Natural fibers are usually used as reinforcement in green composites. Especially, the use of slivers of natural fibers is anticipated for increasing composites' stiffness and strength. However the slivers have various fiber orientation angles and often involve fiber fluctuation. This paper describes effects of fiber orientation angle and fluctuation on Young's modulus and tensile strength of the so-called fully green composites. The composites were reinforced with slivers of high-strength natural fibers extracted from curaua plants. For this study, a fabrication method called 'direct method' was applied for obtaining sliver-based green composites with various fiber orientation angles and fluctuation. Then optical micrographs of composites with fiber fluctuation were obtained. After the micrographs were divided into many fine segments, the fiber orientation angle in each segment was measured. Results show that the tensile strength depends on autocorrelation coefficients expressing the degree of fluctuation in fiber orientation, as well as the fiber orientation angles. However, the Young's modulus was dependent only on the angles. Furthermore, the Young's modulus of the composites was predicted using classical lamination theory. In addition, a statistical concept was applied to an orthotropic analysis for prediction of the Young's modulus. The predicted Young's moduli showed better agreement with the experimental results, than that of the predicted values based on a definite orthotropic theory.
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