Cruciform Joints Research Articles

Fatigue Assessment and Lefm Analysis of Cruciform Joints Fabricated with Different Welding Processes

In this study fatigue testing and defect assessment were carried out on specimens welded with robotic and manual welding using flux cored (FCAW) and metal cored (MCAW) filler materials in order to study the effect of the welding method on the fatigue strength and weld quality. Thirteen different batches were investigated of which two was shot peened before fatigue testing. The local weld geometry was measured for all the specimens before testing. The specimens welded with flux cored weld wire showed the best fatigue strength, small defects and low residual stresses. Large scatter in the fatigue data is observed, especially when manual welding is employed. The few largest defects were removed by the shot peening process, although small defects survived. This led to a smaller scatter in fatigue live for the shot peened specimens. Linear elastic fracture mechanics, LEFM, was employed for analysis of the fatigue test results. The fatigue life predictions using a 2D LEFM FE-model for simulating a continuous cold lap defect along the weld toe showed a qualitative agreement with the fatigue test results. The 2D analysis showed that a continuous cold lap defect should be no more than 0.5 mm deep in order to comply with the requirement of fatigue lives for normal weld quality according to the IIW design rules. For larger defects (> 0.8 mm) an increased toe radius will have a small effect on the fatigue strength. A 3D LEFM analysis of crack growth from a spatter-induced cold lap defect was also carried out. This showed similar trends in crack growth compared to the 2D analysis of a continuous cold lap, although the spatter-induced cold lap defect (semi-elliptical) had a longer fatigue life (x2.7), and hence is less dangerous from a fatigue point of view.

A procedure for fatigue design of web-stiffened cruciform connections

Fatigue design of dynamic loaded ship structures is normally based on finite element analysis using shell elements where the welds are not included in the analysis models. Finite element analyses of a fatigue-tested hopper knuckle type specimen were performed for calibration of analysis methodology in a joint industry project. The test specimen was analyzed using solid and shell elements. Only the solid element models included the weld geometry. In the present work, the finite element analyses were extended to other geometries at web-stiffened cruciform joints such as typical found at hopper connections, at stringer heels, and at joints connecting deck structures to vertical members in ship structures. Based on the result from these analyses, a methodology for derivation of hot spot stress at welded connections accounting for the weld size using shell finite element models was developed. The basis for this procedure is presented in this paper.

A simplified fatigue assessment method for high quality welded cruciform joints

The primary goal of this study was to develop an equation relating the geometric parameters to fatigue strength which can be used is routine design assessment. To attain this, the influence of local geometrical weld variations on the fatigue strength of non-load-carrying cruciform fillet welded joints were systematically studied using plane strain linear elastic fracture mechanics (LEFM). The effects of weld toe radius, flank angle and weld size were considered. Both continuous weld toe cracks and semi-elliptical toe cracks with alternate pre-existing defect depths were considered. A previously developed experimental crack aspect ratio development curve was used for assessing the growth of the semi-elliptical cracks using 2D FE models. A total of 152 experimental fatigue data points from six published studies of welded cruciform joints were evaluated. Details of the actual weld toe radius, flank angle and weld size were available for these joints. For the high quality welds evaluated, an assumed initial crack depth of 0.05 mm was found to correlate best with the experimental data. Of all the geometric parameters considered analytically, weld toe radius was found to have the most dramatic influence on fatigue life. A simple equation is proposed which relates welded joint fatigue strength to the ratio weld toe radius/plate thickness for high quality welds.

Assessment of stress intensity factors for load-carrying fillet welded cruciform joints using a digital camera

This paper presents an experimental method to determine the stress intensity factors (SIFs) of load-carrying fillet welded cruciform joints. This experimental method measured the crack opening displacements (CODs) of the cruciform joint detail by a common digital camera, and utilized the acquired COD data to derive the SIF for the joint detail by a suggested least-squares procedure. A total of fifteen fillet welded cruciform joint specimens were tested and analyzed in this study. The test results were compared with the SIF formula provided in British Standard 7910, and showed that the demonstrated experimental method is able to give satisfactory SIF evaluation for fillet welded cruciform joints. In fact, this method can be applied to determine the SIF of any detail with surface crack where the Irwin’s series solution for crack tip displacement fields is valid. Moreover, the simplicity and portability of this method make the field measurement of SIF possible and provide a good means to acquire SIFs directly from details at the site. This is very useful for fatigue evaluation of details in structures, such as steel bridges or offshore structures.

Improving fatigue life for aluminium cruciform joints by weld toe grinding

ABSTRACTThe increase of fatigue life in aluminium cruciform joints by weld toe grinding was the focus of the current study. The test data are presented by both a nominal stress range approach and by the more refined structural and notch stress range approaches. The influence of the weld toe angle, weld leg length and weld toe radius on the structural and notch stress concentration factor (SCF) was systematically studied by means of finite element analysis. Experimental data based on 18 pieces of as‐welded and 13 pieces of weld toe‐ground specimens made of 12 mm thick plates showed a significant improvement in fatigue life in aluminium by grinding the weld toe and confirmed the permitted improvement in fatigue life by design codes.

Difference of fatigue strength of cruciform welded joints due to specimen configuration

Fatigue strength obtained from cruciform welded joint specimens may be different from that of actual joints such as welded connections of lateral girder flange to main girder web or vertical stiffener to main girder flange, because main girder web is mush wider than lateral girder flange or vertical stiffener is narrower than main girder flange. For the purpose of examining the influence of specimen configuration on the fatigue strength, fatigue tests and stress analyses have been performed using model specimens on the connections of lateral girder flanges and usual cruciform welded joint specimens. Fatigue test results suggest that fatigue strength of the model specimens has been nearly equal to that of cruciform welded joint specimens. This fact has been also confirmed from the stress analyses considering weld shape at the corner of main plate.

DEVELOPMENT OF HIGH STRENGTH REINFORCED CONCRETE FLAT BEAM SYSTEM

RC flat beam system, which was rigid frame construction that consisted with RC flat beams and normal RC columns, were researched and developed in the previous study by the authors. The flat beam of this system has wider width than columns have and shorter depth than normal beams have. In this study, high strength RC was used for the flat beam system. In order to investigate performance of high strength RC flat beam system, cyclic loading tests of two beam-column joint specimens were carried out, which one is cruciform joint specimen, the other is corner joint specimen. As results, it can be said that there were no serious problem in this system. All the specimens showed restoring force characteristics of flexural type, and flexural strength could be estimated by traditional ways. An estimating method of stiffness of RC flat beam and column joint was shown in this paper.

DEVELOPMENT OF STEEL REINFORCED CONCRETE FLAT BEAM SYSTEM

In this paper, a SRC flat beam system, which was rigid frame construction that consisted with SRC flat beams and normal SRC columns, was researched. The flat beam of this system has wider width than columns have and shorter depth than nomal beams have. Steel is placed eccentrically in a beam section if the flat beam and the column were jointed eccentrically. In this case, it is necessary to consider unsymmetrical bending when flexural strength of the SRC flat beam section is estimated. In this study, cyclic loading test of a SRC cruciform joint specimen was carried out to investigate performance of the SRC flat beam system. As a result, the specimen showed restoring force characteristics of flexural type, and an estimating method of flexural strength of SRC section that had eccentricity of steel was shown in this paper.

Fatigue assessment of large-size welded joints based on the effective notch stress approach

In this paper the applicability of the effective notch stress approach to fatigue assessment of existing large-size specimens for cruciform joints, diaphragm joints and out-of-plane gusset joints has been investigated, by considering fatigue crack initiation points, stress distributions and fatigue strengths. By using the maximum principal stress, it is possible to distinguish whether fatigue cracks will initiate at the weld toe or the weld root. Fatigue strengths of the existing test results using the effective notch stress approach are above the SN curve of FAT300. For out-of-plane gusset joints, the effective notch stresses along the weld root line depend significantly on weld size and weld penetration size.

A study on fatigue crack initiation point of load-carrying fillet welded cruciform joints

Load-carrying fillet welded cruciform joints have two fatigue crack initiating points. These are located at the weld toes and weld roots, where high stress concentrations occur. The location of the fatigue crack initiation depends on the weld size, plate thickness and weld penetration. These mechanisms are easily understood. Previous experimental results have indicated that the fatigue crack originating point is also influenced by the magnitude of the stress range, but the reason for this phenomenon is not yet clear. To better understand this phenomenon, fatigue tests on welded joint specimens were performed in this study. In addition, the welding residual stresses were analyzed with a thermo-elastic–plastic finite element method. Finally, a fatigue crack propagation analysis that considered the residual stresses was performed.

Fatigue Design of Welded Joints by Local Approaches: Comparison between Fictitious Notch Rounding and Strain Energy Averaging

The paper demonstrates the close correspondence between two local approaches to assess the fatigue strength of welded joints: Radaj’s approach based on fictitious notch rounding and a recently proposed approach based on the local strain energy density (SED) averaged over a given control volume. This volume surrounds the weld root or weld toe, both modelled as sharp (zero radius) V-notches with different opening angles. The two approaches are applied to load carrying and non-load carrying cruciform joints and the theoretical fatigue notch factors Kf are compared. The SED averaged over the control volume is determined from finite element models with very fine meshes, as typically designed to evaluate the intensity of the asymptotic stress distributions, and also from coarse meshes, showing a surprisingly good correspondence.

Modified Wöhler curve method, theory of critical distances and Eurocode 3: A novel engineering procedure to predict the lifetime of steel welded joints subjected to both uniaxial and multiaxial fatigue loading

In order to propose a procedure suitable for assessing steel welded joints by post-processing simple linear-elastic finite element (FE) models, this paper summarises an attempt to formalise a novel approach based on the use of the modified Wöhler curve method (MWCM) applied along with the theory of critical distances (TCD). This engineering procedure was initially calibrated by using the standard curves supplied by Eurocode 3. Subsequently, an unifying value for the multiaxial critical distance was calculated by taking full advantage from the notch-stress intensity factor (N-SIF) approach. The accuracy and reliability of the devised method was systematically checked by using a large amount of data taken from the literature and generated by testing steel welded details both under uniaxial and multiaxial nominal loading. In particular, both standard cruciform joints and structural details having complex geometries were considered and the devised procedure was applied to specimens tested both in as-welded and in stress-relieved condition. This approach was seen to be highly accurate, giving estimates falling within the widest standard scatter band between the two used to calibrate the approach itself. In particular, our method proved to be capable of correctly taking into account the scale effect as well as the degree of non-proportionality of the stress field damaging the fatigue process zone. Such results are very interesting and promising, especially in light of the fact that the numerical effort which has to be made to apply the proposed approach is the same as the one needed to apply other existing linear-elastic approaches.

Round-Robin Study on Stress Analysis for the Effective Notch Stress Approach

The effective notch stress approach for the fatigue strength assessment of welded structures as included in the Fatigue Design Recommendations of the IIW requires the numerical analysis of the elastic notch stress in the weld toe and/or weld root which is fictitiously rounded with a radius of 1 mm. In order to establish guidelines for modelling the structure and evaluating the results, several numerical analyses have been performed in a round robin. This covers three welded details of different complexity where either the weld toe or the weld root is the potential crack location. The first example is a cruciform joint with non-load carrying fillet welds in one load case and load carrying in the other. The second is a complex T-joint of rectangular hollow section (RHS) members which was also subject of the so-called SAE-FDE weld challenge to predict fatigue life. The third is the fillet-welded end connection of a RHS joint being prone to fatigue failure at the weld root with non-fused root faces. The results are compared with each other and conclusions are drawn with regard to appropriate modelling and to the scatter of results to be expected.

Local strain energy approach applied to fatigue analysis of welded rectangular hollow section joints

This paper deals with fatigue life assessments of welded rectangular hollow sections made of steels and aluminium alloys, in a T-joint configuration, carried out by means of a local stress approach. The welded toe region is modelled like a sharp V-notch with an opening angle of 135 and the intensity of the asymptotic stress distributions is quantified by means of the Notch Stress Intensity Factors (NSIFs). In order to carefully evaluate the NSIFs of hollow sections welded in a T-joint configuration, some Three-Dimensional (3D) Finite Element (FE) analyses are firstly carried out. Afterwards, the NSIFs are used to evaluate the mean value of the strain energy density over a semicircular sector of radius RC at the welded toe. The radius RC is thought of as a material-dependent parameter. The experimental values of fatigue life are compared with those predicted by two previously scatter bands curves recently reported in the literature for welded joints made of structural steel and aluminium alloy. These curves were drawn by using mainly fatigue data obtained from cruciform and T-shaped welded joints. Although the analyses reported here represent only a first attempt to extend an energy-based approach to 3D hollow section joints, the comparison shows that a quite satisfactory assessment of the fatigue life can also be obtained for this type of welded joints. The agreement is good for welded joints made of structural steel, whereas the predictions are found to be in the safe direction for welded joints made of aluminium.

Study of Improvement of Fatigue Properties of Welded Joints by Compressive Stress Induced by Plasma Spraying

In order to increase the fatigue strengths of welded joints, a new kind of metal powder spray was developed based on the volume expansion principle, such that the coating would be left in residual compression. Fatigue tests were performed on cruciform welded joints and plates with longitudinal fillet welded attachments in 1Cr18Ni9Ti stainless steel under stress ratio R = 0.1. The specimens were tested in the as-welded condition and after being plasma sprayed with low carbon steel or low transformation temperature metal powder spray. Previous results had indicated that plasma spraying was an effective improvement technique, producing a clear increase in fatigue life over the range of the S-N curve. When the new metal powder spray was used, the improvement was even higher. Compared with specimens sprayed with low carbon steel, the increase in fatigue strength at 2 × 106 cycles and fatigue life over the range investigated of the new metal powder spray were 3.6 to 5.3 % and 16 to 63 % respectively. The extra improvement in fatigue properties was attributed to the presence of a compressive residual stress in the coating.

WES2805における溶接継手止端部き裂のCTOD評価に用いる応力集中とひずみの計算

WES 2805, a Japanese fitness-for-service procedure, proposes a crack tip opening displacement (CTOD) design curve for the brittle fracture assessment of welded joints, where the local strain acting on the cracked region, ε, is calculated using the stress concentration factor, Kt, and ε is converted to CTOD. However, in WES 2805, Kt, is shown as a few constants that depend on the ratio of crack depth, α, to the welded joint thickness, t, and the constants can be excessively conservative. In this study, the relation between Kt and the stress intensity magnification factor, Mk, was theoretically indicated, and the utility of Mk in the design curve was investigated for three kinds of welded joints. In addition, the local average strain by area was calculated by finite element analysis, and was used for the design curve instead of ε. It was demonstrated that constant Kt was sometimes excessively high and Mk was helpful in modifying Kt. Moreover, the local average strain by area contributes to good CTOD estimation, especially for a toe crack in a load-carrying fillet welded cruciform joint.

OS4-7-4 Weld geometry effect on the fatigue strength of non-load-carrying fillet welded cruciform joints

This study investigated the effect of weld geometry on the fatigue strength of non-load- carrying fillet welded cruciform joints. The weld geometry of the cruciform specimens was intentionally varied. Fatigue tests were carried out on various weld geometry configurations. The configurations included weld flank angle, weld toe radius and weld throat thickness. Fatigue tests were conducted using a servo hydraulic controlled 250 kN capacity UTM with a frequency of 5 Hz under constant amplitude loading. From the experimental results, relationship between the weld geometry and the fatigue strength of non-load-carrying fillet welded cruciform joints was analyzed in detail.

Fatigue Assessment of Cruciform Joints Welded with Different Methods

The advantages of the relatively new laser welding method are low welding distortions and high productivity, and it opens new opportunities for welding of steel structures in the automotive and heavy machinery industries. There is a need for understanding the fatigue strength and to develop fatigue design curves for laser welded joints. In this paper fatigue testing and weld quality of Hybrid Nd: YAG laser/MAG and MAG welded non‐load carrying cruciform joints is examined. Four batches were produced, tested and the results were compared. The local weld geometry of the cruciform welded joints was measured and analysed. The measured parameters were used to calculate the stress concentration factors by implementing FE‐models, in order to anticipate the influence of size of weld geometrical parameters on the stress concentrations. Residual stress measurement was carried out close to the toe region using the X‐ray diffraction method and weld defects (cold laps) in the cracked specimens was measured.

FATIGUE STRENGTH ASSESSMENT OF THE FILLET WELDED CRUCIFORM JOINTS

In this study, fatigue tests to obtain S - N curves and FE analyses to obtain structural stress concentration factors were conducted for two types of fillet welded cruciform joints, that is, load-carrying and non load-carrying types. The obtained S - N curve of the load-carrying joint was changed to that based on hot-spot stress. As a result, the S - N curve of the load-carrying joint based on hot-spot stress almost coincided with that of the non load-carrying joint based on nominal stress. The fatigue strength of a welded joint which has a different geometry from that of the non load-carrying cruciform joint but the same bead profile as that of the non load-carrying cruciform joint could be estimated by using both the structural stress concentration factor at the weld toe position obtained from FEM and the nominal S - N curve of the non load-carrying cruciform joint from experiment.

Experimental behaviour of extended end-plate composite beam-to-column joints subjected to reversal of loading

This paper is concerned with the behaviour of steel and concrete composite joints subjected to reversal of loading. Three cruciform composite joint specimens and one bare steel joint specimen were tested so that one side of the beam-to-column connection was under negative moment and another side under positive moment. The steelwork beam-to-column connections were made of bolted end plate with an extended haunch section. Composite slabs employing metal decking were used for all the composite joint specimens. The moment-rotation relationships for the joints were obtained experimentally. Details of the experimental observations and results were reported.