![]() ![]() However their specific strength performance (tensile, flexural and impact) remains poor. PFCs can compete with E-glass composites in terms of specific stiffness (tensile and flexural) and inter-laminar shear strength. The mechanical performance of vacuum infused unidirectional plant fibre composites (PFCs) against E-glass composites is presented in this paper. Experimental data from other studies are also used for further verification. The model is validated with extensive experimental data from Goutianos and Peijs (Goutianos S, Peijs T (2003) Adv Compos Lett 12(6):237) and is found to be a near-perfect fit (R2 = 0.960). The developed model includes a corrected orientation efficiency factor of cos2(2α), where α is the yarn surface twist angle. The model is based on (i) a modified rule of mixtures for plant fibre composites, (ii) well-defined structure-property relationships in an idealised twisted staple fibre yarn and (iii) the Krenchel orientation efficiency factor. In this paper, a novel mathematical model is developed. No analytical model currently exists to accurately predict the effect of yarn twist on aligned plant fibre composite tensile strength. This results in a drastic drop in composite mechanical properties. Although twist facilitates yarn processability, it has several detrimental effects on the composites produced from such twisted yarn reinforcements one of which is fibre obliquity and misalignment. However, due to the short length of technical plant fibres, the reinforcement needs to be in the form of staple fibre yarns, which have a twisted structure. Hence, aligned plant fibre composites are of interest. The structural potential of plant fibres as reinforcing agents can only be realized when the highest reinforcement efficiency is employed. Off-axis loaded PFCs fail by three distinct fracture modes in three different off-axis ranges each fracture mode produces a unique fracture surface. The application of such models has also enabled the determination of, otherwise difficult to measure, material properties, such as fiber shear and transverse modulus. Through comparison with experimental data, conventional composite micromechanical models are found to be adequate in quantitatively describing the tensile behavior of off-axis loaded PFCs. Consequently, it is proposed that the tensile modulus for PFCs should be measured in the strain range of 0.025 to 0.100%. ![]() This has major implications on the strain range to be used for the determination of the composite elastic Young's modulus. ![]() In addition, through cyclic tests on the composites, the elastic strain limit is found to be only ∼0.15%. A key finding of this study is that due to the nonlinear stress–strain response of PFCs, the apparent stiffness of the composite reduces by ∼30% in the strain range of 0.05 to 0.25%. This article (i) characterizes the stress–strain response, (ii) investigates the tensile properties, and (iii) analyses the fracture modes, of unidirectional flax-polyester composites subjected to off-axis tensile loading. For plant fiber composites (PFCs) to be seriously and readily considered in structural applications, knowledge and reliable prediction of their response to off-axis loads is critical. Composites in load-bearing applications are often exposed to off-axis loads.
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