Mechanical Properties And Water-induced Interfacial Failure Of Ramie Fiber Reinforced Composites

Natural cellulose fiber reinforced composites have attracted increasing attention due to environmental concerns and sustainable development concerns.For the advantages of low density and sustainability,the natural cellulose fiber reinforced composites have been used in the sub-structural and non-structural applications,such as packing and gardening.However,with relatively low mechanical properties and short fiber length of natural cellulose fibers,the natural cellulose fiber reinforced composites,especially the short fiber reinforced composites,have much lower mechanical properties than the glass fiber reinforced composites.Recently,lots of works on water aging performance of natural cellulose fiber reinforced composites have found that the water swelling of fibers would break the interface,which decreases the mechanical properties of the composites.However,few works have systematically reported the water-induced interfacial failure mechanism.All these would prevent natural cellulose fiber reinforced composites from the development in structural applications.With the development in new energy vehicles,it is urgent to develop composites with low density,high strength and low cost for structural applications.In order to further develop the industrial utilization of natural cellulose fiber reinforced composites,improve the mechanical properties of the composites and study the water-induced interfacial failure mechanism of the composites,this thesis focuses on the improvement of mechanical properties of ramie fabric reinforced composites and the understanding of water-induced interfacial failure micromechanical mechanism of ramie fiber reinforced composites,studies the synergistic effect of alkali solution pretreatment and cyclic loading treatment on the mechanical properties of ramie fabric reinforced laminate composites and the influence of hybrid structures on mechanical properties of the 3D orthogonal ramie/Kevlar fabric reinforced composites,builds the micromechanical model on water-induced interfacial failure of natural cellulose fiber reinforced composites and verifies the model with the ramie fiber/polypropylene micro-composites.The details of the studies are described as follows:Firstly,the synergistic effect of two recently developed techniques to maximize the mechanical performance of ramie/poly(lactic acid)laminated composites,namely alkali solution pretreatment and cyclic loading treatment on fabrics,is studied.FTIR,XRD and optical microscope are used to analyze the treatment effect on the chemical properties of ramie fibers,physical properties of ramie fibers and surface structures of ramie fabrics,respectively.The tensile and flexural properties of the composites are tested and the postmortem analyses of the composite fracture surfaces are done by SEM.The results show that the treated fabrics have increased crystallinity and crystal orientation factor as well as better orientation of fibers and more uniform structures,leading to 11% improvement in fabric tensile strength and 57% enhancement of tensile strength,49% higher tensile modulus,18% higher flexural strength,and 91% higher flexural modulus for the corresponding composites.Meanwhile,postmortem analysis shows that better interfacial adhesion is achieved using this approach.The combination of alkali pretreatment and cyclic loading treatment on ramie fabrics would improve the mechanical properties of ramie fabric reinforced composites.Alkali pretreatment would break the hydrogen bonds on the fiber surface,which would remove hemicellulose and/or lignin from the fibers and improve the interfacial bonding.Meanwhile,alkali pretreatment would break the hydrogen bonds in the fiber,thus promote the crystalline orientation of the fibers during the cyclic loading process and improve the mechanical properties of ramie fabrics.Secondly,3D orthogonal woven fabric reinforced composite exhibit higher delamination resistance than the 2D fabric reinforced composites due to the existence of yarns in the thickness direction.In addition,the hybridization of natural and manmade fibers provides not only partial environment friendly benefit but also efficient compensation for the relatively low mechanical properties from pure natural fibers.Thus in order to further improve the mechanical properties of natural cellulose fiber reinforced composites and develop the high-performance textile structure composites,the 3D orthogonal woven hybrid fabrics are adopted as the reinforcements.Ramie yarns and/or Kevlar multi-filaments are used as weft yarns,and eight kinds of 3D orthogonal woven fabrics with four warp layers and five weft layers are prepared.The aforementioned hybrid composites are fabricated.The tensile,flexural and impact properties of the composites are tested and the postmortem analyses of the composite fracture surfaces are done by SEM.The results show that as the volume fraction of Kevlar increases,the tensile properties increase;the impact strength is enhanced as the volume fraction of Kevlar is increased to 5.5% and then levels off when Kevlar yarns are continuously increased.The flexural properties are predominantly dependent on the weft yarns properties in the first and second layers from the upper and bottom surfaces.The flexural and impact failure modes of the composites with relatively higher content of Kevlar fibers are mainly the matrix deformation and interfacial failure.The composites reinforced by fabrics with outermost weft Kevlar yarn layers and the second outermost Kevlar/ramie yarn layers exhibit the largest flexural strength and modulus of 225.8 MPa and 9.1 GPa,respectively and comparable impact strength of 116.2 KJ/m2 to the composites reinforced by fabrics with Kevlar weft yarns.Thirdly,the interface of the natural cellulose fiber reinforced composites might failure due to the difference between swelling of hydrophilic natural cellulose fibers and that of hydrophobic matrix.In order to study the water-induced interfacial failure mechanism of natural cellulose fiber reinforced composites,this thesis proposes a water-induced interfacial failure micromechanical model with the consideration of matrix deformation and fiber deformation in wet composites and the shear lag effect.The matrix would expand in the radial direction due to fiber swelling.When the fiber and the matrix are in the equilibrium state,the axial stain of fibers in the composites is determined with Lamé Equation and Poisson's ratio.Finally,the water-induced theoretical interfacial shear strength(IFSS)is deduced based on the shear lag effect of short fiber reinforced composites.When the water-induced theoretical IFSS is larger than the IFSS of dry composites,the interface of the composites is considered to be damaged.This model would well describe the water-induced interfacial failure mechanism.Lastly,ramie fiber/polypropylene micro-composites are used to verify the model.The ramie fibers and micro-composites are put in different environment(the environment with relative humidity of 65% and 90% and liquid water environment).The water regain,swelling strain and tensile modulus of fibers are tested.The residual IFSS of wet ramie/PP micro-composites is determined by the debonding tests.Then SEM tests of the debonded and undebonded micro-composites are done.Finally,the water-induced theoretical IFSS is figured out.The model is verified by the comparison of the theoretical and the experimental results.It is found that the experimental results agree well with the theoretical analysis results and the micromechanical model would well explain the water-induced interfacial failure of the micro-composites.The thesis mainly studies the mechanical properties and water-induced interfacial failure mechanism of natural cellulose fiber reinforced composites.This would provide experimental and theoretical basis on the improvement of composite mechanical properties,the design of the reinforcements and the mechanism of water-induced interfacial failure and provide guidance to the research and development of natural fiber reinforced composites.