Cruciform Joints Research Articles

Optimization of Welded Joints under Fatigue Loadings

In this paper, the notch effect in weldments has been investigated, and the optimal configuration of different types of welded joints has been analysed using the implicit gradient approach. By considering this implicit gradient method, it is possible to calculate the effective stress related to fatigue damage, with the effective stress being a continuous scalar function of the real stress tensor components, even in the presence of sharp edges. Hence, the search for the optimal configuration that maximises fatigue life can be tackled as the condition of minimum effective stress obtained by changing the weld shape and geometrical parameters. Both load-carrying cruciform joints and spot welds made of steel have been considered. The structural details have been studied by modelling actual shapes without any geometric simplification. Moreover, the same numerical procedure has been considered independently of the size, shape or load condition without imposing restrictive rules on the FE mesh.

Effect of Axial Misalignments in Fillet Welded Cruciform Joint Under Static Loading

Effect of Axial Misalignments in Fillet Welded Cruciform Joint Under Static Loading

Stress Concentration Factors for Non-Load-Carrying Welded Cruciform Joints Subjected to Tension, Bending, and Shear.

This paper deals with the problem of stress concentration at the weld toe of non-load-carrying-type plate cruciform joints under tension, bending, and shear. Theoretical stress concentration factors were derived using the finite element method. Five of the most important geometrical parameters: the thickness of the main plate and the attachments, the weld throat thickness, the weld toe radius, and the weld face inclination angle were treated as independent variables. For each loading mode-tension, bending, and shear-parametric expression of high accuracy was obtained, covering the range used in real structures for cruciform connections. The maximum percentage error was lower than 2.5% as compared to numerical values. The presented solutions proved to be valid for the toe radius ρ tending to zero.

Effect of angular distortion and axial misalignment on the fatigue strength of welded and ground mild steel cruciform joints

AbstractThe focus of this work is to investigate the fatigue strength of a cruciform joint one-sided through-welded by a HV seam with a backing layer made of a 10-mm-thick S355 mild steel. The aim is firstly to analyse the effect of the deformation state incorporating angular distortion and axial misalignment on the fatigue strength of the tested specimens and secondly, the analysis of the benefit by grinding as post-treatment method on the resulting fatigue performance. The deformation impact is numerically investigated utilizing geometry measurements of the specimens as well as strain gauge measurements for validation. Thereby, the near notch stress under clamping condition without any load and under an additional static nominal load is analysed. Based on these investigations, the applied stress range for each tested specimen is converted to a “deformation-free” condition by means of the IIW-recommended km factor. The statistical analyses of the data points with and without this modification reveal a significant impact by the deformation state on both the evaluated fatigue strength and the scatter band. Moreover, this affects the evaluated benefit by grinding as post-treatment, whereas an increase of more than 40% in the fatigue strength at 2 million cycles is observed using the modified data points.

Fatigue performance analysis of stainless steel cruciform joint with angular misalignment

AbstractWelding distortion causes the change of stress distribution of load‐carrying cruciform stainless steel welded joints, and the weld with angle less than 90° between steel plates became the fatigue performance control weld because of stress magnification. The effect of angular misalignment on fatigue performance of stainless steel cruciform welded joint was experimentally and numerically studied. The fatigue damage evolution equations, under different stress amplitudes, were derived using relative deformation increment as damage variable based on continuum damage mechanics to explain the process of damage transformation from micro to macro crack and finally rapid fracture. The hot spot stress magnification factor was positively correlated with the welding distortion angle and exhibited two discernible stages of rise and fall with increasing nominal stress taking the proportional limit as boundary. A proposed formula considering both misalignment angle and applied stress aimed to enhance accuracy in fatigue performance evaluation compared to current methods.

Research on fatigue curve fitting methods based on the notch stress approach

PurposeThis study investigates the fitting techniques for notch fatigue curves, seeking a more reliable method to predict the lifespan of welded structures.Design/methodology/approachBuilding on the fatigue test results of butt and cruciform joints, this research delves into the selection of fitting methods for the notch fatigue curve of welded joints. Both empirical formula and finite element methods (FEMs) were employed to assess the notch stress concentration factor at the toe and root of the two types of welded joints. Considering the mean stress correction and weld misalignment coefficients, the notch fatigue life curves were established using both direct and indirect methods.FindingsAn engineering example was employed to discern the differences between the direct and indirect approaches. The findings highlight the enhanced reliability of the indirect method for fitting the fatigue life curve.Originality/valueWhile the notch stress approach is extensively adopted due to its accurate prediction of component fatigue life, most scholars have overlooked the importance of its curve fitting methods. Existing literature scantily addresses the establishment of these curves. This paper offers a focused examination of fatigue curve fitting techniques, delivering valuable perspectives on method selection.

Experimental study on seismic performance of internal cruciform joints of grouting sleeve connection for modular integrated construction

Modular integrated construction (MiC) adopts room units that integrate maintenance structure, decoration and equipment pipelines in the factory-made modules, resulting in a lack of operating space for connecting modules at the construction site. In the previous research, the joints between modules are usually hinged or semi-rigid connections and partial strength joints. In this paper, a new type of joint with the grouting sleeve connection is proposed to connect modules horizontally and vertically, which is more convenient for construction and does not conflict with interior decoration. Six full-scale cruciform joints were tested to study the effect of axial compression ratio and different structural details. The test results presented the failure mode, stiffness, ductility, and energy dissipation of the specimens. The ultimate inter-story drift ratio is between 4.78 % rad and 5.96 % rad, which is larger than the requirement of 4 % rad in the special moment frame (SMF) system. The horizontal connection plate between modules can cooperate the deformation of left and right beams and columns, and significantly improve the lateral resistance of the joint. The vertical connection between modules did not slip or open and the grouting material is not broken. The inner and outer reinforced plates in the beam-to-column connection can transfer the tensile force of the beam flange well so the outer sleeve does not tear, and the wedge-shaped stiffener can inhibit the cracking of the beam-to-column connection. EC3 Part 1–8 was adopted to evaluate the stiffness characteristic of the proposed connection, indicating that the proposed joint can be classified as rigid joint for the non-sway frame. Finally, the theoretical ultimate resistance of the joint under the ideal failure mode is obtained, which is lower than the test results, indicating that the plastic development of the joint is sufficient and can be classified as full-strength joint.

Fatigue strength of cruciform joints with weld imperfections: A comprehensive numerical study

The paper presents an investigation into the fatigue resistance of load-carrying cruciform joints that exhibit weld imperfections. To accomplish this, a finite-element model of the joint is validated by a comparison to experimental investigations. The model includes both a global and a submodel that integrates the effective notch stress concept into its design. This enables the simulation of various types of weld imperfections, such as lack of penetration, lack of root fusion, undercut, excessive convexity, incorrect weld toe, excessive asymmetry of fillet welds and linear misalignment, to assess their impact on the fatigue strength of the cruciform joint. The analysis reveals that these imperfections lead to an increase in stress at the notches of the cruciform joint, which can be characterized as a function of the joint's geometrical properties. Global geometry influences on the fatigue strength of different cruciform join geometries can be regarded independently from the influence of the weld imperfections that is given by stress concentration factors. The normalised stress concentration factor enables a comparison between imperfection influences and shows, that the notch-softer design with full penetration welding at the cruciform joint is in a relative relation more strongly influenced by imperfections than the design with fillet welds. Also, the undercut represents the imperfection with the strongest effect in a relative relation within the sizes considered.Being able to evaluate the fatigue strength values of steel components with weld imperfections eliminates the need for costly repairs and new fabrications, which would otherwise result in a loss of resources and economic efficiency.

Fatigue performance assessment of thick TIG-Dressing cruciform welded joints made by Q355D structural steel

TIG-Dressing treatment is often used for steel welded joints in construction machinery manufacturing as an effective and reliable method for post-weld fatigue strength improvement. This paper investigates the fatigue performance of thick Q355D cruciform joints in heavy load-carrying steel structures under different TIG-Dressing treatments. Two TIG-Dressing treatments for the full-penetration welded joints were used for fatigue tests considering filling metal during this post-welded processing. Experimental tests studied the fatigue strength of TIG-Dressing cruciform welded joints of Q355D structural steel. The geometric parameters and relevant statistical analyses were performed by actual 3D optical measurement. Moreover, the stress concentration factors are calculated according to the actual geometrics of the welded joint. On the other hand, the effective notch stress and hot spot stress methods were employed to evaluate the fatigue data of Load-Carrying Welded joints (LCWJs) under different notch radius. The collected fatigue data from the literature was compared with the tested data by the above methods. The results show that the fatigue strength of LCWJs under TIG-Dressing processing without filler metal is higher than those under the same TIG-Dressing treatment with filler metal. The effective notch stress method with a fictitious notch radius r = rtrue + 1 mm (rtrue is the actual notch radius at weld toe) could obtain smaller scatter bands for the tested fatigue data.

Stress intensity factors of elliptical cracks at the weld toe

The paper takes into account the assessment of the stress intensity factor (SIF) of defects similar to elliptical cracks located at the weld toe. By assuming a second order approximation of the Oore-Burns equations the SIF has been evaluated with simplified equations. The analytical results are compared with those evaluated by means of the finite elements method. As an example, the SIF is calculated for a defect located in the neighborhood of the weld toe of full penetration cruciform joints under mode I loading.

Misalignment factors to affect the fatigue of welded load-carrying joints

To assess the effects of misalignment on the weld fatigue, we present experimental fatigue test results of load- carrying cruciform joints which are subjected to axial variable amplitude loading. The welds were produced from steel AH36 grade and fatigue strength improved by high-frequently mechanical impact treatment. We identify several misalignment factors affecting the fatigue by considering the real and reference welded joint geometries. Our methodology includes proposals for the empirical calibration functions and local stress analyses by using the Structural Hot Spot Stress, Effective Notch Stress and Peak Stress Method. Additionally, we apply our methodology to other sixteen data sets which are extracted from the literature for load-carrying cruciform and butt welded joints subjected to constant amplitude loading. Finally, we present the resultant calibration functions with respect to corresponding fatigue test data.

Fatigue life prediction of welded joints at sub-zero temperatures using modified Paris–Erdogan parameters

The fatigue life of welded steel components is usually determined by the weldment; in these cases, fracture mechanical approaches are widely used for their prediction. Ferritic steels are known to have a fatigue strength that is dependent on temperature. Therefore, this study evaluates fatigue tests of cruciform joints and transverse stiffeners at different sub-zero temperature levels regarding fatigue life. Simultaneously, the stress intensity factors over the crack length are calculated for the individual experiments using analytical solutions. Then, using the Paris–Erdogan relation with temperature- and material-specific C and m parameters as well as tabular values, the fatigue lives are calculated with analytical solutions and compared with the experimental results. It is shown that the prediction accuracy is significantly increased for the sub-zero temperature range by using temperature-adjusted Paris–Erdogan parameters, as long as the temperature is above the fatigue transition temperature.

High-cycle fatigue life assessment of welded cruciform joints of Q460D steel

In this paper, a reliable high-cycle fatigue life assessment method based on unified crack growth approach is presented for welded cruciform joints. Considering the ellipsoidal fracture model of structural steel as the instability propagation (fracture) criterion of fatigue crack, this study investigates the failure mode of the welded cruciform joint of Q460D steel subjected to a high-cycle fatigue test. Following the ellipsoidal fracture model, a set of theoretical calculation and numerical simulation were established to investigate the fatigue cracking of the welded cruciform joints. The theoretical model was further proposed for estimating the fatigue life of the welded cruciform joints of Q460D steel using the unified crack growth approach. Based on the theoretical and simulated results, the unified crack growth approach and the allowable stress fatigue life formula recommended in China's current code “Standard for design of steel structures, GB50017-2017” were then evaluated and compared. The calculation errors of the unified crack growth approach and the fatigue life formula recommended in China's current code are −48.2% ∼ +8.3% and −60.2% ∼ −34.8%, respectively.

An analytical approach for evaluation of linear elastic stress fields around sharp V-shaped notches in plates

In the context of the theory of elasticity, the stress state at a sharp notch is singular, and the degree of stress singularity depends on the notch opening angle. Most existing analytical approaches for evaluating asymptotic linear elastic stress fields ahead of V-notches rely on notch stress intensity factors (N-SIFs). This paper presents an alternative analytical approach for evaluating notch stress fields by taking advantage of traction-based structural stress and Williams’ eigenfunction methods. To better represent notch stress fields, the first order and higher order terms of eigenfunction are included in the present study. The analytical solutions are then compared to the stress fields resulting from finite element analysis for validation purposes. The stress contours evaluated by the analytical approach are nearly identical to the numerical results corresponding to three distinguishing notch opening angles under pure Mode I loading, which signifies the effectiveness and robustness of the analytical solution methodology developed in the present study. Subsequently, the approach is extended to accommodate Mode II loading by introducing a skew-symmetric term in the stress function. The generalized method is then applied on a welded cruciform joint, which precisely captures the notch fields compared to FEA results.

Stress range in the welded cruciform joint in the shell model using the Hot Spot Stress Method

AbstractThis paper focuses on the linear extrapolation procedure for the Hot Spot Stress Method on a platés surface using shell models. Published papers have shown the benefits of solving the fatigue life by methodologies that integrate changes of the stress near the weld toe. This procedure is based on modelling the joints by solid elements. Current advanced connection models for steel structures are based on shell models which save computational time by an order of magnitude and allow the modelling of structural joints in a sufficiently short time using second‐order theory.Seven models for making shell welded connections are presented here, each of which was prepared for a specific joint. Many researchers have reported that non‐universality and deviations of the stress peak value in the first extrapolation point are major questions. A sensitivity study has indicated that there is a conditional stable ratio between the values of the stress ranges in the two evaluation points. A comparison between cases, that use experimental parameters and results taken from the literature, has shown that there is a reduction in the differences of the stress ranges between solid and shell models.

On the fatigue behaviour and reliability assessment of high strength welded details

This research focusses on the fatigue behaviour of welded components made of high strength duplex (EN 1.4162) and carbon steel. It starts by investigating the base material alongside welded cruciform joints submitted to cyclic loading through experiments and finite element models. The obtained results are then compared with existing research on high strength (carbon and stainless) steel equivalents. This collated database of >500 reference test results is then used to assess the applicability of the current Eurocode and IIW provisions to predict the fatigue life of higher strength steels and to propose new rules that include the benefit of their higher material strength. Proposals are made to upgrade certain fatigue classes (e.g., transverse stiffeners and butt welds) when high strength steel grades are employed. The reliability of these proposals is then supported by the use Weibull models, commonly used in survival analysis.

Low-cycle fatigue in ultra-high-strength steel welded joints in the as-welded and post-weld-treated conditions

This study investigates four different joints under fluctuating tension cycles in the low-cycle fatigue (LCF) regime. The selected material was S960 ultra-high-strength steel, manufactured via the direct-quenching manufacturing route. Based on typical engineering solutions, the following joints were chosen to be investigated in the experimental tests: metal active gas and laser welded butt joints, fillet-welded load-carrying cruciform joints, and non-load-carrying double-sided transverse attachment joints. To evaluate the improvement gained by the post-weld treatment, both as-welded condition and high-frequency mechanical impact treated joints from all joint types were tested. In addition to the experimental tests, some of the joints were analyzed using finite element (FE) models, with the aim of comparing the accuracy of the different fatigue assessment methods and the effect of the toe radius on the computational fatigue lives. The results obtained from the experimental tests and FE analyses were compared with each other and with IIW recommendations using the nominal stress, structural hot-spot (HS) stress, effective notch stress (ENS) and 4R methods. Based on the results, the structural HS and 4R methods are very suitable for the fatigue strength assessment in the LCF regime. With the nominal stress method, the computational fatigue lives remained uniformly conservative, while with the ENS method, the results varied depending on the calculation method. Using the value r = 1 mm as the weld toe rounding is justified in the FE analysis.

Seismic performance of cruciform bolted joints with various reinforcing details in modular steel construction

Extensive research has been done on exterior T-shaped joints of modular structures. The modular structure consists of stacked modules that are not only connected vertically but also need to be connected horizontally. The horizontal connection between modules is another key for the modules to form an integral structure and resist lateral forces. This paper proposes a novel bolted joint that can be applied to multi-column module internal and external connections. By designing special loading supports, three internal cruciform joints and three external cruciform joints of the modular structure were tested under column end loading. The effects of different strengthening measures and the ratio of beam to column flexural bearing capacity on the seismic performance of the joints were compared. The results showed that all the joints had good seismic performance and satisfied the requirements of SMF systems in terms of inter-story drift ratio. The stiffness and bearing capacity of the internal joints after the beam-beam combination increased by 33.2% and 17%, respectively. However, the stiffness and bearing capacity of the cruciform joints strengthened by the cover plate are not significantly improved. For joints that meet the conditions of strong columns and weak beams, modular beams can reach full-section plasticity and can meet the requirements of full-strength joints.

Fatigue performance of Q420C steel fillet-welded joints at low temperatures

This paper focuses on the fatigue performance of Q420C steel fillet-welded joints at low temperatures. The tensile tests and Charpy V-notch tests were first performed to evaluate the mechanical properties of Q420C steel fillet-welded joints under room and low temperatures. It reveals that the yield and ultimate strengths increased with reducing temperature from 20 °C to −50 °C. The Charpy impact energy transition temperature (ETT50) was –33.67 °C. Subsequently, the fatigue test was conducted on the fillet-welded cruciform joint specimens of Q420C steel at 20 °C, 0 °C, −15 °C, −30 °C, 40 °C and −50 °C considering three stress ranges (i.e., 0.0–0.5 ftw, 0.0–0.7 ftw and 0.0–0.9 ftw), where ftw was the design strength of fillet weld. The micro-morphologies of fatigue fracture surfaces were observed by scanning electron microscope (SEM), and the river patterns indicating brittle fracture occurred at −50 °C. The S-N curves and P-S-N curves with a survival probability of 95% were fitted from experimental fatigue life. With decreasing temperature from room temperature (20 °C) to −40 °C, the fatigue life increased at low-stress range (0.0–0.5 ftw), while shrank at high-stress range (0.0–0.9 ftw). Then, the fatigue life presented a declination trend at −50 °C for these three stress ranges, compared with that at −40 °C.

Fatigue properties evaluation of fillet weld joints in full-scale steel marine structures

This study investigated the fatigue behavior of fillet weld steel joints (including cruciform and T-welded joints) in marine structures based on experimental tests and finite element analysis (FEA). In cyclic tensile loading, fatigue failure of full-scale specimens consistent with in-plane failure mode of marine structures was validated with ultrasonic phased array testing (UPAT). The effect of incomplete penetration and weld profile on fatigue performance were analyzed based on traction structural stress method. The results showed that the failure mode of load-carrying cruciform joints would change when the incomplete penetration ratio p’/tr reached 0.91. And the concave weld profile is better choice for welded structures anti-fatigue design. In view of the complexity of fatigue performance evaluation of fillet weld joints in IIW and BS7608 standards, a unified master S–N curve covering different plate thicknesses, materials, and weld profiles was proposed.