This study investigates stress-based fatigue assessment methods to determine their applicability to welded cover plate joints subject to axial and bending loading. The nominal stress (NS), hot spot stress (HSS), 1 mm stress (OM), effective notch stress (ENS), theory of critical distances (TCD), and stress averaging (SA) methods are covered, and their accuracy and reliability are evaluated. To the best of the authors' knowledge, there is limited fatigue test data available for cover plate joints subjected to bending loading. In this study, fatigue tests are performed with cover plate joints under axial and bending loading. Evaluation of the fatigue assessment methods is based on the test results. It is observed that for axial loading, the ENS and OM method have the highest accuracy. For bending, the OM method is non-conservative, and the other methods are overly conservative. Using design curves recommended for thin-walled welded joints subjected to bending highly improves accuracy.

This thesis work is concerned with the mechanical response of welded steel structures, which are distortions, residual stresses, and fatigue. The accuracy of fatigue assessment, distortion and residual stress analysis using Computational weld mechanics (CWM) is focused. The following studies are performed; welding simulations of residual stresses and distortions, weld quality estimation, evaluation of local stress-based fatigue strength assessment methods.

The following CWM methods: thermo-elastic-plastic, inherent strain (local-global), and sub structuring have been implemented both on T-type fillet weld and butt weld specimens to estimate angular distortion and residual stresses. In regard to large welded structures, the CWM methods using lumping method, together with prescribed temperature method, is implemented on a welded box structures to estimate welding residual stress state. The welding distortion analysis has been carried out on a large full scale beam structure experimentally and numerically using CWM techniques such as the inherent strain (local–global) method and the shrinkage method, together with the lumping approach. The accuracy of CWM methods is studied, and improvements are proposed.

A probabilistic model is proposed to estimate the probability of a targeted weld penetration depth. The uncertain process parameters are voltage, current, travel speed, and torch travel angle which were studied based on an experimental investigation. The weld penetration depth is evaluated from macrographs using a digital tool developed in MatLab. The epistemic measurement uncertainty related to this evaluation is quantified and incorporated in the probabilistic model.

Monte Carlo simulation is implemented to consider the weld geometry variations in the ENS methods. The stochastic variability in toe radius, toe angle and leg length are considered. The influence of weld geometry variations on the ENS methods using deterministic and stochastic SCF distribution is assessed.

The Hot Spot Stress (HSS), 1-mm stress (OM), Theory of Critical Distances (TCD), Stress Averaging (SA), and Effective Notch Stress (ENS) methods are evaluated for cover plates and T-joints subjected to axial and bending loading, in terms of accuracy and reliability. The evaluation is based on fatigue test data extracted from the literature and carried out in this study. The fatigue design curves applicable for T-joints under bending are discussed, which can be used in the TCD method and SA method.

The studies above contribute to increasing the accuracy in the estimation of residual stresses and distortions using simplified CWM methods, achieving a required reliability level in manufacturing, and improving accuracy and reliability of local stress-based fatigue assessment methods.  

This study aims to find suitable fatigue assessment methods for welded structures (cover plates and T-joints) subjected to axial and bending loading. The Hot Spot Stress (HSS), 1-mm stress (OM), Theory of Critical Distances (TCD), Stress Averaging (SA), and Effective Notch Stress (ENS) methods are evaluated in terms of accuracy and reliability. The evaluation is based on fatigue test data extracted from the literature and carried out in this study. It is found that the SA method can be used to assess the fatigue strength of cover plate joints under axial loading with relatively good accuracy and low scatter, followed by the ENS method. The HSS, TCD, SA, and ENS methods are conservative estimation methods for T-joints under bending, while the accuracy is low. Furthermore, fatigue design curves applicable for T-joints under bending are discussed, which can be used in the TCD method and SA method.

Idealized geometry is typically used in standards for the fatigue life assessment of welded joints. In the presence of stochastic geometrical variations along the weld, the choice of the idealized geometry is however ambiguous. In the notch stress (NS) method with a fictitious notch radius rref = 1 mm, the FAT 225 curve is derived for welds with relatively good quality in toe profiles. In the NS method with rref = ractual + 1 mm, a lower FAT 200 curve is recommended. Both approaches neglect the stochastic variability in toe radius, toe angle and leg length along the weld. The aim of this paper is two-fold. First, a numerical comparison between both approaches in terms of their predicted fatigue life is performed for a non-load carrying fillet cruciform joints. The results show that the NS method with rref = 1 mm and FAT 225 is substantially more conservative. Second, these methods are enhanced by replacing the deterministic stress concentration factor by a probability distribution computed using Monte Carlo simulation. It is shown that NS with rref = 1 mm and FAT 225 does not predict any substantial influence of the stochastic variability in process parameters since the actual toe radius is not included in the analysis. However, the NS method with rref = ractual + 1 mm and FAT 200 predicts a decrease in fatigue life when uncertainties in geometrical parameters is included. This numerical study paves the way for an experimental validation of the predicted influence of stochastic variability of geometrical parameters based on the stochastic notch stress analysis.

Unwanted distortions are typically observed in components after the welding process. Physical trial tests and extra post-treatments are being widely utilized in industries to minimize and correct the out of tolerance distortions. These methods are time-consuming and costly. There has been growing interest in digital tools which have great potential to minimize the physical test loops and corrections. In this study welding distortions analysis has been carried out on a large beam structure experimentally and numerically using computational welding mechanics (CWM) techniques such as the inherent strain (local?global) method and the shrinkage method, together with the lumping approach. The estimated distortions from the shrinkage together with lumping approaches were in good agreement with the experimental measurements and the computational time affordable. The inherent strain (local?global) method captured the trend of distortion with an underestimation of distortions. The accuracy of the estimated residuals stresses from the inherent strain (local?global) approach is higher than the one from shrinkage together with lumping approaches. Moreover, the effects of various welding process parameters (i.e. welding sequence, fixture, and weld pool size) on welding distortions were investigated. It is found that following the proper welding sequence could minimize the welding distortion of the beam structure. Increasing the constraints of fixtures can prevent welding distortion effectively and reducing weld pool size results in less welding distortions of the beam structure.

In welded structures using robotized metal active gas (MAG) welding, unwanted variation in penetration depth is typically observed. This is due to uncertainties in the process parameters which cannot be fully controlled. In this work, an analytical probabilistic model is developed to predict the probability of satisfying a target penetration, in the presence of these uncertainties. The proposed probabilistic model incorporates both aleatory process parameter uncertainties and epistemic measurement uncertainties. The latter is evaluated using a novel digital tool for weld penetration measurement. The applicability of the model is demonstrated on fillet welds based on an experimental investigation. The studied input process parameters are voltage, current, travel speed, and torch travel angle. The uncertainties in these parameters are modelled using adequate probability distributions and a statistical correlation based on the volt-ampere characteristic of the power source. Using the proposed probabilistic model, it is shown that a traditional deterministic approach in setting the input process parameters typically results in only a 50% probability of satisfying a target penetration level. It is also shown that, using the proposed expressions, process parameter set-ups satisfying a desired probability level can be simply identified. Furthermore, the contribution of the input uncertainties to the variation of weld penetration is quantified. This work paves the way to make effective use of the robotic welding, by targeting a specified probability of satisfying a desired weld penetration depth as well as predicting its variation.

This study estimates the angular distortion and residual stresses due to welding using the following methodologies: thermo-elastic-plastic, inherent strain (local-global), and substructuring on two types of welded joints (T-type fillet weld and butt weld). The numerical results are compared with the experimental measurements and these methodologies are evaluated in terms of accuracy and computational time. In addition, the influence of welding sequence on distortion and transverse residual stresses has been studied numerically by implementing the thermo-elastic-plastic and inherent strain (local-global) methods on the T-type fillet weld. For the T-type fillet weld, the estimated angular distortion from these methods is much the same and in good agreement with the experimental measurements. For the butt weld, the angular distortion calculated by the inherent strain (local-global) method is largely underestimated. In order to gain a better understanding of where the underestimation of angular distortion in the inherent strain (local-global) method comes from, the study discusses the influence of block length and welding speed on angular distortion. It is found that for long weld length or slow welding speed, activating the plastic strain gradually by dividing the weld bead into an appropriate number of blocks can reduce the level of underestimation of angular distortion.

In this study finite element simulation approaches (lumping and prescribed temperature) are implemented to study residual stress distribution in a welded box type structure. This component is a vital part in several load carrying structural applications and the residual stresses are important to quantify from a structural integrity point of view. The thermal history from simulations has been verified with experimental measurements. The residual stresses at the weld toe side were measured, using X-ray diffraction technique. It is shown that a similar trend of residual stress state was captured by the simulation, compared to experimental measurements. The estimated residual stresses from the cases of welds with full penetration and partial penetration are slightly different along the crack path. Compressive residual stress was near the area of both weld toe and root while tensile residual stress was in the center of the weld with the magnitude up to 820 MPa. Moreover, a sub model of the welded box type structure is studied using the following computational weld mechanics concepts: Thermo -elastic -plastic, lumping and prescribed temperature, in order to assess the computational time and the magnitude of estimated residual stresses.