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In order to clarify the fracture behavior of a unidirectional CFRP under proportional loading along the fiber (0°) and fiber vertical (90°) directions, a biaxial tensile test was carried out using a cruciform specimen with two symmetric flat indentations in the thickness direction. Three fracture modes were observed in the specimens after the test. The first mode was a transverse crack (TC), and the second was fiber breakage (FB). The third mode was a mixture mode of TC and FB (TC&FB). According to the measured fracture strains, regardless of the magnitude of the normal strain in the 0° direction, TC and TC&FB modes occurred when the normal strain in the 90° direction, εy, ranged from 0.08% to 1.26% (positive values), and the FB mode occurred when εy ranged from −0.19% to −0.79% (negative values). The TC&FB mode is a unique mode that does not appear as a failure mode under uniaxial tension; it only occurs under biaxial tensile loading. Biaxial tensile tests were also conducted under non-proportional loading. The result showed three fracture modes similarly to the proportional loading case, each of which was also determined by the positive or negative value of εy. Thus, this study reveals that the occurrence of each fracture mode in a unidirectional CFRP is characterized by only one parameter, namely εy.

Mechanical properties of plasma-irradiated and surface-coated wood plastic composites (WPCs) have been investigated in this paper. WPCs were developed by injection molding technique using wood fiber (WF) as reinforcement and polypropylene (PP) as matrix. The short, discontinuous WF was compounded with thermoplastic PP at varying weight fractions of 0 wt%, 25 wt% (WP25), and 50 wt% (WP50) to yield tensile test specimens in accordance with JIS K7139-A32 standards. Subsequently, plasma treatment was performed on the test-pieces, followed by surface coating by immersion in acrylic resin liquid containing homogeneously dispersed TEMPO-oxidized cellulose nanofibers (CNF). The results indicate an increase in surface roughness after plasma irradiation, but surface coating of the specimens with acrylic paint and CNF decreased their surface roughness by ∼50% in comparison to the untreated specimens. Plasma treatment and surface coating also increased the tensile strength of neat PP, WP25 and WP50 specimens by 5.4–7.1%, 3.5–3.7% and 3.0–3.6%, respectively, whereas their fracture strains tended to decrease. Compared to the untreated specimens, the surface-coated specimens generally displayed higher tensile strength. This finding is a corroboration that the observed increase in strength is highly contingent on the adhesion between the specimen surface and the coating layer than on the improvement in surface roughness. Thus, it is inferable that surface coating could be of great importance in enhancing the mechanical performance of WPCs.

Fatigue analysis and fatigue reliability of polypropylene (PP)/wood flour (WF) composites were studied. The composites were prepared by incorporating wood flour with and without maleic anhydride and peroxide (MAPO) into cellulose nanofibres impregnated polypropylene. The prepared composites were then characterized by tensile and fatigue analyses. The tensile strength and fatigue life of MAPO mixed composites were lower compared with the composites without MAPO. It was also found that the fatigue experimental data of the composites were widely scattered regardless of the type of composites. Hence, in this study, a fatigue reliability of the composites was sought. Based on the fatigue experimental data, 95% confidence band were created. Since 95% confidence lower band ensure 95% survivability and only 5% failure of the composites, hence, from this study, it is suggested that the fatigue life obtained from 95% confidence lower band can be used as a material reliability index for safe fatigue design of the composites. Keywords: Melt-viscosity, Wood plastic composite (WPC), Fatigue behavior, Wood flour, Polypropylene

This study analyses the effect of pulverization of wood flour with and without water to the melt-viscosity and mechanical behaviour of polypropylene (PP)/wood flour (WF) composites. The composites were processed in an extruder and subsequent injection moulding. The effects of initial wood flour particle size, type of trees (namely cypress and European scots pine trees) as well as plate gap (200 μm and 350 μm) in pulverization were also evaluated in terms of melt-viscosity, tensile properties, Izod impact strength and fatigue behaviour of the prepared composites. PP/WF composites with pulverised wood flour with water exhibited higher melt-viscosity than that of nonpulverized and pulverized wood flour without water reinforced PP composites. Tensile strength values of composites were slightly affected by the pulverization of wood flour and the enhancement of tensile strength values were depended on the type of initial WF. From Izod impact test results of composites, it was found that the composites reinforced with pulverized wood flour with water displayed higher values of impact energy compared with the composites of pulverized WF without water. Depending on the type of initial WF particle, the positive effect of pulverization of wood flour with water to the fatigue life of the composites was also observed. Keywords: Composites, Mechanical properties, Melt-viscosity, Pulverization, Wood flour

The tensile strength of injection-molded semi-crystalline resin (polyamide) matrix composites was improved to higher than the values predicted from the rule of mixture merely by adding 1.0–5.0 wt% of ramie fibers to the resin. This improvement was attributed to faster crystallization on the ramie fiber surface: so-called ‘transcrystalline’. The resin strength became equivalent to the composite strength after isothermal heat treatment. To elucidate this phenomenon, we observed transcrystallization on the fiber surface in a single fiber composite specimen as a function of heating time. Results show that, despite the short heating time, the transcrystalline layer occurs earlier than spherulites. However, with longer heating time, spherulite radius reached a level equivalent to the transcrystalline layer thickness, eventually exceeding it. Although the heat treatment applied for the low-fiber-content composites or the neat resin improves their strength, premature crystallization on the fiber surface can efficiently enhance the composites’ tensile strength without heat treatment. Keywords: Green composites, Cellulosic natural fibers, Transcrystalline, Spherulite, Fiber reinforcement, Tensile strength

In this paper, we exclusively studied the effects of dry and wet pulverization of different wood flours on the fatigue performance of polypropylene (PP)/wood flour (WF) composites. Wood flours obtained from cypress and Scots pine trees were pulverized in both dry and wet conditions at two different mill-plate gaps, 200 µm and 350 µm, and were used as reinforcement in PP matrices. Master batches of PP with different types of pulverized WF were compounded before processing in an extruder. The PP/WF composites of initial WF were also prepared for comparison. The prepared composites were analyzed by tensile and fatigue tests. It was found that the tensile properties of wood/polypropylene composites were affected by the pulverization of WF. Fatigue test results displayed that wet pulverization of short cypress flour had a negative effect on the fatigue life of PP/WF composites, while wet pulverization of long cypress flour and pine flour had a positive effect on the fatigue life of PP/WF composites.

Recently, a composite material made from natural fibers and biodegradable resin, “green composite,” is attracting attention as an alternative composite material for the replacement of glass fiber-reinforced plastics. Plant-based natural fibers such as kenaf and flax have already been used as composite reinforcement materials because they are more environmentally friendly and costless fibers than artificial fibers. A problem of using natural fibers is the fiber waviness, which affects the tensile properties. Fiber waviness is fluctuation in the fiber orientation that is inherent in the sliver morphology of plant-based natural fibers. This study was conducted to clarify the relation between quantified parameters of fiber waviness and a composite’s tensile strength. First, the fiber orientation angles on a flax-sliver-reinforced composite were measured. Then the angle distribution was quantified through spatial autocorrelation analysis methods: Local Moran’s I and Local Geary’s c. Finally, the relation between the resultant tensile strength and quantified parameters was discussed.

Ceramics are excellent in heat, abrasion and corrosion resistances, but their strength reliability for structural materials is not enough due to their brittle nature. On the other hand, ceramic matrix composites (CMCs) reinforced with ceramic fibers attract attention as a damage tolerating material, because toughness of CMCs is improved through various mechanisms such as fiber bridging, fiber breakage, pullout, interfacial debonding and crack deflection. Thus, clarification of the mechanical behavior of CMCs including such damages deepens its engineering significance. In this study, a new finite element (FE) model for the damage analysis of this material is proposed, in which the effect of Coulomb friction at the interface after fiber breaking or matrix cracking followed by the fiber/matrix debonding is taken into account. Although a limitation condition is incorporated into the model, the advantage of this model is that stress and strain distributions can be obtained by merely one calculation without iteration. The accuracy of this FE model was validated by comparing with the results of both the general-purpose FE analysis software and theoretical matrix crack model. In addition, we proposed a new interfacial contact state to apply this model to the hysteresis behavior of CMC under cyclic loading, and discussed its causes about changes in the hysteresis loop. Results of the present FE model showed that the fiber and matrix stress distributions behaved non-linearly in the interfacial debonding area, while these displayed a constant in the bonding area. This model also showed a good compatibility with the theoretical model and the general-purpose FE analysis. Thus, it is expected that this model can be applied for damage states such as matrix crack deflection, indeed difficult through the conventional theoretical models or general-purpose FE analysis. The present model also simulated well the shift and shape of the hysteresis loop with cyclic loading.

This paper deals with long fiber-reinforced composite strands using continuous ramie yarns, in which polypropylene including MA-PP (maleic anhydride-grafted polypropylene) was used as a matrix material. The composite strands were fabricated by a new method called multi-pin-assisted resin impregnation (M-PaRI) process, for which the equipment was newly applied after conventional extrusion process. The composite strands were then pelletized and injection molded. Tensile strength and Young's modulus of the resultant short ramie/PP reinforced composites were investigated. Results show that this new process improved the mechanical properties of injection-molded specimens.