FATIGUE CRACK INITIATION LIFE PREDICTION FOR WELDED JOINTS BY LOW CYCLE FATIGUE APPROACH

Abstract— Analytical procedures based on low cycle fatigue theory are used to estimate the fatigue crack initiation life (N_i) for a cruciform welded joint in mild steel under constant amplitude tensile cyclic loading; the fatigue crack initiating at the weld toe. Effects due to welding such as residual stresses, geometrical variability and changes in material properties are handled. It is shown that for high mean stresses the discrepancies observed between the N _i estimates provided by commonly used analytical procedures exceed an order of magnitude. For the base metal (BM) the discrepancies become negligible if cyclic relaxation of notch mean stress is taken into consideration. The differences betwen the N _i estimates for heat affected zone (HAZ) material (where fatigue cracks at the weld toe usually initiate) and for BM are quantified. The applicability of HAZ material properties, estimated from hardness, to N _i prediction is evaluated.     

DEFECTS AND CRACK SHAPE DEVELOPMENT IN FILLET WELDED JOINTS

Abstract— Accurate knowledge of defect sizes and crack shape development from initiating defects is essential for a realistic application of fracture mechanic? to the fatigue life prediction of welded joints. Defects at the toes of high quality longitudinal non-load-carrying fillet welded joints have been examined using two methods, and compared with those observed in plain unwelded plate. The defects observed at the weld toe were, on average, four times deeper than the largest observed in the unwelded plate but had similar root radii. For this particular geometry the defects could be considered continuous at the expected initiation site. A simple estimate of the stress concentrating effect of a single composite defect, coupled with experimental evidence, led to the conclusion that the proportion of total life spent in initiation is negligible for all stress levels which cause fatigue failure. Subsequent growth and crack shape development was observed to depend on the initial defect distribution and the complex stress field at the weld toe.     

PREDICTION OF THE FATIGUE LIFE OF LASER WELDED STAINLESS STEEL JOINTS

Abstract— A fatigue scatter band has been computed for laser welded austenitic stainless steel joints. These laser welded steels have a very small heat affected zone. The unified scatter band provided by standards for welded structural steels does not adequately describe the trend of the experimental data of laser welded steels and this makes their design parameters scarcely realistic. The scatter band proposed in this paper has been computed by re-sorting experimental data relative to joints with high stress concentration factors and has subsequently been assessed with data relevant to butt welded joints.     

FATIGUE ANALYSIS OF SURFACE CRACKS AT FILLET WELDED TOES

This paper reports a study of fatigue crack growth and coalescing behaviour at semi-elliptical cracks in the stress concentration region of steel plates with fillet shoulders or fillet welds. Fatigue tests were carried out on machined plate specimens with a fillet geometry similar to a fillet welded joint. These specimens were notched and precracked to provide single and multiple coplanar semi-elliptical surface cracks at the fillet toe region. Finite element stress analysis results were used to obtain approximate Mk factors (i.e.: stress concentration magnification factors) for the fillet toe geometry with a semi-elliptical surface crack. An analytical model was developed to simulate crack shape development and growth to failure in the case of multiple coplanar semi-elliptical cracks. In this model, a simple crack coalescing procedure is applied to merge coplanar cracks when they meet by recharacterising the coplanar cracks into a single semi-elliptical crack. Alternative crack growth laws were investigated and comparisons made between actual and predicted shape developments and lives.     

FATIGUE CRACK INITIATION LIFE PREDICTION IN HIGH STRENGTH STRUCTURAL STEEL WELDED JOINTS

The local approach method is used to calculate the fatigue crack initiation/early crack growth lives (N_i) in high strength structural steel weldments. Weld-toe geometries, welding residual stresses and HAZ (heat affected zone) cyclic mechanical properties are taken into account in the N_i estimation procedure. Fatigue crack initiation lives are calculated from either a Basquin type or a Manson-Collin type equation. The local (HAZ) stress and strain amplitudes and the local mean stress are determined from an analysis based on the Neuber rule and the Molski-Glinka energy approach. The accuracy of the different methods is evaluated and discussed. Finally the previous methods are used with HAZ cyclic mechanical properties estimated from hardness measurements.     

FATIGUE STRENGTH OF NON-LOAD-CARRYING FILLET WELDED JOINTS: EFFECTS OF WELD RESIDUAL STRESSES AND STRESS CONCENTRATION

The fatigue strength of non-load-carrying fillet welded joints of KE36(TMCP) steel was studied. Both residual stress measurements and fatigue tests were carried out, with the plate thickness, the plate width and the heat input being varied. Specimens given a Post Weld Heat Treatment (PWHT) were also prepared. The plate width had no effect on the fatigue strength, because it hardly affected the transverse residual stresses at the weld toe. However, the heat input influenced the transverse residual stress distribution, and a significant difference in fatigue strength due to the heat input was observed, especially when N≥ 10⁶ cycles. It was also found that PWHT removed almost all the residual stresses at the weld toe, improving the fatigue strength drastically. In this study, the values of stress concentration factor K₂ were estimated by Machida's method and it was concluded that the thickness effect resulted from a combination of both stress concentration and residual stresses with the contribution of the latter being particularly significant for N≥ 10⁶ cycles.     

A PROCEDURE FOR FATIGUE LIFE PREDICTION OF SPOT WELDED JOINTS

Abstract— The problem of predicting the fatigue life of spot welded joints has not yet been satisfactorily resolved. Several approaches exist, but many of them are difficult to apply to general structures, which are widely different from simple laboratory specimens. A general criterion is proposed in this paper, based on a theoretical extrapolation of the radial stress. The criterion must be used together with a Finite Element model of the joint and so this paper presents the steps required to get the necessary FE data; steps that are also applicable to the case of complex joints. The advantage of the proposed method is its independence of notch effects near the spot-weld edge. The suggested procedure is applied to several specimens of different materials and the results obtained are successfully compared with experimental data.     

FRACTURE MECHANICS ANALYSIS OF FATIGUE RESISTANCE OF SPOT WELDED COACH-PEEL JOINTS

Resistance spot welding is the most widely used joining method in automobile manufacture. The number, location, and quality of welds are some of the factors that influence the performance of welded subassemblies, and body panel structures. Therefore, design optimization requires knowledge of not only sheet metal behavior, but also weld behavior under service loadings. A linear elastic fracture mechanics approach was employed in this study to estimate the fatigue lives of spot welds subjected to tearing loads in a coach-peel specimen. Using a finite element method (FEM), the initial J-integral values for five coach-peel joints, each with different geometries, were calculated. Fatigue tests conducted on the same weld geometries provided life data. The experimental data were used to derive a relationship between the initial elastic J-integral values (ΔJ_e) and the fatigue life. It was found that the total fatigue life (N_f) of a weld at one applied stress range is related to its range of J-integral value such that a ΔJ_e vs N_f log-log plot gives a straight line relationship. This relationship can be used to evaluate the effects of geometrical variables on the fatigue life of coach-peel joints. The results show that, within the dimension range studied here, the effects of geometrical variables on the fatigue resistance can be ranked in the following decreasing order: weld eccentricity, sheet thickness and weld nugget diameter.     

管节点表面裂纹疲劳扩展的数值模拟

本文采用线弹簧单元与ＡＤＩＮＡ程序结合计算了表面裂纹在交变载荷作用下的应力强度因子，首次提出了预埋线弹簧单元法模拟表面裂纹疲劳扩展，并发展了相应的技术和软件。     

ON THE RELATIONSHIP BETWEEN LOW CYCLE FATIGUE AND CRACK GROWTH RATE PROPERTIES IN WELDED STEEL COMPONENTS

Previous investigations have shown that for plain materials, the crack growth rate constants applied in an elastic-plastic crack growth model may be obtained indirectly from the constants describing the low cycle fatigue capacity of the material. In the present study a simple EPFM crack growth model, based on bulk strain amplitude as the governing parameter, is assumed. The crack growth constants derived from the linear regression constants describing the LCF capacity of manually butwelded components are determined, and the validity of the resulting EPFM crack growth model is checked against measured LCF crack growth data.     

THE EFFECT OF MISALIGNMENT ON THE FATIGUE STRENGTH OF WELDED CRUCIFORM JOINTS

Abstract— The effect of axial misalignment on the fatigue strength of load-carrying transverse cruciform welded joints was investigated using experimental and fracture mechanics methods. Where failure occurred by cracking from the weld toe, misalignment significantly reduced the fatigue strength. The reduction could be predicted using a nominal stress concentration factor (SCF). Misalignment had less effect where failure was due to cracking through the weld metal; an expression was deduced for the SCF in this case. For fracture mechanics assessments, an expression for an effective stress intensity factor using the SCF and stress intensity factors for aligned welds was shown to agree with the finite element (FE) results. Predictions of the effect of misalignment using the FE results agreed with experimental data. Misaligned transverse load-carrying cruciform joints should be assessed for fatigue failure from the toe using the same SCF as for a butt weld with the same misalignment. For failure through the throat, an alternative expression for the SCF is recommended. Fracture mechanics assessments of misaligned joints should be carried out using an effective stress intensity factor derived from the SCF and stress intensity factors for aligned joints. These recommendations are now incorporated in British Standard PD 6493:1991.     

转向架构架焊接接头的组织与性能

采用工艺试验,力学性能试验以及光镜、电镜等显微分析等方法对转向架构架焊接接头的组织形态和力学性能进行了研究,确定了焊接接头各区的组织形态和力学性能的变化情况。研究结果表明,利用(Ar CO2)混合气体保护焊焊接Q345E钢时,焊缝组织主要为先共析体素体、针状铁素体和珠光体,热影响区组织主要为先共析铁素体、珠光体和粒状贝氏体,焊接接头具有良好的力学性能。     

THE EFFECT OF UNDERCUT, MISALIGNMENT AND RESIDUAL STRESSES ON THE FATIGUE BEHAVIOUR OF BUTT WELDED JOINTS

Abstract— A mathematical model is developed to predict the effect of weld toe undercut, misalignment and residual stresses on the fatigue behaviour of butt welded joints subjected to zero-to-tensile loading. Linear Elastic Fracture Mechanics (LEFM), Finite Element Analysis (FEA) and superposition approaches have been used for the modelling. It has been found that an undercut at the toes of welded joints is one of the most important weld geometry parameters. The reduction of fatigue strength of welded joints with a weld toe undercut is at least twice that of joints without an undercut in comparison with flush-ground welded plate. A misalignment of 5% of plate thickness and an undercut of 2% of plate thickness are fairly representative for the lower boundary of S-N curves of butt joints. The improvement of fatigue limits by means of surface treatments is shown to be effective for both undercut and misaligned joints. This approach is practical for a "Fitness-for-Purpose" assessment of welded joints subject to fatigue conditions.     

EFFECT OF YIELD STRENGTH ON THE BASIC FATIGUE STRENGTH OF WELDED JOINTS

Abstract— The basic fatigue strength of welded joints in four steels having different yield strengths has been obtained by tests in which the maximum applied stress was held constant and equal to the yield strength, to simulate the tensile residual stress in real large-scale structures. In the long-life region superior properties occurred with a decrease in the yield strength. It is therefore suggested that both low yield strength steel, which can be produced by a thermo-mechanical control process without affecting the tensile strength, and steels or welding consumables which show a low transformation temperature, may have a high fatigue strength.     

COMPARISON OF FATIGUE LIFE PREDICTIONS IN WELDED JOINTS USING 2D AND 3D K SOLUTIONS

The paper presents the results of fatigue life predictions in non-loaded carrying tee and cruciform joints and also longitudinal seams of tubes, obtained with 2D and 3D stress intensity factor solutions. The 2D computational method uses the standard Bueckner weight function solution to obtain the 2D values of the stress magnifying factor M_K. In the 3D approach a modified 3D weight function solution was used in conjunction with the Raju and Newman base stress intensity formulation. Comparison of results is shown between the 2D and 3D fatigue life predictions and experimental results of fatigue strength obtained by the authors. Results are also given showing the influence of crack aspect ratio, thickness and the ratio of tube radius to tube thickness.     

THE THRESHOLD STRESS INTENSITY RANGE IN FATIGUE

Abstract— The parameters are considered that control the threshold stress intensity in fatigue and relationships existing between these parameters are examined. A new relation is derived which is shown to be consistent with experimental data for a 0·15%C, 1·5%Mn steel.     

FATIGUE LIFE PREDICTION OF WELDED JOINTS—A RE-ASSESSMENT

Abstract— Fatigue life prediction of welded joints needs an accurate and exhaustive theoretical Fracture Mechanics characterization of weld toe crack propagation. The method proposed by Albrecht et al. leads rapidly to accurate solutions of the LEFM δK-parameter. However, non-LEFM short crack behavior within the notch (weld toe) plastic zone must be taken into account. Available information on notch fatigue is surveyed, and practical cases where short crack growth is likely to occur are identified. Based on an elastoplastic finite element analysis, the LEFM validity limits and errors resulting from the misuse of LEFM in fatigue life prediction are quantified.     

INFLUENCE OF STRESS RELIEVING BY VIBRATION ON THE FATIGUE BEHAVIOUR OF WELDED JOINTS IN COMPARISON TO POST-WELD HEAT TREATMENT

Abstract— Constant amplitude fatigue tests with welded specimens under fully reversed four-point bending as well as under axial loading have shown that vibration stress relieving does not lead to a fatigue life improvement of welded parts when compared to the as-welded state. Thus, a substitution of thermal stress relieving by a vibration treatment is not successful. This was also proved by residual stress measurements in the welded parts studied in this paper.     

焊接构件焊接质量及疲劳强度分析

通过对典型焊接构件焊缝的宏观、微观组织分析及力学性能测试,确定该构件的疲劳强度不足主要是焊接中存在的未焊透,起弧处、熄弧处缺陷较多造成的,热影响区并非薄弱点。     

正交异性钢桥面板肋-面板焊缝疲劳验算的应力分析模型评估

以天津塘沽海河大桥的正交异性钢桥面板为例,建立钢箱梁节段的有限元分析(FEA)模型、简化FEA模型和基于钢箱梁节段的子模型进行肋-面板焊缝疲劳应力分析模型的评估。计算结果表明:对于远离纵腹板处的肋-面板焊缝,通过提高简化FEA模型中横隔板高度和约束横隔板底部翼缘的方式可减小与钢箱梁节段FEA模型结果的误差;对于靠近纵腹板处的肋-面板焊缝,所有简化FEA模型的应力计算结果均低于钢箱梁节段FEA模型的结果,且误差超过23%。基于钢箱梁节段的三跨子模型可以准确地给出钢桥面板任意位置的肋-面板焊缝的疲劳应力。它具有钢箱梁节段FEA模型的计算精度和简化FEA模型的计算效率,因此它可作为正交异性钢桥面板任意位置处肋-面板焊缝疲劳应力分析的准确和高效计算模型。