Study on the multi-scale structure and interfacial properties of plant fiber reinforced composites

Abstract

Plant fibers used as reinforcing materials for green composites have become a common concern among scholars in recent years. However, cellulose, the main chemical composition of plant fibers, contains a large number of hydroxyl groups which leads to poor interfacial properties of plant fibers with hydrophobic polymeric matrices, thus low mechanical properties of plant-fiber-reinforced composites (PFRCs). To promote the mechanical performances of PFRCs and extend their large-scale industrial applications, it is highly desirable to comprehensively quantify the interfacial properties of PFRCs in the multi-scale by considering the distinct multi-layer microstructure of plant fibers. To serve the task of interfacial design of PFRCs, a series of experimental techniques (nanoindentation and nano-scale dynamic mechanical analysis (nano-DMA), single fiber pull-out measurement and acoustic emission (AE) characterization) and analysis methods (multiple interfaces modelling and ABAQUS simulation) have been systematically developed, based on the multi­layer structure of PFRCs. In this Ph.D. study, experimental investigations were firstly conducted to facilitate understanding of the multi-layer structure of plant fibers. Elastic modulus and hardness of the epoxy matrix and cell wall layers of sisal fibers (a typical plant fiber) along with interfacial mechanical properties in the sisal-fiber-reinforced composites (SFRCs) were measured from the nanoindentation technique. Single-step and multi-step nanoindentation methods were respectively employed on the multi-layer interfaces of SFRCs to present their distinct mechanical properties upon compressive loading. Specifically, this study measured the transition zones of the multi-layer interfaces regarding modulus and hardness and the interfacial failure loads, which consequently facilitated quantitative analysis of fracture mechanisms for SFRCs with a multi-layer and multi-scale structure. Fatigue performance of multiple interfaces in SFRCs in the nano-scale was evaluated with nano-DMA technique by using the cyclic loading. Subsequently, interfacial failure behaviors of SFRCs during the single fiber pull-out test were studied experimentally with theoretical analysis and simulation. The residual pull-out strength of the single SFRCs was observed to gradually decrease during the test and the corresponding fracture mechanisms were characterized by in-situ AE technique. The single SFRCs were found to present multiple failure modes at three interfaces, namely interfacial failure between technical fiber and matrix, that between elementary fibers and that between cell walls. Meantime, the failure mechanisms of the interfaces in the single SFRCs were described with the help of AE. Statistical analysis was employed to evaluate the failure probability of technical fiber, elementary fiber and micro-fibrils pull-out. The embedded fiber length was concluded to play a critical role in determining the failure modes of the single SFRCs. To further gain insight in the failure mechanisms of single SFRCs, a double-interface model using the traditional shear lag model and a triple-interface finite element (FE) model based on the cohesive zone model (CZM), tailored to the unique multi-layer structure of plant fibers, were developed to describe the fiber pull-out behavior with two and three failure stages, respectively. Quantitative comparisons between the numerical predictions from the single-, double-and triple-interface models and the experimental results, using the applied stress as reference, surmised that the single-, double-and triple-interface models need to be comprehensively considered to accurately describe the pull-out behaviors of single SFRCs. With nanoindentation and single fiber pull-out technique, interface failure mechanisms of plant fibers and single PFRCs at nanoscopic and microscopic scale were revealed through both experiment and analysis. The effects of hierarchical structure of plant fibers on the interfacial failure behaviors of laminated PFRCs in the macro-scale were further investigated using double cantilever beam (DCB) experiments. Compared with unidirectional AFRCs (especially glass fiber), the PFRCs possess higher Mode I interlaminar fracture toughness, which was because the existence of the hierarchical structure of plant fibers and the multiple interfaces of PFRCs made the crack propagation path tortuous, further bringing in a more pronounced phenomenon of fiber bridging and fiber entanglement. To model the multi-scale interfacial regions of laminated PFRCs and to further simulate their multiple interfacial fracture behaviors, FE model was developed in ABAQUS with designed CZM in the crack front. Good consistency between the numerical simulation and the experiment results verified the efficiency of CZM in modelling the multi-layer failure behaviors of laminated PFRCs. Using the micromechanics theory and cohesion model of composite materials, a quantitative relationship among the microstructure characteristic, interlaminar fracture toughness and parameters of the mechanical model was studied based on the design principle of composite structures in this thesis. Conclusively, through experimental investigations, theoretical modelling and numerical simulation, a series of characterization techniques from nanoscopic to macroscopic scale for identifying multiple interfacial failure modes in PFRCs were employed in this thesis by considering the hierarchical structural features of plant fibers. The presented thesis provides a solid theoretical foundation, whereby theoretical and numerical analysis can be accurately conducted to achieve the goal of interface design in laminated PFRCs with multi-layer and multi-scale structures. Research achievements in this Ph.D. study are expected to serve for improving the mechanical performances of PFRCs, achieving large-scale applications of PFRCs in the fields of aerospace, automotive engineering and civil infrastructures and expanding the theories on multi-scale mechanics of composite materials to some extent.