Cruciform Joints Research Articles

On the fatigue behavior of duplex and high-strength welded cruciform joints

This paper presents outcome of a research which investigates the fatigue behaviour of duplex stainless steel grade (EN 1.4162) via cyclic tests on welded cruciform joints and base plates. A database of experimental tests for similar high-strength steel details was also collated from the literature. The obtained experimental S-N curve was then compared to the current European design ones and re-evaluated according to the hot spot stress method. A good agreement was observed between the studied detail performance and the collated reference data. Improvement to the fatigue design rules for the studied duplex and similar high-strength steel details is then proposed.

Cover Picture: Steel Construction 3/2021

AbstractAs part of the AiF‐FOSTA research project P1020 „Economical welded joints of high‐strength steels”, extensive tests were carried out at the Institute of Steel and Timber Construction at the TU Dresden. Tested were lap joints, cruciform joints and butt joints of steels in grades S500 to S960. The picture shows the flat tensile test specimens made from butt joints with a thickness of 20 mm and a width of 35 mm in the weld area to be tested. Fully and partially penetrated butt welds were tested. In order to determine the strains in the course of the tests, strain measurements were carried out using photogrammetry in addition to inductive measurements. For this purpose, a stochastic pattern of graphite spray was applied to the test specimens and the high‐frequency image recordings were digitally evaluated (s. article pp. 138–149. Foto: TU Dresden, Institute of Steel and Timber Construction).

A new design model for welded joints

AbstractIn the AiF‐FOSTA research project P1020 [1], a new design model for welded joints was developed. In this context, a small scale test on flat tensile specimens was designed, with the help of which various influences on the strength and ductility of the welds were investigated. Furthermore, extensive tests were carried out on overlap joints, cruciform joints with double fillet welds as well as partially and fully penetrated butt joints. This was done to calibrate the weld construction factor αw, which takes into account the influence of the type of joint on the load‐bearing capacity. In the following article, after a short summary of the current state of research, the design model, and the results from the parameter studies on flat tensile tests are described in more detail in [2–4]. Subsequently, the test program on welded joints and the calibration of the weld construction factor αw are presented. Finally, the results of the design model and the tests carried out are compared with test results from other research projects and the design model of prEN 1993‐1‐8 [5].

Experimental and Numerical Study of the Elastic SCF of Tubular Joints.

This paper provides data on stress concentration factors (SCFs) from experimental measurements on cruciform tubular joints of a chord and brace intersection under axial loading. High-fidelity finite element models were generated and validated against these measurements. Further, the statistical variation and the uncertainty in both experiments and finite element analysis (FEA) are studied, including the effect of finite element modelling of the weld profile, mesh size, element type and the method for deriving the SCF. A method is proposed for modelling such uncertainties in order to determine a reasonable SCF. Traditionally, SCF are determined by parametric formulae found in codes and standards and the paper also provides these for comparison. Results from the FEA generally show that the SCF increases with a finer mesh, 2nd order brick elements, linear extrapolation and a larger weld profile. Comparison between experimental SCFs indicates that a very fine mesh and the use of 2nd order elements is required to provide SCF on the safe side. It is further found that the parametric SCF equations in codes are reasonably on the safe side and a detailed finite element analysis could be beneficial if small gains in fatigue life need to be justified.

Numerical and databased investigations on the fatigue of cruciform joints with gaps

The paper deals with investigations on the fatigue resistance of the load-carrying cruciform joint with gaps. The aim is to define limits of specific acceptable imperfections, since these do not always necessarily affect the fatigue strength. Different geometrical influences on the fatigue strength of the load-carrying cruciform joint with gaps are analysed numerically. Numerical simulations comprise effective notch stress calculations, which have been compared to fatigue test results from literature, documented in a fatigue test database. A larger gap in combination with a smaller weld size leads to the unfavourable failure at the weld root. For the specific geometrical configurations, borderline functions defining the failure location on the basis of local weld geometry have been defined. Finally, the most interesting configurations are presented, which will be tested experimentally in a subsequent fatigue test program.

Fatigue behaviour of load-carrying fillet-welded cruciform joints of austenitic stainless steel

Recently, considerable advances have been globally reported on stainless-steel bridges. Fatigue performance is an important design factor. The fatigue damage of stainless-steel bridges mostly occurs in fillet-welded joints, especially in load-carrying fillet-welded (LCFW) cruciform joints. In this study, the fatigue behaviour of LCFW cruciform joints, manifested by fatigue strength, S-N curve, crack shape, fatigue crack growth rate, and the fatigue life predicted method, were investigated. Constant range fatigue and beach-marking fatigue tests were conducted. The fatigue experimental data were regressed to the S-N curve, and were compared with the fatigue classification references from common standards. The crack shape and evolution were studied based on fractographical analysis, and the crack depth and width were then determined. Furthermore, the fatigue crack growth rate and the coefficients of the Paris equation were determined based on crack sizes, and fatigue life was predicted based on fracture mechanics. The results of the fatigue behaviour of LCFW cruciform joints of austenitic stainless steel were as follows: (1) The free slope regression S-N curve is more suitable for the test data. (2) The crack growth rate is lower than that in the structural steel weld and is similar to that of the base metal of structural steel. (3) The fatigue crack of LCFW cruciform joints of austenitic stainless steel is semi-elliptic crack, which is different from that of LCFW structural steel. Semi-elliptic crack has lower stress intensity factor and higher fatigue life. (4) The fatigue strength of the LCFW cruciform joints of austenitic stainless steel (55 MPa) is higher than that of structural steel (36 MPa) according to the Eurocode 3 (EC 3) and International Institute of Welding (IIW) recommendations.

New Weldable Steel for Rebars

A new steel grade, developed at the CELSA steelworks in Ostrowiec Świętokrzyski and used in the production of rebars, contributes greatly to the development of industrial and civil engineering. Steel B600B is characterised by yield point fyk = 600 MPa and immediate tensile strength ftk = 700 MPa. Tests revealed that the steel satisfies all requirements of related standards, both in terms of strength and processing properties. The mechanical properties of the new steel grade are higher by 20% than those of currently produced steels characterised by the highest mechanical properties (characteristic yield point Re= 500 MPa). As a result, the application of the new steel provides notable technical and economic advantages. The new steel grade meets requirements concerning technical class C in accordance with PN-EN 1992-1-1, which indicates that the steel has a significant yield point margin (being an important advantage in terms of limit state design). Plastic steels are easier to weld and less susceptible to welding crack formation. Technological (research-related) tests revealed the favourable welding properties of the new steel. Welding tests were performed using the manual metal arc welding method, i.e. the most common welding process used when making structural reinforcements. The welding tests involved the making of butt, overlap and cruciform joints. The strength and technological tests revealed that the steel satisfied the requirements specified in the PN-EN ISO 17660-1 standard.

Fatigue fracture assessment of 10CrNi3MoV welded load‐carrying cruciform joints considering mismatch effect

AbstractFatigue experiments and numerical simulations based on the linear elastic fracture mechanics (LEFM) theory were conducted on the evenmatched (EM) and undermatched (UM) 10CrNi3MoV load‐carrying cruciform welded joints (LCWJs). The study firstly experimentally investigated the fatigue crack growth rate (FCGR) of base metal, EM, and UM weldments. The corresponding Paris parameters as essential input data are provided to assess the fatigue crack propagation behavior for weld toe and weld root failure caused by notch stress concentration variations. On the one hand, the stress intensity factors (SIFs) at weld toe and weld root were calculated considering the effects of LCWJ specimen geometries, initial crack types, and sizes. The comparisons between simulated results and standards analytical solutions were executed, which exhibit good accordance. It proved that the fatigue fracture simulation procedure based on LEFM is appropriate for the fatigue assessment of LCWJs. Eventually, it conducted the parametric analysis by predicted S–N curves, which included in the weld length, initial crack shape, initial crack size, penetration length, and materials fracture parameter, to explore some safety assessment reference lines for both failure modes of LCWJ.

Fatigue Life of 7005 Aluminum Alloy Cruciform Joint Considering Welding Residual Stress.

An evaluation method is proposed for determining the full fatigue life of aluminum alloy cruciform joint, including the crack initiation and propagation with welding residual stress. The results of simulations have shown that the boundary between the initiation and propagation stage is not constant, but a variable value. The residual stress leads to a significant reduction in both stages, which is more severe on initiation. With considering residual stress, the ratio of crack initiation to total life is below 7%. The effect of residual stress varies with external loading; when external load is lower, the residual stress has a greater effect.

Influence of porosity on the fatigue behaviour of welded joints

In this paper, the influence of a single pore on the fatigue behaviour of welded joints has been examined by means of the implicit gradient approach. The pore has been considered a stress raiser that interacts with the weld toe or weld root. As an example, in the case of a cruciform joint with a load-carrying fillet weld, the influence of a single spherical pore is examined. Furthermore, the fatigue behaviour of an aluminium laser welded joint with ellipsoid equivalent pores is considered and a comparison is made with the fatigue strength of an aluminium arc welded joint.

Paper Review of “Effect of weld penetration on low cycle fatigue strength of load-carrying cruciform joints”

Paper Review of “Effect of weld penetration on low cycle fatigue strength of load-carrying cruciform joints”

Fatigue assessment of cruciform joints: Comparison between Strain Energy Density predictions and current standards and recommendations

The main aim of the present work is to investigate the effects of different design parameters on the fatigue strength of cruciform welded joints through an energy based approach, the Strain Energy Density. The most important design parameters taken into account in the present work for investigating the fatigue strength are the plates dimensions, the scale effect, the lack of welding bead penetration and the weld leg length.The results of the numerical analysis carried out have been also compared with the fatigue strength predicted by the most known design guidance for the fatigue assessment of steel welded joints: Eurocode 3; British Standard; International Institute of Welding; DNV-GL.The attached plate thickness severely affects the fatigue strength of the joint compared to the transverse plate one, as well as the weld leg length. The scale effect for this joint class can be predicted adopting the mode I Williams’ eigenvalues. The welding bead penetration, besides the codes provide fragmented information, has been predicted and the optimal design dimensions have been found. Several differences between the codes, with overestimation or underestimation of the cruciform joint fatigue strength, have been found due to their lack of consideration of the aforementioned design parameters.

Fatigue analysis of cruciform welded joint with weld penetration defects

For the fabrication of steel structures such as steel bridges, offshore structure etc, welding is nowadays considered as an efficient metal joining process. The type of fillet welded cruciform joint are widely used in construction of long spanned bridges with improved design and increased weld quality. However, the presence of geometrical discontinuities and metallurgical nonuniformities lead to crack initiations from different positions which are difficult to detect. Moreover, when these structures are subjected to repeated loading these cracks start propagating in different directions and lead to fatigue failure of welded structures. Also, the presence of locked in stresses and other factors affect the fatigue life and fatigue crack initiation. In the present work, the cruciform fillet welded joint with full and incomplete penetration were analyzed for both load types i.e load carrying and non-load carrying by using 3D fatigue FEM analysis. The experimental study of non-load carrying cruciform joint was also carried out. The effect of incomplete penetration depth, welding deformation, residual stresses and stress magnitude on fatigue crack initiation and fatigue life of fillet welded cruciform joints was investigated. Also, the S-N curves plotted for comparing with the results of hotspot stress and experiments. The results of 3D fatigue FEM analysis were compared with S-N curves recommended by IIW and DNV GL.

Fracture analysis of load-carrying cruciform welded joint with a surface crack at weld toe

The load-carrying cruciform welded joint usually fails in the weld root or the weld toe. Determining the failure location in terms of the stress intensity factor (SIF) of the crack at the weld root and the weld toe is an alternative. Various SIF equations have been proposed and incorporated in the British Standard (BS) 7910:2013, including the equations for the welded cruciform joint with a weld toe crack. However, the SIF equations for the weld toe crack are based on 2-D finite element (FE) analysis, which predicts an over-conservative result. In fact, a crack at the weld toe often resembles a semi-elliptical shape. Hence, it is necessary to propose new SIF equations for the load-carrying cruciform welded joint containing a weld toe crack by 3-D FE analysis. The new equations are built up based on multiple regression analysis and the goodness of fit is confirmed via error distribution and statistical analyses by comparing with the FE results. The proposed SIF equations have a wide applicability in the failure analysis of the load-carrying cruciform welded joint.

Effect of cutting process on the residual stress and fatigue life of the welded joint treated by ultrasonic impact treatment

To obtain the final shape and ensure quality in the manufacturing of welded structural components, a cutting process is usually applied to remove the weld edges. Comparative tests were conducted with replicas of non-load-carrying cruciform joints in the as-welded and ultrasonic impact peened conditions, which were before or after cutting process at the two ends. Those contained the measurement of three types of residual stress distributions of various weld lengths by the cut-release method and relevant fatigue tests. The locations of fatigue crack initiation and fracture surfaces were also determined. The experimental results demonstrated that in the welded joint peened by the ultrasonic impact treatment, the welding residual stress component among the superimposed residual stresses relaxes after cutting. Owing to the original compressive welding residual stress in the peened welded joint, the cutting process has a distinct effect on the change of the superimposed residual stress distribution near the cutting edges. Furthermore, the cutting process reduces the fatigue life improvement of the peened welded joint and transfers the positions of the fatigue crack source. For the fatigue life improvement of welded structures, arranging ultrasonic impact treatment after the cutting process is better. Besides, the compressive superimposed residual stress in the UIT-welded joint will not relax completely under cyclic fatigue loading.

Estimation of the Structural Reliability for Fatigue of Welded Bridge Details Using Advanced Resistance Models

This present paper concerns the application of probabilistic fatigue life prediction models based on S–N curves aiming at the estimation of the safety level for a non-load carrying cruciform joint. The safety level is expressed in terms of the reliability index as a function of the number of applied cycles. Estimating the reliability of a structural component against fatigue failure is relevant for both new and existing structures. The analysis is executed considering three models. For each of them, the input random variables are given in a probabilistic manner, based on fatigue test data from the scientific literature. The effect of the model uncertainty on the model response is quantified for both the load and the resistance side, demonstrating its relevance. Moreover, it has been shown that a 0.5 to 1 increase in the reliability index is achievable by applying the newly proposed, more complex model.

Parametric Formula for Stress Concentration Factor of Fillet Weld Joints with Spline Bead Profile.

The existing parametric formulae to calculate the notch stress concentration factor of fillet welds often result in reduced accuracy due to an oversimplification of the real weld geometry. The present work proposes a parametric formula for the evaluation of the notch SCF based on the spline weld model that offers a better approximation of the real shape of the fillet weld. The spline model was adopted in FE analyses on T-shape joints and cruciform joints models, under different loading conditions, to propose a parametric formula for the calculation of the SCF by regression analysis. In addition, the precision of parametric formulae based on the line model was examined. The magnitude of the stress concentration was also analyzed by means of its probability distribution. The results show that the line model is not accurate enough to calculate the SCF of fillet weld if the weld profile is considered. The error of the SCF by the proposed parametric formulae is proven to be smaller than 5% according to the testing data system. The stress concentration of cruciform joints under tensile stress represents the worst case scenario if assessed by the confidence interval of 95% survival probability.

Improved effective notch strain approach for fatigue reliability assessment of load-carrying fillet welded cruciform joints in low and high cycle fatigue

An improved effective notch strain (ENSN) approach is developed for the fatigue reliability assessment of load-carrying fillet welded cruciform joints in low and high cycle fatigue regions. The ENSN approach is improved in the ENSN estimation using an analytical approach, and the improvement is verified using some published experiments performed in the low cycle fatigue regime. The cruciform joint is modelled as a correlated two-component series-system, and various sources of uncertainties are considered. The fillet weld geometry uncertainty is modelled by correlated position variations of geometrical control points. To avoid iterative calls of the finite element analysis in the fatigue reliability assessment, response surface models are constructed, and they are used to approximate the influence of position changes. A critical fatigue notch factor is proposed to represent the fatigue resistance for a given fatigue loading and material, which allows a direct reliability approximation. The critical fatigue notch factor for an ice-induced fatigue loading is derived. The ENSN approach is more reasonable to be used for this type of fatigue loading than the high cycle fatigue approach. A cruciform joint subjected to ice-induced fatigue loading is assessed to demonstrate the application of the fatigue reliability assessment procedures.

Fatigue strength of misaligned non-load-carrying cruciform joints made of ultra-high-strength steel

Misalignments and distortions in welded plate components act as stress raisers and can significantly decrease the fatigue strength of welded connections. This paper investigates the effect of the plate misalignment of transverse attachments on the fatigue behavior of axially-loaded non-load-carrying cruciform (NLCX) joints. Experimental fatigue tests with and without the plate misalignment are carried out for fillet-welded NLCX joints. The test specimens were fabricated of S1100 ultra-high-strength steel grade, and the fatigue tests were conducted using an applied stress ratio of R = 0.1. Numerical finite element analyses were conducted to obtain the stress concentrations induced by the misalignment. In addition, varied geometry parameters were applied to investigate their effect on the magnitude of stress concentrations. Stress concentrations were obtained using the structural hot spot stress method applying linear surface extrapolation and 1 mm below depth methods, and the effective notch stress concept with the reference radius of 1 mm. Experimental fatigue tests showed a decrease of up to 12% in fatigue strength depending on the degree of misalignment. The highest stress concentration was induced when the misalignment to the joint width ratio was e/L = 0.2–0.4. Structural stresses cannot be estimated using the linear surface extrapolation. Instead, structural stress at the 1 mm depth and effective notch stress concept accurately evaluated the misalignment effect on the fatigue performance of NLCX joints, and provided a good correspondence between the theoretical and experimental fatigue strength estimations.

Stress concentration at cruciform welded joints under axial and bending loading modes

The paper is concerned with the problem of stress concentration in cruciform fillet welded joints subjected to axial and bending load. Extended numerical analyses were carried out with the help of the finite element method. It made it possible to estimate stress concentration factors Kt for a variety of geometrical parameters defining the geometry of cruciform welded joints. It has been found that approximate Kt formulas, available in the literature, have two disadvantages, i.e. an unknown accuracy and small range of application with respect to geometrical parameters defining the weld shape. For these reasons, more general and accurate new formulas for stress concentration factors Kt have been derived. Even though the present approach is applicable to all types of welded joints, the analysis presented below has been conducted for a cruciform joint with the weld flank angle of θ = 45°. Final solutions have been given in the form of polynomial expressions, and they can be easily used in computer-aided design procedures.