Engineering Sustainable Composites for Lightweight Automotive Applications: Experimental and Theoretical Insights on Hybrid Fibre-reinforced Polymers

Abstract

This study investigates the enhancement of mechanical properties in cellulose microfibre/carbon fibre hybrid composites on PA11-based matrices, aiming to achieve lightweight and sustainable applications in the automotive sector. The use of a PA11-PP blend matrix led to composites with improved thermo-mechanical performance, elevated heat deflection temperatures, and reduced densities, key attributes for semi-structural automotive components where both weight reduction and mechanical reliability are critical.To support and interpret the experimental findings, a progressive modelling strategy was employed, combining semi-empirical and analytical methods. Classical models such as HROM, Halpin–Tsai, and shear-lag theory were adapted and mathematically refined to predict the tensile modulus of the composites. Using the effective matrix approach, the modified Halpin–Tsai model accurately captured the behaviour of randomly oriented fibres, with best-fit orientation factors (λL = 0.481, λT = 0.736) aligning closely with theoretical values (0.375 and 0.625), confirming its applicability for reinforcement efficiency in hybrid composites. To obtain further insight into stress transfer mechanisms and fibre-length-dependent behaviour, particularly relevant in short-fibre composites, the study advanced toward the shear-lag model. Unlike the Halpin-Tsai formulation, which is primarily geometry-based, the shear-lag approach provides a mechanistic description of load transfer between fibres and matrix. The classical shear-lag model was refined through the introduction of a stress transfer efficiency factor (α), defined as a non-linear function of the ratio of fibre length to its critical length. This analytically modified formulation enabled more realistic and accurate predictions of the tensile modulus in composites reinforced with microfibrillated cellulose and short carbon fibres. The combined use of semi-empirical and analytically modified models, supported by an effective matrix approach, enabled a deeper understanding of the reinforcing mechanisms in hybrid fibre systems. This framework validates experimental observations and offers a versatile design tool for optimizing the performance of bio-based hybrid composites across a wide range of applications.