Accounting for the growing numbers of injuries, fatalities, and property damage, rear-end crashes are an urgent and serious topic nowadays. The vehicle number involved in one crash significantly affected the injury severity outcomes of rear-end crashes. To examine the transferability and heterogeneity across crash types (two-vehicle versus multi-vehicle) and spatiotemporal stability of determinants affecting the injury severity of freeway rear-end crashes, this study modeled the data of crashes on the Beijing-Shanghai Freeway and Changchun-Shenzhen Freeway across 2014–2019. Accommodating the heterogeneity in the means and variances, the random parameters logit model was proposed to estimate three potential crash injury severity outcomes (no injury, minor injury, and severe injury) and identify the determinants in terms of the driver, vehicle, roadway, environment, temporal, spatial, traffic, and crash characteristics. The likelihood ratio tests revealed that the effects of factors differed significantly depending on crash type, time, and freeway. Significant variations were observed in the marginal effects of determinants between two-vehicle and multi-vehicle freeway rear-end crashes. Then, spatiotemporal instability was reported in several determinants, including trucks early morning. In addition, the heterogeneity in means and variances of the random parameters revealing the interactions of random parameters and other insignificant variables suggested the higher risk of determinants including speeding indicators, early morning, evening time, and rainy weather conditions. The current finding accounting for spatiotemporal instability could help freeway designers, decision-makers, management strategies to understand the contributing mechanisms of the factors to develop effective management strategies and measurements.

Although there are lots of researches about the application of FRP composites on the external bonding and reinforcement of timber structures, they are mostly aimed at Carbon FRP, which has a large difference in mechanical properties and inconsistent deformation with timber. Meanwhile, research about the interface between timber and Flax Fiber Reinforced Polymer (FFRP) with similar properties with timber is limited; hence, relevant models have not been established yet. Similar to linear elastic FRP confined timber structure, the failure of FFRP-timber structure interface is a process in which the interfacial stress is gradually transferred from the loaded end to the free end, and the interfacial stress transfer area, that is an effective bond length of the FFRP-timber interface. In order to study the interfacial stress transfer characteristics between FFRP and timber structure, based on the existing CFRP-timber structure interface bonding theory and mechanism, a single lap shear test is used to conduct a systematic study on bonding length and width are studied to optimized the application of FFRP. The bond-slip behavior is studied and fitted to provide theory support for the application of FFRP in the restoration of ancient timber structures.

This paper based on the study of steel plate-fiber reinforced concrete coupling beam components, designs a basic model of composite coupled shear walls with steel plate-fiber reinforced concrete coupling beams. The seismic performance of this model was numerically simulated using the ABAQUS finite element software, analyzing the stress distribution of various parts and the development pattern of plastic hinges in the structure. It also examines how factors such as axial compression ratio, coupling beam cross-sectional dimensions, single-sided wall limb aspect ratio, and total floor height affect the seismic performance of this type of coupled shear wall, providing a reasonable range for axial compression ratios and coupling ratios for composite coupled shear walls with steel plate-fiber reinforced concrete coupling beams. Based on the obtained reasonable axial compression ratio and coupling ratio, and using the analytical solution of the continuum method, a method for controlling the seismic performance of coupled shear wall structures was proposed. Results show that the composite coupled shear walls with steel plate-fiber reinforced concrete coupling beams exhibit high load-bearing capacity and lateral stiffness under inverted triangular horizontal loads, with good displacement ductility and strong energy dissipation capability, making it an excellent seismic performance coupled shear wall system. As the axial compression ratio increases, the displacement corresponding to the yield point and peak point of the structure gradually decreases, and the displacement ductility coefficient shows a trend of first increasing and then decreasing; it is recommended that when designing steel plate-fiber reinforced concrete coupling beam-coupled shear walls, the axial compression ratio should not exceed 0.3. Numerical analysis of coupling ratio-related parameters indicates that in high-intensity seismic design areas, the range of coupling ratios should be between 45% and 60%.

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.

Fiber reinforced polymers (FRPs) have gained widespread application in civil engineering due to their light weight, high strength, and excellent corrosion resistance. Among various fibers, natural fibers have recently attracted increasing attention because of their renewability, low cost, and reduced environmental impact. However, natural fiber reinforced polymer composites (NFRPs) suffer from high moisture absorption and inferior mechanical properties, which are further deteriorated under hygrothermal environments, severely limiting their service conditions. This study focuses on flax fiber reinforced polymer composites (FFRPs) and, through systematic experimental testing, investigates their durability under hygrothermal conditions. Hygrothermal aging was conducted at 90% relative humidity and 40 °C for up to 30 days. Water uptake, mechanical properties, and thermomechanical behavior were evaluated for composites fabricated by two molding processes (autoclave and hand-laying) with varying ply numbers (10, 20, 30, 40, and 50 layers). The optimal ply number for mechanical performance was identified, and the influence of the different resin systems and fiber forms between the two processes on the cross-process comparison, as well as the rationale for the 30-layer laminate and the effect of laminate thickness, were clarified. Microstructural changes were examined via scanning electron microscopy. This work provides systematic experimental evidence and data for assessing the long-term durability of FFRP composites in humid and warm environments, offering valuable guidance for their practical application in civil infrastructure.

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.