Dagang Li

Tensile strength of windmill palm (Trachycarpus fortunei) fiber bundles and its structural implications

AbstractsTrachycarpus fortunei (windmill palm) is one of the most widely distributed and widely used palms in East Asia. In order to find further uses for the palm’s fibers, however, more information on their mechanical and anatomical properties is needed. With this in mind, tensile strength and Young’s modulus of windmill palm fiber bundles were investigated and the structural implications considered. The anatomical features in cross-section, the fracture mode, and the microfibril angle (MFA) of natural fiber bundles were determined. The transverse sectional area occupied by fibers in a fiber bundles (SF) contributes to mechanical strength in practice. It was found that the ratio of SF to the transverse sectional area of a fiber bundle dramatically increases with a decrease in bundle diameter. Therefore, tensile strength and Young’s modulus of an individual fiber bundle in this species increase in parallel with a decrease in fiber bundle diameter. The observed MFA features might have a relationship with the biomechanical movements of fiber bundles in the windmill palm. The potential uses of windmill palm fibers have been discussed.

Excellent rheological performance and impact toughness of cellulose nanofibers/PLA/ionomer composite

Cellulose nanofibers (CNFs) with high aspect ratio were used to reinforce polylactic acid (PLA) by extrusion. Surlyn ionomer was adopted to improve the interfacial adhesion and impact toughness of the CNFs/PLA composite. To overcome the aggregation of CNFs, a water preblending process was carried out before the extrusion. The rheological results showed the ionomer can decrease the viscosity of the CNFs/PLA composite and improve the flowability due to the lubrication of the ionomer. The bending strength (MOR) and bending modulus (MOE) of the composite with ionomer were much higher than those of a composite with maleic anhydride grafted polyethylene (MAPE). In addition, the CNFs/PLA/ionomer composite showed extremely high impact toughness, which was up to 34.2 J m−1 when the loading of CNFs was 30 wt%, increasing by 302.3% as compared to neat PLA. FE-SEM images demonstrated the homogenous dispersion of CNFs in PLA by using the water preblending method.

Homogeneous dispersion of chitin nanofibers in polylactic acid with different pretreatment methods

Chitin nanofibers extracted from crab shell were used to reinforce polylactic acid (PLA) by extrusion molding. The dispersion problem of nanofibers in PLA matrix was solved by three pretreatment methods, including water pretreatment, polyethylene glycol (PEG) pretreatment, and polyethylene oxide (PEO) pretreatment. The results demonstrated that chitin nanofibers were distributed uniformly on the fracture surface of the PLA matrix with three different pretreatment methods. However, the aspect ratio of nanofibers with was reduced with the PEG and PEO pretreatment methods. Therefore, the bending modulus (MOE), bending strength (MOR) and impact toughness of the chitin nanofibers/PLA composites prepared by the water pretreatment method were much higher than those of the composites prepared by the PEG and the PEO pretreatment method. Furthermore, the reinforcing effect with the PEG method is slightly better than that with the PEO method. Although it was found that both PEG and PEO were good interfacial compatibilizers for nanofibers and PLA, the reinforcing effect of the composites prepared by PEG and PEO pretreat methods was suppressed due to the decrease of the aspect ratio for chitin nanofibers.

Exploratory study on fatigue behaviour of laterally loaded, nailed timber joints, based on a dissipated energy criterion

Abstract Exploration of damage accumulation and reliable prediction of the fatigue lives of laterally loaded, nailed timber joints, are important to proper engineering design of wood structural systems subjected to earthquakes, cyclones/hurricanes or other loads causing fluctuating force flows in such joints. Failure of nailed timber joints typically involves the combination of yielding at plastic hinges in nail and/or the crushing of wood under nails. Force-based criteria can predict the static strength of such joints but cannot reliably predict fatigue behaviour because that depends on loading history and the dissipation of energy within plastic nail hinges and/or crushed wood. In this study, the failure modes, damage accumulation and fatigue life of nailed timber joints subjected to reversed cyclic loads under load-control condition were studied. The results showed that there are two failure modes of nailed timber joints, i.e., ductile failure of nails when the applied load level was higher than 85% of the static maximum load to failure (Pmax) and brittle failure when the applied load level was <85% of Pmax. The damage accumulation involved three phases, i.e., damage initiation, damage propagation and failure. Fatigue life of nailed joints under reversed cyclic loads was modelled by an energy criterion that separated dissipated energy into non-damaging and damaging components demarcated by the fatigue limit. This approach replicated the behaviour of nailed joints tested at 20 load levels of fully reversed repetitive cyclic loading.

Fabrication of a flexible free-standing film electrode composed of polypyrrole coated cellulose nanofibers/multi-walled carbon nanotubes composite for supercapacitors

Today, flexible energy storage systems have become attractive alternatives for applications in portable electronic devices. This account reports a simple and low-cost “dipping and polymerization” method for preparing flexible and free-standing supercapacitor film electrodes. The process consisted of depositing polypyrrole (PPy) coating on the surface and inside the network of an entangled cellulose nanofibers (CNFs)/multi-walled carbon nanotubes (MWCNTs) film. The electrochemical performance of the resulting CNFs/MWCNTs/PPy hybrid electrode was evaluated and compared with that of CNFs/MWCNTs film electrodes. The CNFs/MWCNTs/PPy nanocomposite electrode exhibited a particular entangled 3D network structure with an elevated specific capacitance of 288 F g−1, obtained at a scan rate of 5 mV s−1. This value is higher than that of the CNFs/MWCNTs electrode, calculated to be 32.2 F g−1. Furthermore, the CNFs/MWCNTs/PPy electrode exhibited excellent redox reversibility and cycle stability. This novel procedure could provide an effective method for achieving flexible, free-standing, high-performance, low-cost, and environmentally friendly materials for use in supercapacitor electrodes.

High strength gelatin-based nanocomposites reinforced by surface-deacetylated chitin nanofiber networks

In this study, chitin nanofiber (ChNF) was deacetylated on the crystalline surface by NaOH treatment, leading to the fibrillation of mostly individualized nanofibers with high aspect ratio. The small diameter and high strength of chitin nanofibers make them promising reinforcing fillers for composites. Herein by introducing into the gelatin, surface-deacetylated chitin nanofiber (S-ChNF)/gelatin nanocomposites were fabricated in different component ratios using immersion method followed with drying. Due to the reinforcing effect attributed to S-ChNF, mechanical properties of the S-ChNF/gelatin were significantly improved in both stress and Youngs modulus while still maintaining high transparency regardless of nanofiber content. Morphology and Fourier-transform infrared characterization revealed that S-ChNF preserved nanonetwork structures in the gelatin matrix and exhibited good compatibility through hydrogen bonding, which further confirmed the improvement in mechanical properties. Therefore, these S-ChNF/gelatin nanocomposites based on biocompatible and biodegradable raw materials have potential applications in biomedical and food packaging industries.

Comparative Study on Properties of Polylactic Acid Nanocomposites with Cellulose and Chitin Nanofibers Extracted from Different Raw Materials

Polylactic acid (PLA) was reinforced with ultralong cellulose and chitin nanofibers extracted from four raw materials by extrusion. The mechanical, rheological, thermal, and viscoelastic performances of four nanocomposites were comparatively studied in detail. The results showed that fibrillation of poplar was much easier than that of cotton, and fibrillation of crab shell was relatively hard as compared to prawn shell. The poplar CNFs/PLA composite exhibited the best mechanical properties among four nanocomposites due to the highest aspect ratio of nanofibers, while both the cotton CNFs/PLA composite and the crab shell CHNFs/PLA composite had low mechanical strength due to the relatively low aspect ratio. FE-SEM images showed that the ultralong nanofibers were uniformly dispersed in PLA matrix for all four samples with the water preblending method. The CTE values of the nanocomposites with 40 wt% nanofibers extracted from poplar, cotton, crab shell, and prawn shell were 69.5 × 10−6 K−1, 79.6 × 10−6 K−1, 77.2 × 10−6 K−1, and 75.3 × 10−6 K−1, respectively. All the results indicated that the aspect ratio of the nanofibers has a great influence on the performance of the composites, irrespective of the composites prepared by cellulose or chitin.