EFFECT OF MEAN STRESS ON FATIGUE PERFORMANCE OF WELDED JOINTS TREATED BY UPT

The fatigue contrast tests of unload longitudinal direction comer joints asoriginal welded and treated by ultrasonic peening of Q235B in various stress ratio are directed. Theimprovements of fatigue performance of unload longitudinal direction corner joints resulted byultrasonic peening are studied. The effect pattern of stress ratio on fatigue performance of weldedjoints that are treated by ultrasonic peening is studied. As tests results indicate that: ① In thecondition of stress ratio .R=-l, the fatigue strength 'of specimen treated by ultrasonic peening isincreased by 165 percent of that of the original welded specimen. And the fatigue life of specimentreated by ultrasonic peening is as much as 75 approx 210 times of that of the latter. When R=0.1,the fatigue strength is increased by 87 percent and the fatigue life is extended by 21 approx 29times. When R=-0.5, the fatigue strength is increased by 123 percent and the fatigue life isextended by 42 approx 59 times. When R=0.45, the fatigue strength is increased by 51 percent and thefatigue life is extended by 3 approx 14 times. ② If the welded joints are treated by ultrasonicpeening, the fatigue strength is no longer independent on the applied mean stress. The more thestress ratio R, the less the fatigue stress range which can be sustained by the joints is. ③Whether the high value residual stress is in the joints or not, the dead load portion of the appliedload must be considered in the design of the joints which should be treated by ultrasonic peening.

Residual stress relaxation in typical weld joints and its effect on fatigue and crack growth

Many factors influence the fatigue and crack growth behavior of welded joints. Some structures often undergo fairly large static loading before they enter service or variable amplitude cyclic loading when they are in service. The combined effect of both applied stress and high initial residual stress is expected to cause the residual stresses relaxation. Only a few papers seem to deal with appropriate procedures for fatigue analysis and crack growth by considering the combined effect of variable amplitude cyclic loading with residual stresses relaxation. In this article, some typical welded connections in ship-shaped structures are investigated with 3-D elastic-plastic finite element analysis. The effect of residual stress relaxation, initial residual stress, and the applied load after variable amplitude cyclic loading is revealed, and a formula for predicting the residual stress at hot spot quantitatively is proposed. Based on the formula, an improved fatigue procedure is introduced. Moreover, crack growth of typical weld joints considering residual stresses relaxation is studied.

Multiaxial Fatigue Analyses of Stress Joints for Deepwater Steel Catenary Risers

In the present study, the dynamic and fatigue characteristics of two types of stress joints are investigated under ocean environmental condition. Connected with the riser and the platform, stress joint at the vessel hang-off position should be one of the main critical design challenges for a steel catenary riser （SCR） in deepwater. When the riser is under a high pressure and deepwater working condition, the stress state for the joint is more complex, and the fatigue damage is easy to occur at this position. Stress joint discussed in this paper includes two types： Tapered Stress Joint （TSJ） and Sleeved Stress Joint （SSJ）, and multiaxial fatigue analysis results are given for comparison. Global dynamic analysis for an SCR is performed first, and then the local boundary conditions obtained from the previous analysis are applied to the stress joint FE model for the later dynamic and multiaxial fatigue analysis. Results indicate that the stress level is far lower than the yield limit of material and the damage induced by fatigue needs more attention. Besides, the damage character of the two types of stress joints differs： for TSJ, the place where the stress joint connects with the riser is easy to occur fatigue damage; for SSJ, the most probable position is at the place where the end of the inner sleeve pipe contacts with the riser body. Compared with SSJ, TSJ shows a higher stress level but better fatigue performance, and it will have a higher material cost. In consideration of various factors, designers should choose the most suitable type and also geometric parameters.

Fatigue life analysis of SMA490BW steel welded butt joint for train bogie based on ABAQUS/FE-SAFE

Stress concentration has a significant effect on fatigue properties of welded butt joint of bogie.The influences of joint geometric parameters {weld edge transition arc radius r,weld toe inclination angle θ） on stress concentration coefficient Kt at weld toe and fatigue life N were studied based on the finite element software ABAQUS/FE-SAFE,and the parametric equations were founded based on regression analysis of Origin software.The results show that weld edge transition arc radius r plays the most important role in stress concentration coefficient Kt as well as weld toe inclination angle 0.For the same inclination angle θ,stress concentration coefficient Kt decreases gradually with transition arc radius r increasing.With inclination angle θ increases,the effect of transition arc radius r on stress concentration factor Kt is increasing continuously,also the effect of change in stress concentration coefficient Kt caused by r on fatigue life N is increasing continuously.While in the case of same weld edge transition arc radius r,stress concentration coefficient Kt increases with inclination angle θincreasing.With transition arc radius r increases,the effect of inclination angle θ on stress concentration factor Kt is decreasing constantly,the effect of change in stress concentration coefficient Kt caused by θ on fatigue life N is decreasing constantly.The associated parametric equations have important instructive significance for the improvement of stress concentration degree and fatigue life of welded butt joint for train bogie.

Design and Analysis of Tapered Stress Joint for Top Tensioned Riser System

Stress Joint （SJ） plays a key role in the Top Tensioned Riser （TTR） system for deep water engineering. A preliminary design method of tapered SJ is proposed in the paper, which could help designers obtain accurate design data. After a further sensitive analysis is carried out, the related parameters choice and control methods are recommended in the engineering practice. By taking the extreme environment conditions into consideration, the effects of bending stress reduction and curve control are analyzed, and the 3-D FE models are established by ABQOUS for numerical evaluation to verify the correctness of design results. At last, dynamic analysis and fatigue analysis, based on actual project, are carried out with designed stress joint. The analysis results prove the feasibility and guidance of this method in the practical engineering applications.

Topology Optimization and Fatigue Analysis of Temporomandibular Joint Prosthesis

Treatment of bone tumors in the mandible often involves extensive excavation of affected bone, followed by mandibular reconstruction. Prosthetic implants may be needed to restore jaw functionality. The challenges of making prosthetic bone implants include stress shielding and extending the mechanical life of the implant. We have developed a design algorithm to improve the efficiency of prosthesis design. A finite element model of the patient case is constructed from a computer tomography scan, and the computer implements topology optimization techniques to design the prosthesis with limited stress shielding affected by highly biomechanical compatibility. Topology optimization facilitates the design of low weight structures by automatically introducing holes into the structure. This is governed by engineering predetermined constraints to meet certain job specifications. Such a design will be tested for fatigue life before it is ready to be manufactured and used. Topology optimization can be performed as a design process to achieve a final design that takes stress shielding into consideration. The problem of stress shielding is solved by matching the stiffness of the orthopedic implant to the original bone that is being replaced. The material we used was titanium alloy (Ti-6Al-7Nb). Volume fraction of the orthodox implant was used (0.2872 for the studied case) as volume constraints. Compliance of the bulk bone was set as a further constraint to match the stiffness of the bone with the designed structure. Our results show a good life expectancy for the designed parts, with 12% higher life expectancy for stress-based topology optimization than for compliance-based topology optimization.

Fatigue Limit Prediction and Estimation for the Crack Size Rendered Harmless by Peening for Welded Joint Containing a Surface Crack

In this study, the effects of portable pneumatic needle-peening (PPP) on the bending fatigue limit of a low-carbon steel SM490A welded joint containing a semi-circular slit on the weld toe were investigated. PPP was applied to the specimens with a semi-circular slit with depths of a = 0.4, 0.8, 1.2, and 1.6 mm. Then, three-point bending fatigue tests were carried out under R = 0.05. The fatigue limits of low-carbon steel welded specimens containing a semi-circular slit were increased for peened specimens compared with non-peened specimens. Peened specimens having a semicircular slit with a depth of a = 1.2 mm had high fatigue limits, almost equal to those of the non-slit peened specimens. It was concluded that a semi-circular slit with a depth of less than a = 1.2 mm can be rendered harmless by peening. Then, the fatigue improvement by peening was predicted. The fatigue limits before and after peening could be estimated accurately by using a modified Goodman diagram considering the effects of residual stress, stress concentration, and Vickers hardness. Moreover, the maximum depth of a semi-circular slit that can be rendered harmless by PPP was estimated based on fracture mechanics assuming that the semi-circular slit was equivalent to a semi-circular crack. The prediction results were almost consistent with the experimental results.

FATIGUESTRENGTHDETERMINATIONOFBUTTWELDEDJOINTS

The influence of welding parameters of butt joints, made by three processes (MMA, MAG and T.I.M.E), on their geometric characteristics has been evaluated, which in turn caused a change of the fatigue strength. That fore the statistic analysis and FEM were used and a method developed in the Institute of Welding, which consist in a direct connection of parameters used during welding with the fatigue strength of joints welded by various processes. It has been proven, among others, which by using more advanced welding processes (T.I.M.E) the fatigue strength of welded joints can be increased in comparison to joints welded by MMA and MAG.

Seawater Corrosion Fatigue of Welded Joints Under Random Variable Amplitude Loading

Offshore platforms are always subjected to wave action which is random variable amplitude cyclic loading. In order to simulate the stressing condition at the 'hot spot' of the tubular joints and the marine environment, random variable amplitude fatigue tests have been carried out on welded plate joints in sea water. The tests have been conducted under the conditions of loading frequency of 0.2 Hz/, stress ratio of -1, seawater temperature of about 20°C and cathodic protection with the potential about -850 mV, SCE. The test results have been compared with the seawater corrosion fatigue life under constant amplitude loading. Miner's linear cumulative damage summation rule has been used to predict the corrosion fatigue life under variable amplitude loading. The predicted life is in good agreement with the test data.

Improving the fatigue performance of longitudinal welded joints by low transformation temperature electrodes

For a longitudinal welded joint, the tensile residual stresses are as high as the yield stress of the metal, so that the weld toes are sensitive to fatigue load. In this case a low transformation temperature electrode (LTTE) is one of the most useful methods used to improve the fatigue strength of the longitudinal welded joint, because the tensile residual stress is reduced or changed into compressive stress. Three kinds of longitudinal welded joints were selected to conduct fatigue tests. The tests results show that the fatigue strengths at 2×10 6 cycles of the joints welded with LTTE were improved by 41%, 47% and 59% respectively compared with those of the joints welded with E5015, and the fatigue lives at 162 MPa were improved by 9.9 times, 9.6 times and 46.8 times respectively. Furthermore, the LTTE method is not necessary to add process after welding and so that it can be valuable method to improve the fatigue performance of longitudinal welded joints.

Fatigue Reliability Assessment of Correlated Welded Web-frame Joints

The objective of this work is to analyze the fatigue reliability of complex welded structures composed of multiple web-frame joints, accounting for correlation effects. A three-dimensional finite element model using the 20-node solid elements is generated. A linear elastic finite element analysis was performed, hotspot stresses in a web-frame joint were analyzed and fatigue damage was quantified employing the S-N approach. The statistical descriptors of the fatigue life of a non-correlated web-frame joint containing several critical hotspots were estimated. The fatigue reliability of a web-frame joint wasmodeled as a series system of correlated components using the Ditlevsen bounds. The fatigue reliability of the entire welded structure with multiple web-frame joints, modeled as a parallel system of non-correlated web-frame joints was also calculated.

Influence of shrinkage porosity on fatigue performance of iron castings and life estimation method

Shrinkage porosity exists more or less in heavy castings, and it plays an important role in the fatigue behavior of cast materials. In this study, fatigue tests were carried out on the QT400-18 cast iron specimens containing random degrees of shrinkage porosity defect. Experimental results showed that the order of magnitude of life scattered from 103 to 106 cycles when the shrinkage percentage ranged from 0.67% to 5.91%. SEM analyses were carried out on the shrinkage porosity region. The inter-granular discontinuous, micro cracks and inclusions interfered with the fatigue sliding or hindering process. The slip in shrinkage porosity region was not as orderly as the ordinary continuous medium. The shrinkage porosity area on fracture surface(SPAFS) and alternating stress intensity factor(ASIF) were applied to evaluate the tendency of residual life distribution; their relationship was fitted by negative exponent functions. Based on the intermediate variable of ASIF, a fatigue life prediction model of nodular cast iron containing shrinkage porosity defects was established. The modeling prediction was in agreement with the experimental results.

New Developed Welding Electrode for Improving the Fatigue Strength of Welded Joints

A new welding electrode, low transformation temperature electrode (LTTE), was introduced in this paper. It was described in design principle, mechanics, chemical compositions of their deposited metal and manufacturing methods. It was proved that the best transformation starting temperature from austenite to martensite of the deposited metal of LTTE was at about 191℃ and it was obtained by adding alloying elements such as Cr, Ni, Mn and Mo. The microstructure of the weld metal of the LTTE was low carbon martensite and residual austenite. The compressive residual stress was induced around the weld of the LTTE and the -145 MPa in compression could be obtained in middle of weld metal. The fatigue tests showed that the fatigue strength of the longitudinal welded joints welded with the LTTE at 2×106 cycles was improved by 59% compared with that of the same type of welded joints welded with conventional E5015 and the fatigue life was increased by 47 times at 162 MPa. It is a very valuable method to improve the fatigue performance of welded joints.

Stress Concentration Factors at Saddle and Crown Positions on the Central Brace of Two-Planar Welded CHS DKT-connections

一套参量的压力分析在不同轴的装载条件下面为二平面的管状的 DKT 关节被执行。分析结果被用来在中央钩住上在内部僵绳，外部僵绳，和王冠位置在压力集中因素(SCF ) 上在几何参数的效果上介绍一般讲话。把分析基于有限元素(FE ) 的结果并且通过非线性的回归分析， SCF 参量的方程的一个新集合为疲劳设计目的被建立。方程的评价研究对试验性的数据和原来的 SCF 数据库被进行。标准由 UK 能源部(英国雌动物) 建议了的接受的满足也被检查。参量的学习的结果证明高度显著的差别在有一样的几何性质的一个相等的位于同一平面的 KT 关节在多平面的 DKT 关节和相应 SCF 在 SCF 价值之间存在。它能清楚地从这观察被结束使用方程建议在多平面的 DKT 关节计算 SCF 的位于同一平面的 KT 连接将导致更加下面预言或过去预言的结果为。因此，开发特殊为多平面的 DKT 关节设计的 SCF 公式是必要的。根据英国雌动物接受，标准，关联系数的高值，和在建议方程和试验性的数据的预言之间的令人满意的协议保证方程的精确性得到的方程评价的好结果。因此，发达方程能可靠地为疲劳设计被使用近海结构。

Evaluation of Fatigue Life of Semiconductor Power Device by Power Cycle Test and Thermal Cycle Test Using Finite Element Analysis

To accurately predict the fatigue life of a power device, a fatigue life evaluation method that is based on the power cycle is presented in terms of an algorithm based on a combination of electrical analysis, heat analysis, and stress analysis. In literature, the fatigue life of power devices has been evaluated on the basis of the thermal cycle. This cycle is alternately repeated within a range from a high temperature to a low temperature. In an actual operating environment, however, a power device works in a power cycle that consists of being switched ON and OFF. To accurately predict the fatigue life cycle of a device, then, the evaluation should take account of this important aspect of the power cycle. To verify the utility of the evaluation method presented in this study, the results for a power cycle based on the combined use of electrical analysis, heat analysis, and stress analysis are compared to the results based on the thermal cycle, as found in the literature. Our conclusion is that the fatigue life cycle as estimated by the thermal cycle test is higher than that estimated by the power cycle.

Laser Shock Processing of Metal Sheet and Welded Joints

In order to study the application of laser shock processing(LSP) as a post weld treatment technology and a strengthening technology, a series experiments and analysis were taken in this paper. The hardness of the laser shock processed zone of Al-Li alloy was measured, and the microstructure and mechanical properties of the welded joints of the Ni-based superalloy GH30 and the Austenitic stainless steel !Crl8Ni9Ti were compared with those without LSP in this paper. The results showed that the size of strengthened zone was similar to that of laser spot and strengthened layer was about 1mm deep, and the high intense dislocations and twins produced in the shocked zone. Plastic strain also gained surface residual compress stress, which is benefit for the fatigue properties of welded zones. In this test , the surface hardness of welding zone of the superalloy GH30 improved obviously and tensile strength increased by 12%, but the improvement of fatigue life was not obvious; Martensite phase is formed in plasma welding !Crl8Ni9Ti, which reduced the effect of strain deformation martensite induced by laser shock processing, but the surface residual compress stress gained by laser shock processing can obviously improve the fatigue life of !Crl8Ni9Ti welded joints.

Mechanic Analysis of the Key Node in Steel Truss Bridge

Steel truss bridges are frequently used in bridge engineering because of their good ability of spanning capacity, construction and light self-weight. Main trusses are the critical component of steel truss bridge and the main truss are made of truss members linked by integral joints. This paper presents the mechanic performance of key joints, and the codified design of joints in steel truss girders according to the latest European norms. The results showed that the fatigue resistance of welded joints evaluation is necessary to predict, detect, and repair the crack in time for the safety service life of the bridge. The stresses of integral joint are greater than that of truss members;the stresses in the center area of the integral joint are greater than the stress at the edge.

基于零点有效缺口应力的薄板焊接接头疲劳评估

文中采用零点有效缺口应力进行了薄板焊接接头的疲劳评估研究.该方法综合考虑整体结构应力以及缺口根部到零点位置的非线性峰值应力的影响,以临界距离法和零点结构应力法为基础,将零点应力位置作为缺口对结构疲劳影响的临界距离对焊接结构进行疲劳强度评估.通过对两种不同类型的焊接接头试样(T形单面焊接头和不同厚度组合搭接接头)的疲劳试验结果进行拟合.结果表明,在对焊根失效的试样进行疲劳评估时,焊根位置不同截面应力线性化之后得到零点有效缺口应力存在较大差异;其中,焊脚截面应力线性化得到的S-N曲线分散带宽度最小;再者,同时考虑焊趾和焊根失效的零点有效缺口应力S-N曲线,与分别考虑焊趾或焊根失效的零点有效缺口应力S-N曲线分散带相差不大,证明该方法可同时用于焊接接头焊趾和焊根疲劳评估.

焊接残余应力对钢桥面疲劳性能的影响与处理措施

为改善正交异性钢桥面的抗疲劳性能,对钢桥面进行退火处理以降低关键焊缝的焊接残余应力。首先系统性地回顾和探讨钢桥面焊接残余应力分布模式,同时就残余应力对疲劳裂纹发展的影响进行了文献分析。随后,归纳和总结了国内外主流焊接残余应力消除方法,并开展对比分析。借鉴压力容器制造方法,建议在正交异性钢桥面制造中引入退火处理工艺以降低焊接残余应力。研究聚焦钢桥面中疲劳问题较突出的顶板与U肋连接焊缝,设计了16个(其中9个退火处理,7未作处理)局部足尺单U肋试件进行残余应力测试与疲劳试验。试验结果表明,退火处理后焊缝残余应力实测值降低约80%,疲劳强度提高约23%。可以看出,退火处理可以大幅降低焊接残余应力,从而有效提升钢桥面抗疲劳性能。

AH32钢T型焊接接头疲劳强度的试验与数值模拟

对AH32钢T型试件进行疲劳试验,并利用有限元软件对T型焊接接头的应力分布及疲劳寿命进行数值模拟,最终将疲劳试验结果与数值模拟结果对比分析。研究发现,T型焊接接头在焊趾附近处应力值最大,寿命最短。本文研究结果为今后船海结构物的疲劳寿命预测提供了有益参考。