The effect of damage and geometrical variability on the tensile strength distribution of flax fibers

Natural fibers of plant origin are finding non-traditional applications as reinforcement of composite materials. The mechanical properties of fibers exhibit considerable scatter, being affected by the natural variability in plant as well as the damage accumulated during processing. For bast fibers, the primary damage mode is kink bands - zones of misaligned cellulose microfibrils extending across the fiber and oriented roughly perpendicularly to its axis. Another feature typical for natural fibers and contributing to the scatter of fiber strength is the variability of diameter along a fiber length and among the fibers. An analytical expression for the distribution of the longitudinal tensile strength of bast fibers has been derived, accounting for the strength variability of intact fibers and the effect of kink bands. Upon determining the relevant parameters from fiber damage and geometry characteristics by means of optical microscopy, the theoretical strength distribution function has been found to agree reasonably well with the test results of elementary flax fibers.