With the improvement of computational capability, finite element simulation is an increasingly practical method to accurately predict the ultimate capacity and ductile fracture behavior of high-strength bolts. However, the mesh size affects the results of FE simulations but related research on mesh size effects is relatively limited. In the present contribution, the mesoscale critical equivalent plastic strain (MCEPS) is used as a failure index for calibrating the parameters of ductile fracture locus of high-strength bolts with different mesh sizes. The identified fracture locus is compared with a large bulk of experimental data taken from the previously published literature. The results showed that mesh size can have high effects on the calibrated parameters of the plastic constitutive relationship after necking and ductile fracture locus of high-strength bolts.@en

Bolted joints in steel structures are generally subject to combined actions. The research on ultimate capacity of bolted joints under combined tension and shear actions, twin shear actions, and combined tension-twin shear actions is relatively limited. The aim of this paper is to calibrate the fracture locus of grade 10.9 bolts using the mesoscale critical equivalent plastic strain (MCEPS). We use ductile fracture simulation to numerically evaluate the ultimate resistance of bolts subject to multiaxial loading and compare the results with the current design standards. The results show that predictions for grade 10.9 bolts subject to multiaxial loading in Chinese code GB 50017 and American code AISC-360 are not on the safe side, while predictions in European code EN1993-1-8:2005 are slightly on the conservative side. We propose two modification factors to improve prediction of bolt strength subject to multiaxial loading; namely the multiaxial loading factor for shear resistance ξv and the multiaxial loading factor for tensile resistance ξt.@en

A welded rectangular hollow section (RHS) X-joint exposed to tension loading has three typical fracture-related failure modes: Punching shear failure (PSF), Brace failure (BF), and Chord side wall failure (CSWF). Prediction of these failure modes by finite element (FE) simulations requires modelling of the material damage. An appropriate damage model accurately predicts the behaviour of the fracture zone and provides the necessary information to improve design rules for welded high-strength steel (HSS) RHS X-joints based on parametric studies using validated model. In this paper, the parameters of the Gurson-Tvergaard-Needleman (GTN) damage model are calibrated for the base material (BM) and the heat-affected zone (HAZ) of butt-welded cold-formed RHS connections, no fracture appeared in the weld. A computational homogenisation analysis is carried out using representative volume element (RVE) models to calibrate the pressure-dependent yield surface parameters of the GTN damage model, considering the different combinations of the accumulated initial hardening strain and the void volume fraction (VVF) due to a varying stress triaxiality. The critical and final VVFs are calibrated against tensile coupon tests. Finally, the GTN damage models calibrated for BM and HAZ are used in the fracture simulation of nine welded cold-formed RHS X-joints in monotonic tension. The FE model successfully predicts the experimental load-displacement relationships and fractured zone, indicating the calibrated GTN models could effectively be used in parametric study of welded cold-formed RHS X-joints. Finally, possible improvements to the used FE model are outlined for future studies.@en

Accurately obtaining the material parameters of the damage model is very important for the ductile fracture simulation of steel structures made of high strength steel (HSS). The combination of micromechanics with the uncoupled phenomenological fracture model can be used to obtain the material parameters of ductile fracture locus only from the uniaxial engineering stress-strain relationship. A micro failure index is generally used to link the material fracture under different multiaxial stress status. It is relatively difficult to describe the irregular micro-void evolution using the microvoid radius, and also difficult to tackle the micro void coalescence of the mesoscale using microvoid volume fraction. The mesoscale critical equivalent plastic strain (MCEPS) is proposed as the failure index at the unit cell level to calibrate the fracture locus of the uncoupled phenomenological model in this paper. The fracture locus of the HSS is calibrated by comparing the FE results with experimental data using the homogenized MCEPS and maximum MCEPS at the microvoid surface, respectively. The identified fracture locus is further validated against five stress status, including butterfly shear specimen, butterfly tension specimen, the tensile specimen with the symmetric round notch, tension specimen with a central hole in the middle, and Sandia fracture challenge specimens in 2014. The comparisons of predictions with the experimental results showed that the proposed MCEPS index can be used to obtain the material parameters of the ductile fracture locus.@en

Strengthening fatigue damage of orthotropic steel decks (OSDs) needs to comprehensively consider the strengthening effect and the dead weight introduced during the strengthening process, especially for OSDs in super large-span bridges or old renovated bridges, where the dead weight cannot be significantly increased during repair and maintenance. This paper proposed a new CFRP prestressed reinforcement method, that does not significantly increase dead weight while effectively inhibiting fatigue crack growth. The strengthening effects of various strengthening methods on the fatigue crack propagation of rib to deck joint in OSDs were compared. Besides, the strengthening effects of different crack sizes were also analyzed. The results showed that the proposed method is effective in reducing fatigue damage at the rib to deck joint in OSDs.@en

The ductile performance of high strength steel (HSS) from different steel grades, producers, manufacturing processes varies a lot. It is also difficult to conduct all kinds of reliable experiments to generate different stress status through different initial specimen geometries or by applying different load combinations in the civil engineering sector. One of the common issues for HSS structures is to identify the parameters of the ductile fracture model conveniently from the uniaxial stress–strain relationship obtained from common coupon specimens. An attempt is made in this paper to use the proposed mesoscale critical equivalent plastic strain (MCEPS) to calibrate the fracture locus of the uncoupled phenomenological model based only on the engineering stress–strain relationship. The proposed method is successfully validated by the Sandia fracture challenge in 2014.@en