Modelling of manufacturing parameters and characterisation on the mechanical properties of sunn hemp (Crotalaria juncea L.) bio-composites with different fibre forms Babatunde Olatunbosun a , Allan Manalo a , Thiru Aravinthan a , Guangnan Chen b , Shanika Kiriella a,* a Centre for Future Materials, University of Southern Queensland, Toowoomba, QLD 4350, Australia b Centre for Agricultural Engineering, University of Southern Queensland, Toowoomba, QLD 4350, Australia A R T I C L E INFO Keywords: Sunn hemp fibres Fibre forms Bio-composites Manufacturing parameters, Response Surface Methodology ABSTRACT The mechanical performance of sunn hemp fibre-reinforced bio-composites varies with fibre form, yet its opti- mization remains underexplored. This study employs Response Surface Methodology (RSM) to optimize manufacturing parameters (temperature, pressure, and time) and assess their impact on mechanical properties. Results show that bast fibre composites exhibited the highest tensile (71.3 MPa), flexural (95.1 MPa), and interlaminar shear strength (9.0 MPa), surpassing hurd and hybrid forms by at least 35 %. Optimal processing conditions (80 ◦ C, 6 MPa, 15 min) achieved a 75 % desirability score. RSM-based quadratic models predicted mechanical properties with 92–93 % accuracy, providing a reliable tool for composite optimization. These findings establish bast fibre-reinforced bio-composites as a high-performance, sustainable material for con- struction applications. 1. Introduction Natural fibres derived from various plant sources have emerged as promising candidates for the development of sustainable composites, offering a renewable alternative to traditional construction and building materials. Among these natural fibres, sunn hemp has garnered signifi- cant attention due to its remarkable mechanical properties, and abun- dance in various regions, rapid growth rate, and potential to be used as a reinforcement in bio-composites manufacturing [1]. Sunn hemp stands out because of its exceptional strength, attributed to its high cellulose content and low microfibril angle. In addition, sunn hemp is corrosion resistant, has high toughness, and is resistant to fatigue [2]. Sunn hemp fibres (SHFs) are extracted from the sunn hemp plant’s stem and ribbon (outer skin), after which fibre bundles and sunn hemp yarns are created (Fig. 1). SHF presents considerable opportunities for utilization as a reinforcing agent in polymer composites because of its beneficial attri- butes [3]. Studies have demonstrated that the addition of sunn hemp fibre to polyester composites has resulted in increased mechanical properties, fatigue life, and penetration resistance [4]. Despite their advantages, natural fibre composites (NFCs) face challenges related to moisture absorption, thermal degradation, and ultra-violent induced aging, which can deteriorate their mechanical properties over time. Exposure to humidity and fluctuating temperatures can cause fibre swelling, matrix softening, and interfacial debonding, leading to reduced composite strength and durability. Studies on kenaf/sisal fibre-reinforced bio-epoxy composites revealed that prolonged weath- ering conditions significantly degrade tensile and flexural properties, primarily due to increased water uptake and surface erosion [5]. How- ever, treatments such as fibre surface modifications and bio-epoxy resin selection have been shown to enhance hydrophobicity and resistance to environmental degradation, making NFCs a more viable alternative for long-term structural applications. In addition, the inherent strength and rigidity of the fibre position it as a practical substitute for glass fibres, which are widely employed synthetic reinforcement materials [6]. The use of bio-epoxy resin as a matrix further enhances the performance, adhesion, and durability of NFCs, positioning them as a sustainable alternative to petroleum-based composites. Unlike traditional epoxies, bio-epoxy offers greater impact resistance, reduced toxicity, and a lower * Corresponding author. E-mail addresses: babatunde.olatunbosun@unisq.edu.au (B. Olatunbosun), allan.manalo@unisq.edu.au (A. Manalo), Thiru.Aravinthan@unisq.edu.au (T. Aravinthan), Guangnan.Chen@unisq.edu.au (G. Chen), Shanika.Kiriella@unisq.edu.au (S. Kiriella). Contents lists available at ScienceDirect Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat https://doi.org/10.1016/j.conbuildmat.2025.142349 Received 7 January 2025; Received in revised form 24 April 2025; Accepted 17 June 2025 Construction and Building Materials 489 (2025) 142349 Available online 20 June 2025 0950-0618/© 2025 Published by Elsevier Ltd.