Development of a Multiscale Constitutive Framework for Predicting the Failure of Agro-Waste Hybrid Fibre-Reinforced Polymer Composites Under Multiaxial Fatigue Loading

The mechanics of failure in hybrid natural fibre polymer composites under realistic multiaxial fatigue loading conditions have not been addressed by an internally consistent constitutive framework that bridges fibre-scale damage mechanisms to macroscopic failure, a gap that precludes reliable structural design with these materials in engineering applications across Nigeria's agricultural and light manufacturing sectors. This research develops a novel multiscale constitutive framework that couples micromechanical models of progressive fibre-matrix debonding and matrix cracking with meso-scale laminate damage accumulation laws and macro-scale fatigue failure envelopes for hybrid composites incorporating sisal, kenaf, and palm fibre reinforcements processed from agricultural waste streams in southern Nigeria. The framework employs representative volume element finite element analysis at the fibre-matrix scale to characterise local stress states and damage onset criteria, with homogenisation linking to a continuum damage mechanics model at the laminate level. Multiaxial fatigue tests under proportional and non-proportional tension-torsion loading are conducted on hybrid composite tubular specimens to generate experimental calibration and validation datasets. The framework incorporates the stochastic variability of natural fibre properties through probabilistic damage threshold distributions. Model predictions of multiaxial fatigue life and failure mode sequence are validated against experimental results, demonstrating mean prediction error below 18 percent across the test matrix. The framework provides a theoretically grounded design tool for hybrid bio-composite structural components and constitutes an original theoretical contribution to composite mechanics in the context of developing economy sustainable engineering. Keywords: hybrid natural fibre composite, multiscale model, fatigue failure, constitutive framework, agro-waste reinforcement.