped yarns were used to manufacture a cellular thermoplastic matrix composite by the hot-pressing approach. The co-wrapped yarns as intermediate products have been developed to achieve better resin distribution and can easily adjust the weight content of resin filament for the co-wrapped yarn. Three dimensional woven composites can not only provide the outstanding delamination resistance but also maintain relatively high in-plane mechanical properties, compared with2D woven laminated composites. In this part, the weight content of resin filaments for the co-wrapped yarns and the warp yarn density of3D woven orthogonal fabrics were varied to manufacture three different composite samples with different fiber volume fraction and porosity. For composite’s surface morphology and cross sectional view, all the composite samples had some small macro-air pockets distributed in the resin on the composite surface and the large interstices between the warp and the weft yarns; Z-yarns were distorted in all the samples although their structural integrity was still maintained; the composites had a better resin infiltration due to the higher weight content of resin filament for the co-wrappe yarn. For composite mechanical properties, the best resin infiltrated composites had the highest tensile strengths and Young’s moduli in the warp and weft directions which respectively reached the highest value of43.3MPa,91.9MPa,8.5GPa and18.5GPa. And its impact load and energy were respectively about134.3N and0.59J. All composite samples could absorb the sound energy in the frequency range from1000to2500Hz. And the sound absorption peak of the composites tended to occur in the lower frequencies with increasing depth of back cavity.(4) Simulation of3D cellular matrix composites was given based on the micro-structural modeling. Much work on the finite element analysis (FEA) of3D orthogonal glass or carbon woven fabric reinforced composites has been done, but few studies on natural fiber reinforced thermoplastic composite have been reported. In this part, FEA of3D cellular matrix composites was done to predict their elastic properties according to the previous experimental results. Firstly, the cross section of warp and weft yarn was assumed as round and natural fiber staple yarn wrapped with the thermoplastic matrix was selected as the resin infiltrated yarn model. Then the unit cell of3D cellular matrix composites was simplified by removing the Z yarn part, due to the fact that the distorted Z yarn had little effect on the in-plane mechanical properties of composite. Lastly, FEA based on ANSYS software was given to predict the elastic modulus of composites along the warp and weft directions. The modulus calculated from the FEA model is14%higher in warp direction and3%lower in weft direction than the corresponding experimental values.