Flax Fiber Reinforced Polymer (FFRP), as a green material with nonlinear large deformation characteristics, is used in the reinforcement of timber structures. Due to the similar elastic moduli of FFRP, adhesive, and timber, stress concentration at the interface is significantly reduced, demonstrating favorable interfacial performance. This study investigates the effects of adhesive layer thickness and FFRP laminate thickness on the strain distribution, bond-slip relationship, and stress distribution at the FFRP-timber interface through two different types of single-lap shear tests, thereby revealing the bonding mechanism at the FFRP-timber interface. The results show that both the ultimate load and the ultimate strain at the loaded end decrease with increasing adhesive thickness. For instance, increasing the adhesive thickness from 0.5 mm to 3 mm led to a 68.6% reduction in peak interfacial shear stress. The thickness of the adhesive has a minor influence on the overall trend of the bond-slip relationship curve for the FFRP-timber interface, with the curve consisting of an ascending branch, a descending branch, and a horizontal plateau. The distribution patterns of interfacial shear stress for different adhesive layer thicknesses are similar: at the initial loading stage, the maximum shear stress appears at the loaded end and gradually decreases toward the free end; as the load increases, the peak shear stress shifts from the loaded end toward the free end. With an increase in the number of fiber layers in the FFRP laminate, the strain transfer efficiency first increases and then decreases, reaching its maximum when the number of fiber layers reaches 30. The maximum stress increases with the number of FFRP fiber layers, and the stress transfer efficiency peaks at 30 layers.

This paper introduces a beam–column joint with an L-shaped component formed by combining two traditional semi-rigid connection methods, namely top and bottom angle steel connections and end-plate connections. Test studies and finite element simulations are performed on two designed specimens (LJD-1 and LJD-2). The reliability of the finite element model is confirmed, and the hysteresis curves are found to be relatively full, indicating excellent energy dissipation capacity. Moreover, it is revealed that the larger the joint length, the better the improvement in load-bearing capacity and ductility. Based on this, a finite element simulation is conducted considering the joint configuration, thickness of the vertical and horizontal plates, length of the horizontal plate, thickness of the end-plate, and the number of haunches. The optimal ranges for each parameter are determined.

The strength of flax fiber-reinforced polymer (FFRP) composites is a critical factor influencing their reliability in structural applications. However, the prediction of FFRP strength remains challenging due to the nonlinear stress–strain relationship of flax fibers. To address this limitation, this study proposes a three-dimensional progressive failure model (PFM) based on the fiber bundle chain method, with a specific focus on investigating the influence of size effects on the model's predictive performance. The PFM was constructed by generating a representative volume element (RVE) in which each fiber element is assigned an independent tensile strength, $${\sigma }_{\text{p,q}}^{\text{u}}$$ , using Weibull statistical theory, enabling the model to simulate progressive damage in unidirectional composites. To enhance the tensile performance of FFRP, flax fibers were surface-modified via grafting with multi-walled carbon nanotubes (MWCNTs). The PFM was subsequently employed to numerically simulate the tensile behavior of unmodified FFRP, MWCNT-grafted flax fiber composites, carbon–flax hybrid composites, and basalt–flax hybrid composites. The results demonstrate that for the MWCNT-grafted flax fiber composites, the PFM coupled with Zhou's stress concentration factor (SCF) calculation model predicted a tensile failure strain of 1.26% and a tensile strength of 406.8 MPa. Compared with experimental data, the relative errors were 5.9% and 10.3%, respectively, indicating that the PFM can effectively predict the tensile stress and strain of the aforementioned composite systems. Furthermore, the model elucidates the influence of size effects: as the cross-sectional dimensions of the RVE increase, or as the total length of the RVE and the length of fiber elements increase, the tensile strain and stress predicted by the PFM exhibit a decreasing trend. Therefore, the proposed PFM is suitable for simulating the tensile properties of a wide range of composite materials.