Thin circular hollow section-to-plate T-joints: Stress concentration factors and fatigue failure under in-plane bending

Welded thin-walled (t<4 mm) tube-to-plate T-joints made up of cold-formed circular hollow sections welded onto a plate to form a moment resistant connection are used in the road transport and agricultural industry to manufacture equipment and other structural systems. Fatigue design of these joints is not available in current standards. An understanding of the stress concentrations and failure in these connections is therefore necessary as a step towards understanding the fatigue behaviour of these connections. Stress concentration factors (SCFs) of welded thin-walled (t<4 mm) circular hollow section (CHS)-to-plate T-joints are determined at different locations along the weld toes on the tubular brace. The distribution of SCFs along the weld toes shows that the highest SCF occurs at the weld toes in the circular brace at the 0° line. The ratio of the end of test fatigue life (N4) to the through-thickness fatigue life (N3) in the thin CHS-plate T-joints is found to fall within the range of N4/N3 found in previous research of both thick and thin-walled joints. Surface crack growth monitoring is used to obtain an approximation of the length of surface crack at the point of occurrence of a through-thickness crack. The relationship between surface crack length and the occurrence of a through-thickness crack is important in that it can be used as a measure of the criticality of a surface crack during structural health monitoring of equipment or structures.     

随机变幅荷载作用下焊接节点海水腐蚀疲劳裂纹扩展试验

本工作采用国产海上平台用钢的T型和十字形板状焊接节点试件,模拟导管架臂节点的弯曲受力状态,在随机变幅荷载下进行了海水腐蚀疲劳裂纹扩展试验,分别测得在海水自由腐蚀和阴极保护条件下的(~—da/dN)-AK~(NM)关系曲线。对于十字型焊接节点试件,根据试验测得的裂纹扩展速率曲线,用断裂力学方法对其海水腐蚀疲劳裂纹扩展寿命(有阴极保护)进行了估算,估算得到的裂纹扩展寿命曲线与试验数据,以及用线性累积损伤理论估算的寿命曲线,进行了分析比较和讨论。     

Effect of stress ratio on the enhancement of fatigue strength in high performance steel welded joints by ultrasonic impact treatment

In this study, we conducted fatigue tests under various stress ratios using cruciform welded joints to confirm the benefit of Ultrasonic Impact Treatment (UIT) for the fatigue strength of welded joints. The material used in the experiment is the JIS SBHS500 high performance steel for bridges. The fatigue life of the peened weld joint by UIT is significantly longer than that of as-welded joints, especially at a low stress ratio. We also estimated the fatigue life of these cruciform joints by crack growth analysis based on the crack opening and closure simulation using the modified strip-yielding model, accounting for the residual stress distribution created by welding or UIT. These estimation results demonstrate good agreement with experimental results obtained at various stress ratios.     

Acoustic emission detection of fatigue damage in cruciform welded joints

Weld seams are critical points for the initiation of fatigue cracks in steel structures subjected to cyclic loads. Semi-elliptical surface cracking at the toes of a fillet weld is not easily found when it is partially through the thickness and subcritical. In this study the acoustic emission (AE) method is used to detect crack propagation in cruciform fillet welded joints that are representative of typical fatigue sensitive details in steel bridge superstructures. The effect of geometry and fatigue load on the AE data is investigated by varying the width of the base plate and the stress ratio. AE data filtering based on load pattern, source location, and waveform feature analysis was implemented to minimize noise-induced AE signals and false indications due to wave reflections. AE time domain features such as amplitude (b-value), counts, signal strength, and absolute energy are employed to study the influence of geometry and fatigue load on the AE data.     

高性能桥梁钢典型焊接接头疲劳性能试验研究

针对Q500qE高性能钢两种典型焊接接头,包括对接焊缝和横向角接焊缝,设计疲劳试件。其中对接焊试件包括两种形式,分别为板厚56mm和8mm的对 接焊试件;横向角接焊缝试件的主板厚56mm、附连件厚20mm。对三组试件进行了有限元分析,掌握了试件的应力分布状况及薄弱环节,验证了试件设计的合 理性。随后针对三组试件展开了疲劳试验,掌握了不同类型焊接接头的破坏位置及破坏形式。根据试验结果拟合出了三组试件的S-N曲线,并与普通钢的疲 劳性能进行比较,表明Q500qE高性能钢的这两种典型焊接接头的疲劳强度略高于普通钢。采用目前规范规定的疲劳强度容许值进行抗疲劳设计是合理可行的,并且具有足够的安全余量。     

Q460D高强钢及其焊缝连接疲劳性能试验研究

在疲劳荷载作用下钢结构焊缝区易发生疲劳断裂,通过Q460D高强钢及其焊缝连接的疲劳性能试验研究,结合试验数据,拟合了Smax-N曲线预测其疲劳寿命,根据疲劳损伤理论分析了疲劳破坏程度,并通过断口形貌揭示了疲劳裂纹扩展规律。研究结果表明：Q460D母材具有较高的疲劳抗力;对接焊缝接头Smax-N的95%保证率曲线与ANSI/AISC 360-10的疲劳设计曲线吻合较好;GB 50017—2003的设计曲线能较好预估循环次数大于40万次以上十字接头的疲劳寿命。损伤指标能够较好地表征疲劳破坏过程中构件内部状态的变化,缺口系数越大,损伤发展越快。瞬断前裂纹扩展规律与损伤发展一致,随着损伤发展疲劳条带间距逐渐变大。     

Fatigue strength improvement of welded joints by blast cleaning for subsequent painting

In the fabrication of steel bridges, prior to painting, blast cleaning is performed to clean the surfaces and to increase the adhesive properties of the surfaces for the subsequent painting. In addition to these objectives, blast cleaning can improve the fatigue strength of welded joints, as a result of the impact of forcibly propelled abrasive materials on the welded joints. A series of fatigue tests were carried out on the five different types of welded joints: one-side out-of-plane gusset fillet welded joints, both-side out-of-plane gusset fillet welded joints, non-load-carrying rib fillet welded cruciform joints, butt welded joints, and a large-scale model beam with out-of-plane gusset welded joints, and under the three different types of load conditions: uniaxial tension, out-of-plane bending, or in-plane bending stress cycles. Drawing on previous fatigue test results that include unpublished data, this paper presents the beneficial effects of blast cleaning for the subsequent painting applications on the fatigue strength improvement of welded joints and also presents a simple quality control method for the blast cleaning process that improves the fatigue strength.     

基于等效结构应力的钢框架焊接梁柱节点低周疲劳评估

在地震作用下钢框架梁柱焊接节点会发生低周疲劳现象,进而导致结构发生破坏。本文基于等效结构应力法提出一种高层钢框架梁柱焊接节点低周疲劳评估方法。首先,结合有限元多尺度模拟方法,建立包含翼缘及腹板焊缝细节的钢框架梁柱焊接节点多尺度模型。其次,利用不同单元尺寸多尺度模型,验证结构应力的网格不敏感特性,进而通过等效结构应力法评估钢框架梁柱焊接节点的疲劳寿命。结果表明,采用等效结构应力法可消除网格的敏感性,在往复荷载作用下,靠近工艺孔焊缝应力水平高于远离工艺孔焊缝,翼缘中心焊缝寿命最低,裂纹最先产生于翼缘焊缝中心处,评估方法为实际工程提供技术支持。     

Square hollow section (SHS) T-joints with concrete-filled chords subjected to in-plane fatigue loading in the brace

In recent years, there has been an increased use of concrete-filled composite tubular joints in bridge construction. The reliability of these joints under fatigue therefore need to be investigated to avoid any catastrophic failures. In this paper, hollow section T-joints made up of square hollow section (SHS) chords and braces are investigated. The chords of the SHS–SHS T-joints were concrete-filled thereby forming welded composite tubular T-joints. The SHS–SHS T-joints with concrete-filled chords were strain gauged and tested under static loading to determine stress concentration factors (SCFs) at hot spot locations, where cracks are likely to propagate. Fatigue tests of the welded composite joints were also carried out under cyclic in-plane bending in the brace to obtain stress range vs. number of cycles (S–N) data. The maximum stress concentration factor (SCF) at hot spot locations in a welded composite tubular T-joint was found to be generally lower than the maximum SCF in an empty (or called unfilled) hollow section SHS–SHS T-joint. Fatigue failure in welded composite tubular T-joints occurred through the initiation and propagation of cracks at weld toes in either the chord or brace member, with the majority of tests exhibiting the first visual crack at weld toes in the chord. The welded composite tubular T-joints were found to have better fatigue strength compared to the empty hollow section SHS–SHS T-joints. Design rules are recommended for the SHS–SHS T-joints with concrete-filled chords through analysis of fatigue test data using the hot spot stress method.     

Fatigue life prognosis study of welded tubular joints in signal support structures

Welded tubular joints have exhibited vulnerability to fatigue cracks. This paper presents the research on fatigue life prognosis study of steel welded tubular joints from traffic signal support structures. Fatigue testing results of six full-scale steel welded tubular joint specimens of signal support structures are first discussed. In order to predict the fatigue behavior for welded tubular structure and estimate its fatigue life, a fatigue life prognosis procedure based on Bayesian updating of the stochastic coefficients of the fatigue growth model is employed. Bayesian updating allows the use of dynamic diagnostic information with prior knowledge for improved prognosis. Surface fatigue crack growth data are recorded during fatigue tests of three specimens and the experimental data is used to demonstrate the fatigue life prognosis procedure for welded tubular structures. The prognosis results of this study provide insight into how fatigue hazard evolves in welded tubular joints of signal support structures.     

Fatigue Behavior Investigation and Stress Analysis for Rib-to-Deck Welded Joints in Orthotropic Steel Decks

The fatigue problems in orthotropic steel decks have raised widely concerns in recent years. This study focused on the root crack mechanism at rib-to-deck welded joints, based on the previous test results of sectional specimens and the matching FE analysis, the fatigue behaviors of structure detail were investigated by considering the effect of root gap shapes, weld penetrations, and plate thicknesses on crack initiation. Besides, various root crack depths were simulated in models to clarify the stress variations occurring during the propagation stage under cyclic loading. The results showed that the root gap shape and penetration rate have an impact on the root cracking direction and fatigue life at the initiation stage, but seem not directly related to the crack propagation mechanism; the higher penetration rate is advantageous for the prevention of root crack initiation. However, although the stiffness increased with the increase in plate thickness, the fatigue life of crack initiation might be reduced owing to the low fatigue strength of the thick deck plate, whereas the U-rib thickness has limited effect on the stress response of the root tip. Moreover, the significant difference between the 8 mm-crack model and other crack models is the high stress concentration around the crack tip. The stress conditions of root tip would be changed under loading cycles when a root crack propagated into half of deck plate thickness. Finally, the effect of structural dimensions on fatigue strength were also compared according to test results and FEA.     

Fatigue performance of tubular X-joints with PJP+ welds: I — Experimental study

This study examines the fatigue performance of tubular joints fabricated using a new type of enhanced partial joint penetration weld details under constant-amplitude brace in-plane bending actions. The experimental program includes four cyclic tests on two large-scale X-joints, each with a different surface treatment near the weld toe. The experimental results confirm the satisfactory fatigue performance of the tubular X-joints welded using enhanced partial joint penetration welds in comparison with the S-N curves developed for tubular joints with complete joint penetration welds, and demonstrate the significant improvement on the fatigue life rendered by the weld surface grinding and toe grinding. Root fatigue cracking occurs only in specimens with post-weld toe-grinding treatment which enhances the fatigue life for toe cracking.     

Prediction of fracture behavior of beam-to-column welded joints using micromechanics damage model

Moment-resisting steel frames often fail by fracture failure of beam-to-column welded joints during a strong earthquake. This paper provides a numerical methodology based on micromechanics damage model instead of traditional fracture analytical methods to investigate the crack initiation and propagation of welded beam-to-column joints subjected to monotonic loading and ultra low cycle loading conditions. Firstly, parameters used in the micromechanics damage model for steel base metal, heat affected zone and weld metal were calibrated, respectively, against uniaxial tension test results and cycle test results of notched specimens. The evolution of void growth in the notched specimens under different loading conditions was compared. Secondly, fracture process of the welded joints subjected to tensile loading was simulated based on the micromechanics damage model. The predicted load displacement response agrees well with the other researcher's test results. Finally, the micromechanics damage model was applied numerically to investigate the ultra low cycle fatigue fracture behavior of the welded joints under constant amplitude as well as variable amplitude inelastic cyclic loading. According to the distribution and evolution of void in the welded joints obtained from finite element analysis, crack initiation and propagation were presented and the number of cycles to fracture was predicted. It is shown that the fatigue life predicted from finite element analysis based on micromechanics damage model agrees well with the other's test results.     

Fatigue crack growth analysis of a square hollow section T-joint

Stress concentration sites are always found at the brace and chord intersection corners of any rectangular welded tubular joint. As a result, fatigue cracks are liable to be initiated and propagated from these corners. In this paper, the 3D fatigue crack growth under the weld toe of a square hollow section welded T-joint is simulated using boundary element method. In accordance with the 3D analyses, fatigue crack growth is predicted using a model based on the Paris’ law and stress intensity factors. Good agreement between the experimental and predicted crack growth and crack shape development is obtained. Based on this crack growth analysis, the fatigue life of a specimen is predicted and compared with the standard S-N curve for hollow section joints. It is found that the standard S-N curve is safe and slightly conservative.     

Fatigue strength evaluation of welded structural details in corrugated steel web girders

Fatigue strength evaluation of welded structural details is of practical significance in the design of corrugated steel web girders in highway bridges and industrial structures. In this paper, the fatigue strengths of corrugated steel web girders with several welded structural details and welding methods are analytically examined by fatigue tests of corrugated web beams and small-size welded joints. The stress concentration & distribution characteristics of corrugated web beams were analysed using finite element analysis. The beam test results showed that the structure with scallops or notches in the flange has lower fatigue strength and that with butt joints is prone to suffer from shear crack on the corrugated web. Within the inside range of the scallop, the stress concentration becomes greater with the increase of scallop radius and the tension flange contributes significantly to the bending capacity of corrugated steel web beams. The tests for small-size welded joints indicated their applicability in the analysis and prediction of S-N relationship of the test corrugated steel web beams. Finally, the fatigue crack propagation lives of weld joints were compared with those of test corrugated steel web beams with respect to related design Categories of the AASHTO LRFD Bridge Design Specifications.     

Low- and high-cycle fatigue behavior of load-carrying cruciform joints with incomplete penetration and strength under-match

The fatigue strengths of load-carrying cruciform joints with incomplete penetration and a strength mismatch between the base metal and the deposit metal were studied. Low- and high-cycle fatigue tests were performed on specimens with five matching conditions and two sizes of incomplete penetration. The test results revealed that the failure life was governed by crack propagation in both the low-cycle and high-cycle fatigue regions, and the crack propagation paths differed according to the matching and loading conditions. In addition, the fatigue strengths of the joints were compared for the degree of strength matching and the size of incomplete penetration in the low- and high-cycle fatigue regions. It was found that the effect of strength matching on the fatigue strength is negligible in the high-cycle fatigue region, but it becomes large in the low-cycle fatigue region and significantly reduces the fatigue life of the specimen in the under-matched joints.     

Fatigue design of welded very thin-walled SHS-to-plate joints under in-plane bending

The existing fatigue design S–N curve for SHS-to-plate T-joints under in-plane bending is given in the Canadian Standard, CAN/CSA-S16.1-M89, in terms of the classification method. That S–N curve is however based on the class of longitudinally loaded plates with welded non-load carrying attachments, which are different from the SHS-to-plate T-joints. The increased use of welded thin-walled (t<4 mm) tubular joints in the road transport and agricultural industry for applications such as lighting poles, traffic sign supports, truck trailers, swing ploughs, haymakers and linkage graders, means that there is a need to develop fatigue design curves for tubular joints where the tube wall thickness is less than 4 mm. This paper aims to determine fatigue design curves for SHS-to-plate T-joints where the thin-walled tubes have a thickness of less than 4 mm. Tube-to-plate T-joints, made up by welding a square hollow section tube to a plate, are tested under fatigue loading. Constant stress-amplitude cyclic loading is applied to these connections as in-plane bending load. Stress concentration factors (SCFs) have been determined from strain distributions obtained using strain gauge measurements. Analysis of the fatigue test data using least squares method is carried out to determine the design curves of the tube-to-plate T-joints under in-plane bending, for both the classification method and the hot spot stress method. A class of 44 is recommended for the classification method. An S_r._hs–N curve is proposed, with a recommended SCF of 2.0 for the hot spot stress method.     

Fatigue Strength and Root-Deck Crack Propagation for U-Rib to Deck Welded Joint in Steel Box Girder

Fatigue tests and numerical analysis were carried out to evaluate the fatigue performance at the U-rib to deck welded joint in steel box girder. Twenty specimens were tested corresponding to different penetration rates (80 and 100%) under fatigue bending load, and the fatigue strength was investigated based on hot spot stress (HSS) method. The detailed stress distribution at U-rib to deck welded joint was analyzed by the finite element method, as well as the stress intensity factor of weld root. The test results show that the specimens with fully penetration rate have longer crack propagation life due to the welding geometry, resulting in higher fatigue failure strength. The classification of FAT-90 is reasonable for evaluating fatigue strength by HSS method. The penetration rate has effect on crack propagation angle near the surface, and the 1-mm stress below weld toe and root approves to be more suitable for fatigue stress assessment, because of its high sensitivity to weld geometry than HSS.     

Fatigue tests and design of welded thin-walled RHS–RHS and RHS–angle cross-beam connections under cyclic bending

Galvanized sections are suitable for use in the road transportation industry, the agricultural industry and for recreational structures. The structural systems in which galvanized sections can be used include chassis boxes, roof frames, base frames and drawbars among others. These structural systems are subjected to cyclic loading in service. The structural systems are therefore prone to fatigue failure. In Australia, galvanized sections, commonly known as DuraGal, are thin-walled with wall thicknesses less than 4 mm. There are currently no fatigue design rules for sections of thicknesses less than 4 mm. The connections under investigation namely cross-beam connections, are not covered in current fatigue design guidelines. This paper reports on fatigue tests of cross-beam connections made up of galvanized rectangular hollow sections (RHS) and angle sections. Two types of connections have been investigated, namely RHS–RHS and RHS–angle cross beams. Cross beams are manufactured by welding one member on top of another. In this investigation, welding procedure qualification was carried out by determining hardness values in the parent metal, weld metal and heat-affected zone with particular emphasis to checking the possibility of heat-affected zone cracking. Stress concentration factors were determined in typical connections to determine hot spot locations and to verify observed crack growth patterns in these connections. The fatigue test data obtained from high cycle fatigue tests are compared to existing fatigue design curves. Design curves for cross-beam connections are recommended based on deterministic methods and statistical analysis.     

Reliability and residual service life of the steampipe welded joints at the temperature of 540°C

In this paper the influence is described of the 1/2Cr-1/2Mo-1/4V steel pipe joints, welded with electrodes of the same composition (with long-term exposure to the temperature of 540°C and the pressure of 14MPa), on life decreases of steampipe with 273 mm in diameter and 32 mm wall thickness. To the prediction of welded joint lifetimes the model of separated diffusion and fatigue failure was applied. The results show that the prolongation of intervals between the putting out of operation tends to welded joints lifetime elongation. It is also indicated that in lower weld structural stability values for fatigue processes are predominant being caused by a weld failure, and in higher structural stability values the failure is caused by creep processed.