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.     

根部成形及错边量对不等壁厚环焊接头局部应变集中行为的影响

目的 探究高钢级管道不等壁厚环焊接头的局部应变集中规律,揭示其局部开裂的根本原因。方法 采用STT半自动根焊+自保护药芯焊丝半自动焊填充、盖面的方式焊接X80管道不等壁厚环,并利用DIC技术对3种不同错边量及根部成形的不等壁厚环焊接头进行全壁厚拉伸,观察其在拉伸过程中的应变规律。结果 不等壁厚环焊接头的应变集中主要发生在薄壁侧根部焊趾与厚壁侧盖面焊趾的连线区域,这是由于不等壁厚环焊接头受拉时会产生附加弯矩,该弯矩与薄壁侧根部焊趾及厚壁侧盖面焊趾处的应力集中相耦合使该区域发生应变集中。另外,错边量的存在会增大附加弯矩并使根部成形变差,因此错边量越大,应变集中区域面积越小、应变集中程度越高。定量分析结果表明,当薄壁侧管体远端应变达到0.5%时,3类成形接头根部最大应变分别为0.83%、9.60%、11.88%,盖面处最大应变分别为1.00%、7.10%、10.60%。结论 大的错边量或差的根部成形会使不等壁厚环焊接头局部出现严重的应变集中,若与焊接缺陷相耦合可能会导致接头局部损伤,为裂纹的萌生与扩展提供条件。因此,在焊接过程中应增加接头的组对精度、减小错边量。     

Residual stress measurements in a cruciform welded joint using hole drilling and strain gauges

Residual stress measurements were made close to the toe of each fillet weld on a load carrying cruciform welded specimen by the hole drilling technique. A standard milling guide with high speed air turbine was used in conjunction with bonded resistance strain gauge rosettes. Experimental calibrations of the method were conducted under known uniform uniaxial tension loads.
A simple incremental drilling technique was used to determine the stress gradient with hole depth. Measurements were also made both at zero load and known tensile loads in order to study the interaction between residual stresses and the applied loads.     

Anwendung des Kerbspannungskonzeptes auf die reale Geometrie von Schweißnahtenden

The process used to manufacture modern weld structures often lead to weld start and end points. Under cyclic loading, crack initiation occurs predominantly at the weld start. In particular, for special investigations into thin sheet structures, no approach for the determination of the fatigue life has been established thus far. Therefore, in this research, we have attempted to find a general approach for the assessment of weld ends using the notch stress concept. The primary aim was to obtain the real geometry of weld ends with high precision using a three dimensional scanner. We were able to find an idealised weld end model with the mean toe radius of r = 0,2 mm that provides almost the same notch stresses like the real geometry. The applied load cases were longitudinal and transverse tension as well as bending. The crack always occurs at the weld toe. The notch stress S‐N‐curve was derived from the fatigue life of the cyclic testing so that the assessment of weld ends is possible for the first time. Moreover, some conversion rules have been developed so that the idealised weld end geometry can be modelled with an arbitrary toe radius. Going one step further, some analyses have been performed regarding to unify the notch stress concept. The existing results of Olivier – who examined long welds with no start and end points – were re‐evaluated to unify the results of long regular welds with the local weld end under one scatter band. Using the scanned geometry a good agreement could be achieved. The use of an idealised model will be validated soon.     

Fatigue behaviour of welded joints with cracks, repaired by hammer peening

Rehabilitation of a welded structure, which involves repair of cracked joints, is achieved when the local treatment for repair gives a fatigue strength in the joint equal or above the fatigue strength of the uncracked original detail. If the treatment is properly applied the rehabilitation of the detail is assured, and the nature of the weld toe improvement methods can produce a joint, after repair, with a fatigue strength and residual life greater than the initial detail. The paper presents the results obtained on a fatigue study on the rehabilitation of non‐load carrying fillet welded joints loaded in bending at the main plate and with fatigue cracking at the weld toes of the attachment in the main plate and though the plate thickness. Residual stresses were measured at the surface, with X‐ray diffraction. The residual stresses induced by hammer peening at the weld toe were found to be greater along the longitudinal direction of the plate than in the transverse direction. The peak residual stresses near the weld toe were found to be close to yield in compression, justifying the great benefit of hammer peening. Results of a derived gain factor, g, in fatigue life were obtained as a function of the crack depth repaired by hammer peening.     

Local strain energy density and fatigue strength of welded joints under uniaxial and multiaxial loading

In the notch stress intensity approach to the fatigue assessment of welded joints, the weld toe is modelled as a sharp V-notch and the local stress distributions in plane problems are given on the basis of the relevant mode I and mode II notch stress intensity factors (N-SIFs). These factors quantify the magnitude of asymptotic stress distribution obeying Williams’ solution. If the V-notch opening angle at the weld toe is constant and the mode II is not singular, the mode I N-SIF can be directly used to summarize the fatigue behaviour of welded joints. In all the other cases, varying the V-notch angle or including multiaxial loading conditions (where typically both Mode I and Mode III stress distributions are singular), the synthesis can be carried out on the basis of the mean value of the strain energy density over a well-defined volume surrounding the weld toe or the weld root. By using this scalar quantity, two fatigue scatterbands are obtained for structural steels and aluminium alloys, respectively. The material-dependent radius R_C of the control volume (area) is carefully identified with reference to conventional arc welding processes.Sometimes the weld toe radius is found to be very different from zero. The local strain energy approach can be extended as it stands also to these cases, providing a gradual transition from a N-SIF-based approach to a K_t-based approach.     

随焊旋转冲击控制焊接变形新方法

提出了旋转冲击控制应力变形的新方法,并研制了一套旋转冲击装置.工作时通过电锤带动装配在其上的压杆施加旋转冲击作用于焊缝及焊趾部位,使其延展产生拉伸塑性应变来抵消焊接过程产生的残余压缩塑性应变,从而有效的控制了工件的焊接变形.应用所研制的装置,对2A12T4铝合金进行控制焊接变形的试验,发现旋转冲击可以起到减小焊后工件中残余变形、细化焊缝晶粒、提高强度的作用.     

A notch stress intensity approach applied to fatigue life predictions of welded joints with different local toe geometry

In the Notch Stress Intensity Factor (N‐SIF) approach the weld toe region is modelled as a sharp V‐shaped corner and local stress distributions in planar problems can be expressed in closed form on the basis of the relevant mode I and mode II N‐SIFs. Initially thought of as parameters suitable for quantifying only the crack initiation life, N‐SIFs were shown able to predict also the total fatigue life, at least when a large part of the life is spent as in the propagation of small cracks in the highly stressed region close to the notch tip. While the assumption of a welded toe radius equal to zero seems to be reasonable in many cases of practical interest, it is well known that some welding procedures are able to assure the presence of a mean value of the weld toe radius substantially different from zero. Under such conditions any N‐SIF‐based prediction is expected to underestimate the fatigue life. In order to investigate the degree of conservatism, a total of 128 fillet welded specimens are re‐analysed in the present work by using an energy‐based N‐SIF approach. The local weld toe geometry, characterised by its angle and radius, has been measured with accuracy for the actual test series. The aim of the work is to determine if the N‐SIF‐based model is capable of taking into account the large variability of the toe angle, and to quantify the inaccuracy in the predictions due to the simplification of setting the toe radius equal to zero.     

Improving fatigue life for aluminium cruciform joints by weld toe grinding

The 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.     

Fatigue strength of steel rollers with failure occurring at the weld root based on the local strain energy values: modelling and fatigue assessment

Weldments geometry with failures occurring at the weld toe or at the weld root cannot, by its nature, be precisely defined. Parameters such as bead shape and toe or root radius vary from joint to joint even in well-controlled manufacturing operations. The worst case configuration can be achieved by modelling as a sharp, zero radius, notch both the toe and the weld root. The intensity of asymptotic stress distributions obeying Williams’ solution is quantified by means of the Notch Stress Intensity Factors (NSIFs). For steel welded joints with failures originated from the weld roots, where the lack of penetration zone is treated as a crack-like notch, units for NSIFs are the same as conventional SIFs used in LEFM. The different dimensionality of NSIFs for different notch opening angles does not allow a direct comparison of failures occurring at the weld toe or at the weld root. In order to overcome the problem related to the variability of the V-notch opening angle, a simple scalar quantity, i.e. the value of the strain energy density (SED) averaged in the structural volume surrounding the notch tip, has been introduced. This energy is given in closed form on the basis of the relevant NSIFs for modes I, II and III. The radius R_c of the averaging zone is carefully identified with reference to conventional arc welding processes being equal to 0.28 mm for welded joints made of steel.The local-energy based criterion is applied here to steel welded rollers produced by Rulmeca and subjected to prevailing mode I (with failures at the weld root). The aim of the paper is firstly to describe the employed methodology for the fatigue assessment and secondly to show the first synthesis of fatigue data by means of local SED for a specific geometry.     

Fatigue analysis of non-load-carrying fillet welded cruciform joints

The goal of this investigation was to study the effect of local geometrical variations of the weld on the fatigue strength. Therefore the fatigue behaviour of non-load-carrying cruciform fillet welded joint under tensile loading has been studied parametrically. Several two-dimensional (2D) finite element models of the joint were analysed using plane strain linear elastic fracture mechanics (LEFM) calculations in order to get the magnification function M_k. A maximum tangential stress criterion was used to predict the crack growth direction under mixed mode K_I-K_II conditions. The derived M_k solution was then applied both for continuous weld toe cracks and also for semi-elliptical toe cracks at the deepest point of the crack front. An experimental crack aspect ratio development curve was used for propagating semi-elliptical cracks. The as-welded condition was assumed with the result that no crack initiation period was considered and stress ranges were fully effective. The Paris crack growth law was used to predict the growth rate. The effects of weld toe radius, flank angle and weld size on the fatigue strength were systematically studied. Finally, predicted fatigue strength values corresponding to different assumed crack sizes were compared with the available test results.     

Importance of toe irregularity for fatigue resistance of automatic welds

The early propagation stages of fatigue cracks along the toe of automatic bead-on-plate welds in a structural steel are studied. Three methods to introduce a controlled degree of waviness on the weld toe, to optimize the degree of crack interaction during fatigue growth, were tested: arc rotation, variable arc voltage and variable weld speed. The development of fatigue cracks in seven specimens was monitored using a strain-gauge method and beach marking. Crack mismatch and depth to coalescence were found to be much smaller in the case of straight welds, which also showed shorter propagation lives. The period of toe waves, as well as local toe geometry, strongly influence fatigue crack initiation and propagation lives. Best fatigue life improvements were obtained with arc rotation techniques.     

Stress intensity factor expression for center-cracked butt joint considering the effect of joint shape

In this paper, the stress intensity factor (SIF) expression for center-cracked butt joint under tensile load was derived and the SIF differences with or without considering weld reinforcement were discussed. For the sake of discussion, the curve of weld reinforcement is regarded as circle, the reinforcement circle and the other two weld toe smooth transition circles are assumed to be tangential. The influences of such joint shape parameters as weld width, weld toe smooth transition radius and base metal thickness on SIF were also studied. The relationship between SIF for center-cracked butt joint and its shape parameters was finally obtained based on the integration of analytical and finite element method. It showed that it is wrong to perform failure analysis on center-cracked butt joint without considering weld reinforcement, the derivation method of SIF developed here can be use to design the shape of the butt joint with defects under static loads or fatigue condition.     

Stress intensity factors for three‐dimensional weld toe cracks using weld toe magnification factors

In welded components, particularly those with complex geometrical shapes, evaluating stress intensity factors is a difficult task. To effectively calculate the stress intensity factors, a weld toe magnification factor is introduced that can be derived from data obtained in a parametric study performed by finite element method (FEM). Although solutions for the weld toe magnification factor have been presented, these are applicable only to non‐load‐carrying cruciform or T‐butt joints, due possibly to the requirement of very complicated calculations. In the majority of cases for various welded joints, the currently used weld toe magnification factors do not adequately describe the behaviour of weld toe cracks. In this study, the weld toe magnification factor solutions for the three types of welded joints such as cruciform, cover plate and longitudinal stiffener joints were provided through a parametric study using three‐dimensional finite elements. The solutions were formed with exponents and fractions that have polynomial functions in terms of a/c and a/t – that is, crack depths normalised by corresponding half crack lengths and specimen thickness. The proposed weld toe magnification factors were applied to evaluate the fatigue crack propagation life considering the propagation mechanisms of multiple‐surface cracks for all welded joints. It showed good agreement within a deviation factor of two between the experimental and calculated results for the fatigue crack propagation life.     

Fatigue failure transition analysis in load‐carrying cruciform welded joints based on strain energy density approach

This paper details a study of the application of notch stress intensity theory to the fatigue failure mode analysis of the transition in load‐carrying cruciform welded joints. The weldment fatigue crack initiation point is difficult to predict precisely because it usually occurs in the vicinity of the weld toe or weld root. To investigate the relationship between fatigue failure location and the geometry of the weldments, we analysed the weld toe and root asymptotic notch stress fields were analysed using the notch stress intensity factors on the basis of the Williams' solution in Linear Elastic Fracture Mechanics (LEFM). Numerous configurations of cruciform joints of various plate thicknesses, transverse plate thickness, weld sizes and incomplete penetration size were used to investigate the location of the fatigue failure. The strain energy density (SED) surrounding the notch tip was introduced to unify the scalar quantity and preclude the inconsistency of the dimensionality of the notch stress intensity factors for various notch opening angles. The results of the investigation showed that the SED approach can be used to determine the transition zone for a variety of joint geometries. The validity of the SED criteria was verified by comparing the experimental results of this study with the complied results for load‐carrying cruciform welded joints reported in literature.     

A notch stress intensity approach to assess the multiaxial fatigue strength of welded tube-to-flange joints subjected to combined loadings

Full penetration T butt weld joints between a tube and its flange are considered, subjected to pure bending, pure torsion and a combination of these loading modes. The model treats the weld toe like a sharp V‐notch, in which mode I and mode III stress distributions are combined to give an equivalent notch stress intensity factor (N‐SIF) and assess the high cycle fatigue strength of the welded joints. The N‐SIF‐based approach is then extended to low/medium cycle fatigue, considering fatigue curves for pure bending and pure torsion having the same slope or, alternatively, different slopes. The expression for the equivalent N‐SIF is justified on the basis of the variation of the deviatoric strain energy in a small volume of material surrounding the weld toe. The energy is averaged in a critical volume of radius R_C and given in closed form as a function of the mode I and mode III N‐SIFs. The value of R_C is explicitly referred to high cycle fatigue conditions, the material being modelled as isotropic and linear elastic. R_C is thought of as a material property, independent in principle of the nominal load ratio. To validate the proposal, several experimental data taken from the literature are re‐analysed. Such data were obtained by testing under pure bending, pure torsion and combined bending and torsion, welded joints made of fine‐grained Fe E 460 steel and of age‐hardened AlSi1MgMn aluminium alloy. Under high cycle fatigue conditions the critical radius R_C was found to be close to 0.40 mm for welded joints made of Fe E 460 steel and close to 0.10 mm for those made of AlSi1MgMn alloy. Under low/medium cycle fatigue, the expression for energy has been modified by using directly the experimental slopes of the pure bending and pure torsion fatigue curves.     

Statistical analysis of the surface fatigue crack growth in welded joints

A method of analysis based on probabilistic theory is presented for statistical modeling of the surface fatigue crack growth (SFCG) at the weld toe. A procedure to estimate parameters in the model is illustrated, using the experimental data of the SFCG at the weld toe obtained from a replicate test program of A131 steel butt joint specimens under constant amplitude loadings. The statistical nature of the SFCG at the weld toe is investigated. The limiting state of the surface fatigue crack growth at the weld toe is constructed, and the residual life distribution function for the SFCG at the weld toe is derived     

Fatigue properties of arc-welded lap joints with weld start and end points

Fatigue properties of arc‐welded lap joints with weld start and end points were investigated through experiments with 2.3‐mm and 3.2‐mm thick 440 MPa‐class steel sheets. Macroscopic fatigue crack‐initiation sites depended on the length of the weld bead to the specimen width. In joints with shorter weld beads, cracks mainly initiated at the toe of the weld start points, while joints with longer beads had initial cracks at the toe of the bead centre. Crack‐propagation analyses, taking stress distribution around the weld toe and residual stress into account, suggested that residual stress distribution could move crack‐initiation sites from the weld start point to the bead centre, although the applied stress at the toe of the weld start point remains the highest.     

The relative effects of residual stresses and weld toe geometry on fatigue life of weldments

The weld toe is one of the most probable fatigue crack initiation sites in welded components. In this paper, the relative influences of residual stresses and weld toe geometry on the fatigue life of cruciform welds was studied. Fatigue strength of cruciform welds produced using Low Transformation Temperature (LTT) filler material has been compared to that of welds produced with a conventional filler material. LTT welds had higher fatigue strength than conventional welds. A moderate decrease in residual stress of about 15% at the 300 MPa stress level had the same effect on fatigue strength as increasing the weld toe radius by approximately 85% from 1.4 mm to 2.6 mm. It was concluded that residual stress had a relatively larger influence than the weld toe geometry on fatigue strength.     

STRESS INTENSITY FACTOR SOLUTIONS FOR SEMI-ELLIPTICAL WELD-TOE CRACKS IN T-BUTT GEOMETRIES

Abstract— Weld toe magnification factors are widely used in the evaluation of stress intensity factors for cracks in welded structures. Traditionally, the weld magnification factor has been determined from 2-D plane strain models containing edge cracks. However, it has long been recognised that a semi-elliptical weld toe crack cannot be accurately represented by a 2-D approximation due to the 3-D nature of the geometry. As a consequence, some recent research has been carried out using 3-D numerical modelling, which highlights the limitations of the 2-D approach. Nevertheless, 3-D solutions are still scarce and are of limited validity due to the difficulties associated with creating the numerical models. This paper reports the most extensive 3-D numerical investigation of semi-elliptical cracks in T-butt geometries to date. Based on the numerical results, new and accurate equations for weld magnification factors were derived, which quantify the 3-D effects present and emphasise the importance of the attachment. The results obtained from these equations are then used in an assessment of existing solutions.