Investigation of technological size effects of welding on the residual stresses and fatigue life of tubular joints made of structural steels S355 and S690

The fabrication procedure changes when changing the size of the structural components. For the weldments, the change of the welding procedure leads to the change in residual stresses which consequently affects the fatigue life of the structure. The weld size and number of welding passes are important factors which have considerable impact on the values and distribution of the welding residual stresses. This technological size effect is investigated in this study by simulating the welding process for tubular joints made of S355 and S690 structural steel. Two types of joints are studied: circumferential weld and planar K-joint. While rapidly developing, coupled thermo-mechanical analysis of welding is a powerful and versatile tool for the study of the development of thermal strains and stresses cause by welding process. In this study, both multi-pass and single-pass procedures with their corresponding simulation parameters are investigated. The resulted residual stress field when superposed into applied stresses at the joint in a crack propagation analysis yield more realistic crack pass and crack propagation life estimation.

Residual stresses in as-welded joints: finite element modeling and neutron diffraction stress measurements

Claire Acevedo, Jean-Marie Drezet, Alain Nussbaumer

This paper describes the numerical analysis method used to estimate welding induced residual stresses in K-shape tubular bridge joints. The knowledge of residual stress distribution is required to design the geometry of K-joints loaded under fatigue stresses. Numerical simulations are focused on the arc welding MAG process, generally used to weld joints in bridge construction. Thermomechanical analyses are performed in 3D using two finite element codes: ABAQUS® and MORFEO®. ABAQUS has the advantage to offer large analysis capabilities (nonlinear, transient, dynamic, etc.) whereas MORFEO is more dedicated to welding processes and offers the possibility to analyze crack propagation under fatigue loads. Computed residual stresses in the region surrounding the weld are compared with measured residual stresses in order to estimate the ability of the codes to reproduce these stresses. Position, orientation and magnitude of the highest residual stress components are discussed.

Trans Tech Publications2012

Through-thickness performance of adhesive connections between FRP bridge decks and steel main girders

Martin Schollmayer

FRP bridge decks offer several advantages compared with conventional concrete bridge decks, particularly their much lower weight, but also their resistance against corrosion as well as easier installation and maintenance. The poorly conceived methods available for connecting FRP bridge decks with their supporting structures – normally steel girders – nonetheless constitute a disadvantage. Connections involving studs or bolts are not appropriate in this case, since FRP is a very brittle material that offers no ductile properties. Bolted connections usually result in much higher stress concentrations, while adhesive bonding is a more material-adapted connection method since larger surfaces can be linked together, thus ensuring reduced stresses. The bridge system investigated in this thesis consists of a pultruded FRP bridge deck bonded to steel main girders, the bridge's main structural components, which have to transmit the dead and traffic loads to the supports, whereas the bridge deck is spanned in the transverse direction perpendicular to the steel girders. Uplift forces caused by the load-bearing behavior of the bridge deck transverse to the bridge axis lead to through-thickness tensile stresses in the adhesive joint. The main objective of this thesis is the description of the structural behavior in the transverse direction. This includes analysis of the stresses in the adhesive connection as well as determination of the strength of the joints. Analytical and experimental investigations were carried out. It was shown, that the tensile stress distribution in the adhesive joint is non-uniform with high stress concentrations below the FRP deck webs of the cellular deck and above the steel girder web. Alternately inclined deck webs thereby induce significantly higher stresses below the vertical webs. A method for the calculation of the stress state in the adhesive layer is proposed which is validated by numerical and experimental results. The material strength of the connection in terms of a combination of tensile through-thickness and shear stresses is established. The total safety factor of the joint was higher than the safety factor of the FRP deck for bending between the main girders. A possible failure process would not start in the adhesive connection between bridge deck and steel girder which eventually could lead to additional failure of other structural members. The system is redundant. Fatigue loading up to 10 million cycles showed no stiffness degradation. The results of this thesis prove the existence of a good load-bearing behavior under static and fatigue loads of adhesively-bonded joints between pultruded FRP bridge decks and structural steel girders, where the adhesive connection is loaded with uplift forces and moments acting in the bridge deck, in addition to the shear in the connection layer due to composite action. The basis for a design method for adhesively-bonded connections between pultruded FRP bridge decks and steel girders is provided.

EPFL2009

Influence of Residual Stresses on Fatigue Response of Welded Tubular K-Joints

Claire Acevedo

Seeking light and transparent bridge designs, engineers and architects have found an efficient and artistic way to fulfill their requirements: steel tubular bridges. In these modern tubular truss bridges, welded K-joints have been shown to be critically susceptible to fatigue failure induced by repeated traffic loads. Despite the research devoted to the study of fatigue during the last 35 years, especially in the offshore oil industry, estimating their fatigue resistance of tubular truss bridges is difficult and solutions are approximate. This is due to the complexity of the parameters influencing bridge fatigue strength, including: 1) complex loadings, 2) applied stress concentrations in joints, 3) welding imperfections, 4) welding residual stresses, and 5) size effects. Some design rules and studies are available to estimate these effects; however, residual stresses in tubular K-joints remain unknown as well as their influence on fatigue crack propagation. Residual stresses are well known to have detrimental influence on fatigue crack propagation in other metallic structures, and further study is a necessary for improving the understanding of K-joint fatigue behaviour. To address this problem, the main objective of this research is to assess, experimentally and numerically, welding residual stresses and their influence on the fatigue response of K-joints. In this thesis, experimental measurements of the residual stress fields were investigated through neutron-diffraction, hole-drilling and X-ray methods. Results have shown that, principal residual stresses are oriented transversely, i.e. perpendicularly to the weld direction, both at and near the surface. Residual stresses can reach the S355 steel yield strength at the weld toe. This orientation is particularly detrimental for fatigue because it is also the orientation of the principal applied stresses, and as a consequence high residual and applied stresses superimpose. Moreover, it is proved that a restraining effect occurs in the gap region in-between the braces remaining the critical residual stresses high in the gap region. Two large scale tubular truss beams were subjected to high-cycle fatigue in order to assess the effect of residual stresses on fatigue response of welded tubular K-joints. These tests revealed that tensile residual stresses do not affect the fatigue crack growth in details loaded in tension (hot-spot 1), whereas they play a significant role in details loaded in compression (hot-spot 1c). Tensile residual stresses enable crack opening, making the fatigue cycle partly or entirely effective to crack propagation. Based on these tests and previous tests from the ICOM test database, it is recommended to design all (tension or compression) K-joints using a strength curve Srhs Srhs – Nf category 100 (for T = 16 mm). A thermo-mechanical finite element model was also developed to numerically reproduce the residual stress creation (using ABAQUS and MORFEO codes). Once the numerical results were validated by comparison with experimental results, the finite element model was used to model other K-joint geometries. A geometrical parametric study was performed to identify the most influential parameters affecting the residual stress distribution (technological size effect). It is found that a raise of the wall chord thickness T (proportional scaling) or of the wall thickness τ = t/T (non-proportional scaling) strongly increases the magnitude of tensile transverse residual stresses. Based on this parametric study, residual stress distributions with depth for transverse, longitudinal and radial directions are proposed. Finally, an extended finite element method (X-FEM) model was used to propagate a fatigue crack under residual and applied stresses. An analytical model based on the effective stress intensity factor range ΔKeff (fracture mechanics) and combined with the proposed distribution for residual stresses was also established to predict fatigue crack propagation in joints loaded in tension and in compression.