Influence of the optical measurement technique and evaluation approach on the determination of local weld geometry parameters for different weld types

Martin Antonio René Garcia, Alain Nussbaumer
2019
Article

Résumé

The joining by welding processes of components is usually related to the creation of additional notches and geometrical peculiarities. Multiple investigations have shown that a clear correlation between the local weld geometry and the fatigue life of welded joints exist. Thereby, the local increase of the local stress can be expressed by a stress concentration factor at the transition from the base material to the filler material, the so-called weld toe. The stress concentration factor can be determined for the most weld types if the geometric parameters such as plate thickness weld toe radius and flank angle are known. However, no standardized method for the determination of these parameters exists. Beside the well-established 2D-measurement methods on cross sections with weld impression analysis, new 3D-methods based on contactless, optical measurement were applied in the last years for the geometrical analysis of welded joints. With these methods, long length of welds can be analyzed in a very short time and with low effort. However, the influence of the measurement system (geometrical accuracy, lateral resolution) was not quantified yet. Additionally, in all known cases of application different evaluation algorithms were used. This does not allow for a straightforward comparison of the investigated parameters and results. In this round robin study, the determination of weld toe radii and flank angles by different evaluation algorithms and 3D-measurement systems and by different institutes are compared. Furthermore, an approach for the direct determination of the stress concentration factors of fillet welds by translating the complex weld shape in a 2D-finite element simulation was implemented. The results of this direct approach are compared to the stress concentration factors determined indirectly using the geometric parameters and those calculated by established approximation formulas.

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https://infoscience.epfl.ch/record/273647?ln=fr

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Martin Antonio René Garcia, Alain Nussbaumer
2019
Article

Résumé

Official source

https://infoscience.epfl.ch/record/273647?ln=fr

À propos de ce résultat

Concepts associés (13)

Concepts associés

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Publications associées (24)

Publications associées

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Influence of geometrical imperfections and flaws at welds of steel liners on fatigue behavior of pressure tunnels and shafts in anisotropic rock

Alexandre Jean Pachoud

High-head steel lined pressure tunnels and shafts may be considered as critical infrastructures especially when rock overburden is small. In the case of lining failure, catastrophic damages can occur when a large amount of water can reach the steep valley slopes and induce dangerous mud and debris flow. In the last decades, high-strength steel has been used more frequently for such high-head pressure tunnels and shafts mainly for economic and construction reasons. Under the more and more rough operation conditions of storage hydropower plants, high-strength steels, which are generally thinner than lower grade steels, are more prone to fail by fatigue. Design guidelines for the application of high-strength steel for steel liners are still missing. With his research project, Dr Alexandre Pachoud made an important and novel contribution for the design of pressure tunnels and shafts considering the influence of anisotropic behavior of rock as well as the geometrical imperfections and flaws at welds on the fatigue resistance of the steel liners. For the first time, Dr Pachoud studied systematically the influence of rock anisotropy on the deformation of the lining system comprising steel liner, backfill concrete and near as well as far field rock mass. He performed an extensive parametric study with the finite element method (FEM) over a wide range of geometrical and material parameters. Normalized stresses and displacements were analyzed in the steel liner and the far-field rock mass and correction factors to be included in the analytical solution for isotropic rock conditions could be derived. For transversely anisotropic rock mass the analytical solution allows a simple and fast estimation of the maximum stresses in the steel liner by a correction applied to the isotropic analytical solution with a high accuracy. Based on extensive FEM simulations Dr Pachoud derived in a further step parametric correction factors, which allow estimating stress concentrations and structural stresses in steel liners considering geometrical imperfections. Dr Pachoud obtained also Stress Intensity Factors (SIF) for axial cracks in the weld material of the longitudinal joints by means of computational Linear Elastic Fracture Mechanics (LEFM) and could propose new parametric equations for the weld shape correction. For fatigue assessment, a probabilistic model for steel liner crack growth and fracture was developed by using the above mentioned new parametric equations for considering geometrical imperfections. Finally, Dr Pachoud illustrated the implementation of all the developed parametric equations in a probabilistic model for crack growth in the steel liner under dynamic loading with a case study for a high-head power plant. The probabilistic model allows determining the acceptable undetected initial crack sizes in the steel liner depending on the choice of the steel grade, which is crucial for engineering practice using high-strength steels for pressure tunnels and shafts.

EPFL-LCH2017

Chargement

Size effects in the fatigue behaviour of tubular bridge joints

Luís Costa Borges

A critical aspect in the design of tubular bridges is the fatigue performance of the structural joints. Economic viability depends on it. Lower fatigue strength for joints with thicker failing members was observed in welded joints typical to the bridge application. Different approaches to this phenomenon, called size effect, have been suggested, all based on the thickness correction for welded plate joints first proposed by Gurney. For the welded tubular joints, few studies on the size effects have been carried out; most of the existing investigations refer to geometries typical to petroleum industry offshore structures. In contrast to offshore structures, bridge structures have different absolute sizes and different member proportions (in particular lower chord radius to thickness ratios, γ). Tubular joints are far more complex than welded plate joints, multiple parameters are needed to describe the geometry (α, β, γ, τ, ζ) and there are several load scenarios. For these reasons, the fatigue behaviour analysis of such joints is a complex task. Current design recommendations combine the use of the structural (hot-spot) stress at the weld toe with a correction factor to take into account the wall thickness of the failing member. This approach oversimplifies the problem and can be very penalising, in particular for joints composed of thicker tubes, as is commonly the case for bridges. Furthermore, the truss member sizes that result from static design are likely to fall out of the validity range of current recommendations. This thesis focuses on a case of commonly used tubular joints: welded steel K-joint made out of circular hollow section (CHS). The main goals of this research are to understand the fatigue behaviour of as-welded CHS K-joints and to clarify the influences/effects of the different geometric parameters on their fatigue strength. In order to carry out a thorough study on the geometric size effects in CHS K-joints for bridges, fatigue tests were conducted for large-scale specimens with crack depth measurements and an advanced 3-D crack propagation model was developed. The first chapters of this thesis provide an introduction and a brief review of the main concepts in tubular joint fatigue and size effects on fatigue behaviour. The experimental tests of two tubular trusses under fatigue loading are then outlined. Crack growth in selected truss joints is monitored using the Alternating Current Potential Drop (ACPD) system. An advanced 3-D modelling of welded K-joint with surface crack is implemented using the boundary element method (BEM). A crack propagation model, based on Linear Elastic Fracture Mechanics (LEFM), is then developed using a step-wise incremental crack growth strategy. This model allows for fatigue strength and life estimations. Furthermore, it considers the influence of all geometric parameters that define CHS K-joints in a realistic way. The validation of the crack propagation model is made by comparisons with experimental data at different levels (i.e. member and joint strains and stresses, ACPD crack growth data). A parametric study is then carried out on joint geometries typical for a bridge application (low chord radius to thickness ratio) considering three basic load cases. Examples of results are shown and analysed on a "geometry cause"/"effect over the stress intensity factor and fatigue strength" basis. Parametric study results are then analysed, highlighting the case where the joint is proportionally scaled. The geometry correction factor, Y, is introduced as a function of the relative crack depth that is common to homothetic joints. The influence of the absolute size of the joint, also known as thickness effect, is determined for the three basic load cases. Parametric results are finally explored bringing to light the effect of non-proportional scaling. It is shown that size correction factors for fatigue strength can be expressed as a function of the non-dimensional geometrical parameters β, γ and τ, chord thickness, T, and different load cases. A new fatigue design method is proposed for welded (CHS) K-joints, based on LEFM and accounting for geometric size effects.

EPFL2008

Chargement

Metal fatigue during cyclic loading puts an endurance limit on most of today's technology. It impacts the reliability of metallic components used for transportation, electronic devices and energy production because fatigue failure can occur without any apparent forewarning. This phenomenon limits the lifetime of many industrially manufactured items, whose periodic replacement affects the cost of everyday products. The problem of predicting fatigue failure in metals has been addressed using the finite element method, which can provide the stress and strain state of the material in complex geometries. Engineering models describing the material behaviour have been used to predict the damage accumulation, but microstructure and material design require constitutive models at the micrometre length scale, incorporating the physical processes in the material. This involves the study of dislocation dynamics and the formation of dislocation structures, which have been recognized as the key phenomena affecting the macroscopic fatigue behaviour of metals. This problem is computationally challenging, first because of the large number of state variables required to describe the dislocation behaviour, and second because the formation of dislocation structures takes place only after many deformation cycles. In this project a continuum dislocation-based model, specific for cyclic fatigue at the micrometre length scale, is developed. The main novelty is the prediction of 3D dislocation structures starting from a random initial dislocation distribution. In single slip deformation, the characteristic length scale and shape of dislocation structures are predicted using only physical parameters, such as the stacking fault energy, and without any fitting procedure. The model is implemented in a crystal plasticity finite element solver, describing all the slip systems. Therefore, it is possible to model polycrystalline structures and to study the orientation of multiple slip dislocation structures with respect to the loading direction. Compared with existing models, the implementation of the dislocation junction formation mechanism in a continuum framework is a step forward. A collaboration with another PhD student, carrying out electron channeling contrast imaging experiments on fatigued 316 stainless steel, has provided a validation of the model by direct comparison of experimental and simulated dislocation structures close to the specimen surface. The crystal lattice rotation is another variable that can be extracted from finite element simulations. The developed model correlates the forming dislocation structures with the lattice rotation around the coordinate axes. This allows the comparison with Laue microdiffraction experiments carried out on copper single crystal specimens by another PhD student at the SLS, the synchrotron at Paul Scherrer Institut. The dislocation-based model can calculate experimental observables, such as the rotation components and rotation gradients, as a function of the simulated dislocation density in the specimen, which is not directly observable. This provides a validation of the constitutive equations at the micrometre length scale.

EPFL2016

Chargement

Size effects in the fatigue behaviour of tubular bridge joints

Luís Costa Borges

EPFL2008

EPFL2016

Influence of geometrical imperfections and flaws at welds of steel liners on fatigue behavior of pressure tunnels and shafts in anisotropic rock

Alexandre Jean Pachoud

EPFL-LCH2017