Design, advanced manufacturing, and performance evaluation of optical sensor-integrated laminated composite monoleaf springs for heavy commercial vehicles: A numerical and experimental study

Abstract

As the global focus shifts toward electric vehicles and alternative fuel technologies, reducing vehicle weight has become increasingly critical. Lightweighting enhances fuel efficiency, reduces CO2 emissions, and improves vehicle performance. For commercial vehicles, lightweighting also increases payload capacity, operational efficiency, and environmental sustainability while offering cost advantages. By reducing vehicle weight, logistics companies and fleet operators gain a competitive edge through higher payload capacities and improved overall efficiency. Polymer Matrix Composites (PMCs) have emerged as a promising solution for achieving lightweighting goals due to their high strength-to-weight ratios compared to traditional materials like steel and aluminium. Despite their extensive use in aerospace and defence industries, the adoption of PMCs in automotive applications has faced challenges. Requirements such as dynamic load-bearing capacity, cost efficiency, and high-volume production have slowed their use in critical structural components. This thesis focuses on developing polymer composite monoleaf springs (MonoLeaf), which are widely used in the suspension systems of heavy commercial vehicles. Leaf springs operate under dynamic loads and often feature variable cross-sectional geometries to optimize material usage. This design requires significant thickness variations, ranging from 50 mm to 100 mm, introducing challenges in both design and manufacturing, particularly for single-piece Composite design using Unidirectional Continuous Glass Fiber Reinforced Polymer (UD-GFRP) prepreg material. The initial phase of this study involved experimental and numerical characterization of constant-thickness laminated UD-GFRP beams (ComBeam) produced using 300 Grams per Square Metre (GSM) prepreg material reinforced with continuous glass fibers and epoxy resin through thermal forming in a hot press. Before production, the cure kinetics of the resin system were analysed using High-Pressure Dynamic Scanning Calorimetry (HP-DSC). Resin samples were heated from 25 °C to 130 °C at a rate of 3 °C per minute and held isothermally at 130 °C for 30 minutes to simulate actual production processes. The process was repeated under varying pressure levels (1, 2, 4, and 6 MPa) to determine the optimal press pressure and temperature. No significant pressure effects were observed on small-scale samples. Although small coupon samples were produced for initial material characterization, they did not fully replicate the critical effects observed during actual part production, such as exothermic reactions, void formation, and fiber wetting issues. To address this limitation, flat specimens with dimensions approximating real part sizes (1,600 mm × 100 mm) and thicknesses of 52, 60, and 70 mm were produced. These specimens provided valuable insights into the challenges of manufacturing structural components for heavy commercial vehicles and formed the foundation for evaluating structural performance and production methodologies. During the production of flat thick beams, prototypes without sensors were fabricated in three different thicknesses, while additional prototypes with embedded sensors were produced in 70 mm thick beams. Fiber Bragg Grating (FBG) sensors and thermocouples (TCs) were embedded to monitor internal stresses and temperature distributions during production. FBG sensors offered high-precision strain measurements, while TCs provided valuable insights into the effects of exothermic reactions during the curing process. Despite the mould temperature being set to 130 °C, the central temperature in thick beams exceeded 200 °C, revealing significant temperature gradients within the structure. These findings informed the optimization of temperature control strategies and contributed significantly to the design of moulds for variable cross-section monoleaf springs. 520 3 Flat laminated PMC beams were tested using a custom three-point bending test setup designed to simulate the operating conditions of leaf springs. Strain variations were monitored using both embedded FBG sensors and surface-mounted strain gauges (SGs). Calibration of the FBG sensors was performed using SGs. The results revealed high strain levels near the surface of the central connection region (e.g., 13101 με) and unexpectedly high strain values near the neutral axis (e.g., 457 με). These findings were validated through finite element method (FEM) simulations incorporating Geometrical Nonlinear Analysis (GNA) to model large deformations, aligning well with experimental results. This validation process was crucial in understanding neutral axis shifts and the internal structural behaviour of thick specimens, providing valuable insights for the design and production of variable cross-section leaf springs. After completing the flat laminated UD-GFRP beam studies, the design and production processes of variable cross-section leaf springs began using 600 GSM prepreg material. The laying processes of prepreg were optimized to improve efficiency. To investigate the effects of exothermic reactions on mechanical properties, coupon samples were extracted along the cross-sections of prototypes and mechanically characterized. The results revealed a pronounced anisotropic structure within thick PMC components. To further enhance production efficiency, the study transitioned to using 1000 GSM prepreg material with 0.2% carbon nanotube-reinforced epoxy resin. Using this new material composition, FBG and thermocouple sensors were employed to measure temperature compensation and internal stresses during production. In addition, DSC based Tg measurements, fiber volume fraction determination, void analysis using optical microscopy, and X ray imaging were jointly evaluated. These analyses quantitatively revealed the mechanical property gradients that developed particularly in the thick central region due to thermal curing nonuniformity. Finally, leaf springs embedded with FBG sensors were subjected to real-time strain monitoring along the ComLeaf structure, enabling accurate mapping of strain distribution, neutral-axis movement, and stiffness variations under load. These tests provided detailed insights into the fatigue behaviour and long-term durability of the leaf springs. Real-time force-displacement data and FBG sensors support offered a critical advantage in early damage detection and service life prediction. The results demonstrated that the exothermic reactions observed during production had a significant impact on material behaviour, particularly on neutral axis shifts and internal stress distributions. This thesis provides a comprehensive framework for the design, production, and performance evaluation of PMCs. It has optimized the production processes of thick and variable cross-section leaf springs, establishing reliable production and performance criteria. The innovative materials and design strategies developed for heavy commercial vehicles offer lighter, more durable, and environmentally sustainable solutions, promoting the adoption of composite materials in the automotive industry. These results pave the way for future research and create new opportunities for broader-scale PMC applications.