New Insights into the Stiffness and Strength of Flax Composites from Tsai’s Modulus and the Area of the Failure Envelope

Abstract There is a growing trend toward the use of natural fibers as reinforcing materials, with flax being a significant part of this market. The mechanical properties of these polymer composites, like those of synthetic fibers, are governed by parameters and material invariants. The challenge is to minimize these parameters, and to reveal these invariants to make stiffness and strength easily comparable with each other and with other composites, while avoiding excessive complexity. To this end, a simple methodology has been developed using the following parameters: Tsai’s modulus or the trace of the stiffness tensor and the area of the Omni Failure Envelope in stress space. Based on the analysis of significant published experimental data on flax composites, new insights were found. The trace-normalized longitudinal Young modulus is a material property that were found to be 0.77 for tension and 0.67 compression with a coefficient of variation of 5.6% and 15%, respectively. The area of the Omni Failure Envelopes and the strength are linearly related. The use of the proposed parameters and some invariants has been discussed and they are used to compare and rank them with each other and with other composites, including carbon, aramid, and glass fiber-reinforced polymer composites.