Welding Residual Stress Field Research Articles

Assessment of uncertainties in life prediction of fatigue crack initiation and propagation in welded rails

The risk for initiation of fatigue cracks in the rail head and web in the weld zone of a rail is studied. The interaction between the welding residual stress field and the stress field caused by service loads is simulated in a nonlinear finite element (FE) analysis where the welding residual stress distribution (shape) and magnitude, the service load magnitude and the material parameters used in the fatigue life estimation are varied. The initiation of fatigue cracks is assessed using the shear-stress-based multiaxial fatigue criterion proposed by Dang Van, and the propagation of fatigue cracks in the rail web is carried out using a Paris-type crack growth law. A discussion is presented of the interpretation of using the Dang Van criterion to assess crack initiation for stress–response load cycles with low shear stress amplitudes and high hydrostatic stress levels. In addition, the accuracy in the fatigue life assessment is evaluated by statistical uncertainty analysis where the variances according to the Gauss approximation formula are studied. The risk for fatigue crack initiation and propagation in the rail head and rail web, respectively, is enhanced due to the welding residual stress, and the uncertainty in load level dominates the uncertainty in the fatigue assessments.

Investigations on welding residual stresses in penetration nozzles by means of 3D thermal elastic plastic FEM and experiment

Recent discoveries of stress corrosion cracking (SCC) in weldments including penetration nozzles at pressurized water reactors (PWRs) and boiling water reactors (BWRs) have raised concerns about safety and integrity of plant components. It is well known that welding residual stress is an important factor resulting in SCC in weldments. In the present work, both experimental method and numerical simulation technology are used to investigate the characteristics of welding residual stress distribution in penetration nozzles welded by multi-pass J-groove joint. An experimental mock-up is fabricated to measure welding residual stress at first. In the experiment, each weld pass is performed using a semi-circle balanced welding procedure. Then, a corresponding finite element models with considering moving heat source, deposition sequence, inter-pass temperature, temperature-dependent thermal and mechanical properties, strain hardening and annealing effect is developed to simulate welding temperature and residual stress fields. The simulation results predicted by the 3D model are generally in good agreement with the measurements. Meanwhile, to clarify the influence of deposition sequence on the welding residual stress, the welding residual stress field in the same geometrical model induced by a continuous welding procedure is also calculated. Finally, the influence of a joint oblique angle on welding residual stress is investigated numerically. The numerical results suggest that both deposition sequence and oblique angles have effect on welding residual stress distribution.

On the Influence of Welding Residual Stresses on the Dynamic Behavior of Structures

It is widely known that welding processes induce the generation of residual stresses, which, through the so-named stress stiffening effect, can influence the static and dynamic behavior of the welded components. Thus, accounting for this influence becomes important for the understanding of experimental observations and accurate modeling of the dynamic behavior. In this study, the numerical and experimental characterization of the influence of welding residual stresses on the flexural dynamic characteristics of rectangular plates is addressed. It is suggested a general modeling methodology based on finite elements comprising three subsequent analyses, namely: a thermal analysis to compute the transient temperature history due to welding thermal loading; a structural analysis accounting for plastic strains to obtain the welding residual stress fields and geometric distortions, and a dynamic analysis to compute the dynamic characteristics taking into account the stress-stiffening effect and geometric distortions. The results demonstrate the importance of considering the influence of welding residual stresses in the prediction of the flexural dynamic behavior of plates and the feasibility and efficiency of the simplified modeling approach, which can readily be extended to more complex situations, for characterizing this influence.

Residual Stress Analysis of Laser Welded Aluminium T-Joints using Neutron Diffraction

In this study the distribution and magnitude of residual stresses in a T-joint of aerospace grade aluminium alloy weldment was determined using neutron diffraction. A 2 mm thick AA 6013 sheet (as clip) was laser beam welded to a 6 mm thick AA 6056 base plate (as skin) to resemble the “short distance” welded clip-skin joints of an airframe. The total length of the weld was 120 mm and it was welded using 3.3 kW Nd:YAG laser source and 12 % Si containing wire from one side only. No post weld heat treatment was applied after the welding. Start (run-in) and end (run-out) locations of the T-joints are generally considered as high risk areas with respect to solidification cracking and crack initiations under external loadings. It is of interest to investigate the weld residual stress fields at these locations to develop optimum joint design. Therefore, strain measurements have been performed not only in the middle of the weld seam, but also at the run-in and run-out locations of these short distance welds. Higher longitudinal tensile residual stresses are detected at the run-out locations than at the run-in locations. The measurements in the clip showed that the clip has a longitudinal tensile stress peak away (about 8 mm) from the weld seam.

Influence of welding sequence on welding stress of T joint

In order to get the optimizing scheme of T type joint's welding sequence, the effect of welding sequence on welding residual stress field of T joint is simulated from the view of multibead welding, subsection welding and multilayer welding on the basis of ensuring reasonable welding heat source and establishing the finite model. For the multibead welding, the residual tensile stress appears in the weld and nearby while the maximum value appears in the weld. Moreover, the method of the different welding direction between adjacent beads during the multibead welding makes the residual tensile stress peak value minimum. The subsection welding in which the prior welding section is the two end section and the latter welding section is the middle section not only can increase the low stress region of weld, but also can effectively decrease welding residual stress of the prior welding section. Moreover, the decreasing effect of subsection welding is related to the welding direction of the latter welding section and the distance between one point of the prior welding section and the end of the latter section. In the multilayer welding process, the method of symmetry welding gets the minimum residual tensile stress.

Stress intensity factors for through-thickness cracks in a wide plate: Derivation and application to arbitrary weld residual stress fields

Exact closed-form stress intensity factor (SIF) solutions have been developed for mode-I, II and III through-thickness cracks in an infinite plate. Centre-crack problems have been analysed comprehensively in the literature, but the focus has been on the effect of simple loading about the crack centre. In the current work, the formula of Sih–Paris–Erdogan was extended to consider the difference in SIF on the left and right crack tips under an asymmetric stress field. Mathematical manipulations were performed to derive exact stress magnification factors for SIF computations and simultaneously circumvent the problem of crack-tip stress singularity. The solutions so obtained are applied to derive the residual SIFs that would act on a crack growing under the influence of the residual stress fields associated with VPPA (variable polarity plasma arc) and friction stir welds, using measured residual stress profiles.

Residual Stress Analysis of Laser Welded Aluminium T-Joints Using Neutron Diffraction

In this study the distribution and magnitude of residual stresses in a T-joint of aerospace grade aluminium alloy weldment was determined using neutron diffraction. A 2 mm thick AA 6013 sheet (as clip) was laser beam welded to a 6 mm thick AA 6056 base plate (as skin) to resemble the “short distance” welded clip-skin joints of an airframe. The total length of the weld was 120 mm and it was welded using 3.3 kW Nd:YAG laser source and 12% Si containing wire from one side only. No post weld heat treatment was applied after the welding. Start (run-in) and end (run-out) locations of the T-joints are generally considered as high risk areas with respect to solidification cracking and crack initiations under external loadings. It is of interest to investigate the weld residual stress fields at these locations to develop optimum joint design. Therefore, strain measurements have been performed not only in the middle of the weld seam but also at the run-in and run-out locations of these short distance welds. Higher longitudinal tensile residual stresses are detected at the run-out locations than the run-in locations. The measurements in the clip showed that the clip has a longitudinal tensile stress peak away (about 8 mm) from the weld seam.

Some Advances on Understanding of High Cycle Thermal Fatigue Crazing

The aim of this paper is to improve the understanding of high cycle thermal fatigue crazing observed in some areas of residual heat removal (RHR) systems made of 304L stainless steel in PWR nuclear plants. High cycle thermal crazing and the absence of crazing under purely mechanical loading are explained through the arrest of cracks initiated at the surface in the thickness of the component for thermal fatigue. This arrest is due to high stress gradients in the case of thermal loading due to the high frequency of the thermal load. It is shown that close to the weld, the crack network configuration is related to the tensile weld residual stress field in accordance with the detrimental effect of tensile mean stress on fatigue life. However, these results are in apparent contradiction with the material uniaxial fatigue tests, where a beneficial effect of tensile mean stress is observed in load-controlled fatigue tests. Moreover, it is shown that near the weld, the absence of a crack network is related to the compressive weld residual stress field in accordance with the beneficial effect of compressive mean stress on fatigue life. This result is however, in apparent contradiction with the observation of crazing far from the weld under a highly compressive stress. These contradictions may be explained by the detrimental effect of prehardening in strain control and the beneficial effect of prehardening in stress control for stainless steels. From these results it is concluded that shot peening which is usually considered to be beneficial for ferritic steel may be detrimental in high cycle thermal fatigue for stainless steels. It may also be concluded that in high cycle thermal fatigue of stainless steel, improvement of fatigue life would be possible with heat treatment by reduction of strain-hardening rather than with shot peening.

An Integrated Approach to the Determination and Consequences of Residual Stress on the Fatigue Performance of Welded Aircraft Structures

Abstract Although residual stress in welded structures and components has long been known to have an effect on their fatigue performance, access to reliable, spatially accurate residual stress field data has been limited. Recent advances in neutron and synchrotron X-ray diffraction allow a far more detailed picture of weld residual stress fields to be obtained that permits the development and use of predictive models that can be used for accurate design against fatigue in aircraft structures. This paper describes a fully integrated study of the three-dimensional residual stress distribution accompanying state-of-the-art fusion welds in 2024-T4 aluminum alloy, and how it is affected by subsequent machining and service loading. A particular feature of this work has been the development of techniques allowing the nondestructive evaluation of the residual stress field in the full range of specimens used to provide the design data required for welded aircraft structures and the integration of this information into all aspects of damage tolerant design.

Numerical simulation of temperature field and residual stress in multi-pass welds in stainless steel pipe and comparison with experimental measurements

A computational procedure is presented for analyzing temperature fields and residual stress states in multi-pass welds in SUS304 stainless steel pipe. Based on the ABAQUS software, uncoupled thermal–mechanical three-dimensional (3-D) and two-dimensional (2-D) finite element models are developed. The finite element models are employed to evaluate the transient temperature and the residual stress fields during welding. Firstly, a 3-D model is developed to simulate the temperature fields and welding residual stresses. Secondly, based on the characteristics of the temperature fields and the welding residual stress fields, a 2-D axisymmetric model is also developed. The simulated result shows that the 2-D axisymmetric model can be effectively used to simulate the thermal cycles and the welding residual stresses for SUS304 stainless steel pipe. Using the 2-D model, a large amount of computational time can be saved. In this study, experiments are also carried out to verify the effectiveness of the proposed numerical models. The results of both 3-D model and 2-D model are in very good with the experimental measurements.

Three dimensional multi-pass repair weld simulations

Full 3-dimensional (3-D) simulation of multi-pass weld repairs is now feasible and practical given the development of improved analysis tools and significantly greater computer power. This paper presents residual stress results from 3-D finite element (FE) analyses simulating a long (arc length of 62°) and a short (arc length of 20°) repair to a girth weld in a 19.6 mm thick, 432 mm outer diameter cylindrical test component. Sensitivity studies are used to illustrate the importance of weld bead inter-pass temperature assumptions and to show where model symmetry can be used to reduce the analysis size. The predicted residual stress results are compared with measured axial, hoop and radial through-wall profiles in the heat affected zone of the test component repairs. A good overall agreement is achieved between neutron diffraction and deep hole drilling measurements and the prediction at the mid-length position of the short repair. These results demonstrate that a coarse 3-D FE model, using a ‘block-dumped’ weld bead deposition approach (rather than progressively depositing weld metal), can accurately capture the important components of a short repair weld residual stress field. However, comparisons of measured with predicted residual stress at mid-length and stop-end positions in the long repair are less satisfactory implying some shortcomings in the FE modelling approach that warrant further investigation.

용접잔류응력의 이완과 재분포 해석 및 실험적 검증

For the precise assessment of the effect of welding residual stresses on structural strength and fatigue crack growth behavior, new FE analysis algorithms for the estimation of residual stress relaxation due to external load and redistribution due to fatigue crack propagation were proposed in this paper. Initial welding residual stress field was obtained by thermal elasto-plastic analysis considering temperature dependent material properties, and the amount of residual stress relaxation and redistribution were assessed by subsequent elasto-plastic analysis In the analysis of fatigue crack propagation, the applied SIF(Stress Intensity Factor) range was evaluated by <TEX>$\frac{1}{4}$</TEX>-point displacement extrapolation method, and the effect of welding residual stresses on crack propagation was considered by introducing the effective SIF concept. The test results of crack propagations were compared with the predicted data obtained by the analysis.

J-integral and local damage fracture analyses for a pump casing containing large weld repairs

Three-dimensional J-integral and two-dimensional Local Approach finite element studies are described for postulated crack-like defects in a large repair weld to the casing of a light water reactor circulation pump. The repair weld residual stress field is simulated and plant operating pressure and thermal transient loads are applied. Crack tip constraint effects are quantified through detailed analysis of the cracked structure and compact tension fracture toughness specimens. Fracture initiation crack sizes are shown to be larger than conceivable fabrication defects that are detectable using modern ultrasonic inspection techniques. The Local Approach study demonstrates the benefits of quantifying crack tip constraint conditions, compared with conventional J-estimation schemes and cracked body J-integral analysis. The method of introducing the crack into the finite element model is shown to have a large effect on calculated crack tip fracture parameters; a slowly developing crack in the residual stress field being more benign.

Uncertainty analysis of welding residual stress fields

A fuzzy-set approach is applied in conjunction with a finite element procedure to study the effect of uncertainty in material properties on welding residual stress fields. The study focuses on steels that undergo martensitic transformations during the cool-down part of the weld thermal cycle. Uncoupled thermo-mechanical analysis is performed. Filler metal deposition, various convective–radiative energy losses, and latent heat generation are included in modeling of the welding process. A rate-independent thermo-elasto-plastic material model with nonlinear kinematic hardening is adopted in the study. Account is taken of the metallurgical transformations that occur during the entire weld thermal cycle. The material model is validated by comparing its predictions with existing experimental measurements of the welding residual mechanical fields. Fuzzy logic is then used to study the variability in the residual stress field due to pre-selected ranges of variation in material parameters associated with the martensitic transformations. It is found that the variations in material parameters considered have a significant effect on the welding residual stress field.

Prediction of Residual Stresses and Distortions in Welded Structures

A good knowledge of the welding residual stress field and the corresponding distortion is needed when the interaction between manufacturing stresses and future (mechanical and/or thermal) design loads is studied, for example, with respect to possible crack initiation and growth in the HAZ, and also with respect to buckling. It is proposed here that a qualitatively good estimate of the welding residual stresses can be obtained by using FEM without following the temperature history during welding and cooling in detail. The procedure proposed is applied to two different problems: multi-pass butt welding of pipes, and spot welding of box beams. For the case of multi-pass butt welding of pipes, experimental results are available and a good agreement with these results is observed.

Effect of plastic deformation on fatigue crack behaviour in weld residual stress field: Murata, M. and Mukai, Y. J. Soc. Mater. Sci. Jpn40 458 (Nov 1991) 1435–1441 (in Japanese)

The suitability of a crossed-cylinders wear test for evaluation of wear behavior and tribological properties of coated forming tools was examined under conditions close to industrial operations. Seven types of coatings were applied on a high speed tool steel which are used as the specimens while a carbon steel, S45C (JIS), and a commercially pure copper are employed as the counter-materials. A parameter, λm, the effective mean wear rate is introduced to express the anti-wear performance of the coating. CrN shows the lowest effective mean wear rate among the tested coating tools. However, the effective mean wear rate of CrN increases with the increase in sliding speed.The results demonstrate that the crossed-cylinders wear test presented in this experiment is a useful testing method for an appraisal of the wear characteristics of the coating.

Computational crack path prediction for brittle fracture in welding residual stress fields

Unstably curved crack paths are often observed for brittle cracks in weldments, because the welding process induces local residual stresses and material inhomogeneities along the welding line. Crack path stabilities are first examined in the present paper. Then, having developed a method of computational crack path prediction, fracture behavior is numerically simulated for brittle cracks propagating in the welding residual stress fields. In the case of an edge crack in a welded square plate which is loaded in the direction perpendicular to the initial crack and parallel to the welding line, the crack sometimes sharply changes direction parallel to the welding line. Although initial increase of the stress intensity factor is obtained along this type of curved trajectory, it is followed by an immediate sharp drop of the stress intensity factor, which may lead to crack arrest. The present computational results are compared with experiments, which show very good agreement with regard to the crack paths and crack arrest behavior.

Analysis of fatigue crack surface elastic contact in compressive residual stress field of compact specimen.

Based on the superposition technique, the effect of residual stress on the fatigue crack growth rate has been estimated to be the same as the effect of the applied stress ratio, considering the effect of the crack surface contact. In this paper, a simpler method to analyze the partial crack surface contact is proposed. The changes of the real stress intensity factor in a load cycle on a compact specimen with a welding residual stress field is analyzed numerically. The effect of a notch machined into the compact specimen is also analyzed. The results obtained are as follows; (1) The simpler method with the superposition technique can be applied. (2) Because of the crack surface contact, the real stress intensity factor range is reduced and the real stress ratio becomes is heigher than O in a compressive residual stress field. (3) The real stress intensity facor is kept constant from the onset of the crack surface contact to the complete unloading on the compact specimen. (4) The notch machined into the compact specimen reduces the effect of the partial crack surface contact.

FE-calculated stresses in a multi-pass butt-welded pipe—A simplified approach

The development of stresses in a multi-pass butt-welded pipe is studied numerically by use of the finite element method (FEM). A simplified approach is employed where rotational symmetry is assumed, where the complex temperature history experienced by the pipe material during welding is not followed in detail, and where effects of phase transformations are disregarded. Two different weld grooves, a narrow gap groove and a conventional single-U groove, are compared with respect to the calculated welding residual stress field and the radial deflection. Somewhat smaller stresses and larger deflections are found for the narrow gap groove. Calculated stresses and displacements are qualitatively in good agreement with experimental results reported elsewhere, especially for the narrow gap groove.

Welding Stress Redistribution in a Butt-Welded Pipe During Later Mechanical and Thermal Loadings

Stress redistribution in a butt-welded pipe during later mechanical and thermal treatments is studied numerically by use of a thermoelastoplastic FE-model. It is proposed that the influence of the welding process on the redistribution can be modeled using a simplified approach. Here the welding residual stress field is simulated with an equivalent radial line load. Two standard treatments, one mechanical and one thermal, are studied with this simplified approach. Both treatments are performed after the welding is completed. The calculations show that both treatments can be used to turn (detrimental) tensile welding residual axial stresses on the inner surface at the weld into compressive ones.