Benjamin Launert

Residual stress influence on the flexural buckling of welded I-girders

Welded plate girders are used in heavy steel construction, industrial buildings and bride construction. Residual stresses are present in all plate structures. They are mainly caused by welding. In addiiton, they influence the load bearing capacity of these welded components. However, Eurocode (EC) does not provide any specific residual stress patterns for consideration of residual stress impact on load capacity. Hence, the decision for a particular problem has to be made by the designer. Many codes, including EC 3, permit the use of non-linear finite element analysis (FEA) for the design of structures. Recent developments in the last years, enabled the use of computerized models instead of laboratory experiments. In this scope, the FE-model should include all relevant factors properly. This important if considering that weld residual stresses can be a critical assessment factor. In addition, measuring of residual stresses is difficult, time consuming and expensive, it is therefore common to use founded distribution functions (e.g. Swedish BSK 99). Welding simulation tools offer new possibilities for a realistic assessment of weld-induced stresses and deformations. However, the modeling and the computational effort for large structural components is still not in a practicable range and a simplified methodology is in needed. As a result, a new approach (suitable for capacity analysis) is presented and detailed in the present contribution.

The buckling resistance of welded plate girders taking into account the influence of post-welding imperfections - Part 1: Parameter study

Abstract Welding is the most important joining technique and offers the advantage of customizable plate thicknesses. On the other hand, welding causes residual stresses and deformations influencing the load carrying capacity. Their consideration in the design requires simple and fast models. Though welding simulation has contributed to accurately access to these values nowadays, their application to large components remains still in a less practicable range. Nevertheless, many studies emphasized the need to make corrections in recently available simplified models. Especially the influence of residual stresses seems somewhat overestimated in many cases if comparing conventional structural steel S355 and high-strength steel S690. In times of computer-aided design, an improved procedure to implement weld-inducted imperfections appears overdue. This will be presented in two parts. The first part illustrates the potential influence of post-welding imperfections exemplified for weak axis buckling in comparison with the general method in accordance with Eurocode 3. Residual stresses and initial crookedness were varied systematically in order to produce a scatter band of capacities. An approach to characterize the borders of these imperfections was untertaken before that. The excessive scattering of reduction factors for the load bearing capacity demonstrates the importance of these variables. Results were finally evaluated against advanced simulation models which will be further detailed in part two of this contribution.

Combining sectioning method and X-ray diffraction for evaluation of residual stresses in welded high strength steel components

Residual stresses and distortions in welded I-girders for steel construction are relevant when evaluating the stability of steel beams and column members. The application of high strength steels allows smaller wall thicknesses compared to conventional steels. Therefore, the risk of buckling has to be considered carefully. Due to the lack of knowledge concerning the residual stresses present after welding in high strength steel components conservative assumptions of their level and distribution is typically applied. In this study I-girders made of steels showing strengths of 355 MPa and 690 MPa were welded with varying heat input. Due to the dimension of the I-girders and the complex geometry the accessibility for residual stress measurement using X-ray diffraction was limited. Therefore, saw cutting accompanied by strain gauge measurement has been used to produce smaller sections appropriate to apply X-ray diffraction. The stress relaxation measured by strain gauges has been added to residual stresses determined by X-ray diffraction to obtain the original stress level and distribution before sectioning. The combination of both techniques can produce robust residual stress values. From practical point of view afford for strain gauge application can be limited to a number of measuring positions solely to record the global amount of stress relaxation. X-ray diffraction can be applied after sectioning to determine the residual stresses with sufficient spatial resolution.

Application of the stochastic finite element method in welding simulation

Due to the uncertain microscopic structure of the material, the strength of the material exhibits strong randomness. This randomness results in uncertain response of the structure in the sequentially coupled thermal-mechanical analysis by welding simulation. Because of the limitations of deterministic welding simulation, the stochastic finite element method with random field will be introduced into the welding simulation, so that the welded structure can be more accurately calculated in the stability and reliability structural analysis. Particularly, it is necessary to propose reasonable distributions of residual stress from welding simulations based on statistical and reliability theories. This paper is intended to implement the stochastic finite element method in the welding simulation using a general-purpose simulation program and to demonstrate the potential of the proposed approach. Furthermore, the statistical distribution function of the welding simulation response is obtained by maximum entropy fitting method. Then, a numerical example is presented by the proposed method.

Strength Calculation of Stiffened Structures Taking Into Consideration Realistic Weld Imperfections

The topic of this article is the application of an analytical numerical hybrid model for a realistic prediction of imperfections induced by welds. At the beginning, the analytical model, its physical basis as well as the physical interrelationships are explained. This is followed by the explanation of the coupling procedure between the analytical model and the numerical calculation. Afterwards, the coupled hybrid model is applied on the investigated stiffened curved structure for the determination of the weld imperfections. An ultimate load analysis gives information about the load carrying behavior under axial loading. The results are compared against the traditional approach using eigenmode-based imperfections. The comparison underlines the potential additional utilization of load bearing capacity by this new approach. Introduction The strength calculation of stiffened plates by the finite element method (FEM) has been part of the state of the art for a long time. Geometrical nonlinearities as well as the nonlinear material behavior are considered within the calculation. To simplify, both types of imperfections, geometrical and structural ones, are mostly combined in these strength calculations being considered as equivalent geometrical imperfections. Values for standard cases are included in EN 1993-1-5 in case of plated structures or slightly curved panels [1]. Because there are no specific rules for considering imperfections and its scaling in a load capacity calculation of stiffened curved panels they may be also assumed according to EN 1993-1-5 [2,3] as a first approach. However, it remains unclear to some extent how accurate these geometrical imperfections represent the actual residual stresses and deformations caused by welds, especially for more complex cases. The significance of numerical load capacity calculations could be increased enormously if these imperfections were known more exactly and could be considered directly during the computation. Nowadays, the residual stresses and deformations can be determined by means of a thermomechanical FE simulation achieving quite realistic values. However, relevant structures and weld length are very large what leads to enormous calculation time and a huge demand of storage capacity [4]. Simplified numerical approaches are available and able to remedy this situation. However, the application of these models partly demands more expertise than a conventional thermomechanical FE calculation [5] or the simplifications are so extensive that the weld imperfections calculated by the approach partially lose their validity [6]. In order to be able to take weld distortions and residual stresses directly into account in a load capacity calculation, Residual Stresses 2018 – ECRS-10 Materials Research Forum LLC Materials Research Proceedings 6 (2018) 245-250 doi: http://dx.doi.org/10.21741/9781945291890-39 246 fast but still sufficiently accurate procedures that, at the same time, are easy in their application are essential. The Coupled Analytical Numerical Hybrid Model The basic idea of the coupled analytical numerical hybrid model [7] is the linking of the major advantages of both, analytical and numerical procedures. On the one hand, the matchless very short calculation time of the analytical shrinkage force model and its simple application, and on the other hand the possibility to conduct a FE simulation to calculate stresses and distortions at any location of complex welded structures. According to this, all the determining factors on quality and quantity of weld imperfections are passed to an analytical calculation program, capturing the mathematical approach of the shrinkage force model. The output is a mechanical load and the point of action in longitudinal and transversal direction, equivalent to the heat effect of welding. The loads are then applied to the FE model of the weld structure and the distortions and stresses are calculated by a nonlinear elastic calculation. The influence of the weld sequence on the arising weld imperfections is captured by a back coupling. The numerically calculated stresses in the regarded weld caused from a previous weld are submitted to the analytical calculation. The results of the application of the hybrid model are subsequently superposed with additional fabrication tolerances followed by the load capacity calculation, Fig. 1. Figure 1: Scheme of the Load Capacity Calculation Taking Into Consideration Realistic Weld Imperfections. Weld imperfections depend significantly on the maximum temperatures that every point perpendicular to the weld direction is exposed to and the stiffness of the structure. Equations for the calculation of the maximum temperatures were derived by Rykalin [8] constituting the basis of the shrinkage force model. For calculating a force alongside the weld, Okerblom [9] considered the border case of a line source in a rigid thin plate and integrated the thermal strains over the zone of plastic deformations:

Welding of Thick Plates under Site Conditions–Evaluation of the Influence on the Structure Behaviour of Welded Assembly Joints

Construction site welds, as usually found e.g. in bridge or steel structures, are made in contrast to factory production under difficult conditions (Fig. 1). Construction welds site provide not only special requirements for the executive operating but also to the designer. There are decision support tools in the form of guidelines nowadays available. Important for the design engineer is the knowledge of the occurring welding shrinkage of the parts, the distortion and residual stresses. The construction of large steel structures is now been increasingly modular factory production. Joining these modules at the site to an overall structure is still necessary. For welding of thick plates multi-layer welding are used (Fig. 2). In previous research on the welding of thick plates and their simulation constant laboratory conditions were used as boundary conditions during the experimental welds examinations. The systematic studies on welding of thick plates under site conditions will be presented for the complete temperature range.