The tensile behavior of off-axis loaded plant fiber composites: An insight on the nonlinear stress-strain response

Composites in load-bearing applications are often exposed to off-axis loads. 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. This article (i) characterizes the stressstrain response, (ii) investigates the tensile properties, and (iii) analyses the fracture modes, of unidirectional flax-polyester composites subjected to off-axis tensile loading. A key finding of this study is that due to the nonlinear stressstrain response of PFCs, the apparent stiffness of the composite reduces by similar to 30% in the strain range of 0.05 to 0.25%. In addition, through cyclic tests on the composites, the elastic strain limit is found to be only similar to 0.15%. This has major implications on the strain range to be used for the determination of the composite elastic Young's modulus. Consequently, it is proposed that the tensile modulus for PFCs should be measured in the strain range of 0.025 to 0.100%. 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. The application of such models has also enabled the determination of, otherwise difficult to measure, material properties, such as fiber shear and transverse modulus. Off-axis loaded PFCs fail by three distinct fracture modes in three different off-axis ranges; each fracture mode produces a unique fracture surface. POLYM. COMPOS. 2012. (c) 2012 Society of Plastics Engineers