Tubular Welded Joints Research Articles

Statics and stability of diagonalless beams of particular kind – preliminary studies

Attractive large-scale glazing is currently an architectural trend. However, achieving adequate stiffness for larger glazing spans requires the use of complex cross-sections, generally aluminium sections of considerable height. Members with openwork webs are sometimes used in order to achieve increased load-bearing capacity and stiffness with reduced weight. The disadvantage is that this solution takes up a lot of space inside the building. A recently patented diagonalless member attempts to solve the above-mentioned problems. The member is fully demountable and allows glass units to be installed in the space between the chords. It consists of two chords spaced apart by metal sleeves with bolts passed through them. In this study, preliminary qualitative experimental tests were carried out to determine the behaviour of the member under load and to identify zones sensitive to local deformation. On this basis, numerical models (bar and 3D solid models, including contact interactions) were created and tested. Subsequently, the optimum sleeve spacing was determined, the effect of rotational and translational stiffness reduction at the nodes was investigated, and stress concentration zones and forms of stability loss were identified. A new form of local loss of stability of the chord facewall was identified, the so-called sliding push effect of the chord walls on the sleeve (within the larger openings). This is a completely different type of chord facewall failure from that found in known tubular welded joints. The research programme focused on identifying the phenomena occurring in the new member in order to provide a basis for further, more advanced analyses.

Ultimate strength and fatigue of stiffened welded tubular joints in floating energy production structures

Monotonic and cyclic loading experiments are performed on 1/6-scale stiffened steel joints representing the welded connection between the tubular braces and the vertical cylinders of a prototype offshore floating structure for combined wind-wave energy production. The structural behavior of the welded joints is investigated to determine their ultimate strength under monotonic loading and their fatigue life. The brace-to-cylinder welded connection is full penetration and semi-automatic, whereas the stiffeners are welded to the inner surface of the cylinder with fillet welds. High-frequency mechanical impact post-weld treatment is applied to five specimens. Experimental observations indicate two critical locations for fatigue crack initiation: the crown of the brace-to-cylinder weld and the fillet weld of the central stiffener. These locations are also verified by finite element simulations and fractographic examination of failed specimens. Post-weld treatment improves the fatigue resistance of the welded joints in the high-cycle fatigue domain but has negligible influence on the low-cycle fatigue response of the connection and its monotonic ultimate strength. Finally, one fatigue-cracked specimen is tested under monotonic loading exhibiting structural performance similar to the intact specimens.

Fatigue life estimation of the weld joint in K-node of the offshore jacket structure using stochastic finite element analysis

Several methods for finite element modelling and analysis of tubular welded joints are described in various design codes. These codes provide specific recommendations for modelling of the welded joints, using simple weld geometries. In this paper, experimental strain range results from two full-scale semi-automatically welded K-nodes tests are compared to corresponding finite element models. Three dimensional (3D) scans of the weld surfaces have been developed for the automatically welded K-joints. These 3D scans are included in the finite element (FE) models to determine the accuracy of the FE models. Some discrepancies are observed between finite element predictions and test results. Finite element model updating has been applied to improve the accuracy of the developed finite element models. The results are compared to a FE model with a simple weld geometry based on common offshore design codes and a FE model without any modelled weld. The results show that the FE model with 3D scanned welds are more accurate than the other FE models. The precise location of weld toe in the 3D scanned weld is difficult to locate in the FE model, thus stochastic finite element modelling is performed to analyse the probabilistic hot-spot stresses. The results show large variations in the hot-spot stresses. It can be concluded that it is necessary to evaluate the probabilistic nature of stresses for calculating fatigue lives of welded tubular joints in offshore structures.

Hot-spot localisation according to the critical plane-based approach

The tubular welded joints in H-shaped structural components of agricultural sprayers are responsible for failure and the localisation of hot-spots is very important in multiaxial fatigue assessment of such components. Therefore, the purpose of the present paper is to develop a novel approach both to localize hot-spots in tubular welded joints and to estimate the initial crack growth direction. Such an approach is based on the application of the hot-spot stress approach together with the multiaxial critical plane-based criterion proposed by Carpinteri et al. Comparison between experimental data and numerical results has shown a satisfactory agreement in terms of both hot-spots localisation and initial crack growth direction evaluation.

Experimental and Numerical Study on Complex Multi-planar Welded Tubular Joints in Umbrella-Type Space Trusses with Long Overhangs

A test rig with multi-functional purposes was specifically designed and manufactured to study the behavior of multi-planar welded tubular joints subjected to multi-planar concurrent axial loading. An experimental investigation was conducted on full-scale welded tubular joints with each consisting of one chord and eight braces under monotonic loading conditions. Two pairs or four representative specimens (two specimens for each joint type) were tested, in which each pair was reinforced with two kinds of different internal stiffeners at the intersections between the chords using welded rectangular hollow steel sections (RHSSs) and the braces using rolled circular hollow steel sections (CHSSs) and welded RHSSs. The effects of different internal stiffeners at the chord–brace intersection on the load capacity of joints under concurrent multi-planar axial compression/tension are discussed. The test results of joint strengths, failure modes, and load–stress curves are presented. Finite element analyses were performed to verify the experimental results. The study results show that the two different joint types with the internal stiffeners at the chord–brace intersection under axial compression/tension significantly increase the corresponding ultimate strength to far exceed the usual design strength. The load carrying capacity of welded tubular joints decreases with a higher degree of the manufacturing imperfection in individual braces at the tubular joints. Furthermore, the interaction effect of the concurrent axial loading applied at the welded tubular joint on member stress is apparent.

Study on SN-based and FCP-based Fatigue Assessment Techniques for T-Shaped Tubular Welded Joint

Full-scale fatigue tests of the land structure's T-shaped tubular welded joints are conducted. Three-dimensional finite element (FE) fatigue crack propagation (FCP) analyses based on VCCM with tetrahedral elements are performed in order to obtain the reference SIF solutions. The preciseness of the FE FCP analyses is demonstrated by comparing calculated and measured crack propagation behaviors. Hot spot stress (HSS)-based fatigue assessments are carried out by using various fatigue design codes for tubular joints, and the safety margin of each HSS determination techniques are compared. The crack propagation lives are also predicted by carrying out FCP analyses based on Mk factor method. HSS determination techniques examined in the fatigue assessment are adopted to determine the stress concentration factor (SCF) and the degree of bending (DOB) in Mk factor calculations. It is found that the Mk factor-based FCP results show good agreement with the FE FCP results and the measurements when the HSS determination technique with the smallest safety margin is chosen. Based on these results, the method for selecting SCF and DOB calculation procedure for Mk factor-based FCP analysis is proposed. It is also shown that the difference between the fatigue test results and the HSS SN-based fatigue assessment results becomes smaller when the SIF estimated by Mk factor method agrees with the SIF calculated by the FE fracture analysis.

Numerical study on fatigue behavior of tubular joints for signal support structures

Welded tubular joints are widely used for the mast arm pole connection in signal support structures. In order to study the fatigue behavior of such welded tubular joints, static loading and fatigue tests were carried out on several full-scale experiments. In order to understand the local behavior and perform parametric study of critical factors such as initial crack location, finite element (FE) models are subsequently built and the numerical simulation results are validated with the experimental test results. Stress intensity factors (SIFs) at the deepest point of fatigue cracks are calculated by the FE method and an empirical equation are examined by comparison. It is found that the numerical simulation results are in good agreement with the experimental test results. The effect of initial crack location on the SIF and the fatigue life are also investigated. The Bowness equation will result in higher SIF value and more conservative fatigue life prediction for the concerned welded tubular joints. The SIFs at the deepest point decrease and fatigue life increases with the increasing initial crack angle.

THE ESTIMATION OF LIFETIME OF THE THICK-WALL TUBES AND THE TUBULAR WELDED JINTS AT THE STAGE OF FETIGUE SURFACE SEMIELLIPTICAL CRACK PROPAGATION

The work is devoted to the problems of assessing the lifetime of tubular structural elements at the stage of surface cracks propagation under cyclic loading effect with account of residual stresses. The mathematical model is based on a partial differential equation to determine the kinetics of fatigue cracks in a three-dimensionаl elastoplatic body under high-cycle or low-cycle fatigue. In this equation, the basic parameter, responsible for crack growth rate, is the effective crack tip opening displacement range Ddef. To investigate the fatigue crack growth kinetics in structural elements, depending on stress ration using the proposed differential equation, it is sufficient to use the diagram of fatigue fracture alone in coordinating the rate – displacement range (V ~Ddef), thus allowing us to save the materials and time of experimental investigations, related with construction of fatigue curves. The analytical method for approximate determination of Ddef with account of the plastic stretches at the fatigue crack edges has been developed. An analytical approach to approximate determination of the d – opening for a surface semi-elliptical crack in tubular structural elements under non-uniform distribution of stresses over the element thickness has been proposed. The method for determination of fatigue surface semielliptical crack kinetics and residual lifetime of the tubular welding joint is proposed based on nonlinear fracture mechanics approaches. The basic parameter, responsible for crack growth rate, is the effective crack tip opening displacement range. Computional results of fatigue surface semielliptical crack kinetics the residual lifetime of tubular structural elements were compared with the known experimental data.The analytical relationships for estimation of material constants, that are used in differential equations of fatigue cracks kinetics have been established. The residual lifetime and growth of fatigue surface semi-elliptical cracks for such structural elements such as thick-wall tubes, and tubular welded joints of marine stationary platforms have been assessed. Numerical results of the residual lifetime of thick-wall tubes and T-shaped tubular welded joints were compared with the known experimental data.

Calculation of the stress concentration factor in the welded tube junction subjected to combined loadings

Fatigue failure caused by stress concentrations in tubular welded joints is observed in off shore platforms subjected to cyclic loading in corrosive marine environments. In some junctions, the stress concentration can induce a stress thirty times the nominal stress, and increase the risk of fatigue failure in tubular joints. Therefore, it is necessary to accurately assess the intensity of the stress concentrations to effectively deal with the problem of fatigue damage and lead to reliable tubular joints. This work aims to study the stress distribution and location of the "hot" spots in a Twelded tubular structure subjected to a combined loading of tension and bending (in-plane bending, out of plane bending and traction) to better simulate the actual loading.

Fatigue strength assessment of tubular welded joints by an alternative structural stress approach

The tubular joints, frequently employed in the offshore industry, are submitted to stresses resulting from elementary loadings: tension/compression, in-plane bending and out-of-plane bending. This work concerns the analysis of the recommendations commonly used for the fatigue design of welded joints submitted to combined loadings. Particularly, it deals with the fatigue behaviour of T-joints submitted to deviated-bending: first, a finite element analysis was developed and a post-processing based on the structural stress approach, as proposed by the International Institute of Welding (IIW). Then, fatigue tests were conducted on T-joints submitted to deviated-bending. Comparisons between experimental and numerical results showed that this kind of recommendations is not systematically conservative. Thus, an alternative approach based on structural stresses and taking into account the multi-axial stress state in the weld toe was developed in order to complete the recommendations for the fatigue design of tubular welded joints.

Fatigue Life of Repaired Welded Tubular Joints

In this study, the effect of repair on fatigue life of tubular joints is investigated. Six cracked specimens precedently subjected to fatigue loading undergone to weld repair. Two of those specimens were shot peened before primary fatigue loading. It is shown that repair gives rise to about 150% increase in fatigue life for original specimens while the increase of fatigue life for shot-peened and repaired specimens is around 105%. The in depth residual stresses are measured on the repaired joints before and after fatigue loading. It can be stated that repair made a remarkable improvement into fatigue behaviour of tubular joints examined in this investigation. However, if repair is not possible, shot peening can be a good alternative to improve the fatigue life of welded joint but if repair is planned, shot peening may not be financially or technically justifiable.

Experimental and Numerical Analysis of Complex Welded Tubular DKYY-Joints

The design of multi-planar welded tubular joints is still not fully developed and very often it is not covered in design codes due to the complexity of such structures. The joint position for multi-planar tubular structures is especially critical because there are many brace members which are welded onto the chord surface. Thus, many external forces which are transferred from the braces are applied directly to the chord. Among the various multi-planar tubular joint configurations, the DKYY-joint configuration is often used in tubular space structures to form long-span roofs. However, its design, failure modes and calculations of its joint capacity are not provided in any design code. As a result, it is necessary to carry out experimental investigation and finite element numerical analysis to understand further the behavior and failure process of this type of joint. This paper details an actual practical application where a full-scale specimen is investigated experimentally in a test programme. To improve its joint capacity, reinforcements such as strengthening plates are placed inside the brace members. The chord at the joint connection is thickened to avoid local buckling. The stresses at some critical positions on the braces and on the chord are measured from strain gauges and strain rosettes to monitor the failure process and the ultimate joint capacity is determined from experimental load-displacement relationship. Experimental results show that local buckling on the chord surface at the joint is avoided effectively due to the reinforcement to the chord by increasing the chord thickness locally and placing inner circular plate inside the chord. As the stiffness of the chord at the joint is improved efficiently, failure position moves to the brace, and thus the chord is protected. Finally, finite element analysis is conducted and the numerical results are compared against those obtained from experimental measurements.

The Peak Stress Method applied to fatigue assessments of steel tubular welded joints subject to mode-I loading

The paper presents the practical aspects involved in structural design of tubular joints mainly employed in roller coasters manufacturing. Different design standards, commonly adopted in fatigue assessments, are considered and compared. Then some constant amplitude fatigue test results concerning typical tubular welded joints are presented. Finally the experimental results and additional test data taken from the literature are compared with the theoretical estimations based on the Peak Stress Method, which has been recently conceived and proposed as a simplified, application-oriented version of the well known Notch-Stress Intensity Factor approach. The results indicate that the Peak Stress Method is suitable to estimate the fatigue life up to crack initiation of tubular welded joints subject to mode-I loading.

Stress relieving and its effect on life of welded tubular joints

This paper describes investigations into the fabrication and repair of large welded tubular joints and how the processes used affect fatigue life. Research has been conducted to determine the benefit of local post weld heat treatment (PWHT) of the connections in terms of hardness and residual stress. The use of hole drilling and trepanning techniques for residual stresses measurements in the clusters are described. The results are compared with fitness-for-service assessments and considered in relation to the fatigue performance of the welded connections. Very substantial theoretical improvement in fatigue life can be expected after PWHT.

Strength of tubular welded joints of roof trusses in Shanghai Qizhong Tennis Center

Static strength of multi-planar welded joints with seven brace members and one chord member made of circular hollow sections is investigated in the paper, based on the engineering practice of moveable roof trusses of Shanghai Qizhong Tennis Center. Firstly, comparative experiments were carried out on two model joints with a scale of 1:3. One joint was not reinforced, whereas the other was reinforced with ring stiffeners inside the chord member. Failure mode, stress distribution, plastic-zone development and ultimate load capacity of the joints were investigated, and effects of the ring stiffeners on the joint behavior were observed through the experiments. Secondly, finite element analysis of both the tested model joints was performed. The calculation results are in a good agreement with the experimental results, which indicated that the numerical analysis was quite effective. Finally, the strategy for enhancing strength of the complicated joint is discussed. Parameters study on the constructional details of ring stiffeners was carried out using FE method. The present research shows the multi-planar circular hollow section joint with high ratio of diameter to thickness of the chord and multiple braces is liable to chord plasticity under axial tension and compression on the braces. For the design of the joint, it is suggested that the ring stiffeners are installed insider the chord to meet needs of enough stiffness and strength. Both position and number of the stiffeners should be carefully determined based on the axial forces on the braces and their diameters. The stiffener thickness should not be less than the chord thickness, and the diameter of the hole at the center of the stiffener should not be greater than half of the chord diameter.

Weight function and stress intensity factor for a semi-elliptical surface saddle crack in a tubular welded joint

Owing to the complex geometry and non-uniform stress distribution, it is extremely difficult to obtain an accurate value for the analytical stress intensity factor (SIF) solution for a surface crack in a tubular welded joint. It is usually predicted using three-dimensional T-butt SIF solutions in conjunction with appropriate load-shedding models. The problem with this approach is that the effect of the tube curvature is ignored. Thus it may not represent the accurate stress intensity at the tip of the crack and stress redistribution resulting from the reduction of member stiffness with crack growth. Assuming that the relative influence of the weld toe for the deepest point SIF of a semi-elliptical surface crack in a thin pipe is the same as that for an edge crack of the same depth in a flat plate under the same stress distribution and using the weight function for longitudinal cracks in thin pipes as the reference solution, a new weight function in a closed form has been derived for the deepest point of a semi-elliptical surface crack at the saddle position in a tubular welded T-joint in this study. Based on this weight function and the uncracked T-butt through-wall stress distribution database, a new set of SIF parametric equations has been derived for membrane and bending loading respectively. This set of equations has incorporated the crack aspect ratio influence and the effects of the weld toe including the weld toe angle, weld attachment length, and weld radius. With the hot-spot stress, degree-of-bending values and appropriate load-shedding models, they can be used to predict the deepest point SIFs for semi-elliptical surface saddle cracks in tubular welded T-joints. The curvature effect of tubes to the SIFs is identified by comparing the predictions with three-dimensional T-butt finite element data and the results from three-dimensional T-butt weight-function-based SIF solutions. A comparison of the predictions with tubular joint fatigue test results, especially early crack growth data, has shown that this new SIF solution can work very well with the non-linear moment release model. Together with the extended Du-Hancock aspect ratio development model, they provide a fast SIF calculation procedure for the deepest points of surface saddle cracks during fatigue crack growth in tubular welded joints.

Hot spot stress approach for Tsing Ma Bridge fatigue evaluation under traffic using finite element method

The hot spot stress approach is usually adopted in the fatigue design and analysis of tubular welded joints. To apply the hot spot stress approach for fatigue evaluation of long span suspension bridges, the FEM is used to determine the hot spot stress of critical fatigue location. Using the local finite element models of the Tsing Ma Bridge, typical joints are developed and the stress concentration factors are determined. As a case for study, the calculated stress concentration factor is combined with the nominal representative stress block cycle to obtain the representative hot spot stress range cycle block under traffic loading from online health monitoring system. A comparison is made between the nominal stress approach and the hot spot stress approach for fatigue life evaluation of the Tsing Ma Bridge. The comparison result shows that the nominal stress approach cannot consider the most critical stress of the fatigue damage location and the hot spot stress approach is more appropriate for fatigue evaluation.

Comparative behaviour of stiffened and unstiffened welded tubular joints of offshore platforms

The paper presents the results of an experimental investigation conducted on welded tubular joints, that are employed in offshore platforms, to study the behaviour and strength of these joints under axial brace compression loading. The geometrical configuration of the joints tested were T and Y. The nominal diameter of the chord and brace members of the joint were 324 and 219 mm respectively. The chord thickness was 12 mm and the brace 8 mm. The tested joints are approximately quarter size when compared to the largest joints in the platforms built in a shallow water depth of 80 m in the Bombay High field. Some of the joints were actually fabricated by a leading offshore agency which firm is directly involved in the fabrication of prototype structures. Strength of the internally ring-stiffened joints was found to be almost twice that of the unstiffened joints of the same configuration and dimensions. Bending of the chord as a whole was observed to be the predominant mode of deformation of the internally ring-stiffened joints in contrast to ovaling and punching shear of the unstiffened joints. It was observed in this investigation that unstiffened joint was stiffer in ovaling mode than in bending and that midspan deflection of unstiffened joint was insignificant when compared to that of the internally ring stiffened joint. The measured midspan deflection of the unstiffened joint in this investigation and its relation with the applied axial load compares very well with that predicted for the brace axial displacement by energy method published in the literature. A comparison of the measured deflection and ovaling of the unstiffened joint was made with that published by the author elsewhere in which numerical prediction of both quantities have been made using ANSYS software package. The agreement was found to be quite good.

Characteristic parameters for stress distribution along the intersection of tubular Y, T, X and DT joints

Analysis of large-scale fatigue testing results has shown that the fatigue strength of offshore welded tubular joints is not dependent on the hot spot stress alone but is also significantly influenced by stress distributions around the intersection which are normally represented by several characteristic parameters. The through-thickness distribution for example is characterized by degree of bending. In order to represent the stress distribution along the intersection, the average stress concentration factor (SCF) concept was suggested and used in empirical stress intennsity factor (SIF) models. However, this parameter alone is not enough to characterize the stress distribution along the intersection. Furthermore, there is no parametric equation available to predict this parameter. For this reason, a new concept, stress distribution concentration factor (SDCF), has been proposed in this study to describe the spread of stress distribution along the intersection. Systematic thin shell finite element analyses have been conducted for 330 different tubular Y, T and 330 X and DT joints, typical of those used in offshore structures, subjected to different modes of loading. On the basis of these results, a set of parametric equations has been derived for the average SCF and SDCF as a function of non-dimensional joint geometric ratios a, β, γ, τ and ϑ for each mode of loading and for both the chord and brace sides of the intersection of tubular welded Y, T, X and DT joints.

Incorporation of residual stresses into the SINTAP defect assessment procedure

Residual stresses are an important consideration in the structural integrity assessment of welded joints and reliable predictions of structural integrity therefore require that the residual stress distribution is adequately accounted for. However, distributions are dependent on the weld geometry and, in the absence of comprehensive information for welded joints, it is usually necessary to make conservative assumptions. This can result in unrealistic predictions, and consequently, further information on the nature and behaviour of residual stress distributions is required to enable the provision of improved guidance on this subject. The further development of the BS 7910 and R6 procedures for the assessment of residual stress effects was a principal task in the EC funded project named structural integrity assessment procedures for European industry, SINTAP, and an extensive investigation of this subject was performed. It entailed an extensive literature review of distributions in the principal weld geometries (including plate butt, pipe butt, pipe to plate, T-butt and tubular welded joints), experimental and numerical investigations and the development and validation of procedures.