Effect of plastic anisotropy on the limit load of highly undermatched welded specimens in bending

Plastic anisotropy is a typical property of many metals and it can have a significant effect of the limit load of structures. It is therefore of importance to evaluate this effect for typical geometries covered by compendia of flaw assessment procedures for limit load solutions. The present paper deals with a typical highly undermatched specimen in pure bending. It is assumed that the mismatch factor (the ratio of the yield stress of weld material to the yield stress of base material) is so small that plastic deformation solely occurs in the weld whereas the base material is elastic (rigid). The weld material obeys Hill’s quadratic orthotropic yield criterion. The variation of the upper bound bending moment with the thickness of the weld and anisotropic properties of the material is illustrated. Comparison with the isotropic case made shows that an effect of plastic anisotropy can be quite large.     

A three dimensional limit load solution for highly undermatched single edge cracked specimens in pure bending

The accuracy of defect assessment methods for cracked structures depends on the accuracy of limit load solutions. The present paper focuses on a new method to construct simple three‐dimensional kinematically admissible velocity fields for highly undermatched welded joints with edge cracks and its application to single edge cracked specimens in pure bending. The objective is to take into account specific features of flow pattern in highly undermatched joints to propose three‐dimensional kinematically admissible velocity fields whose level of complexity as well as accuracy of limit load predictions are comparable with plane strain fields. The three‐dimensional solution found for single edge cracked specimens in pure bending is compared to available semi‐analytical plane strain solutions. Several feasible ways to improve the solution found are proposed.     

A limit load solution for a highly undermatched scarf‐joint specimen with an arbitrary crack

A new plane strain upper bound solution for highly undermatched scarf‐joined specimens with a crack is proposed. A distinguished feature of this solution is that the crack is arbitrarily located within the weld and its shape is also arbitrary, though some restrictions do apply. The latter are explained in detail such that the class of structures for which the solution is applicable is precisely specified. The solution is given in a form of several ordinary integrals.     

Effect of plastic anisotropy of weld on limit load of undermatched middle cracked tension specimens

Plastic anisotropy may have a significant effect on the limit load of structures. The present paper shows its effect on the limit load of well‐undermatched middle cracked tension specimens assuming that the base material is elastic and the weld material obeys Hill's quadratic orthotropic yield criterion in rigid‐perfect plasticity. The solution is based on a kinematically admissible velocity field, which is compatible with a stress field satisfying the equilibrium equations and the yield criterion in the plastic zone. The velocity field is singular in the vicinity of the bi‐material interface, which is typical for isotropic materials.     

A limit load solution for highly weld strength undermatched DE(T) specimens

A new upper bound solution for highly undermatched welded DE(T) specimens is proposed. A distinguished feature of this solution is that it accounts for the thickness of the specimen. Even though this case is more general than plane-strain and plane-stress solutions, the final result is given in terms of ordinary and double integrals. Comparison with a plane strain solution shows that the thickness of the specimen has a great effect of the limit load.     

Fatigue crack growth analysis of welded aluminium RHS T-joints with manipulated residual stress level

The fatigue life of a welded aluminium T‐joint made from beams with rectangular hollow section (RHS) has been predicted using a crack propagation analysis and compared with experimental results from joints with different residual stress levels. To include the effect of the residual stresses, the stress ratio was calculated at the weld toe and, via Walker's equation, introduced into the analysis. How to obtain the Walker exponent has been discussed in detail. The introduction of a stress ratio at the weld toe provides good agreement between the experimentally and analytically found S–N curves. The effect of the residual stress was successfully included in the analysis.     

A note on the limit load of welded joints of a rectangular cross-section

A flat layer of a rigid perfectly plastic material subjected to tension between two rigid blocks is considered. The layer is assumed to have a rectangular cross-section. The influence of the ratio of the in-plane layer dimensions on the limit load and the maximum tensile stress is studied. Comparison with numerical calculations of the limit load based on the assumption of the plane strain conditions is made.     

Fatigue strength of welded joints under multiaxial loading: experiments and calculations

In reality most welded components are loaded with a combination of different variable forces and moments that often cause a state of multiaxial stress in the fatigue-critical areas. If the multiaxial loading is non-proportional, traditional deformation-based hypotheses are not able to give a reliable lifetime prediction. This investigation is a cooperation between three German research institutes to build an experimental database for the verification of different concepts of lifetime prediction. In accordance with former investigations, a flange-tube connection made of steel P460 was used. The test program was divided into constant amplitude and variable amplitude tests. The ratio between the nominal bending and shear stress is 1. For the variable amplitude tests, a Gaussian-standard is used. A lifetime prediction software for multiaxial state of cyclic stress was developed. The software has a modular structure and allows calculations with different hypotheses and methods. The calculations are based on the local elastic stresses. This is an acceptable method for high-cycle fatigue. In this work, two general types of calculation, the Integral Approach and Critical Plane Approach and a local stress-based modification of the von Mises Criterion, the hypothesis of effective equivalent stress (EESH) are shown. The damage accumulation is performed with the elementary Miner's rule ( S – N curve without fatigue limit). The statistical distributions of the damage sums are also shown.     

Fatigue strength analysis of welded joints in closed steel sections in rail vehicles

A great need exists for practicability and reliability analyses of the various dimensioning concepts in railway vehicle production, as currently parts are commonly sized according to the nominal stress concept. Although Finite Element Analysis is used for the dimensioning of parts according to the nominal stress concept, no use is made of the locally resolved information from these calculation methods. Concepts based on local stress are highly applicable, as the Finite Element Analysis allows detailed modelling of critical areas of parts.     

Fatigue life assessment of welded joints under sequences of bending and torsion loading blocks of different lengths

In this work, the nominal stress concept, the notch stress approach and two critical plane approaches are used to analyse the fatigue endurance of a pipe‐to‐plate welded joint subjected to complex loading histories. Both the pipe and the plate were made of S355JR steel. Starting from already known fatigue endurance curves obtained for the same specimens under pure bending and pure torsion, a first series of tests was conducted, in which specimens were loaded in bending for a given fraction of the estimated life and then in torsion until failure. A similar series of tests was then carried out by changing the loading order: specimens were firstly loaded in torsion for a given fraction of the estimated endurance and then in bending until failure. The whole test campaign was repeated for two different fractions of the estimated life, that is, 0.3 and 0.45, respectively. After that, additional three series of tests were carried out, in which the specimens were subjected to consecutive sequences of bending and torsion blocks of different lengths (short, medium and long, respectively); the relative length of the bending and torsion block in each series was determined in order to produce the same damage. The experimental results, in terms of total damage at failure, were analysed using the Palmgren–Miner hypothesis. For all the assessment methods, the characteristic endurance curves were firstly calibrated on the basis of finite element (FE) analyses and of the experimental results obtained under pure bending and pure torsion loadings. The observed damage at failure resulted always greater than 0.5 for all the employed methods and greater than 1 for most of the tests. The different methods gave similar results, with the critical plane methods giving a slightly more stable damage at failure and a correct determination of the failure location. For all the methods, the damage at failure slightly reduces as the block length shortens.     

Numerical analysis of ductile failure of undermatched interleaf in tension

Ductile failure of an interleaf tension specimen consisting of a metal interleaf bonded between two elastic substrates, with a crack located in the centre of the metal, is studied by means of detailed finite element (FE) analyses. The rate-independent version of the Gurson model is used. This accounts for ductile failure mechanisms of micro-void nucleation, growth and coalescence within the framework of a finite deformation plasticity theory. Also, the rapid evolution of void density due to coalescence, which leads to ultimate failure, is considered. The effect of the interleaf thickness on failure (crack initiation and limited amount of crack growth) is investigated. The results show that the interleaf thickness affects crack initiation only slightly. For all specimens considered, crack initiation takes place at the crack tip. However, after crack initiation, the interleaf thickness affects stress and strain distributions significantly. Reducing the interleaf thickness significantly increases the load-carrying capacity. Moreover, reducing the interleaf thickness increases the maximum hydrostatic stress in the interleaf, which is no longer developed at the crack tip but at a distance far away from the crack tip. The resulting fracture toughness thus decreases as the interleaf thickness decreases. The shielding of the crack tip due to constrained plasticity is observed at higher load levels for interleaf specimens. This revised version was published online in August 2006 with corrections to the Cover Date.     

Rupture strength of welded T-joints on pipes

Results are presented from experimental studies of the rupture strength of models of welded T-joints made of heat-resistant steel. Rupture strength is determined while the joint is subjected to internal pressure. The finite-elements method and a modification of the Neuber method are used as a basis for a new approach to predicting the strength of T-joints under internal pressure during steady-state creep. The calculated results agree satisfactorily with experimental findings.Translated from Problemy Prochnosti, No. 2, pp. 40–45, February, 1992.     

拉伸载荷下碳纤维复合材料T型接头脱粘特性与FBG监测分析

研究了拉伸载荷下碳纤维复合材料(CFRP)T型接头的界面脱粘与裂纹扩展过程。对拉伸载荷下T型接头破坏过程进行数值模拟;在模拟敏感区域布置光纤布拉格光栅(FBG),实时监测界面脱粘的产生及扩展应变特征;使用高速摄像机,捕捉脱粘及裂纹扩展的图像数据。结果表明:T型接头的三角填料区首先出现损伤,裂纹向两个方向扩展。水平方向:向L型层与一型层之间的胶层扩展;竖直方向:向两个L型层之间的胶层扩展。裂纹扩展最终引起结构失效。光纤布拉格光栅中心波长的变化能够在非视觉条件下记录损伤的出现、积累与扩展,可正确预警结构内部损伤的产生,还原裂纹扩展路径。     

强动载荷下焊接钢板力学性能及本构模型研究

为研究强动载荷下船用焊接钢板的力学性能.开展了典型船用焊接钢板母材、焊缝和热影响区的准静态拉伸试验、高温拉伸试验及SHPB动态压缩试验,分析了焊接钢板材料在不同应力状态下的力学行为,基于力学性能试验结果拟合了焊接钢板母材、焊缝和热影响区材料的本构模型.结果 表明:准静态条件下,与母材相比,焊缝和热影响区材料的屈服强度与...     

Numerical analysis of low cycle fatigue for welded joints considering welding residual stress and plastic damage under combined bending and local compressive loads

The numerical analysis of low cycle fatigue of HTS‐A steel welded joints under combined bending and local compressive loads are implemented using the damage mechanics approach. First, a finite element numerical simulation of the welding process is employed to extract the welding residual stresses, which are then imported as initial stresses in the subsequent fatigue analysis. Second, a multiaxial fatigue damage model including damage coupled elasto‐plastic constitutive equations and plastic damage evolution formulation is applied to evaluate the mechanical degradation of the material under biaxial fatigue loads. Further, the fatigue lives of the HTS‐A steel welded joints are computed and compared with the experimental results from literature. A series of predicted load‐life curves clearly illustrates the variation of fatigue lives along with the combined loadings. Finally, the effects of local compression on accumulated plastic strain and fatigue damage are studied in detail. It is revealed that the local compression induces a damage competition between two critical zones.     

Fatigue strength assessment of laser stake‐welded T‐joints subjected to reversed bending

The paper investigates the fatigue strength of laser stake‐welded T‐joints subjected to reversed bending. The fatigue tests are carried out with the load ratio, R ≈ ?0.8. The experimental data is firstly analysed using the nominal stress approach and then by the J‐integral as the local fatigue strength parameter in the finite element (FE) assessment. The nominal stress approach demonstrated that the fatigue strength of the investigated T‐joints is lower than encountered for any other steel joint under reversed tensile loading. The results also showed that the fatigue strength of this joint under the load ratio R ≈ ?0.8 increases with respect to R = 0 bending by 22.6% in the case of the nominal stress approach and 13% in the case of the J‐integral approach. However, the slopes of the fatigue resistance curves for different load ratios appear very similar, suggesting that the load ratio has an insignificant influence to the slope. In contrast to the similar slopes, the scatter indexes were different. The nominal stress approach shows that the scatter index is 3.4 times larger for R ≈ ?0.8 than R = 0 bending. The J‐integral approach showed that the scatter index for R ≈ ?0.8 is only 67% larger than in the R = 0 case because the weld geometry is modelled in the FE analysis.     

Cruciform fillet welded joint fatigue strength improvements by weld metal phase transformations

Arc welding typically generates residual tensile stresses in welded joints, leading to deteriorated fatigue performance of these joints. Volume expansion of the weld metal at high temperatures followed by contraction during cooling induces a local tensile residual stress state. A new type of welding wire capable of inducing a local compressive residual stress state by means of controlled martensitic transformation at relatively low temperatures has been studied, and the effects of the transformation temperature and residual stresses on fatigue strength are discussed. In this study, several LTTW (Low Transformation‐Temperature Welding) wires have been developed and investigated to better characterize the effect of phase transformation on residual stress management in welded joints. Non‐load‐carrying cruciform fillet welded joints were prepared for measurement of residual stresses and fatigue testing. The measurement of the residual stresses of the three designed wires reveals a compressive residual stress near the weld toe. The fatigue properties of the new wires are enhanced compared to a commercially available wire.     

A limit load solution for a highly weld strength undermatched tensile panel with an arbitrary crack

A new plane strain upper bound solution for highly undermatched welded tensile panels with a crack is proposed. A distinguished feature of this solution is that the crack is arbitrarily located within the weld and its shape is also arbitrary, though some restrictions do apply. The latter are explained in detail such that the class of structures for which the solution is applicable is precisely specified. The solution is given in a very simple closed form.     

Interim fatigue design recommendations for fillet welded joints under complex loading

Current fatigue design methods for assessing welded steel structures under complex combined or multiaxial loading are known to be potentially unsafe. This has led to a number of research projects over the past 10 years. Some progress has been made in developing better methods, but they are not yet suitable for general design. This paper presents an interim solution based on a review and analysis of relevant published data; all referring to fatigue failure from a fillet weld toe. These indicate that Eurocode 3/IIW S – N curve FAT80/3 (negative inverse slope of 3) is suitable for combined normal and shear stresses acting in phase, and possibly for out-of-phase (i.e. non-proportional loading) bending and shear if the shear stress is not due to torsion. However, a shallower curve FAT80/5 is necessary for out-of-phase torsion and bending or tension. Both curves are used in conjunction with the nominal maximum principal stress range occurring during the loading cycle.     

Stress concentration factor analysis for welded,notched tubular T-joints under combined axial,bending and dynamic loading

The finite element analysis will be used in this study to predict the location of hot-spot stresses in a welded tubular T-joint. The fillet weld has been modeled all around the joint. Using symmetry, the tubular T-joint is submitted to axial, in-plane bending (IPB) and out-of-plane bending (OPB) loadings. The finite element method analysis shows that stresses are very high on the brace member in the vicinity of the fillet weld and gradually decrease, with a quasi-stable difference, in the direction of the brace extremity. Both on the brace member and along the fillet weld (from crown to saddle), stresses are high at the crown toe, decrease in the middle and increase once again at the saddle point. From a general perspective, this stress distribution analysis reveals that hot-spot stresses (HSS) are located at the crown and saddle points. Dynamic loading greatly increases the stress concentration factor at the hot-spot stress (HSS) located on the brace member where fatigue damage is capable of appearing quickly. In the U-notch, this stress concentration factor (SCF) increases as notch width decreases. In a general way therefore, stress concentration factors decrease on the brace and chord members (in the vicinity of the weld) and increase considerably in the notch, which underscores the deleterious nature of such a defect. Consequently, these zones (HSS) require reinforcement solutions in order to ensure a sufficiently long fatigue life for offshore structures.