Strain amplitude controlled fatigue of Flax-epoxy laminates

Fatigue longevity and evolving material properties of Flax-epoxy laminates is examined under constant strain amplitude cycling. Several recent fatigue studies on Flax-composites, all of which were conducted under stress-control, found that certain laminates demonstrate a modulus increase over fatigue life, even while accumulating internal damage and permanent deformation. This study investigates whether such fatigue-stiffening phenomena are also observed under strain-control. Specimens of four layups ([0](16), [0/90](4S), [+/- 45](4S), and (0/45/90/-45](2S)) are tested under 5 Hz and strain ratio R-epsilon epsilon(min)/epsilon(max) =0.1. Strain-life (epsilon-N) plots are generated, which are found to follow a consistent trend that can be modelled by a linearised relationship with identifiable parameters. No evidence of stiffening is observed. All specimens demonstrate stiffness degradation, thereby contradicting previous studies. The extent of degradation is proportional to loading applied. A directly proportional relationship is observed between strain-rate and measured modulus. It is proposed that the reported modulus increase in existing literature does not reflect any physical improvement of material stiffness, but is a consequence of stress-amplitude controlled loading conditions. As such, strain-controlled cycling may be more appropriate for fatigue studies on natural fibre composites.