Numerical Optimisation of Structural Behaviour of Hollow Box Pulwound Fibre Composite Profiles

Numerical Optimisation of Structural Behaviour of Hollow Box Pulwound Fibre Composite Profiles

Hollow box Pultruded Fibre-reinforced Polymers (PFRP) profiles are increasingly used as structural elements in many civil infrastructure applications due to their cost-effective manufacturing process, excellent mechanical properties-to-weight ratios, and superior corrosion resistance. These structural composite elements manufactured by the pulwinding technology are governed by layup parameters (winding angle, axial-to-wound fibres ratio, and stacking sequence) and geometric parameters (wall slenderness, cross-sectional aspect ratio, and corner radius). However, there is still a lack of knowledge and guidelines in the design for manufacturing against local buckling, which deprives this novel construction material of a large market share compared to conventional construction materials. Investigating these design parameters and their interactions under structural loadings is going to enhance the current standards, provide reliable and economic design guidelines, and optimise the current designs for hollow box PFRP profiles. Therefore, this research investigated the local buckling behaviour of hollow box PFRP profiles under different load applications (compression and bending) and facilitated practical design guidelines for the manufacturing parameters of hollow box PFRP profiles to optimise their structural performance against local buckling.

First, experimental and numerical studies were undertaken under axial compression to characterise the local buckling of hollow box PFRP profiles and compare it to the compressive behaviour of hollow circular PFRP profiles. A numerical modelling approach was developed to simulate the local buckling, post-buckling, and progressive failure of hollow box PFRP profiles using the Finite Element Method (FEM). This approach used the Newton method along with the adaptive automatic stabilisation scheme and a controlled increment size in Abaqus 2019, to overcome the numerical difficulties in simulating local buckling. The numerical predictions were validated against the experimental data. The energy parameters and the constituent failure modes of the FEM models were used to explain the effect of dimension, layup, and slenderness ratio on the post-buckling behaviour and failure modes of hollow PFRP columns.

Secondly, the effect and contribution of the layup and geometric parameters were investigated under axial compression. The developed numerical approach based on FEM was used to perform an extensive parametric study of these parameters. Each geometric parameter was studied individually to obtain the failure map of hollow PFRP stub columns and to assess the applicable levels for each parameter in the interactive study. A full factorial design of experiment was applied to capture the critical parametric interactions with over 135 numerical models. The corner (flange-web junction) geometry was the dominant design parameter in shaping the compressive strength of hollow box PFRP profiles. Supporting this critical zone obtained more reliable and economic designs. Guidelines and recommendations on the design for manufacturing were derived for the optimal compressive behaviour of hollow PFRP profiles to overcome local buckling and achieve material compressive failure.

Thirdly, a combined experimental and numerical methodology was used to investigate the failure modes of hollow box PFRP profiles under four-point bending. Two different profiles, each with 10 samples, were tested until failure and were used to validate the numerical model. The previous FEM approach was extended to suit flexural loading and reduce the computational cost. The validated model was used to study the failure sequence thoroughly and perform an extensive parametric study on the design parameters. Each geometric parameter was studied individually first to determine the relevant levels for each parameter in the full factorial study. A full factorial design of experiment was used to capture the critical parametric interactions with over 81 numerical models. The design rules and recommendations were established for the optimal flexural behaviour of hollow box PFRP profiles to withstand the local buckling of the top flange.

Finally, a fast-converging numerical approach combining the Finite Element Modelling (FEM) and the Genetic Algorithm (GA) was implemented to design the optimal configuration of the geometry and layup design parameters against local buckling under different structural loadings (compression and bending). The objective of the mixed-integer nonlinear-constrained optimisation problem was to minimise the manufacturing cost per metre of pultrusion while maintaining the same stiffness and strength properties of the control profile. The Kriging model, which is a geostatistical prediction tool capable of handling such design problems, was used to interpolate the design space based on the intermediate optimisation data output and produce a practical design chart linking the profile geometry to the local buckling capacity. An experimental case study on the design of a hollow rectangular PFRP girder demonstrated the proposed optimisation approach. The new design saved 10.6% of the cost per metre of pultrusion and enhanced the local buckling strength by 41%.

This research resulted in a comprehensive understanding regarding the design for manufacturing of hollow box PFRP profiles. The effect and significance of each design parameter on the structural behaviour of hollow box PFRP profiles were studied and analysed. This study outlines the importance of the interactions in obtaining optimised, economic, and reliable designs of these profiles and broadening their use in civil infrastructure applications.

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