Stiffness prediction of flax fibers-epoxy composite materials

Abstract

For many years additives of organic origin have been combined with thermoplastic polymers in order to modify their properties: wood flour and fibers including jute, sisal and flax. These natural fibers are particularly interesting owing to their low cost. Polypropylene reinforced with flax and wheat straw fibers was particularly studied [1]. Fiber damage is induced during melt processing operations, addition of fibers caused however a significant increase in mechanical properties. In these processing operations fibers act essentially as fillers. Investigations were conducted in order to analyse properties of a unidirectional composite material displaying a thermoset matrix: epoxy resin reinforced with flax fibers. It appears first necessary to determine precisely mechanical properties of the fibers. These properties depend in fact on defects and dispersion in geometry of fibers. Mechanical properties of the unidirectional composite was then determined. Results are described below. The fibers have a polygonal cross-sectional geometry. They also contain many defects along their length, running transversely across the fibers [2]. These defects may occur when the fiber cells unite [3], but could also result from mechanical damage of the fiber walls. Flax fibers are also subjected to mechanical and chemical treatment before their introduction into polymeric matrix. Influences of these treatments on mechanical properties of fibers have also to be taken into account. If the cross-section is assumed as circular, the fibers diameter is not constant. The diameter changes also along the length of the fiber. It was then necessary to determine mechanical properties of fibers versus their diameters and versus the different treatments. A wide range of modulus values is proposed from 30 000 MPa to 110 000 MPa [2] and it is thereby difficult to predict elastic properties of flax fibers reinforced composite materials. Investigations were conducted in order to determine the effective modulus of a sample of flax-fibers representative of a given production process. Fibers diameters were measured by microscopy.